S. Derler

Empa - Swiss Federal Laboratories for Materials Science and Technology, Duebendorf, Zurich, Switzerland

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Publications (38)79.55 Total impact

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    ABSTRACT: 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.
    Tribology International 09/2015; 89. DOI:10.1016/j.triboint.2014.12.023 · 2.12 Impact Factor
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    ABSTRACT: Adequate slip resistance of floor coverings is a prerequisite for safe walking. The incidence of slips and falls, however, indicates that insufficient slip resistance is a problem frequently arising in practice. Poor slip resistance can be a consequence of inadequate floor material and surface properties, or it can be caused by wear and maintenance. Various treatments and products are commercially available in order to improve or restore the slip resistance of problematic surfaces. In this study, the effectiveness and durability of different categories of anti-slip treatments for resilient floor coverings were investigated in a long-term field test. Repeated measurements of friction coefficients showed that anti-slip coatings initially improved the slip resistance before gradually losing their positive effect, typically with a half-life period of about 3 months. Safety coatings as well as slip-resistant adhesive tapes increased the slip resistance effectively and proved to be relatively durable, although being affected by significant mechanical wear. Scanning electron microscopy showed that the predominant wear mechanisms of the investigated floor coverings and anti-slip treatments are roughening of smooth surfaces, smoothing of rough surfaces, and intrusion of hard particles into polymers. The main and probably general result of our study is that all kinds of anti-slip treatments (coatings, tapes) gradually lose their positive effect due to wear during use.
    Safety Science 07/2015; 76. DOI:10.1016/j.ssci.2015.02.002 · 1.67 Impact Factor
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    ABSTRACT: 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. 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. 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. 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. 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. © 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.
    Skin Research and Technology 06/2015; DOI:10.1111/srt.12235 · 1.54 Impact Factor
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    ABSTRACT: To combine measurement methods of biophysical skin properties in a clinical setting and to measure baseline values in the unloaded sacral region of healthy persons after lying 30 min in supine position. Hydration (Corneometer® CM 825), redness (Mexameter® MX 18), elasticity (Cutometer® MPA 580) and perfusion (PeriFlux System 5000) of the skin in the sacral region of 10 healthy participants (median age: 26.9 years) were measured consecutively in the laying position by two trained examiners. The assessment duration for all four parameters lasted about 15 min. Intra-class correlation coefficients were overall moderate to strong (hydration r = 0.594, redness r = 0.817, elasticity r = 0.719, perfusion r = 0.591). Hydration (median 27.7 arbitrary units (AU)) mainly indicated dry skin conditions. Redness (median 158.5 AU) was low. Elasticity (median 0.880 AU) showed similar values as in the neck region. Perfusion (median 17.1 AU) showed values in the range of results reported in the literature. Biophysical skin properties in the sacral region after supine position can be measured within periods of 15 min. The results provide baseline data for the skin of healthy persons as well as insights into skin-physiological variations. But it remains challenging to optimize measurement procedures and test protocols when transferring preclinical tests in a clinical application. Copyright © 2015 Tissue Viability Society. Published by Elsevier Ltd. All rights reserved.
    Journal of tissue viability 03/2015; 24(2). DOI:10.1016/j.jtv.2015.03.002 · 1.81 Impact Factor
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    ABSTRACT: Pile can be found on different kinds of surfaces on textiles or carpet surfaces for example. This pile exhibits complex contact and friction behaviour. In this study, model surfaces of pile entities produced from PET fibres have been lab-made and their friction behaviour was investigated. The influence of the penetration depth of the slider into the pile, the sliding velocity, the pile density, and the surface tension of the fibre constituting the pile have been studied. The mechanisms involved during friction with a single pile entity – bending in front of the slider and then friction underneath the slider – have been identified and studied. In the first mechanism, pile entity deformation in predominant, and in the second, friction with the slider is significant. In the case of a pile entity assembly, friction also occurs between bristles/loops. The depth penetration into the pile increases the bending contribution. The surface tension of the fibre has a significant influence on fibre-to-fibre friction. The sliding velocity influence comes from both the friction with the slider and between fibres. The coefficient of friction due to the mechanisms underneath the slider is constant whatever the depth of penetration into the pile.
    Wear 01/2015; 328-329. DOI:10.1016/j.wear.2015.01.039 · 1.86 Impact Factor
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    ABSTRACT: 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 50 µm. 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.
    Journal of the Mechanical Behavior of Biomedical Materials 10/2014; 38:114–125. DOI:10.1016/j.jmbbm.2014.06.014 · 3.05 Impact Factor
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    ABSTRACT: In order to develop new advanced physical skin models it is necessary to investigate, in detail, frictional behavior of human skin against textiles. The aim of this work was to investigate friction of skin and the applicability of an existing skin model to simulate friction of skin in a systematic way.
    Swiss Japan Tribology Meeting 2014; 09/2014
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    ABSTRACT: 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.
