S. Derler

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

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Publications (29)44.41 Total impact

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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 01/2014; 38:114–125. · 2.37 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.
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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. · 1.14 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. · 1.54 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. · 3.55 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. 01/2013; 301(s 1–2):324–329.
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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.
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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; · 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. · 1.54 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). · 1.74 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 01/2010; 80(8). · 1.14 Impact Factor
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    ABSTRACT: Sports activities may cause different types of injuries induced by friction between the skin and sport textiles. Focusing on runners who are often bothered with blisters, we studied the textile–foot skin interface in order to measure and predict friction. We first determined the characteristics of mechanical contacts between foot, sock and shoe during running. On this basis, we developed a reciprocating linear tribometer to simulate friction contacts between skin and sport socks. Two different types of cotton socks were studied (basic jersey and terry jersey). To predict the friction of terry jersey socks, we developed a model which takes into account basic textile parameters and assumes that dynamic friction primarily results from the movement of terries (rotation of loops). For the investigated cases, a good correspondence was found between calculated and measured friction forces.
    Wear. 01/2010; 268:1103-1110.
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    ABSTRACT: While hip protectors are effective in some clinical trials, many, including all in community settings, have been unable to demonstrate effectiveness. This is due partly to differences in the design and analysis. The aim of this report is to develop recommendations for subsequent clinical research. In November of 2007, the International Hip Protector Research Group met to address barriers to the clinical effectiveness of hip protectors. This paper represents a consensus statement from the group on recommended methods for conducting future clinical trials of hip protectors. Consensus recommendations include the following: the use of a hip protector that has undergone adequate biomechanical testing, the use of sham hip protectors, the conduct of clinical trials in populations with annual hip fracture incidence of at least 3%, a run-in period with demonstration of adequate adherence, surveillance of falls and adherence, and the inclusion of economic analyses. Larger and more costly clinical trials are required to definitively investigate effectiveness of hip protectors.
    Osteoporosis International 10/2009; 21(1):1-10. · 4.04 Impact Factor
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    ABSTRACT: Hip protectors represent a promising strategy for preventing fall-related hip fractures. However, clinical trials have yielded conflicting results due, in part, to lack of agreement on techniques for measuring and optimizing the biomechanical performance of hip protectors as a prerequisite to clinical trials. In November 2007, the International Hip Protector Research Group met in Copenhagen to address barriers to the clinical effectiveness of hip protectors. This paper represents an evidence-based consensus statement from the group on recommended methods for evaluating the biomechanical performance of hip protectors. The primary outcome of testing should be the percent reduction (compared with the unpadded condition) in peak value of the axial compressive force applied to the femoral neck during a simulated fall on the greater trochanter. To provide reasonable results, the test system should accurately simulate the pelvic anatomy, and the impact velocity (3.4 m/s), pelvic stiffness (acceptable range: 39-55 kN/m), and effective mass of the body (acceptable range: 22-33 kg) during impact. Given the current lack of clear evidence regarding the clinical efficacy of specific hip protectors, the primary value of biomechanical testing at present is to compare the protective value of different products, as opposed to rejecting or accepting specific devices for market use.
    Osteoporosis International 10/2009; 20(12):1977-88. · 4.04 Impact Factor
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    ABSTRACT: The mechanical properties of human skin are known to change with ageing, rendering skin less resistant to friction and shear forces, as well as more vulnerable to wounds. Until now, only few and contradictory results on the age-dependent friction properties of skin have been reported. This study has investigated in detail the influence of age on the friction of human skin against textiles. In vivo skin-friction measurements on a force plate were combined with skin analyses concerning elasticity, hydration, pH value and sebum content. Thirty-two young and 28 aged persons rubbed their volar forearm in a reciprocating motion against various textiles on the force plate, using defined normal loads and sliding velocities, representing clinically relevant contact conditions. Mean friction coefficients ranged from 0.30 +/- 0.04 (polytetrafluoroethylene) to 0.43 +/- 0.04 (cotton/polyester). No significant differences in the friction properties of skin were found between the age groups despite skin elasticity being significantly lower in the aged persons. Skin hydration was significantly higher in the elderly, whereas no significant differences were observed in either skin pH value or sebum content. Adhesion is usually assumed to be the dominant factor in skin friction, but our observations imply that deformation is also an important factor in the friction of aged skin. In the elderly, lower skin elasticity and skin turgor are associated with more pronounced skin tissue displacements and greater shear forces during frictional contact, emphasizing the importance of friction reduction in wound-prevention programmes.
