Poster Session BTP2

Introduction:

The cause of osteolysis in a hard-on-polymer artificial joint is the discharge of fine polyethylene wear particles with micron or submicron sizes from the polyethylene that stimulates macrophage activity and promotes the secretion of cytokines. In general assay procedures, the macrophages were cultured in the presence of the UHMWPE particles in the well plates and the culture supernatants were then used for the measurement of cytokines based on the enzyme-linked immunosorbent assay (ELISA).

In this report, for a highly dense culture between the macrophages and the microscopic particles, the fabrication of the microfluidic device for secretions of inflammatory cytokines from human monocyte-derived macrophage (HMDM) is explained, and a sequence of reliability verification tests is performed.

Methods:

Figure 1 shows the fabrication process of the microfluidic device. The device consists of a narrow flow channel (dimensions of 200 × 500 μm), wherein a highly dense culture of human monocyte-derived macrophages (HMDM) and microscopic particles is expected to be formed. Fluorescent polystyrene particles (0.051, 0.20, 0.49, 1.1, 3.2 μm in diameter) were used as the microscopic particles in order to confirm the macrophage phagocytosis. The culture medium mixed with the microscopic particles was injected into the microfluidic device. The culture medium was collected, and an enzyme-linked immunosorbent assay (ELISA) was performed to investigate the production of pro-inflammatory cytokines by HMDMs that phagocytosed the microscopic particles. Pro-inflammatory cytokines, namely, TNF-α and IL-6, were measured with the sandwich method with ELISA kits.

Results and discussion:

Despite the smaller volume of the culture medium with microscopic particles compared with that used in a conventional well-plate, the secretions patterns of TNF-α and IL-6 were confirmed. I t was confirmed that the microfluidic device was able to provide the culture environment for the secretion of inflammatory cytokines.

Figure 1: Fabrication of microfluidic device used for the secretion of inflammatory cytokines.

 Introduction:

The cause of osteolysis in a hard-on-polymer artificial joint is the discharge of fine polyethylene wear particles with micron or submicron sizes from the polyethylene that stimulates macrophage activity and promotes the secretion of cytokines. In general assay procedures, the macrophages were cultured in the presence of the UHMWPE particles in the well plates and the culture supernatants were then used for the measurement of cytokines based on the enzyme-linked immunosorbent assay (ELISA).

In this report, for a highly dense culture between the macrophages and the microscopic particles, the fabrication of the microfluidic device for secretions of inflammatory cytokines from human monocyte-derived macrophage (HMDM) is explained, and a sequence of reliability verification tests is performed.

Methods:

Figure 1 shows the fabrication process of the microfluidic device. The device consists of a narrow flow channel (dimensions of 200 × 500 μm), wherein a highly dense culture of human monocyte-derived macrophages (HMDM) and microscopic particles is expected to be formed. Fluorescent polystyrene particles (0.051, 0.20, 0.49, 1.1, 3.2 μm in diameter) were used as the microscopic particles in order to confirm the macrophage phagocytosis. The culture medium mixed with the microscopic particles was injected into the microfluidic device. The culture medium was collected, and an enzyme-linked immunosorbent assay (ELISA) was performed to investigate the production of pro-inflammatory cytokines by HMDMs that phagocytosed the microscopic particles. Pro-inflammatory cytokines, namely, TNF-α and IL-6, were measured with the sandwich method with ELISA kits.

Results and discussion:

Despite the smaller volume of the culture medium with microscopic particles compared with that used in a conventional well-plate, the secretions patterns of TNF-α and IL-6 were confirmed. I t was confirmed that the microfluidic device was able to provide the culture environment for the secretion of inflammatory cytokines.

Often due to physical discomfort, workers wear hearing protectors incorrectly or only part of the time they are exposed to hazardous noise. Thus, the effective level of protection is greatly reduced. It has been reported that the primary source of physical discomfort when wearing polymer foam earplugs is friction.

When a polymer foam earplug is inserted into the ear it only contacts skin of the cartilaginous portion of the ear canal. The skin of this portion of the ear is covered in tiny hairs that contain modified apocrine glands that produce sweat and sebaceous glands that produce sebum. Together, these substances make up cerumen which serves to clean, lubricate and protect the ear from bacteria and fungus.

The objective of this study was to investigate whether the presence of cerumen reduces the amount of friction at the earplug-skin interface.

Anhydrous lanolin was used to simulate cerumen because it exhibits similar shear thinning properties. An existing test set-up was adapted to measure the dynamic friction coefficient (µ) of the volar forearm sliding against four different polymer foam earplugs that were mounted on a Perspex block, attached to a multi-axis force plate. This allowed normal and shear loading to be recorded during a reciprocating motion of the forearm.

Data presented in Figure 1 displays the normal force applied and frictional forces measured as the earplugs were rubbed against the volar forearm. This particular test (5N normal force) resulted in the lowest µ due to the shear thinning behaviour of lanolin.

At each applied load (1N, 2N and 5N) the presence of 1g of anhydrous lanolin reduced µ significantly. At 1N, 2N and 5N the recorded percentage reduction in µ was 51%, 72% and 75% respectively. As displayed by this marked improvement in friction coefficient measurements the outcome of this study was the determination that cerumen is an effective lubricant.

Often due to physical discomfort, workers wear hearing protectors incorrectly or only part of the time they are exposed to hazardous noise. Thus, the effective level of protection is greatly reduced. It has been reported that the primary source of physical discomfort when wearing polymer foam earplugs is friction.

When a polymer foam earplug is inserted into the ear it only contacts skin of the cartilaginous portion of the ear canal. The skin of this portion of the ear is covered in tiny hairs that contain apocrine glands that produce sweat and sebaceous glands that produce sebum. Together, these substances make up cerumen which serves to clean, lubricate and protect the ear from bacteria and fungus.

The objective of this study was to investigate whether the presence of cerumen reduces the amount of friction at the earplug-skin interface.

Anhydrous lanolin was used to simulate cerumen because it exhibits similar shear thinning properties. An existing test set-up was adapted to measure the dynamic friction coefficient (µ) of the volar forearm sliding against different polymer foam earplugs that were mounted on a Perspex block, attached to a multi-axis force plate. This allowed normal and shear loading to be recorded during a reciprocating motion of the forearm.

Data presented in Figure 1 displays the normal force applied and frictional forces measured as the earplugs were rubbed against the volar forearm. This particular test (5N normal force) resulted in the lowest µ due to the shear thinning behaviour of lanolin.

At each applied load (1N, 2N and 5N) the presence of 1g of anhydrous lanolin reduced µ significantly. At 1N, 2N and 5N the percentage reduction in µ was 51%, 72% and 75% respectively. As displayed by the improvement in friction coefficient measurements the outcome of this study was that cerumen is an effective lubricant.

Introduction/Purpose: Extrinsic tooth discoloration is a result of build-up of stain compounds (chromogens from e.g. coffee, tea, tobacco, red wine), which are incorporated into the pellicle (a protein-rich film covering the tooth enamel surface). To support the development of next generation stain removal products with superior performance over existing solutions (Fig. 1, [1, 2]), there is a need for an objective test methodology to investigate the mechanical mechanisms of pellicle-stain removal and to identify critical design parameters driving the stain removal process. Objectives of the work were (1) to develop a suitable pellicle-stain model for brushing tribology experiments and (2) to study whether pellicle-stain removal can be explained by friction parameters.

Fig. 1. Sonicare DiamondClean power toothbrush with clinically proven stain removal effectiveness superior to a manual toothbrush [1]. Example of surface stain removal after 2 weeks [2].

Methods:

A unique in vitro pellicle-stain model (Fig. 2) and test capabilities were developed to assess the removal of stain in brushing tribology experiments using a single tuft in a sliding regime (Fig. 3). The model allows studying friction interactions with toothpaste slurries. Cleaning efficacy is expressed as color difference as measured using a CCD camera. Friction coefficients were determined from the ratio of friction to normal forces. Mean frictional power was calculated by numerical time integration of transient friction forces over transient sliding velocity (i.e., total sliding distance divided by total sliding time).

Results:

• The developed pellicle-stain model is based on black tea extract and human saliva which is coated on polymer sheets. Samples showed a large range of biologically realistic shades (Fig. 2).
• The measured color difference between pre- and post-brushing tribology experiments was used to assess the stain removal (cleaning efficacy) of different brush configurations (nylon tufts varying in angle, bristle length, diameter) and sliding conditions (number of rotations) (Fig. 3).
• The results show that stain removal can be systematically explained by friction parameters. Measured friction coefficients ranged from 0.2-0.7. The colour difference was found to increase linearly with the friction coefficient (R² > 0.65) and frictional power (energy dissipation rate) (Fig. 4).

Fig. 2. Left: In vitro stain model compared to a clinical shade guides. Right: Schematic of stain model.

Fig. 3. Stain removal in single tuft brushing experiment using a CETR UMT tribometer with rotary stage

Fig. 4. Relationship between frictional power and stain removal (overall R²=0.72, black line, condition 1: R²=0.93, condition 2: R²= 0.95, p <0.05).

Discussion/Conclusion:

A first-of-its-kind tribology method for studying interactions between a single tuft and pellicle-stain was successfully implemented. The presented method can be further used to generate understanding of stain-bristle interactions to identify critical parameters and derive design guidelines for novel stain removal brush heads. In the future, the pellicle-stain model system may be also used as a valuable tool to generate in vitro claims for new oral healthcare devices.

[1] Colgan et al., Evaluation of stain removal by Philips Sonicare DiamondClean power toothbrush and manual toothbrushes, J Dent Res 92(Spec Iss A):177745, 2013.

[2] Master et al., Evaluation of tooth shade change following stain induction and Sonicare FlexCare use, J Dent Res 88 (spec Iss A): 2581, 2009.

Introduction: Extrinsic tooth discoloration is a result of build-up of stain compounds, which are incorporated into the pellicle (a protein-rich film covering the tooth surface). To support the development of next generation stain removal products with superior performance, there is a need for an objective test methodology to investigate the mechanical mechanisms of pellicle-stain removal and to identify critical design parameters driving the stain removal process.
Methods:
A unique in vitro pellicle-stain model was developed to assess the removal of stain in single tuft sliding brushing tribology experiments. The model allows studying friction interactions with toothpaste slurries. Cleaning efficacy is expressed as color difference measured using a CCD camera. Friction coefficients were determined from the ratio of friction to normal forces. Mean frictional power was calculated by numerical time integration of transient friction forces over sliding velocity.
Results:
The developed pellicle-stain model is based on black tea extract and human saliva which is coated on polymer sheets. Samples showed a large range of biologically realistic shades. The measured color difference between pre- and post-brushing tribology experiments was used to assess the stain removal of different Nylon tuft configurations (varying in angle, bristle length, diameter) and sliding conditions. The results show that stain removal can be systematically explained by friction parameters. Measured friction coefficients ranged from 0.2-0.7. Colour difference was found to increase linearly with the friction coefficient and frictional power.
Discussion/Conclusion:
A first-of-its-kind tribology method for studying interactions between a single tuft and pellicle-stain was successfully implemented. This method can be further used to generate understanding of stain-bristle interactions to identify critical parameters and derive design guidelines for novel stain removal brush heads. In the future, the pellicle-stain model system may be also used as a valuable tool to generate in vitro claims for new oral healthcare devices.

Soft surfaces and lubricants play a key role in situations where food particles are lubricating the in-mouth surfaces, in skin lubrication by skin care products and even in human joints. Soft tribology is known to be rather complex, especially due to the deformable nature of surfaces and of (semi) solid lubricants which directly affects the contact area. In many of these cases there are particles between the surfaces, so deformation of both the surfaces and the particles may take place. To better understand what happens when deformation in both particles and substrates is present in sliding, lubricated surfaces, we have designed two model systems: (I) soft hydrogel microparticles between relatively hard surfaces (Rudge et al., Soft Matter, 2020). (II) hard glass particles between relatively soft elastomeric surfaces. We vary particle deformability (hardness), particle packing fraction and particle size along with measuring parameters such as normal force, speed and type of frictional motion (e.g. rotational, linear). To change such measuring parameters, we use different commercially available tribometers (Anton Paar, Bruker, PCS Instruments) and a custom-made 3D printed tribotool.

Our tribosystems do not display the classical speed-dependent Stribeck behavior. Often we find four different regimes instead of the usual three (boundary, mixed, hydrodynamic), and we attribute the emergence of a fourth regime to surface and particle deformation. We conjecture that when particles are able to roll between the surfaces, the friction coefficients remain low (rolling friction). A sudden increase in friction is found when particles or surfaces are deformed in such a way that surface contact and sliding friction occurs. This transition from rolling ball bearings to sliding surfaces becomes even more evident when rough surfaces are used. In this case, particle entrapment takes place, leading to larger contact areas and lower rolling ability, which gives higher friction coefficients. Our work thus provides new insights into how surface-surface and particle-surface interactions can be used to control soft material friction.

Soft surfaces and lubricants play a key role in situations where food particles are lubricating the in-mouth surfaces, in skin lubrication by skin care products and even in human joints. Soft tribology is known to be complex, especially due to the deformable nature of surfaces and of (semi) solid lubricants which directly affects the contact area. In many of these cases there are particles between the surfaces, so deformation of both the surfaces and particles may take place. To better understand what happens when deformation in both particles and substrates is present in sliding, lubricated surfaces, we designed two model systems: (I) soft hydrogel microparticles between relatively hard surfaces (Rudge et al., Soft Matter, 2020). (II) hard glass particles between relatively soft surfaces. We vary particle deformability (hardness), particle packing fraction and particle size along with measuring parameters such as normal force, speed and type of frictional motion. We change measuring parameters using different commercial tribometers (Anton Paar, Bruker, PCS Instruments) and a custom-made 3D printed tribotool.

Our tribosystems do not display classical Stribeck behavior. Often we find four different regimes instead of the usual three (boundary, mixed, hydrodynamic), and we attribute the emergence of a fourth regime to surface and particle deformation. We conjecture that when particles are able to roll between the surfaces, the friction coefficients remain low (rolling friction). A sudden increase in friction is found when particles or surfaces are deformed in such a way that surface contact and sliding friction occurs. This transition from rolling ball bearings to sliding surfaces becomes more evident with rough surfaces. In this case, particle entrapment takes place, leading to larger contact areas and lower rolling ability, and higher friction coefficients. Our work provides new insights into how surface-surface and particle-surface interactions can be used to control soft material friction.

