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The epidermis constitutes a continuous external layer covering the body, offering protection against bacteria, the most abundant living organisms that come into contact with this barrier. The epidermis is heavily colonized by commensal bacterial organisms that help protect against pathogenic bacteria. The highly regulated and dynamic interaction between the epidermis and commensals involves the host's production of nutritional factors promoting bacterial growth together to chemical and immunological bacterial inhibitors. Signal trafficking ensures the system's homeostasis; conditions that favor colonization by pathogens frequently foster commensal growth, thereby increasing the bacterial population size and inducing the skin's antibacterial response, eliminating the pathogens and re-establishing the normal density of commensals. The microecological conditions of the epidermis favors Gram-positive organisms and are unsuitable for long-term Gram-negative colonization. However, the epidermis acts as the most important host-to-host transmission platform for bacteria, including those that colonize human mucous membranes. Bacteria are frequently shared by relatives, partners, and coworkers. The epidermal bacterial transmission platform of healthcare workers and visitors can contaminate hospitalized patients, eventually contributing to cross-infections. Epidermal transmission occurs mostly via the hands and particularly through fingers. The three-dimensional physical structure of the epidermis, particularly the fingertips, which have frictional ridges, multiplies the possibilities for bacterial adhesion and release. Research into the biology of bacterial transmission via the hands is still in its infancy; however, tribology, the science of interacting surfaces in relative motion, including friction, wear and lubrication, will certainly be an important part of it. Experiments on finger-to-finger transmission of microorganisms have shown significant interindividual differences in the ability to transmit microorganisms, presumably due to genetics, age, sex, and the gland density, which determines the physical, chemical, adhesive, nutritional, and immunological status of the epidermal surface. These studies are needed to optimize interventions and strategies for preventing the hand transmission of microorganisms. [ABSTRACT FROM AUTHOR]

The epidermis constitutes a continuous external layer covering the body, offering protection against bacteria, the most abundant living organisms that come into contact with this barrier. The epidermis is heavily colonized by commensal bacterial organisms that help protect against pathogenic bacteria. The highly regulated and dynamic interaction between the epidermis and commensals involves the host’s production of nutritional factors promoting bacterial growth together to chemical and immunological bacterial inhibitors. Signal trafficking ensures the system’s homeostasis; conditions that favor colonization by pathogens frequently foster commensal growth, thereby increasing the bacterial population size and inducing the skin’s antibacterial response, eliminating the pathogens and re-establishing the normal density of commensals. The microecological conditions of the epidermis favors Gram-positive organisms and are unsuitable for long-term Gram-negative colonization. However, the epidermis acts as the most important host-to-host transmission platform for bacteria, including those that colonize human mucous membranes. Bacteria are frequently shared by relatives, partners, and coworkers. The epidermal bacterial transmission platform of healthcare workers and visitors can contaminate hospitalized patients, eventually contributing to cross-infections. Epidermal transmission occurs mostly via the hands and particularly through fingers. The three-dimensional physical structure of the epidermis, particularly the fingertips, which have frictional ridges, multiplies the possibilities for bacterial adhesion and release. Research into the biology of bacterial transmission via the hands is still in its infancy; however, tribology, the science of interacting surfaces in relative motion, including friction, wear and lubrication, will certainly be an important part of it. Experiments on finger-to-finger transmission of microorganisms have shown significant interindividual differences in the ability to transmit microorganisms, presumably due to genetics, age, sex, and the gland density, which determines the physical, chemical, adhesive, nutritional, and immunological status of the epidermal surface. These studies are needed to optimize interventions and strategies for preventing the hand transmission of microorganisms.

Tactile interactions with product surfaces are essential to how such products are used and the value that users ascribe to them. It has been shown that contact mechanisms and frictional interactions are central to the haptic attributes perceived by the users. While there are established and well-studied models that describe the adhesive and deformation components of friction, interlocking behavior—while shown to be dominant in some tactile applications—has not been thoroughly studied as a frictional mechanism for soft materials such as skin, partly because of the complex contact mechanics involved. This investigation involved a computational parametric study of interlocking behavior between a soft material and a parallel-ridge surface texture, across a range of ridge widths, spacing and applied pressure. The goal of the work was to determine if a relatively simple contact model could be developed to predict the number of potential points of interlocking between the soft body and the ridge features. The results showed that a standard Hertzian approach predicted the number of ridge edge contacts (potential interlocking locations) with acceptable accuracy, so long as the ratio of ridge width to ridge spacing was large. However, a hybrid model that also incorporated the ridge width performed remarkably well across a wide range of ridge widths and spacing. This model may provide utility as an input into various proposed models of frictional interlocking to explain haptic perception of textured surfaces. [ABSTRACT FROM AUTHOR]

