Skin Tribology Research Articles

Multidimensional outline of experimental techniques for human skin tribology: A scoping review

Human skin tribology involves the study of friction behaviour of skin, with other materials in contact. Numerous studies have been conducted based both on the simulations and experimentations for the tribological investigations. Present study reports key experimental findings post year 2000 in which human skin (in vivo) is taken as test specimen. This review study highlights the latest technologies and trends that is in practice for tribological investigations on the human skin. For the selection of articles, the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) technique is adapted. Articles were screened out using various inclusion and exclusion criteria. Classifications were made based on the site of investigations viz. fingertip (22%), finger pad (36%), volar forearm (32%), and other regions (10%) as the anatomical regions. Study concludes that the maximum investigations have been performed on fabric/ textile followed by polymers, metals, plastics, and many others. Shearing has come up as the prominent factor for the erosion of soft surfaces. Study concludes the stick- slip friction to be the foremost contributor to the adhesion friction.

Impact of Indenter Size and Microrelief Anisotropy on the Tribological Behavior of Human Skin.

Everyday, we interact with screens, sensors, and many other devices through contact with the skin. Experimental efforts have increased our knowledge of skin tribology but are challenged by the fact that skin has a complex structure, undergoes finite deformations, has nonlinear material response, and has properties that vary with anatomical location, age, sex, and environmental conditions. Computational models are powerful tools to dissect the individual contribution of these variables to the overall frictional response. Here, we present a three-dimensional high-fidelity multilayer skin computational model including a detailed surface topography or skin microrelief. Four variables are explored: local coefficient of friction (COF), indenter size, mechanical properties of the stratum corneum, and displacement direction. The results indicate that the global COF depends nonlinearly on the local COF, implying a role for skin deformation on the friction response. The global COF is also influenced by the ratio of the indenter size to the microrelief features, with larger indenters smoothing out the role of skin topography. Changes in stiffness of the uppermost layer of skin associated with humidity have a substantial effect on both the contact area and the reaction forces, but the overall changes in the COF are small. Finally, for the microrelief tested, the response can be considered isotropic. We anticipate that this model and results will enable the design of materials and devices for a desired interaction against skin.

Analysis on Characteristics of Implants and Surface Bionic Prosthetic Materials

According to the 19902020 Internet for literature on materials commonly used in prosthetics and their compatibility with skin tribology and biology, it found that with the large number of amputees affected by the disease, traffic accidents, work-related injuries, and natural disasters, the relatively high manufacturing costs of prostheses could place a financial burden on patients. In addition, the choice of prosthetic material itself affects the comfort and ease of use for the patient. Whether it is an osseointegrated prosthesis or a cavity-receiving prosthesis, the use of appropriate materials is an important issue in prosthetic engineering. This paper analyses the research conducted over the last 20 years on materials for implantable prostheses and receptive cavity materials and presents some of the findings on the preferred material selection for each type of material, as well as the associated biomechanical effects and skin tribology. Through the analysis, the paper concludes that for implantable materials, biocompatible materials such as metallic materials, bioceramics, and composites are preferred. For receptive cavity materials, polymeric materials such as thermoplastic sheets, resin-based composites, and low-temperature thermoplastics are preferred.

A Brief Review on Factors Affecting the Tribological Interaction between Human Skin and Different Textile Materials.

The application of tribology is not just limited to mechanical components of engineering systems. As a matter of fact, the understanding of friction and wear can be applied to everyday life. One of the important fields is skin tribology, as human skin interacts with various surfaces of different materials. This paper focuses on the friction behavior of the skin when in contact with the fabric and other materials in relative motion. The excessive friction at the fabric-skin interface may lead to discomfort, blistering, chafing, and pressure ulcers especially in athletes who experience higher friction due to rapid movement for an extended period. Other than understanding the fabric properties, it is equally important to understand the structure and properties of the skin to evaluate its function and interaction with the different fabric materials. The identification of the contributing factors of skin friction can help to design suitable fabric materials. An overview of skin functions and the factors that affect the friction on the skin–textile material interface are presented in this review article.

Skin tribology in sport

This review describes the principles of skin friction and wear for the benefit of sports scientists, engineers and clinicians. Skin exhibits complex behaviour, defying tribological laws for dry contact; hence, its friction and wear characteristics are affected by sliding speed, normal load, and contact area. Some sports seek to increase skin friction to enhance performance; however, this needs to be offset against injury risk given that skin abrades when slid across a rough and hard surface, delaminates when slid across a smooth and hard surface, and chafes or blisters when repeatedly rubbed against some fabrics. Whilst skin interactions can both define and hinder athlete performance, there exists a need to better understand skin biomechanics to optimise the balance of risk versus reward.