X-ray tomography, atomic force microscopy (AFM), a nanotribometer and a micromanipulator were used to quantitatively characterize the artificial cartilage samples that were compare with cartilage samples collected from human patients. Evaluated by X-ray tomography and scanning electron microscopy, the porosity of PDMS platform was increased from 54.6%±1.1% to 81.7%±0.7% upon the addition of NaCl micro-crystals, which is consistent with the porosity of natural cartilage, as reported in the literature. More importantly, the synergistic effect of porosity and crosslinking density enabled us to vary the Young’s modulus of PDMS matrix from 1.503±0.026 MPa to 0.070±0.003 MPa, which covers the entire range of modulus acquired from articular cartilage previously^[1]. Tribological evaluated with coefficient of friction range from 0.0335 ± 0.0025 to 0.1333 ± 0.0097. We demonstrated the feasibility of fabricating a suite of artificial cartilage samples by manipulating the fraction of crosslinking agent and foaming agent, which results in PDMS samples possessing a broad range of structural and mechanical properties. This would server satisfactorily a synthetic platform to replicate cartilage of different stages of OA, and develop corresponding physical intervention strategies with pre-determined characteristic.

Figure: 3D structures of PDMS ratio of NaCl 2:1(left) and 8:1(right), and PDMS ratio of crosslinking are both 5:1.

1.Kleemann, R.U., et al., Altered cartilage mechanics and histology in knee osteoarthritis: relation to clinical assessment (ICRS Grade). (1063-4584 (Print)).

We have developed an artificial cartilage platform, building upon polydimethylsiloxane (PDMS) matrix, with controlled porosity and mechanical properties, to replicate the natural cartilages of different stages of osteoarthritis (OA). This was achieved by manipulating the molar ratio of PDMS with crosslinking agent and weight ratio of PDMS with forming agent respectively.

X-ray tomography, atomic force microscopy (AFM), a nanotribometer and a micromanipulator were used to quantitatively characterize the artificial cartilage samples that were compare with cartilage samples collected from human patients. Evaluated by X-ray tomography and scanning electron microscopy, the porosity of PDMS platform was increased from 54.6%±1.1% to 81.7%±0.7% upon the addition of NaCl micro-crystals, which is consistent with the porosity of natural cartilage, as reported in the literature. More importantly, the synergistic effect of porosity and crosslinking density enabled us to vary the Young's modulus of PDMS matrix from 1.503±0.026 MPa to 0.070±0.003 MPa, which covers the entire range of modulus acquired from articular cartilage previously[1]. Tribological evaluated with coefficient of friction range from 0.0335 ± 0.0025 to 0.1333 ± 0.0097. We demonstrated the feasibility of fabricating a suite of artificial cartilage samples by manipulating the fraction of crosslinking agent and foaming agent, which results in PDMS samples possessing a broad range of structural and mechanical properties. This would server satisfactorily a synthetic platform to replicate cartilage of different stages of OA, and develop corresponding physical intervention strategies with pre-determined characteristic.

The replacement of oil-based lubricants with more sustainable water-based lubricants has been a long-standing unfulfilled ambition. A water-based lubricant can provide several advantages over traditional lubricants such as a high thermal conductivity, high electrical conductivity, high cooling capacity and low viscosity However, the physical instabilities and poor wear performance associated with water-based lubricants has led to minimal adoption within mechanical systems. This need for stability has led to the exploration of nanoencapsulation as a means to provide longevity and improved lubrication performance for water-based lubricants.

The tribological performance of water-based lubricants was investigated using the High Frequency Reciprocation Rig and the Elastohydrodynamic Rig, to ascertain the friction levels attainable, the resulting wear rates and to visualise the lubrication mechanisms occurring.

Figure 1: Comparison of friction coefficients for 50Hz HFRR tests

Figure 2: Wear volume comparisons for test lubricants

Figures 1 and 2 show the range of biolubricant compositions investigated to determine suitable encapsulation compositions. These investigations provide a baseline insight into potential avenues to explore to decrease the performance gap between water- based lubricants and oil-based lubricants. Though currently no pure solution is shown to rival oil, with further development and novel implementation the overall performance for wear and friction coefficients may be improved.

It was hypothesised that through encapsulating biodegradable polymeric/biological additives within micelles a local lubrication system can be implemented to improve and sustain long-term lubrication. As the additives that adhere to the contacting surfaces gradually breakdown and wear away by shear, the release of further additives from sheared nanocapsules at the contact provides a replenishing source to continue lubrication. This novel mechanism allows for the creation of a stable, effective and sustainable biolubricant for application in multiple systems.

The replacement of oil-based lubricants with more sustainable water-based lubricants has been a long-standing unfulfilled ambition. A water-based lubricant can provide several advantages over traditional lubricants such as a high thermal conductivity, high electrical conductivity, high cooling capacity and low viscosity However, the physical instabilities and poor wear performance associated with water-based lubricants has led to minimal adoption within mechanical systems. This need for stability has led to the exploration of nanoencapsulation as a means to provide longevity and improved lubrication performance for water-based lubricants.

The tribological performance of water-based lubricants was investigated using the High Frequency Reciprocation Rig and the Elastohydrodynamic Rig, to ascertain the friction levels attainable, the resulting wear rates and to visualise the lubrication mechanisms occurring

A range of biolubricant compositions were investigated to determine suitable encapsulation compositions. These investigations provide a baseline insight into potential avenues to explore to decrease the performance gap between water- based lubricants and oil-based lubricants. Though currently no pure solution is shown to rival oil, with further development and novel implementation the overall performance for wear and friction coefficients may be improved.
It was hypothesised that through encapsulating biodegradable polymeric/biological additives within micelles a local lubrication system can be implemented to improve and sustain long-term lubrication. As the additives that adhere to the contacting surfaces gradually breakdown and wear away by shear, the release of further additives from sheared nanocapsules at the contact provides a replenishing source to continue lubrication. This novel mechanism allows for the creation of a stable, effective and sustainable biolubricant for application in multiple systems.

Food industries have a long-standing interest in increasing the protein content of food products, however, increasing protein concentration often generates undesirable mouthfeel. Although the specific mechanisms of such adverse sensorial perception remain poorly understood, we hypothesize that it is mechanistically linked to electrostatic interaction between protein and saliva during oral processing. To test this hypothesis, we used varying concentrations (0.1-10 wt%) of model dairy proteins and investigated their physicochemical and material properties in the presence of model saliva containing purified bovine submaxillary mucin1 (BSM) at 37 ○C. Soft tribology was performed using glass-polydimethylsiloxane (PDMS) contact surfaces to analyse the lubrication behaviour of proteins, model saliva and protein-saliva mixtures (1:1 to 1:4 w/w) at 2 N load. Both model saliva and proteins reduced friction coefficients in the boundary regime as compared to the buffer. Interestingly, an enhanced lubrication behaviour was observed in protein + model saliva mixture, possibly associated with electrostatic repulsion of saliva (ζ-potential – 5 mV) with the protein at neutral pH (ζ-potential – 23.5 mV), resulting in forming a thicker locally phase-separated lubricating film in the contact zone. Ongoing experiments will assess these interactions further.

Acknowledgement: Funding from the BBSRC-Mondelez DTP is acknowledged.

Reference:

¹ A. Sarkar, F. Xu, S. Lee (2019). Advances in Colloid and Interface Science, 273, Article No. 102034

² F. Xu, E. Liamas, M. Bryant, A. F. Adedeji, E. Andablo-Reyes, M. Castronovo, R. Ettelaie, T. V. J. Charpentier, S. Lee A. Sarkar, (2020). Advanced Materials Interfaces, 7, Article No. 1901549

Food industries have a long-standing interest in increasing the protein content of food products, however, increasing protein concentration often generates undesirable mouthfeel. Although the specific mechanisms of such adverse sensorial perception remain poorly understood, we hypothesize that it is mechanistically linked to electrostatic interaction between protein and saliva during oral processing. To test this hypothesis, we used varying concentrations (0.1-10 wt%) of model dairy proteins and investigated their physicochemical and material properties in the presence of model saliva containing purified bovine submaxillary mucin1 (BSM) at 37 ○C. Soft tribology was performed using glass-polydimethylsiloxane (PDMS) contact surfaces to analyse the lubrication behaviour of proteins, model saliva and protein-saliva mixtures (1:1 to 1:4 w/w) at 2 N load. Both model saliva and proteins reduced friction coefficients in the boundary regime as compared to the buffer. Interestingly, an enhanced lubrication behaviour was observed in protein + model saliva mixture, possibly associated with electrostatic repulsion of saliva (ζ-potential – 5 mV) with the protein at neutral pH (ζ-potential – 23.5 mV), resulting in forming a thicker locally phase-separated lubricating film in the contact zone. Ongoing experiments will assess these interactions further.

Acknowledgement: Funding from the BBSRC-Mondelez DTP is acknowledged.

Reference:

¹ A. Sarkar, F. Xu, S. Lee (2019). Advances in Colloid and Interface Science, 273, Article No. 102034

² F. Xu, E. Liamas, M. Bryant, A. F. Adedeji, E. Andablo-Reyes, M. Castronovo, R. Ettelaie, T. V. J. Charpentier, S. Lee A. Sarkar, (2020). Advanced Materials Interfaces, 7, Article No. 1901549

Introduction

Repeated friction of the skin against objects can generate unpleasant sensations due to nervous receptors located in the first millimetres of skin.

Though lots of studies aimed at better describing the mechanical behaviour of the skin, a few did record through-layers strains. Shear and compression responses were often deduced from observations at the surface [1].

The aim of this work was to design an experimental setup able to measure skin layers deformations during indentation and friction testing using digital image correlation.

Methods

Porcine skin samples were die-cut with 2-5mm width. Speckle patterns were created on their lateral surface using HE staining, their quality was estimated using indicators defined in [2]. Images were acquired with a CCD camera, pre-processed to remove noise and enhance outlines before applying the DIC (16-bit images, subset radius: 45px, strain radius: 25px). Couples of images of the samples in the same position and sets of 20 synthetic images with subpixel translation helped determine the accuracy of the measurements. Indentation trials gave insights into the through-layers mode of deformation of the skin.

Results and discussion

Though speckle quality indicators were lower than in [2], DIC on the two sets of images allowed estimating that the maximum overall error on strain measurement was lower than 0.15%, up to 4mm sample width. This result is in agreement with the literature and even without physical deformations it proves the suitability of this setup to capture skin through-layers deformations.

The strain map obtained with the indentation test highlights how this setup could help supplement existing data about skin mechanical behaviour.

References

[1]J. Weickenmeier, M. Jabareen, and E. Mazza, J. Biomech., vol. 48, no. 16, pp. 4279–4286, 2015.

[2]G. Crammond, S. Boyd, and J. Dulieu-Barton, Opt. Lasers Eng., vol. 51, no. 12, pp. 1368–1378, 2013.

Introduction

Repeated friction of the skin against objects can generate unpleasant sensations due to nervous receptors located in the first millimetres of skin.

Though lots of studies aimed at better describing the mechanical behaviour of the skin, a few did record through-layers strains. Shear and compression responses were often deduced from observations at the surface [1].

The aim of this work was to design an experimental setup able to measure skin layers deformations during indentation and friction testing using digital image correlation.

Methods

Porcine skin samples were die-cut with 2-5mm width. Speckle patterns were created on their lateral surface using HE staining, their quality was estimated using indicators defined in [2]. Images were acquired with a CCD camera, pre-processed to remove noise and enhance outlines before applying the DIC (16-bit images, subset radius: 45px, strain radius: 25px). Couples of images of the samples in the same position and sets of 20 synthetic images with subpixel translation helped determine the accuracy of the measurements. Indentation trials gave insights into the through-layers mode of deformation of the skin.

Results and discussion

Though speckle quality indicators were lower than in [2], DIC on the two sets of images allowed estimating that the maximum overall error on strain measurement was lower than 0.15%, up to 4mm sample width. This result is in agreement with the literature and even without physical deformations it proves the suitability of this setup to capture skin through-layers deformations.

The strain map obtained with the indentation test highlights how this setup could help supplement existing data about skin mechanical behaviour.

References

[1]J. Weickenmeier, M. Jabareen, and E. Mazza, J. Biomech., vol. 48, no. 16, pp. 4279–4286, 2015.

[2]G. Crammond, S. Boyd, and J. Dulieu-Barton, Opt. Lasers Eng., vol. 51, no. 12, pp. 1368–1378, 2013.

Wear and corrosion at modular taper junctions of orthopedic endoprosthesis are associated with adverse clinical reactions. Micromotion is the cause for wear generation and corrosion processes initiated at the surface. The subsurface microstructure of the alloy determines the tribological implant properties. Therefore, this study aims to generate an enhanced wear resistance by modification of the surface microstructure of a CoCr28Mo6 wrought alloy by applying heat treatment and deep rolling. Ten retrieved CoCr28Mo6 hip explants of different manufacturers were analyzed regarding their surface properties. CoCr28Mo6 wrought alloy samples (DIN EN ISO 5832-12) were heat treated at 750°C for 24 hours and/or plastically deformed by deep rolling with varying axial forces (170N, 230N and 250N). The samples were metallographically prepared and investigated using optical and scanning electron microscopy with EDS and EBSD, micro hardness testing, XRD and tribological testing.

The investigation of the retrieved components revealed that, independent of the manufacturer, neither the head taper nor the trunnion exhibited a defined surface condition. The initial microstructure of CoCr28Mo6 wrought alloy exhibited a face-centered cubic (fcc) crystal structure. After heat treatment, the matrix entirely converted to hexagonal close-packed (hcp) structure. In the initial fcc-condition, deep rolling generated a plastically deformed surface layer within the first 100 µm and stress-induced phase transformation to hcp was observed. The micro hardness was increased by 24% for the hcp-matrix and by 39% by deep rolling in comparison to the initial fcc-matrix. This trend was confirmed by an increase in residual compressive stresses. In oscillating ball-on-plate tests under lubrication, the modified samples generated lower wear volumes in comparison to fcc-matrix samples. This study shows that the surface integrity can be modified to enhance the wear resistance of a CoCr28Mo6 wrought alloy. Target-oriented adaption of the microstructure should be applied as it reduces the risk of fretting wear.