This study focused on active light touch with predefined textures specially-designed for tactile perception. The counter-body material is stainless steel sheet. Three geometric structures (grid, crater and groove) were fabricated by pulsed laser surface texturing. A total number of twenty volunteers participated in the research which contains two parts: perception tests and skin friction measurements. The perception tests focused mainly on the participants׳ perceptual attributes: perceived roughness and perceived stickiness. For the skin friction measurements, a multi-axis force/torque transducer was used to measure the normal force and friction force between skin and counter-surface along with the fingertip position. The results of the predefined textures showed the ability to reduce skin friction due to the reduction of contact area. Moreover, the participants׳ perceptual attributes were greatly influenced by the predefined micro-structures in the light touch regime.

Human skin interacts with various materials in our daily life. The interaction between human skin and contacting materials is very important for the development of skin contacting products. Owing to the ethic and different testing results because of the using of in vivo or ex vivo skin, it is important to develop an artificial skin model (ASM) for the study. Therefore, an ASM mimicking the deformation and friction behavior of in vivo human skin was designed based on the evaluation of in vivo human skin behavior, and its frictional interaction with wound dressings was studied. The ASM was prepared by the combination of hydrophilic network carboxyl chitosan (CC) and hydrophobic network polydimethylsiloxane (PDMS). The influence of ingredient ratio, including PDMS/CC and curing agent/PDMS ratio, on the mechanical property of ASM was determined firstly. By adjusting the curing agent/PDMS ratio, the water absorption swelling rate (WASR) of ASM could be controlled to mimic different hydration status of human skin. With the changing of ingredient ratio and hydration level, the elastic modulus and viscoelasticity of ASM can be tailored to be similar to that of in vivo human skin. When the PDMS/CC ratio was 7:3, and PDMS/curing agent ratio was smaller than 1:30, the elastic modulus of ASM was in the range of in vivo inner forearm, and with the increasing of indentation depth, the elastic modulus decreased. Based on the ASM, the frictional interaction between skin/wound dressing/mattress was studied. It was found that although the friction using ASM was slightly higher than that using in vivo inner forearm, but the friction decreasing trend was the same for different kinds of wound dressings. In addition, the friction tested with ASM was less fluctuation, more reliable and reproducible than that tested with in vivo skin, indicating that the ASM was suitable to be used for the studying of frictional interaction between skin and product, such as wound dressings. fx1 • An artificial skin model (ASM) was prepared by the combination of hydrogel and PDMS. • The water swelling rate of ASM could be adjusted to mimic different hydration status of human skin. • The mechanical property of ASM could be tailored to be similar to that of in vivo human skin by changing the formulation. • The frictional interaction between the ASM and wound dressings showed the similar trend as that of in vivo human skin. [ABSTRACT FROM AUTHOR]

Friction, shear forces and moisture between the human skin and textiles are important factors affecting skin injuries such as blisters, abrasions and decubitus ulcers. Whilst much research has been conducted to study the friction of skin-textile couples, the interactions between contact mechanics and incontinence management products are not well understood. This study addresses some tribological issues at the skin-textile interface using skin care products. It was observed that the use of skin care products and moisture can increase friction. This is due to changes in the mechanical properties of materials such as their compliance and the higher forces required to shear the interfaces in wet conditions. It was concluded that not only does the coefficient of friction vary, but also the mechanism of slip at the interface can be modified through the addition of medicated creams. This research highlights the importance of understanding the principles of contact mechanics of interfaces which can lead to significant improvement in incontinence management. [ABSTRACT FROM AUTHOR]

