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A simple model for speed-invariant shear stress in biological aqueous lubrication has been developed based on previous observations of these fluid films and our foundational understanding of these complex fluids. The model revealed that the exponent of the shear-thinning behavior is not a critical parameter in speed invariance. Additionally, due to the localization of fluidization as a result of gradients in concentration through the aqueous gel network, the shear-thinning behavior results in a monotonic decline in viscosity to the high shear-rate viscosity plateau, η∞. Finally, and perhaps somewhat surprising, was the finding that the optimal gradient was the weakest possible gradient. These findings are consistent with our understanding of aqueous lubrication across soft biological interfaces, which is sensitive to the macromolecular quality of the gel-spanning networks and the water content but not the sliding speed. The finding that this speed-invariant shear stress does not require a unique concentration profile reveals a built-in mechanism of biological resilience to maintain lubricity over a wide range of conditions. Gradients in concentration localize fluidization at the lowest concentration regime during sliding. This leads to an expanding shear zone with proportionally thicker fluidized zones for increasing values of sliding speed resulting in roughly equivalent shear rate and shear stress for all sliding speeds [ABSTRACT FROM AUTHOR]

To understand the dissipative mechanisms in soft hydrogel lubrication, polyacrylamide (PAAm) hydrogels with two distinct surface structures were examined under various contact conditions. The characteristic speed-dependent friction of the self-mated, crosslinked hydrogel surfaces could be explained by hydrodynamic shearing of a thin water layer between two rather impermeable bodies. On the other hand, the frictional response of brushy hydrogel surfaces is dependent on the contact conditions and the level of surface hydration. In a migrating contact, brushy hydrogels showed low, speed-independent friction (µ ~ 0.01) likely due to a thick layer of shearing liquid trapped within the sparse surface network. In stationary contact, however, brushy hydrogel surfaces can partially exude water from the near-surface region over time, as shown by time-resolved Fourier-transform infrared (FTIR) spectroscopy. This is assumed to be reflected in a friction increase over time. Interfacial shearing appears to shorten the characteristic exudation times compared to those observed under static loading. Once fluid has been exuded, brushy surfaces were shown to reach similar friction values as their crosslinked analogs. The results thus indicate that the dominating dissipation mechanism during sliding at low contact pressures is shearing of the interfacial liquid film, rather than poro-elastic dissipation within the bulk. Maintenance of surface hydration is therefore crucial, in order to take advantage of the low friction of such systems. [ABSTRACT FROM AUTHOR]

This study focuses mainly on potential skin abrasion risks that are likely to be caused by the interactive force on a contact surface between a human body and a physical assistant robot's cuff, particularly when these robots are misused. In this study, we develop a novel method for evaluating skin contact states in order to estimate abrasion risks. First, a simplified cuff model is designed to conduct abrasion tests on a piece of dummy skin. The skin is pasted onto a dummy's leg to simulate the human skin's viscoelastic properties. Then, the edge condition of the contact surface is measured while the cuff model slides on the dummy skin. We conduct an analysis of the interactive force and displacement between the cuff model and dummy skin and several principal influential factors are examined during the evolution of abrasion-induced skin damage. These results are applied to evaluate the contact conditions between the robot's cuff and human skin, which makes a significant contribution to estimating the risk of abrasion injuries for a physical assistant robot user. [ABSTRACT FROM AUTHOR]

Biotribology and tribocorrosion are often not included in numerical or computational modeling efforts to predict wear because of the apparent complexity in the geometry, the variability in removal rates, and the challenge associated with mixing time-dependent removal processes such as corrosion with cyclic material removal from wear. The lollipop is an accessible bio-tribocorrosion problem that is well known but underexplored scientifically as a tribocorrosion process. Stress-assisted dissolution was found to be the dominant tribocorrosion process driving material removal in this system. A model of material removal was described and approached by lumping the intrinsically time-dependent process with a mechanically driven process into a single cyclic volumetric material removal rate. This required the collection of self-reported wear data from 58 participants that were used in conjunction with statistical analysis of actual lollipop cross-sectional information. Thousands of repeated numerical simulations of material removal and shape evolution were conducted using a simple Monte Carlo process that varied the input parameters and geometries to match the measured variability. The resulting computations were analyzed to calculate both the average number of licks required to reach the Tootsie Roll center of a Tootsie Roll pop, as well as the expected variation thereof. [ABSTRACT FROM AUTHOR]