    Skin Research and Technology 09/2009; 15(3):288-98. · 1.41 Impact Factor
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    ABSTRACT: This study investigated how the sliding friction coefficients of wet foot skin depend on the microscopic surface properties of contacted hard substrates. Fourteen subjects conducted repeated slip experiments with one foot on each of 28 different wet surfaces. The friction and normal forces transmitted to the substrates were measured using a tri-axial force plate, and coefficients of friction were determined over periods of stationary sliding. The surface structures of the substrates, characterised by an optical 3D profilometer, varied from roughness asperities in the range of micrometers to macroscopic surface elements in the range of millimetres. The analysis of the load dependence of the skin friction behaviour based on a two-term model provided information on the adhesion and deformation components of friction. Both adhesion and deformation were found to increase with the surface roughness of the substrates. Adhesion mechanisms seemed to predominate on smooth surfaces showing low friction coefficients (<0.3), while the contributions due to skin deformations were found to be up to 0.4 on rough surfaces with high friction coefficients (>0.5). Independent of friction measurements, the analysis of the surface microstructure of the substrates indicated that ploughing as a skin deformation mechanism could contribute around 50% to the deformation component of friction.
    Wear. 01/2009;
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    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 50 N, leading to skin contact pressures of up to 120 kPa. 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.
    Tribology International. 01/2009;
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    ABSTRACT: In this study, the potential of currently available padded soccer goalkeeper shorts to prevent hip injuries sustained in side jumps was investigated. Within the scope of this study, a survey among professional and amateur goalkeepers, trainers and the medical staff was performed. In addition, commercially available shorts were impact-tested using an artificial hip model. The results were compared with requirements established for hip protectors for elderly. The results of the survey found that contusions and abrasions were observed most often for all players. Fractures and arthritis were only reported by amateur players, whereas bursitis was more frequently observed by professional players. Amateurs have a significantly higher risk of injury than professional players, and the higher injury risk during training is significantly higher compared with the injury risk during a match. No difference of the injury risk was found between goalkeepers wearing padded shorts and those who did not. Impact tests indicated a wide range of performance of the currently available products. The padded shorts generally reduce impact forces, but mostly perform poorly. Only shorts that were made of visco-elastic foam fulfilled the basic requirements requested for hip protectors for elderly. The quality of most of the currently available goalkeeper shorts needs to be improved to effectively prevent hip injury sustained in side jumps.
    British journal of sports medicine 07/2008; 44(6):426-9. · 3.67 Impact Factor
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    ABSTRACT: The rate of hip fractures due to falls increases with age. External hip protectors placed over the greater trochanter can prevent hip fractures, but the willingness to wear such protective devices is rather low. Most of the commercially available hip protectors consist either of an energy-absorbing foam pad or of a hard shell that distributes the load to the surrounding tissue. In the present study, a fibre-reinforced shell composed of three curved strips bonded with a ring, was designed and lined with shock absorbing foam. The development of the new shell design was based on quasi-static and impact tests of manufactured shells in combination with finite element simulations. The results of the numerical analysis showed the potential protection effect of the shell and indicated how the design can be further improved. First impact tests on an anatomical hip model showed promising results of the new protector shell in combination with a foam pad.
    Medical Engineering & Physics 05/2008; 30(9):1186-92. · 1.78 Impact Factor
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    ABSTRACT: Friction and shear forces, as well as moisture between the human skin and textiles are critical factors in the formation of skin injuries such as blisters, abrasions and decubitus. This study investigated how epidermal hydration affects the friction between skin and textiles.The friction between the inner forearm and a hospital fabric was measured in the natural skin condition and in different hydration states using a force plate. Eleven males and eleven females rubbed their forearm against the textile on the force plate using defined normal loads and friction movements. Skin hydration and viscoelasticity were assessed by corneometry and the suction chamber method, respectively.In each individual, a highly positive linear correlation was found between skin moisture and friction coefficient (COF). No correlation was observed between moisture and elasticity, as well as between elasticity and friction. Skin viscoelasticity was comparable for women and men. The friction of female skin showed significantly higher moisture sensitivity. COFs increased typically by 43% (women) and 26% (men) when skin hydration varied between very dry and normally moist skin. The COFs between skin and completely wet fabric were more than twofold higher than the values for natural skin rubbed on a dry textile surface.Increasing skin hydration seems to cause gender-specific changes in the mechanical properties and/or surface topography of human skin, leading to skin softening and increased real contact area and adhesion.
    Journal of The Royal Society Interface 04/2008; 5(28):1317-28. · 4.91 Impact Factor