Wear and corrosion at modular taper junctions of orthopedic endoprosthesis are associated with adverse clinical reactions. Micromotion is the cause for wear generation and corrosion processes initiated at the surface. The subsurface microstructure of alloy determines the tribological implant properties. Therefore, this study aims to generate an enhanced wear resistance by modification of the surface microstructure of a CoCr28Mo6 wrought alloy by applying heat treatment and deep rolling.
Ten retrieved CoCr28Mo6 hip explants of different manufacturers were analyzed regarding their surface properties. CoCr28Mo6 wrought alloy samples (DIN EN ISO 5832-12) were heat treated at 750°C for 24 hours and/or plastically deformed by deep rolling with varying axial forces (170N, 230N and 250N). The samples were metallographically prepared and investigated using optical and scanning electron microscopy with EDS and EBSD, micro hardness testing, XRD and tribological testing.
The investigation of the retrieved components revealed that, independent of the manufacturer, neither the head taper nor the trunnion exhibited a defined surface condition. The initial microstructure of CoCr28Mo6 wrought alloy exhibited a face-centered cubic (fcc) crystal structure. After heat treatment, the matrix entirely converted to hexagonal close-packed (hcp) structure. In the initial fcc-condition, deep rolling generated a plastically deformed surface layer within the first 100 µm and stress-induced phase transformation to hcp was observed. The micro hardness was increased by 24% for the hcp-matrix and by 39% by deep rolling in comparison to the initial fcc-matrix. This trend was confirmed by an increase in residual compressive stresses. In oscillating ball-on-plate tests under lubrication, the modified samples generated lower wear volumes in comparison to fcc-matrix samples.
This study shows that the surface integrity can be modified to enhance the wear resistance of a CoCr28Mo6 wrought alloy. Target-oriented adaption of the microstructure should be applied as it reduces the risk of fretting wear.

Discomfort is common among contact lens (CL) wearers, but little is known on the underlying mechanisms. The eye is a delicate and sensitive organ to study. Therefore, a 3D model allows difficult, or even impossible, tests on the ocular surface. To understand better the interaction between CL, the ocular surface and the eyelid, a multi-body dynamic simulation (MBDS) was developed.

Using a CAD software, the three parts of the system eye-CL-eyelid were modelled. MSC Adams was then used to articulate the parts and simulate a blinking pattern. The analysis resulting from this simulation allowed a contact map to be produced and applied forces determined for each step of the blinking movement. Next steps in the work will be to develop an experimental simulation based on the outputs of the model.

Discomfort is common among contact lens (CL) wearers, but little is known on the underlying mechanisms. The eye is a delicate and sensitive organ to study. Therefore, a 3D model allows difficult, or even impossible, tests on the ocular surface. To understand better the interaction between CL, the ocular surface and the eyelid, a multi-body dynamic simulation (MBDS) was developed.

Using a CAD software, the three parts of the system eye-CL-eyelid were modelled. MSC Adams was then used to articulate the parts and simulate a blinking pattern. The analysis resulting from this simulation allowed a contact map to be produced and applied forces determined for each step of the blinking movement. Next steps in the work will be to develop an experimental simulation based on the outputs of the model.

Discomfort is common among contact lens (CL) wearers, but little is known on the underlying mechanisms. The eye is a delicate and sensitive organ to study. Therefore, a 3D model allows difficult, or even impossible, tests on the ocular surface. To understand better the interaction between CL, the ocular surface and the eyelid, a multi-body dynamic simulation (MBDS) was developed.

Using a CAD software, the three parts of the system eye-CL-eyelid were modelled. MSC Adams was then used to articulate the parts and simulate a blinking pattern. The analysis resulting from this simulation allowed a contact map to be produced and applied forces determined for each step of the blinking movement. Next steps in the work will be to develop an experimental simulation based on the outputs of the model.

Introduction: Oral mucositis is a painful inflammation of mouth and throat mucosal tissues with or without lesions. Chemotherapy is one of the main causes of oral mucositis which can limit nutritional intake of patients and may lead to stopping the treatment. Method: We aimed to prepare the protective oromuco-adhesive hydrogel film to overcome the mucositis and oral lesion pain with additional anti-bacterial efficacy.  For this purpose, the biocompatible chitosan/ Carbopol oral gel was prepared and its mechanical properties, swelling, and rheological behaviors, surface muco-adhision property, and erosion time were physicochemically characterized. Also, the feelings of pain-reduction and swallowing-comfort were assessed by Randomized clinical trials (RCT). Result: The result data showed that the prepared oral gel has a proper mucoadhesive property with a well-comfort feeling for mucositis patients by three times a day administration. Conclusion: Thus, it was concluded that oral mucositis can be relieved by tribological improved biopolymers.

Introduction: Oral mucositis is a painful inflammation of mouth and throat mucosal tissues with or without lesions. Chemotherapy is one of the main causes of oral mucositis which can limit nutritional intake of patients and may lead to stopping the treatment. Method: We aimed to prepare the protective oromuco-adhesive hydrogel film to overcome the mucositis and oral lesion pain with additional anti-bacterial efficacy.  For this purpose, the biocompatible chitosan/ Carbopol oral gel was prepared and its mechanical properties, swelling, and rheological behaviors, surface muco-adhision property, and erosion time were physicochemically characterized. Also, the feelings of pain-reduction and swallowing-comfort were assessed by Randomized clinical trials (RCT). Result: The result data showed that the prepared oral gel has a proper mucoadhesive property with a well-comfort feeling for mucositis patients by three times a day administration. Conclusion: Thus, it was concluded that oral mucositis can be relieved by tribological improved biopolymers.

Replacement arthroplasty is a surgical procedure performed to replace with prosthetic devices human synovial joints that have lost functionality due to trauma or illness. These, are complex tribological systems that, once implanted, operate under the biological conditions of the human body, lubricated by synovial fluid. To improve upon their performance, several experimental techniques and apparatuses exist to characterize the tribological behavior of prosthetics; particularly, wear of the bearing surfaces, usually made of high-density plastic materials like ultra-high molecular weight polyethylene (UHMWPE). For the testing of total knee replacements, for instance, the ISO 14243-3 standard provides the guidelines for the loading and displacement parameters under which wear-testing machines of these devices must operate. The standard recommends the use of bovine serum diluted with deionized water to have a protein mass concentration of 20 g/l. Under some circumstances, protein mediated mechanisms significantly contribute to friction, either through viscous shearing or protein aggregation. Some authors have reported the observation of gel-like phases formed at the contact entrance of a ball-on-disc experiment. This phase explains the behavior of the coefficient of friction (COF), but does not provide clues as to the interaction of parameters that give rise to this phenomenon. Since bovine serum is a standard fluid for in-vitro testing, it is important to characterize its tribological behavior. Thus, we investigate the role of proteins in the lubrication of an ASI 316L sphere loaded against an UHMWPE disk, at high and low  entrainment speed, using diluted fetal bovine serum (FBS) at protein mass concentration of 20 g/l. We compare the experimental behavior of the COF of both FBS and pure deionized water and elaborate prediction models for both cases for low (20 mm/s) and high (80 mm/s) entrainment speed. From this statistical analysis, we isolate protein contribution.

Replacement arthroplasty is a surgical procedure performed to replace with prosthetic devices human synovial joints that have lost functionality due to trauma or illness. These, are complex tribological systems that, once implanted, operate under the biological conditions of the human body, lubricated by synovial fluid. To improve upon their performance, several experimental techniques and apparatuses exist to characterize the tribological behavior of prosthetics; particularly, wear of the bearing surfaces, usually made of high-density plastic materials like ultra-high molecular weight polyethylene (UHMWPE). For the testing of total knee replacements, for instance, the ISO 14243-3 standard provides the guidelines for the loading and displacement parameters under which wear-testing machines of these devices must operate. The standard recommends the use of bovine serum diluted with deionized water to have a protein mass concentration of 20 g/l. Under some circumstances, protein mediated mechanisms significantly contribute to friction, either through viscous shearing or protein aggregation. Some authors have reported the observation of gel-like phases formed at the contact entrance of a ball-on-disc experiment. This phase explains the behavior of the coefficient of friction (COF), but does not provide clues as to the interaction of parameters that give rise to this phenomenon. Since bovine serum is a standard fluid for in-vitro testing, it is important to characterize its tribological behavior. Thus, we investigate the role of proteins in the lubrication of an ASI 316L sphere loaded against an UHMWPE disk, at high and low  entrainment speed, using diluted fetal bovine serum (FBS) at protein mass concentration of 20 g/l. We compare the experimental behavior of the COF of both FBS and pure deionized water and elaborate prediction models for both cases for low (20 mm/s) and high (80 mm/s) entrainment speed. From this statistical analysis, we isolate protein contribution.

Total hip arthroplasty implants count on the performance of metallic components, currently made of CoCrMo- and TiAlV- alloys. Over 683’000 hip arthroplasties performed in the United States from 2012 to 2019, substantial 8.5% consisted of revision procedures, mostly due to infections and inflammatory reactions (AJRR). This study aimed to investigate the tribocorrosion behavior of a Ni-free high nitrogen steel in healthy and inflammatory simulated synovial fluids, in comparison with CoCrMo-alloy.

Low carbon CoCrMo medical alloy (ASTM F1537) and X13CrMnMoN18-14-3 (brand name: P2000) samples were tested against a ceramic ball (Al₂O₃) under reciprocating sliding wear conditions (37 N, 40° amplitude, 2 Hz, 86400 cycles, n = 3), with continuous monitoring of the metal electrochemical potential (OCP). Tests were run in four simulated synovial fluids with addition of H₂O₂ (0, 3, 10, and 30 mM), representing increasing levels of inflammation. Wear scars were analyzed by white light confocal profilometry (OrthoLux, Redlux), and related to OCP results and SEM/EDS observations. Factorial AVONA was used to assess differences at a=0.05.

The P2000 steel performed comparably to CoCrMo-alloy at 0 mM H₂O₂, better at 3 and 10 mM H₂O₂, and a worse at 30 mM H₂O₂. Ratios of mean wear scar volumes of P2000 to CoCrMo were 1, 0.5, 0.4, and 1.9 with 0, 3, 10 mM, and 30 mM H₂O₂, respectively. Still, OCP drops due to wear were significantly lower for P2000 than for CoCrMo at 10 and 30 mM H₂O₂ (respectively: 220 ± 29 mV and 176 ± 49 mV for P2000, 446 ± 89 mV and 559 ± 38 mV for CoCrMo). Further investigations are needed to understand the different interaction mechanisms between H₂O₂ and the oxide films of the two metals, and the impact of N on passivation and repassivation, for future implant material developments.

Total hip arthroplasty implants count on the performance of metallic components, currently made of CoCrMo- and TiAlV- alloys. Over 683’000 hip arthroplasties performed in the United States from 2012 to 2019, substantial 8.5% consisted of revision procedures, mostly due to infections and inflammatory reactions (AJRR). This study aimed to investigate the tribocorrosion behavior of a Ni-free high nitrogen steel in healthy and inflammatory simulated synovial fluids, in comparison with CoCrMo-alloy.

Low carbon CoCrMo medical alloy (ASTM F1537) and X13CrMnMoN18-14-3 (brand name: P2000) samples were tested against a ceramic ball (Al₂O₃) under reciprocating sliding wear conditions (37 N, 40° amplitude, 2 Hz, 86400 cycles, n = 3), with continuous monitoring of the metal electrochemical potential (OCP). Tests were run in four simulated synovial fluids with addition of H₂O₂ (0, 3, 10, and 30 mM), representing increasing levels of inflammation. Wear scars were analyzed by white light confocal profilometry (OrthoLux, Redlux), and related to OCP results and SEM/EDS observations. Factorial AVONA was used to assess differences at a=0.05.

The P2000 steel performed comparably to CoCrMo-alloy at 0 mM H₂O₂, better at 3 and 10 mM H₂O₂, and a worse at 30 mM H₂O₂. Ratios of mean wear scar volumes of P2000 to CoCrMo were 1, 0.5, 0.4, and 1.9 with 0, 3, 10 mM, and 30 mM H₂O₂, respectively. Still, OCP drops due to wear were significantly lower for P2000 than for CoCrMo at 10 and 30 mM H₂O₂ (respectively: 220 ± 29 mV and 176 ± 49 mV for P2000, 446 ± 89 mV and 559 ± 38 mV for CoCrMo). Further investigations are needed to understand the different interaction mechanisms between H₂O₂ and the oxide films of the two metals, and the impact of N on passivation and repassivation, for future implant material developments.

Introduction: Idiopathic low back pain is one of the most common medical concerns in the world. Recent research points to  the role of thoracolumbar fascia (TLF) in proprioception and pain perception in lower back. TLF is formed by layers of parallel bundles of collagen fibres, separated by loose connective tissue containing hyaluronic acid (HA) improving the lubrication. Pathologic changes of TLF cause alteration of HA production, which eventually leads to decreased mobility and pain. The present study aims to identify the effect of kinematics, molecular weight, and concentration of HA on friction of laboratory model of human fascia.

Methods: Pair of two fascia layers was substituted by the contact of PDMS samples. A Bruker UMT TriboLab in pin-on-plate configuration was used for friction analysis, while the contact zone was lubricated by HA solutions of various concentrations and molecular weights, and by phosphate buffered saline (PBS) as a reference. The subsequent phase of testing introduced the effect of velocity on coefficient of friction (COF). The experiments were carried out considering the speed range between 1 and 4 Hz while the applied load was 1 N.

Results: The measured values of COF in PDMS-on-PDMS sliding pair showed clear importance in terms of molecular weight and HA concentration. With increasing molecular weight friction decreased while further lowering was observed for reduced HA concentration. Additionally, it was found that COF is stabilized with increasing velocity. 

Discussion: The present study offers an insight into the effect of various properties of HA on friction of human TLF model. Based on the experiments with various samples of HA, it may be assumed that lubrication of human fascia may be improved by application of suitable lubricant. Future study should focus on measurements with ex vivo animal TLF, describing the effect of the lubricant in conditions closer to the physiological and/or pathological state. 

Introduction: Idiopathic low back pain is one of the most common medical concerns in the world. Recent research points to  the role of thoracolumbar fascia (TLF) in proprioception and pain perception in lower back. TLF is formed by layers of parallel bundles of collagen fibres, separated by loose connective tissue containing hyaluronic acid (HA) improving the lubrication. Pathologic changes of TLF cause alteration of HA production, which eventually leads to decreased mobility and pain. The present study aims to identify the effect of kinematics, molecular weight, and concentration of HA on friction of laboratory model of human fascia.

Methods: Pair of two fascia layers was substituted by the contact of PDMS samples. A Bruker UMT TriboLab in pin-on-plate configuration was used for friction analysis, while the contact zone was lubricated by HA solutions of various concentrations and molecular weights, and by phosphate buffered saline (PBS) as a reference. The subsequent phase of testing introduced the effect of velocity on coefficient of friction (COF). The experiments were carried out considering the speed range between 1 and 4 Hz while the applied load was 1 N.

Results: The measured values of COF in PDMS-on-PDMS sliding pair showed clear importance in terms of molecular weight and HA concentration. With increasing molecular weight friction decreased while further lowering was observed for reduced HA concentration. Additionally, it was found that COF is stabilized with increasing velocity. 

Discussion: The present study offers an insight into the effect of various properties of HA on friction of human TLF model. Based on the experiments with various samples of HA, it may be assumed that lubrication of human fascia may be improved by application of suitable lubricant. Future study should focus on measurements with ex vivo animal TLF, describing the effect of the lubricant in conditions closer to the physiological and/or pathological state. 

