Variables influencing the frictional behaviour of in vivo human skin

In the past decades, skin friction research has focused on determining which variables are important to affect the frictional behaviour of in vivo human skin. Until now, there is still limited knowledge on these variables. This study has used a large dataset to identify the effect of variables on the human skin, subject characteristics and environmental conditions on skin friction. The data are obtained on 50 subjects (34 male, 16 female). Friction measurements represent the friction between in vivo human skin and an aluminium sample, assessed on three anatomical locations. The coefficient of friction increased significantly (p<0.05) with increasing age, increasing ambient temperature and increasing relative air humidity. A significant inversely proportional relationship was found between friction and both the amount of hair present on the skin and the height of the subject. Other outcome variables in this study were the hydration of the skin and the skin temperatur

A systems based experimental approach to tactile friction

This work focuses on the friction in contacts where the human finger pad is one of the interacting surfaces. This ‘tactile friction’ requires a full understanding of the contact mechanics and the behaviour of human skin. The coefficient of friction cannot be considered as a property of the skin alone, but depends on the entire tribo-system. In this work, frictional forces were measured using a commercially available load cell. Parameters such as the hydration of the skin, the normal load on the contact and the roughness of the contacting surfaces were varied, whilst keeping the other parameters constant. The tests were performed under controlled environmental conditions. The total friction force is a combination of forces related to adhesion and to deformation.\ud \ud A commonly made assumption is that, to describe the friction of human skin, the deformation component can be ignored and only the adhesive behaviour has to be taken into account. However, in this study it was found that the forces related to the (micro-scale) deformation of skin can have a significant contribution to the total friction force; this is valid both for dry conditions and in the presence of water, when hydration of the skin causes softening\u

Abrasive wear between rough surfaces in deep drawing

In tribology, many surface contact models are based on the assumption that surfaces are composed of a collection of small asperities of which the tips are equally sized and spherically shaped and have some kind of statistical height distribution. This approach was used in 1966 by Greenwood and Williamson and was successfully followed by many researchers during the following decades. The statistical representation of surface topography enables calculation of contact forces and asperity deformations with reasonable accuracy using well established equations. Although this approach has proven to be suitable for static contact situations, alternative representations of the surface topography are required when modelling abrasive wear. In the current work an elastoplastic contact model is developed in which a representation of the surface topography is obtained by best fit approximations of the micro-contacts, obtained from real, measured surface height data. In this deterministic surface representation, the tips of the contacting asperities are assumed to have an ellipsoidal shape. Given the material parameters and contact conditions, the load and deformation of a single asperity can be computed. Subsequently, the wear induced by each individual asperity is obtained by inserting its size and shape and the conditions into a “single asperity micro-abrasion model”. By summing the contributions of all individual asperities, the total abrasive wear volume is obtained. The results of the developed abrasive wear model are compared with results obtained using a statistical approach

Tribologically modified surfaces on elastomeric materials

Proliferation of meniscal fibrochondrocytes cultured on a new polyurethane scaffold is stimulated by TGF-

A contact model for orthotropic-viscoelastic materials

In many industrial applications, fibre-reinforced polymers are in contact with rigid surfaces. The contact behaviour of such polymers is both anisotropic and viscoelastic: the polymers exhibit viscoelastic behaviour and the addition of short fibres results in a directionality in the material behaviour of the composite. This work focuses on the contact problem of an orthotropic viscoelastic material in contact with a spherical rigid indenter, in which the radius of the contact area is much larger than the fibre size.\ud \ud A general form of anisotropic behaviour has been considered in order to describe materials with a high degree of anisotropy. By using separation of variables a solution for the quasi-static contact problem for viscoelastic and generally anisotropic materials is obtained by combining the linear theory of viscoelasticity with the Hertz solution for elastic anisotropic materials as derived by Willis. The developed contact model requires nine elastic material parameters in combination with one time dependent material parameter as input, in order to characterise the behaviour of the material. It is shown that the results of the developed model show good agreement with both isotropic and anisotropic materials described in literature. Furthermore, it is shown that ignoring the anisotropy of a viscoelastic material will result in an overestimate of the stiffness of the material

A model for the contact behaviour of weakly orthotropic viscoelastic materials

Fibre reinforced elastomers behave anisotropically as well as viscoelastically. Yang and Sun (1982) developed an elastic contact model for anisotropic materials which, in the present work, is extended to account for viscoelastic effects. The developed viscoelastic contact model uses the creep compliance function of the material in the direction of indentation. The results of the model agree with experimental results obtained on short fibre reinforced EPDM. Furthermore, a parameter study of the coefficients of the creep compliance function on the real contact area has been made. The results show that, at short time scales, the viscoelastic real area of contact can be significantly smaller than when assuming fully elastic behaviour. At long time scales the results of the viscoelastic contact model equal those of the elastic model of Yang and Su

Interpersonal differences in the friction response of skin relate to FTIR measures for skin lipids and hydration

Understanding the mechanical response of skin to contact is of importance when developing products that interact with the skin. The shear forces that arise due to friction in the interface are a key aspect of skin interactions, because shear is known to contribute to discomfort and tissue injury. However, the frictional response of skin shows large variations between people. It has been hypothesised that these variations relate to differences between people in the physiological properties of their skin, but the underlying mechanisms are not well understood. In order to gain new insights into these interpersonal differences in friction behaviour, in vivo FTIR measurements and in vivo friction measurements were performed on the same patch of skin. Quantitative analysis of the various peaks in the FTIR spectra provided information on the moisture content of the stratum corneum and the amount and mechanical properties of the lipids on the skin. The lipid viscosity, as characterised by the width of the 2920 cm−1 peak, correlates with the friction, whilst, interestingly, no relationship was found between the quantity of lipids on the skin surface and the coefficient of friction. Additionally, and as expected, a fairly strong correlation was obtained between the moisture content, as characterised by the height of the Amide I peak and the coefficient of friction. The presented results show that spectroscopy techniques can be used in as a non-invasive method to identify people who may show elevated levels of friction and thus are at increased risk of developing shear induced tissue injury