The understanding of the tactile perception mechanism implies the reproduction and measurement of friction forces and vibrations induced by the contact between the skin of human fingers and object surfaces. When a finger moves to scan the surface of an object, it activates the receptors located under the skin allowing the brain to identify surfaces and information about their properties. The information concerning the object surface is affected by the forces and vibrations induced by the friction between the skin and the rubbed object. The vibrations propagate in the finger skin and are converted into electric impulses sent to the brain by the mechanoreceptors. Because of the low amplitude of the induced vibrations, it results quite hard to reproduce the tactile surface scanning and measuring it without affecting measurements by external noise coming from the experimental test-bench. In fact the reproduction of the sliding contact between two surfaces implies the relative motion between them, which is obtained by appropriate mechanisms having a more or less complicated kinematics and including several sliding surfaces (bearings, sliders, etc.). It results quite difficult to distinguish between the vibrations coming from the reproduced sliding and the parasitic noise coming from the other sliding contact pairs. This paper presents the design and validation of a tribometer, named TRIBOTOUCH, allowing for reproducing and measuring friction forces and friction induced vibrations that are basilar for a clear understanding of the mechanisms of the tactile sense. [ABSTRACT FROM AUTHOR]

Background/Purpose The work outlined in this paper was aimed at achieving further understanding of skin frictional behaviour by investigating the contact area between human finger-pads and flat surfaces. Methods Both the static and the dynamic contact areas (in macro- and micro-scales) were measured using various techniques, including ink printing, optical coherence tomography (OCT) and Digital Image Correlation (DIC). Results In the studies of the static measurements using ink printing, the experimental results showed that the apparent and the real contact area increased with load following a piecewise linear correlation function for a finger-pad in contact with paper sheets. Comparisons indicated that the OCT method is a reliable and effective method to investigate the real contact area of a finger-pad and allow micro-scale analysis. The apparent contact area (from the DIC measurements) was found to reduce with time in the transition from the static phase to the dynamic phase while the real area of contact (from OCT) increased. Conclusions The results from this study enable the interaction between finger-pads and contact object surface to be better analysed, and hence improve the understanding of skin friction. [ABSTRACT FROM AUTHOR]

Tactual encoding of information is an essential aspect of communication in many aspects of life, but especially so for persons with visual impairments. The most common encoding is the use of topological features on a static medium such as paper. While Braille is widely used and effective for text transcription, the effective use of tactile graphics to communicate spatial or graphical information is less standardized. Tactile graphical techniques typically employ collections of individual topological features or texture patterns to communicate spatial information (maps, plots, diagrams), with the goal of minimizing reader confusion among the elements. In this study, the authors investigated the tribology and perception aspects of this complex issue in order to identify fundamental surface parameters which play a dominant role. Using texture specification guidelines published by the Braille Authority of North America (BANA), the investigators produced raised parallel-ridge textures on thermally activated tactile paper. The texture parameters modified included the width of ridges, spacing between ridges, and the inclination of the ridges to the pattern boundaries. Subjects reported their perception of tactile similarity or difference between members of texture pairs that were presented under a blind. Measurements of friction force were also collected for these textures to determine if friction mechanisms were most responsible for perceptive ability, or if other surface attributes played a dominant role. The study found that modifying pattern pitch improved perceptive ability to differentiate between adjacent mismatched texture patterns, but that ridge width had a deleterious effect. The results suggested that perceptive ability was not strongly related to various friction mechanisms proposed, but was closely correlated with changes in the number of pattern ridges in contact with the fingertip during tactile graphic exploration. • This psychophysical study involved the perceptive ability of sighted people to differentiate among tactile graphic textures. • Results provided evidence for texture grouping based on perceptive similarity. • Pattern pitch had a substantial positive impact in perceiving differences, while ridge width had a deleterious effect. • Friction mechanisms were ruled out as primary drivers of perceptive ability. [ABSTRACT FROM AUTHOR]

The surface topography of the human wrist skin is studied by using optical and atomic force microscopy (AFM) methods. By using these techniques the surface roughness power spectrum is obtained. The Persson contact mechanics theory is used to calculate the contact area for different magnifications, for the dry and wet skin. The measured friction coefficient between a glass ball and dry and wet skin can be explained assuming that a frictional shear stress σf ≈ 13 MPa and σf ≈ 5 MPa, respectively, act in the area of real contact during sliding. These frictional shear stresses are typical for sliding on surfaces of elastic bodies. The big increase in friction, which has been observed for glass sliding on wet skin as the skin dries up, can be explained as result of the increase in the contact area arising from the attraction of capillary bridges. Finally, we demonstrated that the real contact area can be properly defined only when a combination of both AFM and optical methods is used for power spectrum calculation.