The lubricant solution used in tribological testing of biocompatible materials employed in total knee replacements must have a total protein concentration of 20 g/L (ISO 14243-3:2014); bovine serums (BSs) are currently used as base fluids to prepare the aforementioned solution. BSs normally have a higher concentration of bovine serum albumin (BSA) and minor concentration of globulins (α, β and γ). It has been observed that these proteins degrade and adsorb onto the surface of the materials during the tests. Therefore, the aim of this study is to analyze protein degradation of four lubricant solutions having a protein concentration of 20 g/L, after 6 hours of testing. The parameters for the tribological tests were: applied load, L, 2 and 8 N; sliding-to-rolling ratio, SRR, 20 and 1800 %; and entrainment speed, V_m, 20 and 80 mm/s. Samples of the solutions were analyzed by the Bradford method, Ellman’s method, UV-Vis spectroscopy (range 200-800 nm), and electrophoresis (SDS-PAGE). This analyzes allowed to determine the degradation mechanism (hydrolysis or denaturation) suffered by the proteins in solution.

The lubricant solution used in tribological testing of biocompatible materials employed in total knee replacements must have a total protein concentration of 20 g/L (ISO 14243-3:2014); bovine serums (BSs) are currently used as base fluids to prepare the aforementioned solution. BSs normally have a higher concentration of bovine serum albumin (BSA) and minor concentration of globulins (α, β and γ). It has been observed that these proteins degrade and adsorb onto the surface of the materials during the tests. Therefore, the aim of this study is to analyze protein degradation of four lubricant solutions having a protein concentration of 20 g/L, after 6 hours of testing. The parameters for the tribological tests were: applied load, L, 2 and 8 N; sliding-to-rolling ratio, SRR, 20 and 1800 %; and entrainment speed, V_m, 20 and 80 mm/s. Samples of the solutions were analyzed by the Bradford method, Ellman’s method, UV-Vis spectroscopy (range 200-800 nm), and electrophoresis (SDS-PAGE). This analyzes allowed to determine the degradation mechanism (hydrolysis or denaturation) suffered by the proteins in solution.

Even after tribological mitigation, biofilm-focused pin site infections are the most frequent complication of external fixation surgery. These infections are hard to treat because the sessile cells of a bacterial biofilm tolerate antibiotics at concentrations far above the planktonic minimum inhibitory concentration (MIC).  Genotypically resistant cells may also be present in the biofilms, reducing local antibiotic concentration and making the infection even more refractory through social resistance. We have grown E. coli biofilms on Kirschner wires in an in vitro model system to analyze the biochemical exploitation of an engineered, b-lactamase-expressing strain by a susceptible, wild-type strain. Co-cultured biofilms were grown from various seeding strain ratios for 24 hours at different concentrations of ampicillin, and we measured the equilibrium strain ratio to determine social protection. Our results show that a minority of resistant cells can sufficiently detoxify the biofilm to allow colonization by susceptible cells, and that the equilibrium strain ratio depends on the ampicillin concentration. This study demonstrates that antibiotic-susceptible bacteria in the presence of b-lactamase-producing counterparts can be protected from antibiotics under clinically realistic biofilm conditions, and we suggest strategies for reducing this protective effect.

Even after tribological mitigation, biofilm-focused pin site infections are the most frequent complication of external fixation surgery. These infections are hard to treat because the sessile cells of a bacterial biofilm tolerate antibiotics at concentrations far above the planktonic minimum inhibitory concentration (MIC).  Genotypically resistant cells may also be present in the biofilms, reducing local antibiotic concentration and making the infection even more refractory through social resistance. We have grown E. coli biofilms on Kirschner wires in an in vitro model system to analyze the biochemical exploitation of an engineered, b-lactamase-expressing strain by a susceptible, wild-type strain. Co-cultured biofilms were grown from various seeding strain ratios for 24 hours at different concentrations of ampicillin, and we measured the equilibrium strain ratio to determine social protection. Our results show that a minority of resistant cells can sufficiently detoxify the biofilm to allow colonization by susceptible cells, and that the equilibrium strain ratio depends on the ampicillin concentration. This study demonstrates that antibiotic-susceptible bacteria in the presence of b-lactamase-producing counterparts can be protected from antibiotics under clinically realistic biofilm conditions, and we suggest strategies for reducing this protective effect.

Introduction:

Polycarbonate-based thermoplastic polyurethanes (PC-TPU) have been investigated and used in medicine due to their excellent mechanical properties (high elasticity and flexibility), good biocompatibility, as well as high resistance to tear, abrasion and oxidation. ChronoFlex AL™75D and Carbothane™ PC75A are family of biodurable aliphatic PC-TPUs, engineered to be used as medical-grade polymers. The aim of this study was to characterize these PC-TPU foils, in terms of their tribological properties for their application as a potential implant material for articulating surfaces.

Methods:

Six rectangular samples were cut out of each foil and tested against bovine cartilage plugs (Ø=6mm, n=6 for each foil) with synovial fluid lubrication using a pin-on-plate testing device. Sixteen different loading-and motions conditions (σ=0.05‑2.5MPa and v=0.33‑50mm/s; n=4 each) were chosen to comprehensively investigate their friction properties. The friction coefficient (µ=F_F/F_N) was determined after a testing duration of 10 minutes (µ_end). Statistical analysis was done by t-test. Afterwards, the cartilage samples were histologically stained and graded according to modified Mankin Histological Histochemical Grading System (HHGS) to detect any signs of wear.

Results

Both foils predominantly behave similarly over a wide range of the parameter space. Their overall obtained friction coefficients ranged between 0.037-0.332. Only at the most severe condition (σ=2.5MPa, v=50mm/s), ChronoFlex AL™75D resulted in a significantly lower friction coefficient (µ_end=0.172) than Carbothane™ PC75A (µ_end=0.332; p=0.312). HHGS conceded that the cartilage samples showed almost no sign of wear, for either foil.

Discussion

With regard to the low and therefore promising friction properties of both foils, they might be appropriate candidates for articulating surface implants. This conclusion is supported when comparing µ of biomaterials that are already in clinical use (Actifit™ µ≈0.4). Additionally, from histological examination, it could be expected that they have acceptable wear protection and therefore might have a chondroprotective effect.

Introduction:

Polycarbonate-based thermoplastic polyurethanes (PC-TPU) have been investigated and used in medicine due to their excellent mechanical properties (high elasticity and flexibility), good biocompatibility, as well as high resistance to tear, abrasion and oxidation. ChronoFlex AL™75D and Carbothane™ PC75A are family of biodurable aliphatic PC-TPUs, engineered to be used as medical-grade polymers. The aim of this study was to characterize these PC-TPU foils, in terms of their tribological properties for their application as a potential implant material for articulating surfaces.

Methods:

Six rectangular samples were cut out of each foil and tested against bovine cartilage plugs (Ø=6mm, n=6 for each foil) with synovial fluid lubrication using a pin-on-plate testing device. Sixteen different loading-and motions conditions (σ=0.05‑2.5MPa and v=0.33‑50mm/s; n=4 each) were chosen to comprehensively investigate their friction properties. The friction coefficient (µ=F_F/F_N) was determined after a testing duration of 10 minutes (µ_end). Statistical analysis was done by t-test. Afterwards, the cartilage samples were histologically stained and graded according to modified Mankin Histological Histochemical Grading System (HHGS) to detect any signs of wear.

Results

Both foils predominantly behave similarly over a wide range of the parameter space. Their overall obtained friction coefficients ranged between 0.037-0.332. Only at the most severe condition (σ=2.5MPa, v=50mm/s), ChronoFlex AL™75D resulted in a significantly lower friction coefficient (µ_end=0.172) than Carbothane™ PC75A (µ_end=0.332; p=0.312). HHGS conceded that the cartilage samples showed almost no sign of wear, for either foil.

Discussion

With regard to the low and therefore promising friction properties of both foils, they might be appropriate candidates for articulating surface implants. This conclusion is supported when comparing µ of biomaterials that are already in clinical use (Actifit™ µ≈0.4). Additionally, from histological examination, it could be expected that they have acceptable wear protection and therefore might have a chondroprotective effect.

Introduction: A healthy synovial joint is very important for painless active movement of natural musculoskeletal system. The right function of nature synovial joints ensures well lubricated contact surfaces with very low friction coefficient and low wear of cartilage tissue. The mechanism of lubrication in natural joints is not explored enough. The understanding of lubrication process can assist in the development and understanding of new suitable medical treatments.

A cartilage together with a natural lubricant (synovial fluid) are very important parts of synovial joint for tribology investigation. There are two main types of studies in this term, those which deal with friction in cartilage contact and others focused on investigation of lubrication in cartilage contact. There are no works dealing with simultaneous measurement of friction and visualization of cartilage contact. Therefore, this study is focused on combination of these two experimental cartilage investigation. In addition, the methodology of simultaneous friction measurement and visualization of cartilage contact is described.

Materials and methods: The specially designed reciprocating tribometer simulating synovial joint was used. The fluorescence microscopy was incorporated as a suitable visualization method. This apparatus allows friction measurement and simultaneous visualization of cartilage contact. 

The schema of experimental apparatus is shown in Fig. 1.

The contact was formed by 9.7 mm diameter cartilage sample removed from pig’s hip joint and the optical glass, which allows insight into the contact. The model synovial fluid corresponding the physiological composition of synovial fluid (20 mg/ml albumin, 3.6 mg/ml ϒ‑globulin and 2.5 mg/ml hyaluronic acid) was used as an experimental lubricant in several modifications of fluorescently labelled component separately. The evaluation of visualization was carried out by the specially designed software, which is based on snaps processing by morphological opening of snaps. All experiments were carried out under the same operating conditions (10 N load, 10 mm/s velocity and 20 mm stroke). The entire evaluation process of experimental data is shown in Fig. 2.

Results and discussion: The influence of individual labelled components on lubricating film forming was evaluated. The trend of friction coefficient and fluorescently labelled particles count in the contact were described and the relationships between them were specified. The albumin protein was found to be the main component responsible for the lubricating film forming and its film was stable film during experiment, (see Fig. 3). It changes particle count in the contact very little. The average size of albumins protein clusters is immutable during the experiment. On the contrary, the ϒ‑globulin forms lubricating film whose particle count trend decline and along with it, the average size of its clusters also decline. The relation between the raising trend of friction coefficient and albumin particle count was indicated. This methodology shows a great potential for understanding the lubrication in human synovial joints, which will help to treat joint diseases.

Fig. 3. Evaluation schema

Introduction: A healthy synovial joint is very important for painless active movement of natural musculoskeletal system. The right function of nature synovial joints ensures well lubricated contact surfaces with very low friction coefficient and low wear of cartilage tissue. The mechanism of lubrication in natural joints is not explored enough. The understanding of lubrication process can assist in the development and understanding of new suitable medical treatments.

A cartilage together with a natural lubricant (synovial fluid) are very important parts of synovial joint for tribology investigation. There are two main types of studies in this term, those which deal with friction in cartilage contact and others focused on investigation of lubrication in cartilage contact. There are no works dealing with simultaneous measurement of friction and visualization of cartilage contact. Therefore, this study is focused on combination of these two experimental cartilage investigation. In addition, the methodology of simultaneous friction measurement and visualization of cartilage contact is described.

Materials and methods: The specially designed reciprocating tribometer simulating synovial joint was used. The fluorescence microscopy was incorporated as a suitable visualization method. This apparatus allows friction measurement and simultaneous visualization of cartilage contact. The schema of experimental apparatus is shown in Fig. 1.

Fig. 1. Schema of experimental apparatus

The contact was formed by 9.7 mm diameter cartilage sample removed from pig’s hip joint and the optical glass, which allows insight into the contact. The model synovial fluid corresponding the physiological composition of synovial fluid (20 mg/ml albumin, 3.6 mg/ml ϒ‑globulin and 2.5 mg/ml hyaluronic acid) was used as an experimental lubricant in several modifications of fluorescently labelled component separately. The evaluation of visualization was carried out by the specially designed software, which is based on snaps processing by morphological opening of snaps. All experiments were carried out under the same operating conditions (10 N load, 10 mm/s velocity and 20 mm stroke). The entire evaluation process of experimental data is shown in Fig. 2.

Fig. 2. Evaluation schema

Results and discussion: The influence of individual labelled components on lubricating film forming was evaluated. The trend of friction coefficient and fluorescently labelled particles count in the contact were described and the relationships between them were specified. The albumin protein was found to be the main component responsible for the lubricating film forming and its film was stable film during experiment, (see Fig. 3). It changes particle count in the contact very little. The average size of albumins protein clusters is immutable during the experiment. On the contrary, the ϒ‑globulin forms lubricating film whose particle count trend decline and along with it, the average size of its clusters also decline. The relation between the raising trend of friction coefficient and albumin particle count was indicated. This methodology shows a great potential for understanding the lubrication in human synovial joints, which will help to treat joint diseases.

Fig. 3. Evaluation schema

Various effects have been observed when a slimy fluid is held in a palm; these include friction control of the skin, and cleansing and moisturizing of the skin. However, there are few reports on the changes in emotional state when a slimy fluid is held in a palm. These emotional changes are affected by the viscosity properties. The purpose of this study is to investigate the emotional changes caused by holding a slimy fluid in palms, by using a psychophysiological index.

Newtonian and non-Newtonian fluids, with viscosities ranging from 0.01 to 100 Pa・s, were prepared (Figure 1). Eight male subjects in their 20s soaked their palms in the slimy fluid without being able to see the fluid. At the room temperature of 25 °C, the subjects could move their palms freely for 1 min and they were allowed to rub their palms together. During the testing, heart rate variability, which is the physiological phenomenon of variation in the time interval between heartbeats, was recorded. A frequency analysis was performed for the estimation of autonomic nerve activity (the sympathetic and parasympathetic nerves). After holding the fluid, the subjects were asked to provide feedback by using the semantic differential method.

Significant changes in the sympathetic and parasympathetic nerve activations were observed when the subjects soaked their palms in the slimy fluid (Figure 2). It was obvious that the high viscosity Newtonian fluid (NF-H) reduced parasympathetic nervous system activity. These changes in the psychophysiological indexes were thought to influence the feelings of the subjects ascertained using the semantic differential method. By elucidating the relationship between the characteristic of the slimy fluid and psychophysiological index, the efficiency of development of products that are exposed to human skin can be expected.

Figure 1: Viscosities of slimy liquids. Newtonian fluids (NF) was a water solution of polyvinyl alcohol, and non-Newtonian fluids (NF) was a water solution of polyethyleneglycol.

Figure 2: Sympathetic and parasympathetic nerve activations, and feelings of subjects ascertained using semantic differential method.