An evaluation of tactile frictional behavior of the wooden material

Skin tribology and contact mechanics have emerged as a recent topic of research, intended to unveil the behavior of human skin. There have been variegated efforts to calculate the coefficient of friction with respect to various parameters such as sliding speed, normal load, and surface roughness. Wood and other timber products have their importance considering the ergonomically designs that have gained importance nowadays. This article shows the relation between normal load, average sliding speed and coefficient of friction (μ) of various types of woods (Rosewood, Sunmica, Teak wood and Pine wood). An experimental investigation is conducted to understand frictional behavior between human skin and different types of woods. Wooden material has been tested with the normal load having the range of 6.5N to 13N. The variation of μ has been remarked as μrosewood>μpine>μteak>μsunmica. Experimental observation shows that the rosewood has the highest COF which suggests its application where grip and strength of high order are required.With decreasing sliding speed, the relation between normal load and μ become more significant.

Mechanisms of tactile sensory deterioration amongst the elderly

It is known that roughness-smoothness, hardness-softness, stickiness-slipperiness and warm-cold are predominant perceptual dimensions in macro-, micro- and nano- texture perception. However, it is not clear to what extent active tactile texture discrimination remains intact with age. The general decrease in tactile ability induces physical and emotional dysfunction in elderly, and has increasing significance for an aging population. We report a method to quantify tactile acuity based on blinded active exploration of systematically varying micro-textured surfaces and a same-different paradigm. It reveals that elderly participants show significantly reduced fine texture discrimination ability. The elderly group also displays statistically lower finger friction coefficient, moisture and elasticity, suggesting a link. However, a subpopulation of the elderly retains discrimination ability irrespective of cutaneous condition and this can be related to a higher density of somatosensory receptors on the finger pads. Skin tribology is thus not the primary reason for decline of tactile discrimination with age. The remediation of cutaneous properties through rehydration, however leads to a significantly improved tactile acuity. This indicates unambiguously that neurological tactile loss can be temporarily compensated by restoring the cutaneous contact mechanics. Such mechanical restoration of tactile ability has the potential to increase the quality of life in elderly.

Implications of Multi-asperity Contact for Shear Stress Distribution in the Viable Epidermis – An Image-based Finite Element Study

Understanding load transfer mechanisms from the surface of the skin to its deeper layers is crucial in gaining a fundamental insight into damage phenomena related to skin tears, blisters and superficial/deep tissue ulcers. It is unknown how shear stresses in the viable epidermis are conditioned by the skin surface topography and internal microstructure and to which extent their propagation is conditioned by the size of a contacting asperities.In this computational study, these questions were addressed by conducting a series of contact finite element analyses simulating normal indentation of an anatomically-based two-dimensional multi-layer model of the skin by rigid indenters of various sizes and sliding of these indenters over the skin surface. Indentation depths, local (i.e. microscopic) coefficients of friction and Young's modulus of the stratum corneum were also varied. For comparison purpose and for isolating effects arising purely from the skin microstructure, a geometrically-idealised equivalent multi-layer model of the skin was also considered.The multi-asperity contact induced by the skin topographic features in combination with a non-idealised geometry of the skin layers lead to levels of shear stresses much higher than those produced in the geometrically-idealised case. These effects are also modulated by other system parameters (e.g. local coefficient of friction, indenter radius).These findings have major implications for the design and analyses of finite element studies aiming at modelling the tribology of skin, particularly if the focus is on how surface shear stress leads to damage initiation which is a process known to occur across several length scales.

A comparison of friction behaviour for ex vivo human, tissue engineered and synthetic skin

Skin tribology is complex and in situ behaviour of skin varies considerably between test subjects. The main influencing factor, elasticity, varies due to structural and moisture differences. To find a more reliable test platform, for the first time, synthetic and biological (tissue engineered) substitutes were compared to ex vivo skin, epidermis and dermis.Friction initially increased with rising hydration, before decreasing beyond a threshold for all samples. Friction for Synthetic skin and dermis increased at a similar rate to the other samples, but from a different starting point, and friction dropped at lower hydration. Tissue engineered skin could provide a reliable test platform, but the synthetic skin could only be used if the offset in the data is accounted for.

Biotribology: Recent progresses and future perspectives

Biotribology deals with all aspects of tribology concerned with biological systems. It is one of the most exciting and rapidly growing areas of tribology. It is recognised as one of the most important considerations in many biological systems as to the understanding of how our natural systems work as well as how diseases are developed and how medical interventions should be applied. Tribological studies associated with biological systems are reviewed in this paper. A brief history, classification as well as current focuses on biotribology research are analysed according to typical papers from selected journals and presentations from a number of important conferences in this area. Progress in joint tribology, skin tribology and oral tribology as well as other representative biological systems is presented. Some remarks are drawn and prospects are discussed.