Beyond the sense of sight, the sense of touch is one of the primary ways that individuals experience their surrounding environment. Fundamentally understanding the relationship of skin-surface tribology and its elicited tactile attributes could provide a breakthrough in improving the ability to efficiently transmit tactile information to those who rely on the sense of touch to interact with their surroundings, such as the blind and visually impaired (BVI) community. The tactile language of braille has been adopted by the BVI community, employing configurations of raised dome-shape dots to convey what is ordinarily presented in text and image form. The coefficient of friction caused by skin sliding across a these dot features is hypothesized to affect the reader's tactile sensitivity, and skin-on-braille coefficient of friction has been investigated in previous work, where macro-scale deformation of the human fingerpad sliding over the dot contour was identified as the dominant friction mechanisms. This investigation succeeds that study by examining a simplified large-scale, two-dimensional representation of skin-on-braille sliding to characterize the underlying contact mechanisms in the loading behaviors that dictate the resulting coefficient of friction. This was accomplished by using a multi-axis tribometer to sliding a 25.4 mm radius cylindrical polyurethane (representing a human fingerpad) rod over a lubricated 3.17 mm aluminum half rod (representing a braille dot) under displacement-displacement-controlled conditions. The results from the tribometer study indicate that the presence of the dot feature drastically affects the vertical and lateral loading behavior by vertically displacing the body's elastic bulk, generating rubber-like Poisson effect contributions. Most importantly, the Poisson effect rapidly increases the lateral load when the body contacts the dot's leading edge, and rapidly decreases when the body rests largely in contact with the dot's trailing edge. This rapid decrease is caused by a “propulsion” effect, where vertical compression expands the material laterally, and when situated on the trailing edge of the dot, propels it into the direction of sliding, virtually negating adhesive surface friction. Computational modeling of this system discovered that while normal contact pressures dominated the fluctuations seen in the vertical loading, effects due to both normal contact pressures and frictional shears nearly equally drove the lateral loading behavior. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The tribological interactions between skin and textured surfaces has profound impact on both the tactile perception of the product being used, as well as the functionality of the product with regards to friction coefficient. Previous work has shown that parallel-ridged textures have vastly different friction coefficients with regards to the direction of skin sliding, and that penetration of the skin into the voids between ridges not only add contact area but also potential for interlocking. The ability to model skin penetration into textural elements would prove to be very useful for predicting friction; however, the mechanics of the problem are incredibly complex such that they rule out a closed-form analytical solution. The authors investigated soft-body penetration using a non-dimensional computational approach based on the elastic properties of skin, as well as the texture ridge geometry parameters, as well as the normal loading. Model results were verified experimentally. The model was applied to a number of different combinations of ridge parameters and it was found that the amount of penetration could be predicted very well using a simple exponential relationship among the nondimensional terms. Texture groove width and applied normal load played a dominant role in penetration. These results yield a quantitative mechanics model which can be integrated into an overarching frictional model to predict skin on texture behavior due to both adhesion and edge interlocking. [ABSTRACT FROM AUTHOR]

The tactile perception of a surface texture originates from scanning a finger on the surface. This kind of sliding contact activates the mechanoreceptors located into the skin, allowing the brain to identify the object and to perceive information about the scanned surface. Perception is collected by mechanoreceptors either by sensing pressure or by sensing vibration: the first mechanism is typical of large spaced surface textures, while the second is necessary to perceive finer textures. These different behaviors are well known in the literature as the duplex perception mechanism. In the present paper a numerical model describing finger-surface scanning is introduced in order to investigate the relationship between contact induced vibrations and scanning conditions. The model has been validated by experimental comparisons in a previous work. The perception model is used to develop a parametric analysis of the vibration induced from the finger-surface scanning as a function of surface geometry, scanning speed and contact force. The proposed parametric analysis points out the minimum number of parameters needed to describe the tactile perception of a periodic texture, and it shows the tribological reasons for which duplex perception mechanism is an effective biological evolution towards optimal tactile perception. [ABSTRACT FROM AUTHOR]

