Background
For many applications in research, material development and testing, physical skin models are preferable to the use of human skin, because more reliable and reproducible results can be ...obtained.
Purpose
This article gives an overview of materials applied to model physical properties of human skin to encourage multidisciplinary approaches for more realistic testing and improved understanding of skin–material interactions.
Methods
The literature databases Web of Science, PubMed and Google Scholar were searched using the terms ‘skin model’, ‘skin phantom’, ‘skin equivalent’, ‘synthetic skin’, ‘skin substitute’, ‘artificial skin’, ‘skin replica’, and ‘skin model substrate.’ Articles addressing material developments or measurements that include the replication of skin properties or behaviour were analysed.
Results
It was found that the most common materials used to simulate skin are liquid suspensions, gelatinous substances, elastomers, epoxy resins, metals and textiles. Nano‐ and micro‐fillers can be incorporated in the skin models to tune their physical properties.
Conclusion
While numerous physical skin models have been reported, most developments are research field‐specific and based on trial‐and‐error methods. As the complexity of advanced measurement techniques increases, new interdisciplinary approaches are needed in future to achieve refined models which realistically simulate multiple properties of human skin.
The stick–slip behaviour of the index finger pad sliding on wet, smooth glass was investigated as a function of normal force and sliding velocity in friction measurements using a tri-axial force ...plate. The friction coefficients during stick–slip (0.53±0.21) were found to be typically 30% lower than those of stationary sliding (0.78±0.35). Stick–slip friction either occurred continuously throughout entire friction measurements or alternated with stationary sliding phases. During stick–slip the amplitude of the friction coefficient generally varied more than ±25% about the mean, whereas stationary sliding was characterized by variations smaller than ±10%. The load-dependence of the overall friction coefficients as well as those for stationary sliding and stick–slip friction was in agreement with the adhesion friction model. In addition, the friction coefficients decreasing with increasing sliding velocity indicated friction in the mixed lubrication regime. Results obtained on stick–slip friction of the finger pad might be useful in connection with the development of artificial skin in robotics and for the control of stick–slip friction between precision tools and soft tissue.
Environmental law, in the current context of society's evolution, is a highly topical discipline. The study of this discipline is not easy, being made difficult by the great diversity of ...environmental protection fields and implicitly the legislation. Also, the teaching of this discipline is challenging, requiring the adaptation of classic teaching methods but also the finding of new tools. The present paper, starting from the presentation of some technical aspects of the discipline that produce barriers in the teachinglearning process, presents the author's experience in using (i) the project, as a personalized teaching method and (ii) complementary online information resources.
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.
► Skin–textile friction can be understood qualitatively based on the adhesion model. ► Hospital bed sheets are not optimal with regard to friction and moisture transport. ► Risk of pressure ulcers could be reduced by improved hospital bed sheets.
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.
•The friction and abrasion of the finger pad on glass and abrasive papers was analysed.•Ploughing and abrasion are important friction mechanisms of skin on abrasive papers.•Friction measurements of skin are combined with microscopic and protein analysis.•A protein assay showed that the amount of abraded skin varies with roughness.
We investigated the influence of the contact pressure distribution on the friction of the index finger pad. The skin contact pressure distributions were characterised by round profiles for low ...forces. At higher loads, the pressure distributions of the finger pad became asymmetric and conical. Additional experiments with the knuckle revealed pressure distributions with sharp peaks. The friction coefficients of the finger pad and the knuckle indicated a common behaviour in accordance with the adhesion friction model. Varying pressure distributions were found to influence the friction coefficients of skin, contributing to the variation of measurement results at comparable normal loads.
► Contact pressure distribution of the finger pad during friction varies with load. ► At small forces, finger pads show pressure distributions close to the Hertz model. ► At higher loads, pressure distributions of the finger pad are asymmetric and conical. ► Pressure distribution influences the friction coefficient of the finger pad.
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.