Contact and frictional properties of stratum corneum of human skin

Abstract Most skin tribology studies gave attention on the examination of frictional properties of normal and healthy skin surfaces. This study aims to investigate the frictional and mechanical properties of skin under different load and friction conditions after removing the stratum corneum from the designated uninjured forearm tissue by a tape stripping method. The influence of water on friction coefficient was explained by using adhesion model of friction. For quantifying the amount of removed stratum corneum from the skin surfaces, an UV/vis spectrum analysis was used to measure the absorbance and pseudo-absorbance of stratum corneum on the tape. To evaluate the influence of water on friction coefficient, trans-epidermal water loss (TEWL) value of different skin conditions was assessed by water evaporation measurements. Frictional and mechanical properties of the skin surfaces were acquired via a tribometer. Results revealed that the elastic modulus of skin decreased and the friction coefficient of skin increased with the increasing of the amount of removed stratum corneum. Friction force versus normal force was presented to explain the influence of water on friction coefficient and section change phenomenon of friction coefficient under different degrees of tape strips.

Special Issue Editorial - Tribology of Skin

Special Issue Editorial - Tribology of Skin

Skin tribology phenomena associated with reading braille print: The influence of cell patterns and skin behavior on coefficient of friction

Efficient transmission of tactile information is vital for individuals who rely on their sense of touch to interact with and navigate their surroundings, including visually impaired persons. Somatosensory phenomena have been investigated with respect to surface topologies and neuron sensitivities in the skin, but there is little knowledge of the specific skin tribology when reading tactual-coded information such as braille. Braille is a tactual code that employs raised dome-shaped dots in six-position cells (2 columns by 3 rows per cell), with various dot patterns representing individual text characters, punctuation or mathematical operators. Due to the hypothesized significance of friction on tactile sensitivity, the authors investigated the effect of basic braille dot configurations on friction coefficient in fingertip sliding. Initial studies investigated the effect of multiple dot-row configurations and media type on friction coefficient, but the tribological effect of individual features and associated skin interactions was ill-defined. Subsequently, the frictional effect of an individual dot of varying radius was investigated and modeled against a multi-term frictional model implementing Hertzian contact, the Greenwood–Tabor hysteresis component of a spherical indenter against a soft surface, as well as Wolfram׳s traditional adhesion model. The results of the study show that macro-scale deformation of the fingerpad during fingertip-on-dot sliding is the primary friction mechanism and suggest that the contribution due to a macroscopic feature is largely independent of sample medium. Based on this understanding, the effect of braille dot spacing on a dot׳s friction contribution was investigated. The results from the spacing study indicate that the fingerpad׳s interaction with dot pairs is highly influenced by dot feature spacing. Further work is necessary to identify the fundamental sliding mechanics at the finger-dot interface, but the ability to identify the frictional mechanisms as well as the sliding interactions will provide a means to understand how much of a role friction plays in braille character recognition, as well as suggest potential friction-based methods to enhance the information density of braille codes.

Skin Friction at the Interface between Hands and Sports Equipment

The friction between the finger pad/palm and items of sports equipment strongly influences how well an athlete is able to perform. It not only determines how well equipment can be gripped and manipulated, but also how the equipment feels to use and the perceived level of performance. In this paper various fundamental aspects of finger pad friction are reviewed, including the effects of applied force, skin moisture, material, surface texture etc., and the influence that they have on friction mechanisms such as adhesion, deformation, interlocking and hysteresis. A number of applied case studies are then outlined. The first is rugby balls and the effect the ball surface pimple pattern has on friction. Initially high speed video was used to establish how the hand interacts with a ball. Friction tests were then carried out with different hand conditions and pimple patterns and the link between friction and pass accuracy was explored. The second study relates to friction modifiers used in sports such as rock climbing and athletics. These can be affected by hand and environmental conditions so a focus was placed on tests with moist hands or wet surfaces. Finally Frisbee interactions were investigated. The impact of loss of feel as a result of wearing gloves was studied to see if any improvements in wet conditions with gloves were offset by the reduced feedback from the Frisbee interface. The fundamentals of skin tribology can play a key role in developing optimised sports equipment, gaps still exist, however, in the understanding and modelling of surface texture and how important feel/comfort are, which are both important for sports equipment design.