Poly(vinyl) alcohol hydrogel (PVA) is a well-known polymer widely used in the medical field due to its biocompatibility properties and easy manufacturing. In this work, the tribo-mechanical properties of PVA-based blocks are studied to evaluate their suitability as a part of a structure simulating the length scale dependence of human skin. Thus, blocks of pure PVA and PVA mixed with Cellulose (PVA-Cel) were synthesised via freezing/thawing cycles and their mechanical properties were determined by Dynamic Mechanical Analysis (DMA) and creep tests. The dynamic tests addressed to elastic moduli between 38 and 50 kPa for the PVA and PVA-Cel, respectively. The fitting of the creep compliance tests in the SLS model confirmed the viscoelastic behaviour of the samples with retardation times of 23 and 16 seconds for the PVA and PVA-Cel, respectively. Micro indentation tests were also achieved and the results indicated elastic moduli in the same range of the dynamic tests. Specifically, values between 45–55 and 56–81 kPa were obtained for the PVA and PVA-Cel samples, respectively. The tribological results indicated values of 0.55 at low forces for the PVA decreasing to 0.13 at higher forces. The PVA-Cel blocks showed lower friction even at low forces with values between 0.2 and 0.07. The implementation of these building blocks in the design of a 2-layered skin model (2LSM) is also presented in this work. The 2LSM was stamped with four different textures and their surface properties were evaluated. The hydration of the 2LSM was also evaluated with a corneometer and the results indicated a gradient of hydration comparable to the human skin. [ABSTRACT FROM AUTHOR]

The friction and abrasion behaviour of the finger pad on abrasive papers was investigated in friction experiments, combined with microscopic analyses and a protein assay to quantify skin particles abraded in friction contacts. Friction measurements at varied normal forces resulted in relatively high and load-independent friction coefficients, pointing to ploughing and abrasion as important friction mechanisms. The microscopic analyses revealed that large numbers of skin particles are abraded in form of single corneocytes, corneocyte fragments and agglomerates of corneocytes. In addition, micro-scratches were observed on the epidermal ridges of the finger pad after friction contacts. In friction measurements at the same conditions, the amount of abraded skin particles varied for abrasive papers with different roughness, while friction coefficients were comparable. [ABSTRACT FROM AUTHOR]

Research on human skin interactions with healthcare and lifestyle products is a topic continuously attracting scientific studies over the past years. It is possible to evaluate skin mechanical properties based on human or animal experimentation, yet in addition to possible ethical issues, these samples are hard to obtain, expensive and give rise to highly variable results. Therefore, the design of a skin equivalent is essential. This paper describes the design and characterization of a new Epidermal Skin Equivalent (ESE). The material resembles the properties of epidermis and is a first approach to mimic the mechanical properties of the human skin structure, variable with the length scale. The ESE is based on a mixture of Polydimethyl Siloxane (PDMS) and Polyvinyl Alcohol (PVA) hydrogel cross-linked with Glutaraldehyde (GA). It was chemically characterized by XPS and FTIR measurements and its cross section was observed by macroscopy and cryoSEM. Confocal Microscope analysis on the surface of the ESE showed an arithmetic roughness ( R a ) between 14–16 μm and contact angle (CA) values between 50–60°, both of which are close to the values of in vivo human skins reported in the literature. The Equilibrium Water Content (ECW) was around 33.8% and Thermo Gravimetric Analysis (TGA) confirmed the composition of the ESE samples. Moreover, the mechanical performance was determined by indentation tests and Dynamo Thermo Mechanical Analysis (DTMA) shear measurements. The indentation results were in good agreement with that of the target epidermis reported in the literature with an elastic modulus between 0.1–1.5 MPa and it showed dependency on the water content. According to the DTMA measurements, the ESE exhibits a viscoelastic behavior, with a shear modulus between 1–2.5 MPa variable with temperature, frequency and the hydration of the samples. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

To control tactile friction, that is the friction between fingertip and counter-body, the role of surface texture is required to be unveiled and defined. In this research, an experimental approach is used based on measuring tactile friction for directional texture (grooved channel) with varying depths. For a reference surface, in this current case a polished surface from the same tool steel is compared. The experimental results are analyzed to explain the observed skin friction behavior as a function of surface texture parameters, sliding direction and applied normal load. Sliding parallel to the groove length shows greater values in COF than sliding perpendicular to the groove direction. Furthermore, parallel sliding reveals a higher dependency of COF on the depth of the grooved channel texture than perpendicular sliding. Application of the two term friction model suggests that the adhesion component of friction has greater impact on parallel than perpendicular sliding direction. According to the observations, grooved channels are well suited to control skin friction in direction dependent sliding, for moderately loaded contact situations. This experimental research contributes to the haptic perception related research, and to the development of other direction-dependent surface structures for touch. [ABSTRACT FROM AUTHOR]