In order to investigate the tribological behavior of medical devices in contact with tissue, friction tests for four kinds of medical metallic alloys (316L stainless steel, CoCr, NiTi and TiMoSn) on soft tissue-mimicking poly(vinyl alcohol) hydrogel (PVA-H) biomodel were carried out at low normal load. XPS analysis and wettability tests for them were prepared to understand the difference in friction. According to the surface oxide compositions, these alloys can be divided into two groups: 'Fe/Cr-oxide-surface alloys' for 316L and CoCr, and 'Ti-oxide-surface alloys' for NiTi and TiMoSn. From the wettability test, Fe/Cr-oxide-surface alloys show lower polar components of surface free energy than Ti-oxide-surface alloys. Fe/Cr-oxide-surface alloys show higher friction coefficients in the elastic friction domain than those of Ti-oxide-surface alloys, while there was no significant difference in the hydrodynamic lubrication. Since elastic friction is governed by the adsorption of hydrogel polymer on counterbody, the surface characteristic of alloys plays an important role in friction. A tentative explanation for this tendency is expressed by linking two different theories describing the adsorption force of hydrogel and wettability of countermaterial. [ABSTRACT FROM AUTHOR]

Ocular tear film mucins and lipids promote lubricity of the corneal surface during ocular movements. The mechanisms of this lubricity are difficult to model and to measure due to the delicate nature of the film itself and the conditions under which it exists. This study describes a kinetic friction coefficient measured between a glass probe and a living mouse eye. A portable custom micro-tribometer was used to prescribe sliding motions and record normal and frictional forces. Friction coefficient measured over both sliding directions resulted in µ = 0.068 under a pressure of approximately 12 kPa. In vivo measurements may enhance the understanding of corneal friction response, as well as provide an empirical friction coefficient for more complex mechanical models. [ABSTRACT FROM AUTHOR]

The human skin is constantly in interaction with materials and products. Therefore, skin friction is relevant to all people. In the literature, the frictional properties of the skin have been linked to a large variety of variables, like age, gender and hydration. The present study compares the data of 450 skin friction measurements with the skin hydration and skin temperature on four locations on the body, measured with four materials: stainless steel (SST), aluminum (Al), PE and PTFE. The median skin temperature was 32.1 °C and the median skin hydration was 25.5 AU, as measured with a Corneometer. The median coefficient of friction was 0.52 for the static coefficient of friction and 0.36 for the dynamic coefficient of friction. There was a linear relationship between those two types of coefficient of friction. The coefficient of friction was highest for SST and lowest for PTFE. The frictional properties depend on the skin hydration and skin temperature. Gender differences were found for both skin hydration and coefficient of friction. Most of the variation in the coefficients of friction could be explained by the differences in hydration. [ABSTRACT FROM AUTHOR]

As a living tissue, human skin has a biological response when it rubs against other external surfaces, among which, the comfort sensation attributes during friction contact make an important contribution to one's quality of life. However, limited quantitative parameters can be used to describe the sensations, and they have rarely been studied scientifically. In this paper, the comfort sensations of human volar forearm skin during friction testing were studied by biofeedback of physiological and psychological responses. A UMT-II tribometer was used to measure tribological parameters of the skin under different normal force of 0.2 and 1.0 N, and corresponding comfort sensations of the skin were assessed quantitatively using BioTrace + for physiological signals monitoring: skin conductance, skin temperature, and electroencephalogram (EEG). The psychological responses were characterized qualitatively according to the volunteers' sensations of pain, drag, and heat. Results showed that the tangential force, amplitudes of EEG signals and psychological responses increased with the normal force increasing. The friction coefficients, differences of skin conductance and temperature, amplitudes of EEG signals, and psychological responses gradually decreased with the number of friction tests increasing. The discomfort sensations of human skin were strongly related to friction conditions, which intensified under the large normal force, and gradually weakened with the number of tests increasing. The physiological responses were in accord with the psychological ones. The comfort sensations during friction testing can be assessed quantitatively by the physiological signals of conductance, temperature, and EEG. [ABSTRACT FROM AUTHOR]