Various effects have been observed when a slimy fluid is held in a palm; these include friction control of the skin, and cleansing and moisturizing of the skin. However, there are few reports on the changes in emotional state when a slimy fluid is held in a palm. These emotional changes are affected by the viscosity properties. The purpose of this study is to investigate the emotional changes caused by holding a slimy fluid in palms, by using a psychophysiological index.

Newtonian and non-Newtonian fluids, with viscosities ranging from 0.01 to 100 Pa・s, were prepared (Figure 1). Eight male subjects in their 20s soaked their palms in the slimy fluid without being able to see the fluid. At the room temperature of 25 °C, the subjects could move their palms freely for 1 min and they were allowed to rub their palms together. During the testing, heart rate variability, which is the physiological phenomenon of variation in the time interval between heartbeats, was recorded. A frequency analysis was performed for the estimation of autonomic nerve activity (the sympathetic and parasympathetic nerves). After holding the fluid, the subjects were asked to provide feedback by using the semantic differential method.

Significant changes in the sympathetic and parasympathetic nerve activations were observed when the subjects soaked their palms in the slimy fluid (Figure 2). It was obvious that the high viscosity Newtonian fluid (NF-H) reduced parasympathetic nervous system activity. These changes in the psychophysiological indexes were thought to influence the feelings of the subjects ascertained using the semantic differential method. By elucidating the relationship between the characteristic of the slimy fluid and psychophysiological index, the efficiency of development of products that are exposed to human skin can be expected.

Introduction: Mouth is not only an important human organ, but also the first organ of the digestive system. Oral health can affect eating, nutrition, speech, and social activity. Saliva is the key component for maintaining oral health, and its functions include lubricating, chewing, resisting bacterial growth, regulating pH value, digestion, and taste perception. For Xerostomia patients, not only oral health is affected by reduced saliva, but also the quality of life. Among all the inconveniences, chewing is the most bearable pain. The main reason is that the lubricating property of saliva is reduced, thus the irritation between oral organs is increased. Xerostomia can be induced by many causes, and patients can only apply artificial saliva to moisture the oral cavity before the cause of disease is identified. Although artificial saliva can reduce the symptoms, it can only last for few hours. In order to understand the bio-tribologicl functions of saliva and its relationship with tongue, we developed an in vitro and ex vivo oral friction testing system.

Methods: We established in vitro and ex vivo oral friction testing system with a universal micro-tribometer-2 (CETR-UMT2). For in vitro oral friction testing system, we used polydimethylsiloxane (PDMS) as pin-on-disc friction testing material. We tested the effects of different normal loads (1, 2, 5 Newton ) and rotation speeds (1, 10, 50, 100 resolution per minute) on the friction coefficient of PDMS under dry or lubricating friction conditions. For ex vivo friction testing establishment, we used porcine tongue-PDMS materials to mimic soft tissue-soft tissue biotribology. We measured the roughness of the tongue, and investigated the relationship between roughness and friction coefficient under dry or lubricating conditions.

Results: We found that under dry condition, bigger normal load resulted in lower friction coefficient of PDMS. However, bigger normal load caused larger friction coefficient of PDMS under lubricating condition. When a PDMS pin was sliding against a PDMS disc under the same normal load, we found that the friction coefficient was increased when the rotation speed was from 1 to 10 rpm but reduced when the rotation speed was from 10 to 100 rpm regardless of dry or lubrication condition. For ex vivo friction testing system, we demonstrated that higher roughness of porcine tongue resulted in lower friction coefficient under dry condition. However, higher roughness of tongue caused higher friction coefficient under lubricating condition.

Conclusions: The results of in vitro oral friction testing system suggested that normal loads had more effects on the friction coefficient of PDMS-PDMS system than rotation speeds under dry condition. It is possible that bigger normal load resulted in deformation of surface asperities and the deformation caused lower resistance between two surfaces. Under lubricating condition, the characteristics of lubricants and the interaction between lubricants and two PDMS surfaces needed to be taken into account. Therefore, there was no positive or negative relationship could be observed. The results of ex vivo oral friction testing system demonstrated that the friction coefficient of tongue was mainly affected by the roughness of tongue surface. Under dry condition, higher roughness might result in fewer contact areas and the friction coefficient was lower. In contrast, the rheological property of lubricant or adsorption of the components in the lubricant might increase the resistance between tongue and PDMS resulting in higher friction coefficient. The in vitro and ex vivo oral friction testing system we established may contribute to develop a longer-lasting artificial saliva that can benefit Xerostomia patients in the future.

Introduction: Mouth is not only an important human organ, but also the first organ of the digestive system. Oral health can affect eating, nutrition, speech, and social activity. Saliva is the key component for maintaining oral health, and its functions include lubricating, chewing, resisting bacterial growth, regulating pH value, digestion, and taste perception. For Xerostomia patients, not only oral health is affected by reduced saliva, but also the quality of life. Among all the inconveniences, chewing is the most bearable pain. The main reason is that the lubricating property of saliva is reduced, thus the irritation between oral organs is increased. Xerostomia can be induced by many causes, and patients can only apply artificial saliva to moisture the oral cavity before the cause of disease is identified. Although artificial saliva can reduce the symptoms, it can only last for few hours. In order to understand the bio-tribologicl functions of saliva and its relationship with tongue, we developed an in vitro and ex vivo oral friction testing system.

Methods: We established in vitro and ex vivo oral friction testing system with a universal micro-tribometer-2 (CETR-UMT2). For in vitro oral friction testing system, we used polydimethylsiloxane (PDMS) as pin-on-disc friction testing material. We tested the effects of different normal loads (1, 2, 5 Newton ) and rotation speeds (1, 10, 50, 100 resolution per minute) on the friction coefficient of PDMS under dry or lubricating friction conditions. For ex vivo friction testing establishment, we used porcine tongue-PDMS materials to mimic soft tissue-soft tissue biotribology. We measured the roughness of the tongue, and investigated the relationship between roughness and friction coefficient under dry or lubricating conditions.Results: We found that under dry condition, bigger normal load resulted in lower friction coefficient of PDMS. However, bigger normal load caused larger friction coefficient of PDMS under lubricating condition. When a PDMS pin was sliding against a PDMS disc under the same normal load, we found that the friction coefficient was increased when the rotation speed was from 1 to 10 rpm but reduced when the rotation speed was from 10 to 100 rpm regardless of dry or lubrication condition. For ex vivo friction testing system, we demonstrated that higher roughness of porcine tongue resulted in lower friction coefficient under dry condition. However, higher roughness of tongue caused higher friction coefficient under lubricating condition.

Conclusions: The results of in vitro oral friction testing system suggested that normal loads had more effects on the friction coefficient of PDMS-PDMS system than rotation speeds under dry condition. It is possible that bigger normal load resulted in deformation of surface asperities and the deformation caused lower resistance between two surfaces. Under lubricating condition, the characteristics of lubricants and the interaction between lubricants and two PDMS surfaces needed to be taken into account. Therefore, there was no positive or negative relationship could be observed. The results of ex vivo oral friction testing system demonstrated that the friction coefficient of tongue was mainly affected by the roughness of tongue surface. Under dry condition, higher roughness might result in fewer contact areas and the friction coefficient was lower. In contrast, the rheological property of lubricant or adsorption of the components in the lubricant might increase the resistance between tongue and PDMS resulting in higher friction coefficient. The in vitro and ex vivo oral friction testing system we established may contribute to develop a longer-lasting artificial saliva that can benefit Xerostomia patients in the future.

Introduction

Due to the COVID-19 pandemic, medical staff wear Personal Protective Equipment (PPE) for much longer than recommended. This can result in a range of skin issues including irritation and injuries such as urticaria, skin tears and pressure injuries, ranging from isolated indentation marks at the locations of contact, to deep tissue level bruising across a larger area (Fig. 1). These injuries have been attributed to friction-related shear stresses at the PPE-skin interface and are correlated with the increased duration of PPE use, heavy sweating and the application of higher-grades of PPE. Additionally, there is a dermal pathway to COVID-19 infection, and therefore preventing these injures in healthcare workers is critical.

Method

This study focusses on computationally modelling and optimising the PPE-skin interface to decrease subsurface stresses, thereby reducing skin injury and the risk of viral exposure. In the short term, this will help healthcare professionals modify the PPE-skin interface to reduce injury, for example, through the use of creams or lubricants. In the long term, this information will help optimise PPE design; for example, by modifying geometric and material parameters. In this study, a finite element model describing the contact between respirator masks and facial tissue was developed to calculate the stresses in the skin resulting from interaction with PPE. Skin bruising is indicative of the rupturing of blood vessels in the dermis and therefore, the main output of the model is the shear stress in this skin layer.A parametric study was performed by varying the stiffness, and Poisson’s ratio of the mask’s inner layer material, which is in contact with the skin, as well as its thickness and contact area. Additionally, the friction coefficient at the PPE-skin interface was also varied to observe the effect on shear stresses in the dermis.

Results Summary 

 A typical stress field involves two isolated stress peaks in the upper dermis (Fig. 2) under the edge of the contact with the face mask. These peaks are consistent with observed facial indentation marks. Deeper into the skin, a larger central peak forms towards the lower/inner dermis, consistent with deep tissue bruising. The results show that a significant reduction of the shear stresses in the dermis can be achieved through optimising the characteristics of the PPE-skin interaction.

Stiffness (E): The use of softer materials decreases shear stresses in the dermis (Fig. 3) and result in lower stress peaks in the upper dermis. In contrast, stiffer materials result in the migration of the maximum shear stress to the upper dermis.

Friction (mu): Lower interfacial friction coefficients between at the skin-PPE boundary results in lower dermal stresses (Fig. 3). They decrease stresses in the upper dermis whilst having a negligible effect on the lower dermis.

Poisson’s ratio (v): For materials with intermediate levels of stiffness, a lower v results in a significant reduction in maximum shear stress in the dermis. For soft materials, a lower v results in a lower average shear stresses and a migration shear stresses from the upper dermis to the lower dermis. 

Thickness (t): For softer materials, the thicker materials resulted in a slight reduction in average shear stresses. 

Contact Area (A): By changing the radius of curvature (r) of the edge of the mask, the contact area was changed.   Larger contact areas resulted in lower dermal shear stresses. However, the effects of changing contact area along with modulus were highly non-linear. This is likely due to competing pressure distribution and frictional mechanisms associated with changes in contact area. 

Conclusion:

Skin injury can be prevented by better designed face masks, and by optimising the interaction of the material and the skin. In the short term, focus should be made on selecting materials with lower frictional coefficients, stiffnesses and Poisson’s ratio, whilst maximising contact area. This will help decrease the risk of discomfort, skin injury and skin failure, thereby reducing the risk of viral exposure.

Introduction & Methods

This study focusses on computationally modelling and optimising the face mask-skin interface to decrease subsurface stresses, thereby reducing skin injury and failure and thus the risk of viral exposure. In the short term, this will help healthcare professionals modify the PPE-skin interface to reduce injury, for example, through the use of creams or lubricants. In the long term, this information will help optimise PPE design; for example, by modifying geometric and material parameters. In this study, a finite element model describing the contact between respirator masks and facial tissue was developed to calculate the stresses in the skin resulting from interaction with PPE. Skin bruising is indicative of the rupturing of blood vessels in the dermis and therefore, the main output of the model is the shear stress in this skin layer. A parametric study was performed by varying material and geometric parameters of the face masks inner layer, which is in contact with the skin, as well as its interfacial properties.

Results & Conclusion

It was found that typical stress fields in the skin consist of isolated peaks in the upper/outer dermis under the edge of the face mask contact, consistent with indentation. Additionally deeper in the skin, a wider stress peak forms in the lower/inner dermis, consistent with deep tissue bruising. The results show that a significant reduction of the shear stresses in the dermis can be achieved through optimising the characteristics of the PPE-skin interaction. In the short term, focus should be made on selecting materials with lower frictional coefficients, stiffnesses and Poisson’s ratio, whilst maximising contact area. This will help decrease the risk of discomfort, skin injury and skin failure, thereby reducing the risk of viral exposure.

A test system has been developed and utilized for the purpose of making comparative wear measurements of dental restorative materials. The method imposes abrasive conditions and small contact forces after which mass changes are determined.  The present results show that the materials made by additive and subtractive methods have equivalent wear characteristics to traditional, Carded denture teeth.  Under the test conditions imposed 3D printed teeth exhibited 30% less wear than Carded while CNC milled teeth exhibited 18% greater wear.  Electron microscopy used to examine wear characteristics of the 3 materials indicates each has different mechanisms by which material is removed under abrasive wear conditions.

A test system has been developed and utilized for the purpose of making comparative wear measurements of dental restorative materials. The method imposes abrasive conditions and small contact forces after which mass changes are determined.  The present results show that the materials made by additive and subtractive methods have equivalent wear characteristics to traditional, Carded denture teeth.  Under the test conditions imposed 3D printed teeth exhibited 30% less wear than Carded while CNC milled teeth exhibited 18% greater wear.  Electron microscopy used to examine wear characteristics of the 3 materials indicates each has different mechanisms by which material is removed under abrasive wear conditions.

Human-made adhesives lose their tack rapidly after first use, while animals such as geckos can reuse their adhesive feet for a lifetime. Nature’s use of fibrillar structures as strong, renewable and self-cleaning adhesives has inspired the development of synthetic adhesives with similarly structured surfaces. More than a decade of research and engineering has culminated in ’gecko tape’: a re-useable adhesive that has a structured surface similar to that of geckos and that outperforms the usual sticky tape. We report experiments that show that, despite its name, a commercial gecko tape shares few adhesive principles with its eponym. In particular, we find no evidence that the micrometric features that are present on the surface of the gecko tape play a role in its adhesive strength. In addition, we find that contrary to the gecko, the tape leaves behind a layer of adhesive after removal from the surface. The fact that the gecko tape outperforms a conventional adhesive tape is due to the fact that the softness of the backing of the gecko tape allows to create a much larger contact area for a given normal force. This effect suggests that one of the reasons that the gecko adheres so well is that the surface features on its feet are very soft. For the gecko tape, the conclusion is that surface features are not necessary to create a superb adhesive; tuning the backing layer elasticity may be enough.

Human-made adhesives lose their tack rapidly after first use, while animals such as geckos can reuse their adhesive feet for a lifetime. Nature’s use of fibrillar structures as strong, renewable and self-cleaning adhesives has inspired the development of synthetic adhesives with similarly structured surfaces. More than a decade of research and engineering has culminated in ’gecko tape’: a re-useable adhesive that has a structured surface similar to that of geckos and that outperforms the usual sticky tape. We report experiments that show that, despite its name, a commercial gecko tape shares few adhesive principles with its eponym. In particular, we find no evidence that the micrometric features that are present on the surface of the gecko tape play a role in its adhesive strength. In addition, we find that contrary to the gecko, the tape leaves behind a layer of adhesive after removal from the surface. The fact that the gecko tape outperforms a conventional adhesive tape is due to the fact that the softness of the backing of the gecko tape allows to create a much larger contact area for a given normal force. This effect suggests that one of the reasons that the gecko adheres so well is that the surface features on its feet are very soft. For the gecko tape, the conclusion is that surface features are not necessary to create a superb adhesive; tuning the backing layer elasticity may be enough.