This paper describes three series of tests that were designed to investigate how skin mechanical and structural properties, measured using a ‘Cutometer’ and optical coherence tomography, affect the frictional behaviour of human finger-pads. First, the skin mechanical properties across all fingers and the palm in participants’ dominant hands were assessed. Results showed that the distensibility of skin (total deformation in a suction test) is associated with stratum corneum thickness and that this in turn affects friction (thicker stratum corneum leads to higher friction), giving a link between distensibility and friction. Tape stripping to remove the superficial layer of the skin led to increased moisture (and/or electric charge on the skin surface) that led to higher friction. No accompanying changes were seen in structural properties, so it was concluded that moisture was the main cause of the adhesion increase. More work is required to isolate moisture and possible changes in electric charge using alternative measurement techniques. When rubbing with sand-paper, the stratum corneum thinned considerably and friction reduced. Moisture was ruled out as a cause of friction changes in this instance. Skin normal stiffness also did not change, but lateral stiffness changes have been seen in previous work when the stratum corneum thickness has been reduced, so this is likely to be the cause of the reduced friction. This will be investigated further in future work using dynamic optical coherence tomography measurements. [ABSTRACT FROM PUBLISHER]

Human skin is characterised by a complex and highly variable friction behaviour. Although the variation of friction coefficients measured for skin depends on numerous parameters related to the skin itself, the surface in contact as well as contact conditions, water or sweat – either bound in the stratum corneum and manifest as skin hydration or in form of liquid films lubricating the interface – is the most important factor. Here, we analyse the variation of previous experimental data on the basis of the adhesion friction model and show how lower and upper bounds, i.e. envelope functions, can be derived for measured skin-friction coefficients. From these envelope functions, essential tribological parameters such as the interfacial shear strength and the real contact area of skin are estimated. [ABSTRACT FROM PUBLISHER]

A laminar skin simulant was constructed to study the incidence of friction blisters. The skin simulant consists of a thin polyurethane top layer and textured gum foam rubber bottom layer adhered to an acrylic-backing plate to emulate the layered structure of the human skin. Friction blisters were produced on the skin simulant by using a dual-axis tribometer. The effect of the applied normal load on the number of cycles required to produce a blister was investigated. The skin simulant was also analyzed as an adhesive-bonded laminar composite to determine the relationship between the applied normal load and the number of cycles for blistering. The normal load and the number of cycles were found to be inversely related and vary by a power law function, as observed in previous work on human subjects in Naylor’s pioneering study. The results obtained from the experimental data and the fracture mechanics modeling of the skin simulant indicate the potential of elastomeric skin simulant in providing useful insight into blister mechanics and other tribological properties of skin. [ABSTRACT FROM PUBLISHER]

The contact and friction behavior of a medical compression stocking (MCS) under different strains was investigated in friction and compression experiments against a mechanical skin model. In addition, the 3D topography of the MCS surfaces was analyzed in order to study the relationship between macroscopic friction and microscopic surface properties. The load dependence of friction coefficients was found to be in accordance with the adhesion friction model. The surface structure of MCS samples was considerably changed when varying the strain state, while friction coefficients remained comparable, indicating real contact areas independent of strain on the microscopic level. The experimental findings could be confirmed and explained on the basis of the microscopic surface analyses, when interpreting the fabric surfaces to be composed of numerous individual round asperities obeying the Hertz contact model. [ABSTRACT FROM AUTHOR]

Abstract: The friction behaviour of the human fingerpad as a function of asperity geometry was investigated experimentally. Surface textures consisting of evenly distributed spherically tipped asperities were used for in vivo testing. Using analytical expressions, a multi-scale model was developed to explain the observed friction behaviour as a function of texture geometry, load and skin properties. Friction is found to increase with increasing tip radius. A minimum value for the coefficient of friction seems to exist as a function of asperity density. A maximum value for tip spacing exists above which the contact is not determined by the texture properties only. According to the model normal adhesion plays an important role in the observed friction behaviour. [Copyright &y& Elsevier]

Abstract: Decubitus ulcers or pressure ulcers are a major health problem. This study investigated the possibility of using low-friction bed sheets with optimized moisture transport properties for the prevention of decubitus ulcers. The tribological behavior of a newly developed prototype in contact with human skin was investigated in vivo. For comparison conventional hospital bed sheets were also investigated. Friction measurements under dry and wet conditions on the inner forearm showed that conventional hospital bed sheets are not optimal; their friction was systematically higher by up to 50% when compared with the prototype. These results suggest that friction and shear stresses on the skin of patients and thus the risk of pressure ulcers could be reduced by improved hospital bed sheets. [Copyright &y& Elsevier]