Local backside wear measurements on ultra-high molecular weight polyethylene (UHMWPE) tibial inserts in LCS mobile bearing knee prostheses have been performed using a new radioisotope tracing technique. The radioisotope tracers Ru and Rh were synthesized via a fusion evaporation reaction and recoil-implanted into cylindrical plugs of UHMWPE. The labelled plugs were carefully fitted into tibial inserts at two relevant locations. With bovine serum acting as a lubricant, the tibial inserts were then worn in vitro for 500,000 and 780,000 cycles, respectively, in a pneumatic knee motion simulator. Results reflect the non-linear change of wear during the wear-in phase and its evolution to a long-term steady-state rate. This new technique shows potential for extracting localized wear rates across the backside of a tibial insert in order to develop a comprehensive backside wear model. [ABSTRACT FROM AUTHOR]

The surface chemical compositions of three types of silicone hydrogel contact lenses, PureVision (balafilcon A), ACUVUE OASYS (senofilcon A), and OOPTIX (lotrafilcon B), were analyzed using X-ray photoelectron spectroscopy prior to and following treatment in a test solution of diblock copolymer of poly(ethylene oxide) and poly(butylene oxide) (EO-BO). Prior to treatment, differences in surface elemental compositions of the lenses were found to reflect known bulk compositions and/or respective surface treatments. Following solution treatment, surface chemical modifications were apparent in balafilcon A and lotrafilcon B, especially in the distribution of chemical functionalities present at the surface. Only modest changes in surface composition were observed for the senofilcon A material. Atomic force microscopy (AFM) was employed to evaluate the surface topography and frictional properties of the lenses prior to and following similar solution treatments. AFM measurements in saline revealed large disparities between the coefficients of friction of the three lenses, with balafilcon A and lotrafilcon B exhibiting coefficients of friction approximately five times greater than that of senofilcon A. Lens surface treatment with the diblock copolymer test solution produced a significant reduction in the coefficients of friction of the two lenses exhibiting higher friction, yet only a small reduction in friction was observed for senofilcon A lens. Together, these results depict a strong correlation between the surface chemistry and frictional response of the lens systems as they relate to solution treatment with this specific diblock copolymer. [ABSTRACT FROM AUTHOR]

In this review, we discuss the current knowledge on the tribology of human skin and present an analysis of the available experimental results for skin friction coefficients. Starting with an overview on the factors influencing the friction behaviour of skin, we discuss the up-to-date existing experimental data and compare the results for different anatomical skin areas and friction measurement techniques. For this purpose, we also estimated and analysed skin contact pressures applied during the various friction measurements. The detailed analyses show that substantial variations are a characteristic feature of friction coefficients measured for skin and that differences in skin hydration are the main cause thereof, followed by the influences of surface and material properties of the contacting materials. When the friction coefficients of skin are plotted as a function of the contact pressure, the majority of the literature data scatter over a wide range that can be explained by the adhesion friction model. The case of dry skin is reflected by relatively low and pressure-independent friction coefficients (greater than 0.2 and typically around 0.5), comparable to the dry friction of solids with rough surfaces. In contrast, the case of moist or wet skin is characterised by significantly higher (typically >1) friction coefficients that increase strongly with decreasing contact pressure and are essentially determined by the mechanical shear properties of wet skin. In several studies, effects of skin deformation mechanisms contributing to the total friction are evident from friction coefficients increasing with contact pressure. However, the corresponding friction coefficients still lie within the range delimited by the adhesion friction model. Further research effort towards the analysis of the microscopic contact area and mechanical properties of the upper skin layers is needed to improve our so far limited understanding of the complex tribological behaviour of human skin. [ABSTRACT FROM AUTHOR]

The current paper describes an experimental study of the friction of the human finger pad. The data highlight the role of sweat secretion and contact occlusion in producing wide-ranging values for the coefficient of friction that are particularly sensitive to the tribological configuration, sliding velocity, surface roughness and porosity of the counterbody. In particular, the large coefficients of friction typically observed on dry smooth surfaces are associated with a relatively damp interface, and can be considerably reduced by either decreasing or increasing the interfacial moisture content or by surface roughening. It is concluded that the very large range in the values of the coefficient of friction reported in the literature mainly result from differences in occlusion time associated with different tribological configurations, as well as from variations in surface roughness and finger pad sweat rates. [ABSTRACT FROM AUTHOR]