Introduction

Metal ion and particle release, particularly cobalt, has become an important subject in total hip arthroplasty, as it has shown to induce metal hypersensitivity, adverse local tissue reactions and systemic ion related diseases. The purpose of the following study was to analyse the ion release barrier function of a zirconium nitride (ZrN) multilayer coated hip stem, designed for patients with metal ion hypersensitivity, within a new ZrN-Multilayer vs bone cement tribo-system. 

Methods

Six hip stems with a ZrN multilayer coating polished surface (CoreHip AS, Aesculap AG, Germany) were tested in comparison with six uncoated cobalt-chrome hip stems with a polished surface (CoreHip, Aesculap AG, Germany). In order to create a worst case scenario, the smallest stem size with the biggest offset in combination with an XL ceramic head was used. The stems were embedded according to ISO 7206-6 in a bone cement sheet. The dynamic test was performed under Ringer solution with an axial force of 3 875 N [1] at 11.5 Hz for 10 million cycles. The test was interrupted after 1, 3 and 10 million cycles in order to collect test medium samples for metal ion concentration analysis using ICP-MS. Finally, the stems were extracted and the surfaces of the stems were analysed through SEM/EDX.

Results

The SEM/EDX analysis demonstrated that the ZrN multilayer coating kept its integrity, as no trace of the substrate material (CoCrMo) could be detected. Moreover, the ion concentration analysis showed a reduction of up to two orders of magnitude in the release of cobalt, chromium and molybdenum in the ZrN coated stems in comparison with the uncoated version.

Discussion

The results showed that the hip stems with a ZrN multilayer coating substantially reduce the release of ions from the substrate material. 

References

[1] Bergmann et al., (2016). PLoS ONE, 11(5): e0155612

Introduction

Metal ion and particle release, particularly cobalt, has become an important subject in total hip arthroplasty, as it has shown to induce metal hypersensitivity, adverse local tissue reactions and systemic ion related diseases. The purpose of the following study was to analyse the ion release barrier function of a zirconium nitride (ZrN) multilayer coated hip stem, designed for patients with metal ion hypersensitivity, within a new ZrN-Multilayer vs bone cement tribo-system. 

Methods

Six hip stems with a ZrN multilayer coating polished surface (CoreHip AS, Aesculap AG, Germany) were tested in comparison with six uncoated cobalt-chrome hip stems with a polished surface (CoreHip, Aesculap AG, Germany). In order to create a worst case scenario, the smallest stem size with the biggest offset in combination with an XL ceramic head was used. The stems were embedded according to ISO 7206-6 in a bone cement sheet. The dynamic test was performed under Ringer solution with an axial force of 3 875 N [1] at 11.5 Hz for 10 million cycles. The test was interrupted after 1, 3 and 10 million cycles in order to collect test medium samples for metal ion concentration analysis using ICP-MS. Finally, the stems were extracted and the surfaces of the stems were analysed through SEM/EDX.

Results

The SEM/EDX analysis demonstrated that the ZrN multilayer coating kept its integrity, as no trace of the substrate material (CoCrMo) could be detected. Moreover, the ion concentration analysis showed a reduction of up to two orders of magnitude in the release of cobalt, chromium and molybdenum in the ZrN coated stems in comparison with the uncoated version.

Discussion

The results showed that the hip stems with a ZrN multilayer coating substantially reduce the release of ions from the substrate material. 

References

[1] Bergmann et al., (2016). PLoS ONE, 11(5): e0155612

The control of slippage between a hand and an object is fundamental to improving athletic performance in sports that require a strong grip, such as baseball and rock-climbing. In order to prevent slippage between a hand and an object, athletes often use grip enhancing powders to maintain grip in both dry and wet conditions. In baseball pitching, rosin powder, which comprises magnesium carbonate powder and pine resin, is often used as a grip-enhancing agent. However, the effect of rosin powder on friction at the baseball–human finger interface remains unclear. The current study investigated the effect of rosin powder on the friction coefficient between a baseball and a finger using sliding friction tests. Ten young adult males participated in this study who were asked to slide the index finger of their dominant hand over the leather skin of a baseball adhered to the triaxial force sensor. The normal load condition was set based on the normal force range applied at the finger during ball releasing process in fastball pitching. The results indicated that there was less dependence of the friction coefficient on the normal force and less variation in the friction coefficient among individuals, meaning that rosin powder application stabilizes friction under both dry and wet conditions. For most participants, the friction coefficient was not necessarily increased by the presence of rosin powder at the finger pad–leather sheet interface under dry conditions. However, under wet conditions, rosin powder application increased the friction coefficient compared with the non-powdered condition in the large normal force condition, indicating the efficacy of rosin powder as a grip-enhancing agent.

The control of slippage between a hand and an object is fundamental to improving athletic performance in sports that require a strong grip, such as baseball and rock-climbing. In order to prevent slippage between a hand and an object, athletes often use grip enhancing powders to maintain grip in both dry and wet conditions. In baseball pitching, rosin powder, which comprises magnesium carbonate powder and pine resin, is often used as a grip-enhancing agent. However, the effect of rosin powder on friction at the baseball–human finger interface remains unclear. The current study investigated the effect of rosin powder on the friction coefficient between a baseball and a finger using sliding friction tests. Ten young adult males participated in this study who were asked to slide the index finger of their dominant hand over the leather skin of a baseball adhered to the triaxial force sensor. The normal load condition was set based on the normal force range applied at the finger during ball releasing process in fastball pitching. The results indicated that there was less dependence of the friction coefficient on the normal force and less variation in the friction coefficient among individuals, meaning that rosin powder application stabilizes friction under both dry and wet conditions. For most participants, the friction coefficient was not necessarily increased by the presence of rosin powder at the finger pad–leather sheet interface under dry conditions. However, under wet conditions, rosin powder application increased the friction coefficient compared with the non-powdered condition in the large normal force condition, indicating the efficacy of rosin powder as a grip-enhancing agent.

Human oral surfaces are coated with a highly hydrated and lubricating protective saliva pellicle that consists of multiple proteinaceous layers. The lubrication function of this pellicle is altered by the interactions with food and beverages and is thought to influence sensory perception during and immediately after consumption. To capture the change in lubricity that occurs due to food-saliva interactions, we develop a dynamic tribology protocol (DTP), which measures the friction response of an ex vivo saliva pellicle as a function of time in the presence of food ingredients and/or beverages. 

In this study, the DTP is used to investigate the friction response of the saliva pellicle exposed to skim milk and whey protein isolate (WPI) mixtures under neutral pH conditions. Descriptive sensory analysis, carried out by an experienced, trained panel ensured the instrumental relationship with textural mouthfeel percepts is valid. 

Four textural sensory attributes, in-mouth thickness and smoothness, and mouthfeel after swallowing attributes smoothness and mouthcoating were found to strongly correlate to the friction response of the samples determined by the DTP. It was shown that the lubricity of the saliva pellicle in contact with skim milk-WPI mixtures decreases with increasing casein:WPI ratio. This decrease is driven by an increase in the mass loss of adsorbed saliva pellicle by interaction with dairy protein measured with Quartz Crystal Microbalance with Dissipation and Atomic Force Microscope. There is a linear correlation between adsorbed mass and friction measured using DTP. The correlation between textural sensory perception and instrumentally measured friction coefficient is higher than the friction measured on bare PDMS surfaces. This supports the importance of using saliva that offers closer mimic to the oral processing conditions, leading to much improved correlations with sensory attributes. 

In conclusion, DTP is an accessible technique to probe the influence of dairy protein-saliva interactions on sensory perception.

Human oral surfaces are coated with a highly hydrated and lubricating protective saliva pellicle that consists of multiple proteinaceous layers. The lubrication function of this pellicle is altered by the interactions with food and beverages and is thought to influence sensory perception during and immediately after consumption. To capture the change in lubricity that occurs due to food-saliva interactions, we develop a dynamic tribology protocol (DTP), which measures the friction response of an ex vivo saliva pellicle as a function of time in the presence of food ingredients and/or beverages. 

In this study, the DTP is used to investigate the friction response of the saliva pellicle exposed to skim milk and whey protein isolate (WPI) mixtures under neutral pH conditions. Descriptive sensory analysis, carried out by an experienced, trained panel ensured the instrumental relationship with textural mouthfeel percepts is valid. 

Four textural sensory attributes, in-mouth thickness and smoothness, and mouthfeel after swallowing attributes smoothness and mouthcoating were found to strongly correlate to the friction response of the samples determined by the DTP. It was shown that the lubricity of the saliva pellicle in contact with skim milk-WPI mixtures decreases with increasing casein:WPI ratio. This decrease is driven by an increase in the mass loss of adsorbed saliva pellicle by interaction with dairy protein measured with Quartz Crystal Microbalance with Dissipation and Atomic Force Microscope. There is a linear correlation between adsorbed mass and friction measured using DTP. The correlation between textural sensory perception and instrumentally measured friction coefficient is higher than the friction measured on bare PDMS surfaces. This supports the importance of using saliva that offers closer mimic to the oral processing conditions, leading to much improved correlations with sensory attributes. 

In conclusion, DTP is an accessible technique to probe the influence of dairy protein-saliva interactions on sensory perception.

With the development of new polymeric materials, such as PEEK (polyetheretherketone) and its modifications, the possibility to create movable connection of small joint implants from polymer-polymer combinations has opened up. In comparison with metal-polymer or ceramics-polymer friction joints which have been long used, the main advantages of such a connection include a reduced generation of potentially toxic abrasion particles and surprisingly, very good friction and abrasion properties. To select a suitable combination of materials, long-term tribological experiments were designed. PEEK-UHMWPE (ultra-high molecular weight polyethylene) combinations were tested using pin-on-disc method in PBS (phosphate buffered saline) and compared to the commonly used CoCrMo-UHMWPE combination and its improved, DLC (diamond-like carbon) coating including, variant CoCrMo/DLC-UHMWPE. The results showed the PEEK-UHMWPE pair having similar low friction coefficient as Al₂O₃-UHMWPE but lower wear. The functionality of the PEEK-UHMWPE friction pair was verified on a motion simulator. The tests were performed on a model of the first metatarsophalangeal (MTP) total joint replacement (figure 1), where the metatarsal head was made of PEEK and the phalangeal cup was made of UHMWPE. It was confirmed that the proposed combination can be suitable for the production of small joint implants.

Fig.1 Model of the first metatarsophalangeal total joint replacement ProSpon

With the development of new polymeric materials, such as PEEK (polyetheretherketone) and its modifications, the possibility to create movable connection of small joint implants from polymer-polymer combinations has opened up. In comparison with metal-polymer or ceramics-polymer friction joints which have been long used, the main advantages of such a connection include a reduced generation of potentially toxic abrasion particles and surprisingly, very good friction and abrasion properties. To select a suitable combination of materials, long-term tribological experiments were designed. PEEK-UHMWPE (ultra-high molecular weight polyethylene) combinations were tested using pin-on-disc method in PBS (phosphate buffered saline) and compared to the commonly used CoCrMo-UHMWPE combination and its improved, DLC (diamond-like carbon) coating including, variant CoCrMo/DLC-UHMWPE. The results showed the PEEK-UHMWPE pair having similar low friction coefficient as Al₂O₃-UHMWPE but lower wear. The functionality of the PEEK-UHMWPE friction pair was verified on a motion simulator. The tests were performed on a model of the first metatarsophalangeal (MTP) total joint replacement (figure 1), where the metatarsal head was made of PEEK and the phalangeal cup was made of UHMWPE. It was confirmed that the proposed combination can be suitable for the production of small joint implants.

Spinal instrumentation systems are widely used for the treatment of traumatic spinal fractures, dislocations, and instability or deformity of the spine. In general, the surgical technique, the patient, the implant design and material affect the clinical outcome.

Various complications are reported associated with spinal instrumentation: malpositioning of instrumentation, pedicle screw loosening and/or breakage, rod breakage, pseudoarthrosis, osteolysis, and granuloma formation. In addition, evidence of corrosion observed on retrieved implants includes surface discoloration, superficial and severe metal loss, and signs of galvanic, crevice and particularly fretting corrosion. Corrosion mainly occurred at the junctions of the construction materials, raising questions about the impact of galvanic corrosion on the fretting-corrosion rates. The implants were made of a variety of materials, such as SS, CoCrMo alloy, and Ti6Al4V alloy.

A wide range of biomechanical and mechanical testing has been reported over the past decade, based on available ASTM and ISO standards. However, only few studies are published which investigate the corrosion and more specifically the fretting and tribo-corrosion behaviour of mixed and same-material spinal instrumentation under laboratory conditions. The experiments performed include combinations of mechanical testing, e.g. ASTM standards F1717 and F2129, with electrochemical corrosion test cells.[1-3]

This review will consolidate the understanding of corrosion that can be gleaned from analysis of retrieved hardware. It will bring forward an analysis of the potential for the role of corrosion in implant failure and implant degradation mechanisms, to be improved through laboratory testing of spinal instrumentation in corrosion cells designed to be able to measure in real-time. How an advanced corrosion analysis can complement the standard testing protocols already in use will be discussed. This review paper will also make comparisons and draw analogies to other tribocorrosion systems in metal implants and other relevant biomedical systems.

References

1.Mali, S.A., V. Singh, and J.L. Gilbert, Effect of mixed alloy combinations on fretting corrosion performance of spinal screw and rod implants. J Biomed Mater Res B Appl Biomater, 2017. 105(5): p. 1169-1177.

2.Singh, V., et al., Material dependent fretting corrosion in spinal fusion devices: Evaluation of onset and long-term response. J Biomed Mater Res B Appl Biomater, 2018. 106(8): p. 2858-2868.

3.Lukina, E., et al., Fretting corrosion behavior of nitinol spinal rods in conjunction with titanium pedicle screws. Mater Sci Eng C Mater Biol Appl, 2017. 72: p. 601-610.

Spinal instrumentation systems are widely used for the treatment of traumatic spinal fractures, dislocations, and instability or deformity of the spine. In general, the surgical technique, the patient, the implant design and material affect the clinical outcome.