Abstract: This paper presents work using an in vivo technique, optical coherence tomography (OCT), to investigate the structure of human finger pad skin and the influences of some related parameters on skin friction, such as contact area, deformation and hydration. The experimental results show that there was no significant relationship between the thickness of the stratum corneum (SC) and the friction coefficient as well as the number of sweat ducts (SD). The real contact length was found to increase with increasing the applied normal force following the power law of . The study of hydration found an increasing linear relationship between the SC moisture and the friction coefficient (up to 72au). [Copyright &y& Elsevier]

Abstract: One way of controlling friction and associated energy losses is to engineer a deterministic structural pattern on the surface of the rubbing parts (i.e., texture engineering). Custom texturing enhances the quality of lubrication, reduces friction, and allows the use of lubricants of lower viscosity. To date, a standardized procedure to generate deterministic texture constructs is virtually non-existent. Many engineers, therefore, study natural species to explore surface construction and to probe the role that surface topography assumes in friction control. Snakes offer rich examples of surfaces where topological features allow the optimization and control of frictional behavior. In this paper, we investigate the frictional behavior of a constrictor type reptile, Python regius. The study employed a specially designed tribo-acoustic probe capable of measuring the coefficient of friction and detecting the acoustical behavior of the skin in vivo. The results confirm the anisotropy of the frictional response of snakeskin. The coefficient of friction depends on the direction of sliding: the value in forward motion is lower than that in the converse direction. Detailed analysis of the surface metrological feature reveals that tuning frictional response in snakes originates from the hierarchical nature of surface topology combined to the profile asymmetry of the surface micro-features, and the variation of the curvature of the contacting scales at different body regions. Such a combination affords the reptile the ability to optimize the frictional response. [Copyright &y& Elsevier]

Abstract: The surfaces of materials differ in topography and compatibility with human skin. Those differences were distinguished by measuring the friction coefficients between a human finger and different types of surfaces. Sample groups used in these tests were glass, plastic, and fabric. In the case of fabric, the following materials were used: linen, canvas, felt, silk, velvet, and cotton. Experiments were conducted using a specially-designed apparatus under constant temperature and relative humidity. Two factors were identified using the finger friction measurements. First was the friction level and second was the variance in friction. In Particular, it was found that different surfaces show significantly different variance in their friction coefficients. Fitting parameters from a linear regression of the relationship between friction coefficient and normal load were used to map materials' responses. [Copyright &y& Elsevier]

Abstract: Decubitus or pressure ulcers in immobile patients are associated with prolonged pressure, shear and friction forces acting on the skin and the soft tissue underneath. We investigated the friction behaviour of hospital bed sheets in comparison with prototypes of low friction textiles in order to assess their potential for decubitus prevention. A specific textile composed of synthetic fibres showed a factor of three lower friction than normal hospital bed sheets under both dry and wet conditions. By absorbing and distributing interfacial water within the textile structure, the prototype additionally exhibited beneficial water transport properties. [Copyright &y& Elsevier]

Human hands sweat in different circumstances and the presence of sweat can alter the friction between the hand and contacting surface. It is, therefore, important to understand how hand moisture varies between people, during different activities and the effect of this on friction. In this study, a survey of fingertip moisture was done. Friction tests were then carried out to investigate the effect of moisture. Moisture was added to the surface of the finger, the finger was soaked in water, and water was added to the counter-surface; the friction of the contact was then measured. It was found that the friction increased, up until a certain level of moisture and then decreased. The increase in friction has previously been explained by viscous shearing, water absorption and capillary adhesion. The results from the experiments enabled the mechanisms to be investigated analytically. This study found that water absorption is the principle mechanism responsible for the increase in friction, followed by capillary adhesion, although it was not conclusively proved that this contributes significantly. Both these mechanisms increase friction by increasing the area of contact and therefore adhesion. Viscous shearing in the liquid bridges has negligible effect. There are, however, many limitations in the modelling that need further exploration. [ABSTRACT FROM AUTHOR]