촉감 감성 해석을 위한 미끄럼 마찰 시험과 신호 분석

Abstract The friction behavior of human skin is determined by the complex interplay of the material and sur-face properties of the skin, as well as the contacting material, and strongly depends on the contact parameters(e.g., pressure and sliding velocity) and the presence of substances such as water, sweat, or skin surface lipidsat the interface. Including a study on the effect of a surface's physical roughness for skin sliding over the surface,various studies have been conducted to understand human tactile sensibility. However, to investigate products inrelation to human tactile sensibility, more objective research is needed. This study performed sliding experimentsbetween the skin and the surfaces of phone cases to understand how the texture, friction, and stick-slip char-acteristics are related. Eight phone case surfaces with different topologies and chemical (or mechanical) com-patibilities with skin were prepared and tested multiple times. Keywords finger(손가락), friction(마찰), skin tribology(피부 트라이볼로지), biotribology(바이오 트라이볼로지), haptics(햅틱스), tactile(촉감), phone case(휴대폰 케이스), touch(터치)

Skin tribology: Science friction?

Abstract The application of tribological knowledge is not just restricted to optimizing mechanical and chemical engineering problems. In fact, effective solutions to friction and wear related questions can be found in our everyday life. An important part is related to skin tribology, as the human skin is frequently one of the interacting surfaces in relative motion. People seem to solve these problems related to skin friction based upon a trial-and-error strategy and based upon on our sense for touch. The question of course rises whether or not a trained tribologist would make different choices based upon a science based strategy? In other words: Is skin friction part of the larger knowledge base that has been generated during the last decades by tribology research groups and which could be referred to as Science Friction? This paper discusses the specific nature of tribological systems that include the human skin and argues that the living nature of skin limits the use of conventional methods. Skin tribology requires in vivo, subject and anatomical location specific test methods. Current predictive friction models can only partially be applied to predict in vivo skin friction. The reason for this is found in limited understanding of the contact mechanics at the asperity level of product-skin interactions. A recently developed model gives the building blocks for enhanced understanding of friction at the micro scale. Only largely simplified power law based equations are currently available as general engineering tools. Finally, the need for friction control is illustrated by elaborating on the role of skin friction on discomfort and comfort. Surface texturing and polymer brush coatings are promising directions as they provide way and means to tailor friction in sliding contacts without the need of major changes to the product.

Tribology of Skin: Review and Analysis of Experimental Results for the Friction Coefficient of Human Skin

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.

Development of a Synthetic Skin Simulant Platform for the Investigation of Dermal Blistering Mechanics

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.

Effect of human ageing on skin rheology and tribology

Abstract Microscopic observations of the human skin have shown that skin topography contains a network of lines whose organization reflects the multidirectional tensions of elastic and collagen fibres in the superficial dermis. This morphology is present at birth and its scale of depth and wavelength increases with age until puberty. The aim of this work is to study how human ageing influences skin morphology, elasticity, loss factor, damping and skin tribology. A complete device to assess the rheology of the skin has been developed. The frequency and strain amplitude range from 10 to 60 Hz and from 1 to 10 μm. An in vivo study including dynamic indentation and tribologic analysis has been performed on 46 subjects aged from 18 to 70 falling into 3 groups. The complex modulus measured by dynamic indentation at 10 Hz frequency stress ranges from 7.17 ± 2.06 kPa for the oldest group to 10.7 ± 2.64 kPa for the youngest group. The combination of the dynamic device with acoustic measurements during friction gives an invaluable help to understand the significance of energy dissipation during friction and shows the impact of skin rheology on acoustic signature during ageing.

Investigation of skin tribology and its effects on the tactile attributes of polymer fabrics

The most fundamental interaction between a user and a product is physical touch. In fact, a significant amount of the perceived value of a product results from the initial touch experience by the potential customer, whether the product is an automobile interior or a portable music player. Therefore, the ability to engineer a product's tactile character to produce favorable sensory perceptions has the potential to revolutionize product design. Another major consideration is the potential for products to produce friction-induced injuries to skin such as blistering. Progress in this field has been hindered by the difficulties in drawing correlations between human sensory outcomes (e.g. softness, smoothness, leather-like feel) and quantitative physical properties (e.g. friction coefficient, elastic modulus). In this paper, a framework is proposed to address this issue with regards to polymer fabrics, which are used everywhere from clothing to protective wound-care products, and results are reported. Human evaluators were used in a modified Quantitative Descriptive Analysis (QDA) approach in order to identify four specific tactile attributes of fabric materials—sensible texture, abrasiveness, slipperiness, and fuzziness. The friction coefficients of the same fabrics against skin were measured by employing a three-axis dynamometer. The results were then correlated to sensory attributes by means of standard statistical methods. It was concluded from this work that human evaluators can score various fabrics along the above four tactile descriptors with significant statistical repeatability if “high” and “low” benchmark reference fabrics are made available during the evaluation. It was also observed that friction between fabric and skin is heavily dependent on both material type and fiber geometry, but that the impact of friction coefficient on tactility is still unclear. However, the methods presented in the paper may be used to further identify key fabric properties that are responsible for tactility.