The friction behaviour of five medical compression stockings (MCSs) against forearm skin was investigated in vivo as a function of normal load under dry and wet conditions. In addition, the ...interfacial moisture transport and skin hydration were analysed to understand the influence on friction. The friction coefficients of dry fabrics showed no significant differences between a traditional MCS and versions with modified material composition and base structure. Under wet conditions, the modified MCS fabrics showed considerably lower friction coefficients than the traditional MCS fabric. Changes in the material composition led to 20–30 % lower friction coefficients, different base structures to 50–60 % lower friction. The water transport properties of the MCS fabrics suggest that removal of interfacial water is useful for reducing the friction between MCSs and skin. The results provide information for the optimisation of MCSs regarding improved comfort and moisture transport properties.
The mechanical contact between medical textiles and skin is relevant in the health care for patients with vulnerable skin or chronic wounds. In order to gain new insights into the skin-textile ...contact on the microscopic level, the 3D surface topography of a normal and a new hospital bed sheet with a regular surface structure was measured using a digital microscope. The topographic data was analysed concerning material distribution and real contact area against smooth surfaces as a function of surface deformations. For contact conditions that are relevant for the skin of patients lying in a hospital bed it was found that the order of magnitude of the ratio of real and apparent contact area between textiles and skin or a mechanical skin model lies between 0.02 and 0.1 and that surface deformations, i.e. penetration of the textile surface asperities into skin or a mechanical skin model, range from 10 to 50µm. The performed analyses of textile 3D surface topographies and comparisons with previous friction measurement results provided information on the relationship between microscopic surface properties and macroscopic friction behaviour of medical textiles. In particular, the new bed sheet was found to be characterised by a trend towards a smaller microscopic contact area (up to a factor of two) and by a larger free interfacial volume (more than a factor of two) in addition to a 1.5 times lower shear strength when in contact with counter-surfaces. The applied methods can be useful to develop improved and skin-adapted materials and surfaces for medical applications.
Relaxor ferroelectrics were discovered in the 1950s but many of their properties are not understood. In this review, we shall concentrate on materials such as PMN (PbMg
1/3
Nb
2/3
O
3
), which ...crystallize in the cubic perovskite structure but with the Mg ion, charge 2+, and the Nb ion, charge 5+, randomly distributed over the B site of the perovskite structure. The peak of the dielectric susceptibility for relaxors is much broader in temperature than that of conventional ferroelectrics, while below the maximum of the susceptibility most relaxors remain cubic and show no electric polarization, unlike that observed for conventional ferroelectrics. Because of the large width of the susceptibility, relaxors are often used as capacitors. Recently, there have been many X-ray and neutron scattering studies of relaxors and the results have enabled a more detailed picture to be obtained. An important conclusion is that relaxors can exist in a random field state, as initially proposed by Westphal, Kleemann and Glinchuk, similar to that which has been studied for diluted antiferromagnets. If a relaxor is cooled from a high temperature, then the Burns temperature is a measure of when slow fluctuations become evident. These fluctuations are connected with the disorder and are known as nano-domains. The Burns temperature is not a well-defined transition temperature. At a lower temperature, there is a well-defined boundary to a so-called random field state when the nano-domains become static but there is no long-range periodic order. This phase may have both history-dependent properties and a skin effect in which the surface of the sample is different from that of the bulk material, as also found in experiments on magnetic systems.
Section 1
is an introduction to the review, to ferroelectricity and to relaxors. Section 2 gives a description of the results obtained by dielectric, optical, specific heat and other macroscopic properties. These long-wavelength properties give a variety of different characteristic temperatures and do not directly probe the random field state. In
Section 3
, we describe the results of neutron and X-ray scattering and show that they strongly support the interpretation that relaxors have a random field state. In
Section 4
, we briefly describe the results for other relaxor systems such as (PMN)
1−x
(PT)
x
for which PMN is mixed with different amounts of the ferroelectric lead titanate (PT), and we show that the existence of a random field state enables us also to describe the experimental results for these mixed materials. We hope that this review will inspire further theoretical and experimental work to understand the nature of the random field states and to compare the experimental results more satisfactorily with theory.
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