    ARCHIVE Proceedings of the Institution of Mechanical Engineers Part J Journal of Engineering Tribology 1994-1996 (vols 208-210) 02/2014; 229(3). DOI:10.1177/1350650114527922 · 0.66 Impact Factor
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    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.
    Tribology International 09/2013; 65:91-96. DOI:10.1016/j.triboint.2013.02.005 · 2.12 Impact Factor
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    ABSTRACT: 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.
    Tribology International 07/2013; 63:14–20. DOI:10.1016/j.triboint.2012.03.001 · 2.12 Impact Factor
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    ABSTRACT: The surfaces of fibrous materials like textiles are characterised by emergent superficial fibres forming hairiness. Hairiness is an important factor for the frictional and tactile properties of fabrics. In this study, we investigated the friction mechanisms of hairy fabrics by sliding probes with varying geometry over the textile surfaces. The total friction between a hairy fabric and a cuboid-shaped slider is given by two main contributions—one due to the contact of hairs with the front of the slider and the other one resulting from the hairiness underneath the slider. By changing the slider geometry, the relative importance of these two contributions can be systematically varied, so that conclusions about adhesion and deformation mechanisms involved in the friction of hairy fabrics can be drawn. For a given slider area and a given normal load, the force underneath the slider was found to be independent of the slider width while the force in front of the slider increased linearly with the slider width. The major contribution of the force underneath the slider is highlighted. This force is influenced by the material and surface roughness of the slider, indicating that adhesion mechanisms play a significant role in the friction.
    Wear 06/2013; 303(s 1–2):343–353. DOI:10.1016/j.wear.2013.03.035 · 1.86 Impact Factor
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    ABSTRACT: In this paper, in-shoe blister formation during running is analyzed and possibilities to reduce these cutaneous lesions by appropriate sock properties are discussed. Six types of cotton-made athletic socks were studied in order to evaluate the influence of the knitted structure on the sock-to-skin friction (terry jersey, plain jersey, and non-raised jersey duffel structures). The contact conditions between a running shoe and foot areas prone to friction blisters were simulated using a linear reciprocating tribometer, and the friction behavior of the socks in contact with a mechanical skin model was investigated. In additional experiments, the shock absorption properties of the socks were measured. Among the investigated socks, the terry jersey fabric knitted in a specific process and a dense, tight jersey structure, i.e. a plain jersey knitted with two yarns, were found to be most suitable for running socks.
    Textile Research Journal 05/2013; 83(8):836-848. DOI:10.1177/0040517512461698 · 1.33 Impact Factor
  • S. Derler, G.-M. Rotaru
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    ABSTRACT: 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.
    Wear 04/2013; 301(s 1–2):324–329. DOI:10.1016/j.wear.2012.11.030 · 1.86 Impact Factor
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    ABSTRACT: Decubitusulcersorpressureulcersareamajorhealthproblem.Thisstudyinvestigatedthepossibility of usinglow-frictionbedsheetswithoptimizedmoisturetransportpropertiesforthepreventionof decubitusulcers.Thetribologicalbehaviorofanewlydevelopedprototypeincontactwithhumanskin was investigatedinvivo.Forcomparisonconventionalhospitalbedsheetswerealsoinvestigated. Friction measurementsunderdryandwetconditionsontheinnerforearmshowedthatconven- tional hospitalbedsheetsarenotoptimal;theirfrictionwassystematicallyhigherbyupto50%when comparedwiththeprototype.Theseresultssuggestthatfrictionandshearstressesontheskinof patientsandthustheriskofpressureulcerscouldbereducedbyimprovedhospitalbedsheets
    Tribology International 02/2013; 65:91-96. · 2.12 Impact Factor
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    ABSTRACT: Textile-based drug delivery systems have a high potential for innovative medical and gerontechnological applications. In this study, the tribological behaviour and lubrication properties of a novel textile with drug delivery function/finishing was investigated by means of friction experiments that simulated cyclic dynamic contacts with skin under dry and wet conditions. The textile drug delivery system is based on a loadable biopolymer dressing on a polyester (PES) woven fabric. The fabrics were finished with low (LC) and highly cross-linked (HC) polysaccharide dressings and investigated in the unloaded condition as well as loaded with phytotherapeutic substances. The mechanical resistance and possible abrasion of the functional coatings on the textile substrate were assessed by friction measurements and scanning electron microscopical analyses. Under dry contact conditions, all investigated fabrics (PES substrate alone and textiles with loaded and unloaded dressings) showed generally low friction coefficients (0.20-0.26). Under wet conditions, the measured friction coefficients were typically higher (0.34-0.51) by a factor of 1.5-2. In the wet condition, both loaded drug delivery textiles exhibited 7-29% lower friction (0.34-0.41) than the PES fabric with unloaded dressings (0.42-0.51), indicating pronounced lubrication effects. The lubrication effects as well as the abrasion resistance of the studied textiles with drug delivery function depended on the degree of dilution of the phytotherapeutic substances. Lubricating formulations of textile-based drug delivery systems which reduce friction against the skin might be promising candidates for advanced medical textile finishes in connection with skin care and wound (decubitus ulcer) prevention.