Various complications are reported associated with spinal instrumentation: malpositioning of instrumentation, pedicle screw loosening and/or breakage, rod breakage, pseudoarthrosis, osteolysis, and granuloma formation. In addition, evidence of corrosion observed on retrieved implants includes surface discoloration, superficial and severe metal loss, and signs of galvanic, crevice and particularly fretting corrosion. Corrosion mainly occurred at the junctions of the construction materials, raising questions about the impact of galvanic corrosion on the fretting-corrosion rates. The implants were made of a variety of materials, such as SS, CoCrMo alloy, and Ti6Al4V alloy.

A wide range of biomechanical and mechanical testing has been reported over the past decade, based on available ASTM and ISO standards. However, only few studies are published which investigate the corrosion and more specifically the fretting and tribo-corrosion behaviour of mixed and same-material spinal instrumentation under laboratory conditions. The experiments performed include combinations of mechanical testing, e.g. ASTM standards F1717 and F2129, with electrochemical corrosion test cells.[1-3]

This review will consolidate the understanding of corrosion that can be gleaned from analysis of retrieved hardware. It will bring forward an analysis of the potential for the role of corrosion in implant failure and implant degradation mechanisms, to be improved through laboratory testing of spinal instrumentation in corrosion cells designed to be able to measure in real-time. How an advanced corrosion analysis can complement the standard testing protocols already in use will be discussed. This review paper will also make comparisons and draw analogies to other tribocorrosion systems in metal implants and other relevant biomedical systems.

References

1.Mali, S.A., V. Singh, and J.L. Gilbert, Effect of mixed alloy combinations on fretting corrosion performance of spinal screw and rod implants. J Biomed Mater Res B Appl Biomater, 2017. 105(5): p. 1169-1177.

2.Singh, V., et al., Material dependent fretting corrosion in spinal fusion devices: Evaluation of onset and long-term response. J Biomed Mater Res B Appl Biomater, 2018. 106(8): p. 2858-2868.

3.Lukina, E., et al., Fretting corrosion behavior of nitinol spinal rods in conjunction with titanium pedicle screws. Mater Sci Eng C Mater Biol Appl, 2017. 72: p. 601-610.

The synovial fluid contains a glycoprotein with good lubricity thanks to surface-active phospholipids (SAPLs) [1]. Due to their polar head group and non-polar moieties, SAPLs become hydrophobic after adsorption to the hydrophilic articular surface. This protects the underlying cartilage against wear and ensures low coefficients of friction [2]. In fact, prosthetic retrieval studies have shown a significant SAPLs content when rinsing common joint replacement materials such as UHMWPE and metal alloys. Although few cases of retrieval pyrolytic carbon (PyC) implants have been performed, it has been established that PyC has a remarkable blood compatibility. Used so far in heart valves, PyC appears to be an ideal candidate for orthopedic applications such as hemi-shoulder arthroplasty [3]; one of the less invasive treatment preserving the glenoid.

It is necessary to establish the affinity of SAPLs or, at least, dipalmitoyl phosphatidylcholine (DPPC) with PyC. The aim of this study is to compare the adsorption of DPPC on several prosthetic implants after friction against articular cartilage.

Tribological tests between various biomaterials (PyC, CoCrMo or ZTA) and live cartilage discs were performed on a simulator [4] for 3 hours per day over 5 days. The lubricant was based on cell culture medium containing hyaluronic acid-phospholipid vesicles. Phospholipids (PLT) adsorbed on the biomaterials were quantified by HPLC after rinsing. Cartilage damage was also assessed by quantifying biochemical markers present in the lubricant. 

Although not significant, on average, PyC showed the lowest amount of saturated PLT (19.9 ug) after immersion in the lubricant. However, after rubbing surface analysis suggests that PyC induces stronger adsorption of saturated PLT than other biomaterials (43.4 ug vs .35.6-36.1 ug). By proving here that the affinity of SAPLs, key elements of lubrication, is strongly dependent on the biomaterial. Further analysis of PyC and its interaction with the SAPLs is necessary.

Figure 1 Masse of adsorbed saturated phospholipids on ceramic (ZTA), Cobalt alloy (CoCr) and Pyrolytic carbon (PyC) after immersion or wear test for 15 hours.

[1] Hills BA, Butler BD. Ann Rheumatic Dis 1984; 48:51–7

[2] Purbach B, Hills BA, Wroblewski BM. Clin Orthop Relat Res 2002; 395:115–8

[3] Garret J et al. J Shoulder Elbow Surg 2017, 26(7):1143-1151

[4] Veselack T et al. Lubricants 2018, 6, 19

The synovial fluid contains a glycoprotein with good lubricity thanks to surface-active phospholipids (SAPLs) [1]. Due to their polar head group and non-polar moieties, SAPLs become hydrophobic after adsorption to the hydrophilic articular surface. This protects the underlying cartilage against wear and ensures low coefficients of friction [2]. In fact, prosthetic retrieval studies have shown a significant SAPLs content when rinsing common joint replacement materials such as UHMWPE and metal alloys. Although few cases of retrieval pyrolytic carbon (PyC) implants have been performed, it has been established that PyC has a remarkable blood compatibility. Used so far in heart valves, PyC appears to be an ideal candidate for orthopedic applications such as hemi-shoulder arthroplasty [3]; one of the less invasive treatment preserving the glenoid.

It is necessary to establish the affinity of SAPLs or, at least, dipalmitoyl phosphatidylcholine (DPPC) with PyC. The aim of this study is to compare the adsorption of DPPC on several prosthetic implants after friction against articular cartilage.

Tribological tests between various biomaterials (PyC, CoCrMo or ZTA) and live cartilage discs were performed on a simulator [4] for 3 hours per day over 5 days. The lubricant was based on cell culture medium containing hyaluronic acid-phospholipid vesicles. Phospholipids (PLT) adsorbed on the biomaterials were quantified by HPLC after rinsing. Cartilage damage was also assessed by quantifying biochemical markers present in the lubricant. 

Although not significant, on average, PyC showed the lowest amount of saturated PLT (19.9 ug) after immersion in the lubricant. However, after rubbing surface analysis suggests that PyC induces stronger adsorption of saturated PLT than other biomaterials (43.4 ug vs .35.6-36.1 ug). By proving here that the affinity of SAPLs, key elements of lubrication, is strongly dependent on the biomaterial. Further analysis of PyC and its interaction with the SAPLs is necessary.

[1] Hills BA, Butler BD. Ann Rheumatic Dis 1984; 48:51–7

[2] Purbach B, Hills BA, Wroblewski BM. Clin Orthop Relat Res 2002; 395:115–8

[3] Garret J et al. J Shoulder Elbow Surg 2017, 26(7):1143-1151

[4] Veselack T et al. Lubricants 2018, 6, 19

Measuring the skin friction of human skin against textiles is essential for preventing friction-related skin discomfort, irritation, and skin injuries.  In addition, the further development of this area with respect to broad base applications such as health care and medical textiles, and for protective clothing such as those used during heavy-duty by firefighters, military or police personnel is very important.  This research aimed to demonstrate the repeatability of a novel device capable of attaining coefficient of friction measurements across six-testing areas located on the volar and dorsal forearm, chest, torso, and upper and lower back.  The experiment was conducted with 10 participants under controlled environmental condition (25^oC-%50RH). The friction probe featured an attached 100% cotton textile and was applied with 2 ± 0.5N normal force with a fixed motor power of 50% across the six previously identified test locations using a mixed counterbalanced order. The repeatability of the static and dynamic friction coefficient between the skin and textile scores was assessed with an interclass coefficient (ICC). The results showed static and dynamic COF skin/textile values of 0.91 ± 0.05 and 0.90 ± 0.06 ICC respectively expressing excellent repeatability (Koo and Li, 2016). Some test regions like lower back recorded lower repeatability than the other test sides, in which value of 0.84 ICC for static COF skin/textile was obtained, but still, it was expressed good repeatability and reliability.

Measuring the skin friction of human skin against textiles is essential for preventing friction-related skin discomfort, irritation, and skin injuries.  In addition, the further development of this area with respect to broad base applications such as health care and medical textiles, and for protective clothing such as those used during heavy-duty by firefighters, military or police personnel is very important.  This research aimed to demonstrate the repeatability of a novel device capable of attaining coefficient of friction measurements across six-testing areas located on the volar and dorsal forearm, chest, torso, and upper and lower back.  The experiment was conducted with 10 participants under controlled environmental condition (25^oC-%50RH). The friction probe featured an attached 100% cotton textile and was applied with 2 ± 0.5N normal force with a fixed motor power of 50% across the six previously identified test locations using a mixed counterbalanced order. The repeatability of the static and dynamic friction coefficient between the skin and textile scores was assessed with an interclass coefficient (ICC). The results showed static and dynamic COF skin/textile values of 0.91 ± 0.05 and 0.90 ± 0.06 ICC respectively expressing excellent repeatability (Koo and Li, 2016). Some test regions like lower back recorded lower repeatability than the other test sides, in which value of 0.84 ICC for static COF skin/textile was obtained, but still, it was expressed good repeatability and reliability.

Various deposition technologies are implemented by the Fast-Moving Consumer Goods industry to meet consumer needs such as sensorial benefits. Actives such as cationic surfactant or polymer, esterquats (quaternary ammonium compounds of two fatty acid chains with weak ester linkages) ¹, are commonly used to enhance fabric characteristics and hence sensorial benefits, as well as longevity of the garment ^2,3. Upon adsorption to the negatively charged fabric surfaces, the deposited actives are believed to adapt an orientation exposing the hydrophobic tail to the ambient, and hence reduce friction, providing a pleasant tactile perception.

The aim of this work is to establish the underpinning principle of textile perception, so as to develop suitable detergent chemistry accordingly to not only control the deposition kinetics of cationic actives on fabrics (natural and synthetic fibers), but also determine the consumer perception. 

A novel experimental approach was developed to correlate and model the physical-chemistry properties of the fabrics to the sensorial benefits for the consumers via tribological characteristics and passive acoustic emission signals, ⁴ linking tribological characteristics over multiple lengths scales with chemical formulation and the acoustic data. Finger-fabric friction data (Figure 1) provides valuable insight on the design of chemical compounds and formulation that can reduce the coefficient of friction measured. Neural Network analysis was used to differentiate acoustic emission data of one fabric to the other, which addresses the complexity associated with skin tribology.

Figure 1 Finger-fabric friction data for different fabric treatments on cotton terry

1.             Mishra, S. & Tyagi, V. K. Esterquats: the novel class of cationic fabric softeners. J. Oleo Sci. 56, 269–276 (2007).

2.             Crutzen, A. Fabric softeners. in Liquid Detergents (ed. Lai, K.-Y.) 487–554 (2006).

3.             Murphy, D. S. Fabric softener technology: A review. J. Surfactants Deterg. 18, 199–204 (2015).

4.             Benabdallah, H. S. & Aguilar, D. A. Acoustic emission and its relationship with friction and wear for sliding contact. Tribol. Trans.51, 738–747 (2008).

Various deposition technologies are implemented by the Fast-Moving Consumer Goods industry to meet consumer needs such as sensorial benefits. Actives such as cationic surfactant or polymer, esterquats (quaternary ammonium compounds of two fatty acid chains with weak ester linkages) ¹, are commonly used to enhance fabric characteristics and hence sensorial benefits, as well as longevity of the garment ^2,3. Upon adsorption to the negatively charged fabric surfaces, the deposited actives are believed to adapt an orientation exposing the hydrophobic tail to the ambient, and hence reduce friction, providing a pleasant tactile perception.

The aim of this work is to establish the underpinning principle of textile perception, so as to develop suitable detergent chemistry accordingly to not only control the deposition kinetics of cationic actives on fabrics (natural and synthetic fibers), but also determine the consumer perception. 

A novel experimental approach was developed to correlate and model the physical-chemistry properties of the fabrics to the sensorial benefits for the consumers via tribological characteristics and passive acoustic emission signals, ⁴ linking tribological characteristics over multiple lengths scales with chemical formulation and the acoustic data. Finger-fabric friction data (Figure 1) provides valuable insight on the design of chemical compounds and formulation that can reduce the coefficient of friction measured. Neural Network analysis was used to differentiate acoustic emission data of one fabric to the other, which addresses the complexity associated with skin tribology.

1.             Mishra, S. & Tyagi, V. K. Esterquats: the novel class of cationic fabric softeners. J. Oleo Sci. 56, 269–276 (2007).

2.             Crutzen, A. Fabric softeners. in Liquid Detergents (ed. Lai, K.-Y.) 487–554 (2006).

3.             Murphy, D. S. Fabric softener technology: A review. J. Surfactants Deterg. 18, 199–204 (2015).

4.             Benabdallah, H. S. & Aguilar, D. A. Acoustic emission and its relationship with friction and wear for sliding contact. Tribol. Trans.51, 738–747 (2008).

In oral cavity, titanium and in general all biomaterials are subjected to a wet microenvironment where microorganisms, glycoproteins, acid metabolites and products such as fluoride solutions present in toothpastes and mouthwashes can interact. In addition, they are in function, being subjected to centric and eccentric loads during chewing. The biomechanical performance of the implant-abutment interface is closely related to the connection geometry. The study of biotribocorrosion allows to evaluate the titanium surface behavior, the micro-leakage of microorganisms and saliva and their influence on corrosion and wear of the material. All these factors are determinants in peri-implant soft-tissues stability and long-term success.

The purpose was to determine in vitro methodologies used by the biomechanics research group of the National University of Colombia for the research of biotribology on the implant-abutment interface connection. 

Methods: The abutments and implants were scanned to obtain a real 3D dataset at 10X by a focus variation microscope (Alicona) before and after cyclic loading (250,000 cycles-120N-2Hz). Images 1 and 2. A medium contaminated with E. coli at 37^oC was applied in the dynamic loading device developed according to ISO 14801:2016. Bacterial cultures were performed to evaluate micro-leakage and quantify them before and after cyclic loading. The real 3D dataset   is then used to obtain the metrological assessment of the implant-abutment connection.

Results: Data of abrasive wear and plastic deformation were quantified. The application of dynamic loading under conditions like the oral cavity allowed to evaluate   aspects such as: bacterial leakage, the surface behavior in different media and   the   implant-abutment micro-gap analysis.

Conclusion: The optical high-resolution measurements enable traceable 3D geometry measurements and connection surface. These in vitro studies allow the evaluation of the impact of micro-leakage at the connections on the role of peri-implantitis as well as identifying design and manufacturing quality of them.

Image 1.  Dynamic Loading Device

Image 2.  Real 3D Dataset

In oral cavity, titanium and in general all biomaterials are subjected to a wet microenvironment where microorganisms, glycoproteins, acid metabolites and products such as fluoride solutions present in toothpastes and mouthwashes can interact. In addition, they are in function, being subjected to centric and eccentric loads during chewing. The biomechanical performance of the implant-abutment interface is closely related to the connection geometry. The study of biotribocorrosion allows to evaluate the titanium surface behavior, the micro-leakage of microorganisms and saliva and their influence on corrosion and wear of the material. All these factors are determinants in peri-implant soft-tissues stability and long-term success.