Excessive frictional loading to the skin often results in the formation of blisters, due to the transmission of shear loading to the interfaces between dermal cell strata. The consequences of blistering range from mild discomfort to serious infection. In some patients, such as those disposed to epidermolysis bullosa or neuropathic diabetes, blisters can severely degrade life quality. Investigation of environmental and application parameters that affect blister formation has occurred primarily as a qualitative, observational pursuit on human subjects, which has often led to confounding of data and lack of repeatability. The authors have developed a Synthetic Skin Simulant Platform (3SP) that reproduces the mechanical behavior of human skin when exposed to tribological loading. The platform is an assembled construct of bonded elastomeric layers that act as surrogates for the epidermis, basement membrane, dermis, and subdermal structure. Epidermal (top layer) materials are typically silicone or polyurethane films with a friction coefficient akin to human skin, while sublayers display mechanical properties similar to their anatomical analogs. Blistering is evident optically by examining the separation voids formed after applying shear loads to the epidermal layer. The 3SP has been used in a two-axis pin-on-flat tribometer with a stainless steel indenter to study the normal load and friction coefficients encountered at the onset of frictional blistering. The 3SP allows for modulation of friction coefficient, interfacial adhesion strength, and subdermal stiffness for investigation of blistering damage to various anatomical sites. Experimental results have been compared to human test data and have shown that the 3SP provides the potential to make significant advances with respect to skin tribology research. [ABSTRACT FROM AUTHOR]

In this study, the friction of eight kinds of poly(vinyl alcohol) hydrogel (PVA-H) samples has been studied under various load and velocity conditions to elucidate the effects of PVA factors such as the degree of polymerization (DP) and the saponification value (SV) on the tribological behavior of PVA-H. Results showed the variations of the friction properties due to the PVA factors in the two friction conditions found for the hydrogels: elastic friction and hydrodynamic lubrication. In the elastic friction, the larger frictions were induced by the higher values of DP and SV. In the hydrodynamic lubrication, on the other hand, PVA-Hs with lower SV showed larger friction. The results can then be used to adjust the parameters of PVA-H in order to get given friction properties, for instance for the friction between catheter or scalpel and PVA-H, which can be used as a biomodel material of artery or oral mucosa, for the training of surgeons. [ABSTRACT FROM AUTHOR]

Previous studies have established that some of the wear damage seen on cast CoCrMo joint surface is caused by entrained third-body hard particles. In this study, wet-cell micro-indentation and nano-scratch tests have been carried out with the direct aim of simulating wear damage induced by single abrasive particles entrained between the surfaces of cast CoCrMo hip implants. In situ electrochemical current noise measurements were uniquely performed to detect and study the wear-induced corrosion as well as the repassivation kinetics under the micro-/nano-scale tribological process. A mathematical model has been explored for the CoCrMo repassivation kinetics after surface oxide film rupture. Greater insights into the nature of the CoCrMo micro-/nano-scale wear-corrosion mechanisms and deformation processes are determined, including the identification of slip band formation, matrix/carbide deformation, nanocrystalline structure formation and strain-induced phase transformation. The electrochemical current noise provides evidence of instantaneous transient corrosion activity at the wearing surface resulting from partial oxide rupturing and stripping, concurrent with the indent/scratch. [ABSTRACT FROM AUTHOR]