    Colloids and surfaces B: Biointerfaces 02/2013; 108C:103-109. DOI:10.1016/j.colsurfb.2013.01.055 · 4.29 Impact Factor
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    ABSTRACT: Many textile surfaces are characterised by emergent fibres, i.e. superficial hairiness. It is well known that hairiness plays an important role for the frictional and tactile properties of fabrics. In the present paper, we describe a brush model to predict the friction of hairy fabrics. The new model is based on the assumption that the deformation of superficial hairs during friction is equivalent to the deformation of the hairy textile structure during surface indentation or compression. The force required to compress a given number of hairs per surface unit can be measured by indentation experiments and used to predict the force required to lay down hairs during dynamic friction. Three cases of hairiness deformation are analysed: purely plastic, purely elastic and visco-elastic. For four different textile fabrics, the theoretical calculations based on visco-elastic material behaviour showed fairly good agreement with the experimental results.
    Wear 08/2012; 296(s 1–2):519–527. DOI:10.1016/j.wear.2012.07.027 · 1.86 Impact Factor
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    ABSTRACT: Background: Skin aging is a risk factor for a decubitus and biophysical skin properties could help to identify persons at risk. Whether such biophysical properties of aged human skin differ between areas is undetermined. Objective: To investigate whether viscoelasticity, hydration or friction differ between important areas for decubitus risk. Methods: Pilot study in 32 (18 female, 14 male) acute and subacute old patients aged 81.9 ± 5.9 years (±SD), without active skin disease after an average of 10 days of stay. Assessment of skin resilience/viscoelasticity (E) and hydration (H) at the volar forearm (VF), trochanter (TR) and the sacrum (SA), nutrition by a Mini Nutritional Assessment (MNA), total body water (TBW), lean body mass (LBM), % body fat (%F) by bioimpedance and routine laboratory parameters (hemoglobin, hematocrit, leukocytes, C-reactive protein, serum proteins and creatinine). Results: Mean body mass index (27 ± 4.2), MNA (22.5 ± 2.9), Braden score (20 ± 2.5), E (68.5 ± 6.0%) and H (38.3 ± 6.7) at any site and laboratory parameters did not differ by sex. Men had more TBW (+12 ± 1.5 liters), LBM (+9 ± 2 kg), less %F (-8.8 ± 2.1%), increased H-TR (+7.11 ± 2.8) and H-SA (+5.68 ± 2.5). Overall E-VF correlated significantly with E-TR (r(2) = 0.40, p < 0.0001) and E-SA (r(2) = 0.40, p < 0.0001). In contrast, skin hydration was not correlated. Conclusion: Results of forearm elasticity experiments can be used as a model for other body sites at risk for the development of pressure ulcers.
    Gerontology 04/2012; 58(6). DOI:10.1159/000336623 · 2.68 Impact Factor
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    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.Highlights► The friction behaviour of a novel prototype of a hospital bed sheet was investigated. ► Low friction was found in contact conditions that are relevant for immobile patients. ► The distribution of interfacial water during friction was measured using thermography. ► Efficient water transport within the textile limited the influence on friction. ► The prototype performed better than normal bed sheets under dry and wet conditions.
    Tribology International 02/2012; 46(1):208-214. DOI:10.1016/j.triboint.2011.03.011 · 2.12 Impact Factor
  • S Derler, LC Gerhardt
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    ABSTRACT: 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.
    Tribology Letters 01/2012; 45(1). DOI:10.1007/s11249-011-9854-y · 2.15 Impact Factor
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    ABSTRACT: This paper explores the influence of textile, physiological, and sensorial parameters on sock comfort. Comfort parameters of sport socks were evaluated in two different sport exercises by a non-trained panel. The criteria of the sensory evaluation were perceived foot temperature, fabric dampness, and pain. No rating scale or common lexicon was needed for the sensory evaluation. Foot temperature and humidity were measured during the sport exercises by sensors integrated in the footwear, while coefficients of friction between sock fabric samples and a mechanical skin model were measured using a previously developed Textile Friction Analyzer. The influence of textile, physiological, and sensorial parameters on the sock comfort was statistically investigated. Significant correlations were found between physiological and sensorial parameters as well as between the fabric friction and perceived comfort. Perceived comfort depended on the fibre content of sock fabrics and on the perceived dampness and temperature of the feet. Surface roughness and water content of the textiles had no influence on sock comfort. The new approach of sensory evaluation using a non-trained panel is suitable for the assessment of sock comfort. Using test parameters simulating the contact conditions between foot and sock in sport activities, the Textile Friction Analyzer is an appropriate device to determine the fabric friction which is related to the sock comfort. The results of this research represent a first step towards a better understanding of the influence of different parameters on sock comfort.
    Textile Research Journal 10/2010; 80(8). DOI:10.1177/0040517510369409 · 1.33 Impact Factor