The purpose was to determine in vitro methodologies used by the biomechanics research group of the National University of Colombia for the research of biotribology on the implant-abutment interface connection. 

Methods: The abutments and implants were scanned to obtain a real 3D dataset at 10X by a focus variation microscope (Alicona) before and after cyclic loading (250,000 cycles-120N-2Hz). Images 1 and 2. A medium contaminated with E. coli at 37^oC was applied in the dynamic loading device developed according to ISO 14801:2016. Bacterial cultures were performed to evaluate micro-leakage and quantify them before and after cyclic loading. The real 3D dataset   is then used to obtain the metrological assessment of the implant-abutment connection.

Results: Data of abrasive wear and plastic deformation were quantified. The application of dynamic loading under conditions like the oral cavity allowed to evaluate   aspects such as: bacterial leakage, the surface behavior in different media and   the   implant-abutment micro-gap analysis. 

Conclusion: The optical high-resolution measurements enable traceable 3D geometry measurements and connection surface. These in vitro studies allow the evaluation of the impact of micro-leakage at the connections on the role of peri-implantitis as well as identifying design and manufacturing quality of them.

Introduction

The high wear resistance of hip prostheses (HPs) is a fundamental requirement for their clinical success. Pre-clinical assessment of novel HPs design typically requires long and costly experimental wear testing, consequently in-silico clinical trials represent an attractive low-cost option.    

Finite element (FE) wear models of HPs are the most widespread: they can predict the geometry changes due to wear, typically entailing user subroutines, to the detriment of high computational costs [1]. Mathematical wear (MAT) models, rarely reported in the literature [2], allow parametric formulations and fast predictions, although not implementing the geometry update.  

In this study we propose a comparison of MAT and FE wear models of HPs, with the aim of evaluate their complementary and optimized use for in-silico clinical trials. 

Methods

MAT and FE wear models of a metal-on-plastic HPs of 28 mm, were developed, reproducing a literature case study [2]. Unilateral wear on the cup was simulated assuming a constant wear coefficient. The Leeds ProSim hip simulator boundary conditions were simulated (Fig. 1). Some innovative aspects were considered in FE modelling: i) use of the intuitive wear toolbox of ANSYS Workbench^®; ii) use of computational strategies such as accelerated wear/   submodeling techniques. 

Results and discussion

MAT and FE wear predictions at 1 Mc were compared demonstrating the validity of the mathematical approach for the running-in phase (same wear volume and similar wear maps, Fig.2). 

On the other hand, the wear evolution in the steady state phase can be captured only by the FE model. As an example, the surface geometrical changes during a wear cycle is shown in Fig.3 (maximum wear at the maximum load). 

Results suggest the combined use of MAT and FE models so to achieve the most accurate wear predictions at the lowest computational cost.

References

1.Mattei,Trib Int,63:66-77,2013.

2.Liu,Proc IMECHe H:J Eng Med,225:16-24,2011.

Introduction

The high wear resistance of hip prostheses (HPs) is a fundamental requirement for their clinical success. Pre-clinical assessment of novel HPs design typically requires long and costly experimental wear testing, consequently in-silico clinical trials represent an attractive low-cost option.    

Finite element (FE) wear models of HPs are the most widespread: they can predict the geometry changes due to wear, typically entailing user subroutines, to the detriment of high computational costs [1]. Mathematical wear (MAT) models, rarely reported in the literature [2], allow parametric formulations and fast predictions, although not implementing the geometry update.  

In this study we propose a comparison of MAT and FE wear models of HPs, with the aim of evaluate their complementary and optimized use for in-silico clinical trials. 

Methods

MAT and FE wear models of a metal-on-plastic HPs of 28 mm, were developed, reproducing a literature case study [2]. Unilateral wear on the cup was simulated assuming a constant wear coefficient. The Leeds ProSim hip simulator boundary conditions were simulated (Fig. 1). Some innovative aspects were considered in FE modelling: i) use of the intuitive wear toolbox of ANSYS Workbench^®; ii) use of computational strategies such as accelerated wear/   submodeling techniques.

Results and discussion

MAT and FE wear predictions at 1 Mc were compared demonstrating the validity of the mathematical approach for the running-in phase (same wear volume and similar wear maps, Fig.2). 

On the other hand, the wear evolution in the steady state phase can be captured only by the FE model. As an example, the surface geometrical changes during a wear cycle is shown in Fig.3 (maximum wear at the maximum load). 

Results suggest the combined use of MAT and FE models so to achieve the most accurate wear predictions at the lowest computational cost.

References

1.Mattei,Trib Int,63:66-77,2013.

2.Liu,Proc IMECHe H:J Eng Med,225:16-24,2011.

It is known from past research that the concentration of biomolecules that constitute synovial fluid (SF) plays a crucial role in boundary lubrication and wear protection of sliding surfaces. In this study, we aim to understand the effects of diluting SF on film formation and wear protection on model oxide surfaces. Our adsorption isotherm curves obtained with quartz crystal microbalance with dissipation (QCM-D) show that model oxide surfaces (silica) are equally saturated with SF components diluted up to a factor of 1:10 (surface mass density, Γ_10%-100% = 900 ng/cm²), with higher dilutions resulting in unsaturated surfaces (Γ_1%-10% < 900 ng/cm²). Interestingly, SF with digested hyaluronan showed a very similar adsorption isotherm curve to that of undigested SF, suggesting that high molecular weight hyaluronan is not required for full film formation. Preliminary micro-tribometer results of high-density polyethylene (HDPE) sliding against silica indicate, however, that there is an inverse relationship between films formed with different concentrations of SF and coefficients of friction. Coefficients of friction decreased from µ_100% = 0.3 to µ_10% = 0.15 for films formed with full (100%) or 1:10 diluted SF (10%), respectively. Our results suggest that film molecular compositions formed with undiluted or diluted SF are different, despite similar surface mass densities. To complement adsorption and micro-tribology characterization, we are currently investigating film morphologies and composition. With these studies, we will contribute to understanding synergistic molecular interactions of SF components that regulate film formation and wear protection of synthetic surfaces.

It is known from past research that the concentration of biomolecules that constitute synovial fluid (SF) plays a crucial role in boundary lubrication and wear protection of sliding surfaces. In this study, we aim to understand the effects of diluting SF on film formation and wear protection on model oxide surfaces. Our adsorption isotherm curves obtained with quartz crystal microbalance with dissipation (QCM-D) show that model oxide surfaces (silica) are equally saturated with SF components diluted up to a factor of 1:10 (surface mass density, Γ_10%-100% = 900 ng/cm²), with higher dilutions resulting in unsaturated surfaces (Γ_1%-10% < 900 ng/cm²). Interestingly, SF with digested hyaluronan showed a very similar adsorption isotherm curve to that of undigested SF, suggesting that high molecular weight hyaluronan is not required for full film formation. Preliminary micro-tribometer results of high-density polyethylene (HDPE) sliding against silica indicate, however, that there is an inverse relationship between films formed with different concentrations of SF and coefficients of friction. Coefficients of friction decreased from µ_100% = 0.3 to µ_10% = 0.15 for films formed with full (100%) or 1:10 diluted SF (10%), respectively. Our results suggest that film molecular compositions formed with undiluted or diluted SF are different, despite similar surface mass densities. To complement adsorption and micro-tribology characterization, we are currently investigating film morphologies and composition. With these studies, we will contribute to understanding synergistic molecular interactions of SF components that regulate film formation and wear protection of synthetic surfaces.

Introduction: A cartilage-on-cartilage motion exhibits very low friction under physiological conditions. This seems to be established by an interaction between cartilage extracellular matrix and synovial fluid. Hyaluronic acid is one of the main components of synovial fluid and also the main constituent which affects the rheology of synovial fluid. Hyaluronic acid is a polymer and differences in its molecular weight can significantly affect its rheology. This study is focused on the rheological analysis of hyaluronic acid solutions and changes in cartilage friction caused by differences in hyaluronic acid molecular weight.

Methods: Viscosity and viscoelastic properties of hyaluronic acid solutions were analyzed with rotational rheometer in cone-plate and plate-plate configuration. In total, four hyaluronic acid solutions with molecular weight between 77 kDa and 2 010 kDa were tested. The frictional measurements were realized on a commercial tribometer Bruker UMT TriboLab, while the coefficient of friction dependency on sliding distance was measured. The contact couple consisted of the intact porcine articular cartilage pin and the plate made from optical glass. Previously tested hyaluronic acid solutions were used as lubricants.

Fig. 1: Scheme of frictional measurements

Results and discussion: Rheological measurements showed a strong dependency between the molecular weight and viscosity or viscoelastic properties of hyaluronic acid solutions. Hyaluronic acid solutions with higher molecular weight exhibited higher viscosity, dynamic moduli and shear thinning ratio. Frictional measurements showed substantial dispersion in the results showing no clear dependency between the hyaluronic acid molecular weight and the friction in the cartilage-on-glass contact. Results showed that mechanical properties and overall conditions of individual cartilage specimen can significantly affect the effectiveness of hyaluronic acid solutions during the sliding motion.

Fig. 2: From the left: Viscosity curves of tested hyaluronic acid solutions; development of coefficient of friction in cartilage contact lubricated by the same solutions

Introduction: A cartilage-on-cartilage motion exhibits very low friction under physiological conditions. This seems to be established by an interaction between cartilage extracellular matrix and synovial fluid. Hyaluronic acid is one of the main components of synovial fluid and also the main constituent which affects the rheology of synovial fluid. Hyaluronic acid is a polymer and differences in its molecular weight can significantly affect its rheology. This study is focused on the rheological analysis of hyaluronic acid solutions and changes in cartilage friction caused by differences in hyaluronic acid molecular weight.

Methods: Viscosity and viscoelastic properties of hyaluronic acid solutions were analyzed with rotational rheometer in cone-plate and plate-plate configuration. In total, four hyaluronic acid solutions with molecular weight between 77 kDa and 2 010 kDa were tested. The frictional measurements were realized on a commercial tribometer Bruker UMT TriboLab, while the coefficient of friction dependency on sliding distance was measured. The contact couple consisted of the intact porcine articular cartilage pin and the plate made from optical glass. Previously tested hyaluronic acid solutions were used as lubricants.

Results and discussion: Rheological measurements showed a strong dependency between the molecular weight and viscosity or viscoelastic properties of hyaluronic acid solutions. Hyaluronic acid solutions with higher molecular weight exhibited higher viscosity, dynamic moduli and shear thinning ratio. Frictional measurements showed substantial dispersion in the results showing no clear dependency between the hyaluronic acid molecular weight and the friction in the cartilage-on-glass contact. Results showed that mechanical properties and overall conditions of individual cartilage specimen can significantly affect the effectiveness of hyaluronic acid solutions during the sliding motion.

Introduction:

The composition, topography, adhesiveness and nanomechanical properties of biomaterials are all factors that affect biotribology and biological processes, e.g., cell differentiation, morphogenesis and tissue formation [1-4]. Atomic Force Microscopy (AFM) is a highly versatile tool, ideal for the characterization of these properties, from single molecules to cells and tissues, on the nm scale.

Methods and Results:

JPK BioAFMs, like NanoWizard^® ULTRA Speed 2, enable fast imaging of challenging biological samples and visualization of dynamic processes with high spatio-temporal resolution under near physiological conditions. The kinetics of collagen type I fibrillogenesis was imaged in situ revealing the formation of the 67 nm D-banding hallmark.

Quantitative Imaging mode (QI™) measures sample properties e.g. stiffness and nanomechanics on the nanometer scale. Complex data like contact point, Young´s modulus or recognition images can also be extracted at the same resolution.

Investigating large, sticky and rough samples such as tissues and hydrogels using AFM has always been a challenge due to the limited z-axis of the AFM. The HybridStage™, equipped with an extended xyz scanner unit up to 300x300x300 µm³, a motorized unit for large sample movements in the mm range and optical tiling, is ideal for investigating such samples. It enables multi-region AFM probing over a large, rough sample area and provides additional correlative optical data sets.

Adhesion dynamics between cells and biomaterials play a crucial role in, e.g. the applicability of potential implant materials. The AFM based Single Cell Force Spectroscopy platform enables quantitative measurement of the interactions between individual cells and any substrate.

Discussion:

We will demonstrate the capabilities of BioAFM to yield complementary information on a variety of biomaterial samples.

[1]          Elter, P. et al., Eur Biophys J, 2011, 40(3):317-27

[2]          Engler AJ. Et al., Cell; 2006, 126(4):677-89

[3]          Cisneros, DA. et al., Small, 2007, 3(6):956-63

[4]          Koser DA. Et al., Nat. Neurosci.; 2016, 19:1592-1598

Fig. Outline of a singe cell force spectroscopy experiment

Introduction:

The composition, topography, adhesiveness and nanomechanical properties of biomaterials are all factors that affect biotribology and biological processes, e.g., cell differentiation, morphogenesis and tissue formation [1-4]. Atomic Force Microscopy (AFM) is a highly versatile tool, ideal for the characterization of these properties, from single molecules to cells and tissues, on the nm scale.

Methods and Results:

JPK BioAFMs, like NanoWizard^® ULTRA Speed 2, enable fast imaging of challenging biological samples and visualization of dynamic processes with high spatio-temporal resolution under near physiological conditions. The kinetics of collagen type I fibrillogenesis was imaged in situ revealing the formation of the 67 nm D-banding hallmark.

Quantitative Imaging mode (QI™) measures sample properties e.g. stiffness and nanomechanics on the nanometer scale. Complex data like contact point, Young´s modulus or recognition images can also be extracted at the same resolution.

Investigating large, sticky and rough samples such as tissues and hydrogels using AFM has always been a challenge due to the limited z-axis of the AFM. The HybridStage™, equipped with an extended xyz scanner unit up to 300x300x300 µm³, a motorized unit for large sample movements in the mm range and optical tiling, is ideal for investigating such samples. It enables multi-region AFM probing over a large, rough sample area and provides additional correlative optical data sets.

Adhesion dynamics between cells and biomaterials play a crucial role in, e.g. the applicability of potential implant materials. The AFM based Single Cell Force Spectroscopy platform enables quantitative measurement of the interactions between individual cells and any substrate.

Discussion:

We will demonstrate the capabilities of BioAFM to yield complementary information on a variety of biomaterial samples.

[1]          Elter, P. et al., Eur Biophys J, 2011, 40(3):317-27

[2]          Engler AJ. Et al., Cell; 2006, 126(4):677-89

[3]          Cisneros, DA. et al., Small, 2007, 3(6):956-63

[4]          Koser DA. Et al., Nat. Neurosci.; 2016, 19:1592-1598