The aim of this article is to propose an analytical approximate squeeze-film lubrication model of the human ankle joint for a quick assessment of the synovial pressure field and the load carrying due to the squeeze motion. The model starts from the theory of boosted lubrication for the human articular joints lubrication (Walker et al., Rheum Dis 27:512-520, 1968; Maroudas, Lubrication and wear in joints. Sector, London, 1969) and takes into account the fluid transport across the articular cartilage using Darcy's equation to depict the synovial fluid motion through a porous cartilage matrix. The human ankle joint is assumed to be cylindrical enabling motion in the sagittal plane only. The proposed model is based on a modified Reynolds equation; its integration allows to obtain a quick assessment on the synovial pressure field showing a good agreement with those obtained numerically (Hlavacek, J Biomech 33:1415-1422, 2000). The analytical integration allows the closed form description of the synovial fluid film force and the calculation of the unsteady gap thickness. [ABSTRACT FROM AUTHOR]

mathematical model has been developed for the understanding of temperature distribution in knee joint. Temperature rises in knee joint as a result of frictional energy. This heated synovial fluid enters into the articular cartilage by the process of filtration and supplies heat to cartilage and bone. This cooled fluid again mixes well with the lubricant in the joint cavity. The problem is formulated as a two-region flow and diffusion model: flow and thermal diffusion within the intra-articular gap; and within the porous matrix covering the approaching bones at the joint. The solution of the coupled mixed boundary value problem is solved by using perturbation method. It has been observed that, in certain diseased and or old synovial joints, the movement of the fluid into or out of the cartilage resisted, and therefore, the temperature does rise. The temperature does rise in old and diseased joints as observed by varying the values of parameters from its normal values. These values refer to old age and/or diseases affecting degeneration of synovial fluid and or cartilage. [ABSTRACT FROM AUTHOR]

Latex versus glass has frequently been used as a model system for the investigation of natural lubrication mechanisms, despite its significant differences from articular cartilage pairings. The differences in surface chemistry account for its different behavior in terms of protein adsorption and lubrication. While cartilage is well known for its protein resistance, most proteins present in synovial fluid can non-specifically adsorb onto latex or glass. We have investigated latex-versus-glass lubrication by means of pin-on-disk tribometry in the presence of synovial-fluid proteins and glycoproteins, focusing on the influence of the glass-cleaning procedure on friction. In order to simulate the effects of possible contamination of glass in previous studies, both hydrophilic and hydrophobic glass substrates were tested. Albumin was shown to impair lubrication (in comparison to PBS) when latex was slid against both types of glass surface, whereas bovine synovial fluid (BSF) and alpha-1-acid glycoprotein (AGP) impaired the lubrication of latex versus hydrophilic glass and improved the lubrication of latex versus hydrophobic glass. Protein adsorption on the surfaces was monitored by means of fluorescence imaging and optical waveguide lightmode spectroscopy (OWLS), which revealed a faster and greater amount of adsorption of AGP on hydrophobic surfaces than on hydrophilic ones. The influence of surface chemistry on the friction behavior of BSF and on the adsorption of AGP suggests that it plays a role in determining the relative amounts of adsorbed synovial fluid proteins. When BSF is used as a lubricant in the latex-versus-hydrophobic-glass system, more of the AGP, relative to albumin, appears to adsorb on both surfaces, counteracting the negative effect of albumin on friction. It therefore seems that latex on glass, while displaying nominal similarities to cartilage on cartilage under certain conditions, is not a useful model system. Moreover, surface contamination of the glass can play a major role in determining the results. [ABSTRACT FROM AUTHOR]

The presence of “proteinaceous β-sheet rich fibrillar structures” and amyloidogenic material, has been alluded to extensively in the literature, in association with natural materials exhibiting superior mechanical strength per unit volume. Here we provide a clear experimental demonstration and explanation for why individual amyloid quaternary structures themselves have beneficial mechanical characteristics. [ABSTRACT FROM AUTHOR]

This paper presents a study on friction and indentation of attachment pads of the great green bush cricket, using a system involving a biological material and a flat surface. The pads are evolutionarily optimized frictional devices that are adapted to attach the insect to a variety of natural surfaces. The attachment pads of both living and dead insects were tested and evaluated using several analysis techniques. These techniques included friction analysis by microtribometry, microindentation, scanning electron microscopy (SEM), histologic sectioning, and shock freezing. Microtribological analysis showed that the friction between pad and surface is anisotropic. The anisotropy increases with increasing normal force. Microindentation revealed that the pads respond very flexibly to applied loads leading to an optimized morphology with respect to contact area. All results make clear that the microtribological properties are a perfect combination of surface structure, lubrication and active interface control. [ABSTRACT FROM AUTHOR]