Abstract: The friction behaviour of human skin was studied by combining friction measurements using a tri-axial force plate with skin contact area measurements using a pressure sensitive film. Four subjects carried out friction measurement series, in which they rubbed the index finger pad and the edge of the hand against a smooth and a rough glass surface under dry and wet conditions. The normal loads were varied up to values of 50N, leading to skin contact pressures of up to 120kPa. The analysis of the pressure dependence of friction coefficients of skin for contrasting sliding conditions allowed to determine the involved friction mechanisms on the basis of theoretical concepts for the friction of elastomers. Adhesion was found to be involved in all investigated cases of friction between skin and glass. If adhesion mechanisms predominated (skin against smooth glass in the dry condition and skin against rough glass in the wet condition), the friction coefficients were generally high (typically >1) and decreased with increasing contact pressure according to power laws with typical exponents between −0.5 and −0.2. Contributions to the friction coefficient due to viscoelastic skin deformations were estimated to be relatively small (<0.2). In those cases where the deformation component of friction played an important role in connection with adhesion (skin against rough glass in the dry condition), the friction coefficients of skin were typically around 0.5 and their pressure dependence showed weak trends characterised by exponents ranging from −0.1 to +0.2. If hydrodynamic lubrication came into play (skin sliding on smooth glass in the wet condition), the friction coefficients were strongly reduced compared to dry friction (<1), and their decrease with increasing contact pressures was characterised by exponents of <−0.7. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Finger pad friction varies significantly between individuals due to personal differences and environmental effects. For this reason, comparison of the absolute friction values becomes non-informative and is rarely reported in literature. This work investigates whether friction data can be normalised to correlate between individuals. A set of textured samples were tested by 10 volunteers in dry sliding conditions with a range of normal loads up to 4 N. A reference smooth sample was used for each participant to normalise the dataset. The proposed approach allows to eliminate one of the unknown parameters, e.g., finger pad ridge contact area function, and discuss the data with respect to other variables. • Friction measurements from 10 volunteers, 4 samples and 9 normal loads were analysed to propose a normalisation method. • Normalisation reduces one of the personal variables and allows friction comparison between individuals. • Friction measurements normalised by a reference represent a change of contact area fraction with normal load. • Volunteers showed similar fractions of real contact area despite differences in skin properties and friction. [ABSTRACT FROM AUTHOR]

Accurate depiction of visual information presents a formidable obstacle to the inclusion of people with visual impairments in standardized educational assessment. This challenge is often addressed by using tactile graphics, such as the use of a collection of tactile patterns representing various zones similar to the use of colors to represent political boundaries on a visual map. This study investigated friction mechanisms involved during fingertip exploration of tactile graphics, as well as the role that friction played in forming perceptions. Perception experiments were run using four texture types published by the Braille Authority of North America (BANA): two parallel-ridge textures of different ridge widths and spacings, and two ordered-dot textures of different dot spacings. To minimize tactile confusion, BANA guidelines prohibit the use of similar textures adjacent to each other. In this study, texture pairs were printed adjacent to each other on thermally activated ('swell touch') tactile paper. Sighted human subjects participated in a blinded task of determining if the two textures in the pair were the same or different. Friction measurements of the fingertip exploration of the texture pairs was recorded using a dynamometer in a range of normal loads from 0.4 to 0.8 N. A generalized linear model (GLM) with a logistic link function was used to predict the influence of texture pair and participant on the likelihood of perceiving each texture pair as the same or different. Results showed that the particular texture pair had a strong correlation to the ability of participants to discern between textures. These quantitative results support the BANA guidelines. The friction data showed that there was not a significant amount of difference among the coefficients of friction of various textures, such that COF was not a strong predictor of perceptive ability. However, there was some evidence that penetration of the fingertip between raised texture elements, and interlocking with these elements, allowed for dissimilar dot patterns to be better discerned than dissimilar ridge patterns. The results indicate that there are other aspects, possibly still tribological in nature, that govern tactile perception when exploring tactile graphics. These phenomena may include such things as finger deformation and vibrational response produced during contact with individual texture elements. • Psychophysical studies were done on sighted people to evaluate the tactile discrimination of tactile graphic textures. • The probability of discerning textures was well predicted using the General Linear Model with a logistic link function. • Sequence order effects in similar ridged textures appeared to be related to finger penetration into the textures. • The ability to discern between these texture types did not appear to be strongly influenced by coefficient of friction. [ABSTRACT FROM AUTHOR]