Wear 263 (2007) 1112–1116

Short communication

Tribology of human skin and mechanical skin equivalents in contact with textiles S. Derler ∗ , U. Schrade, L.-C. Gerhardt Empa-Materials Science and Technology, Lerchenfeldstrasse 5, CH-9014 St. Gallen, Switzerland Received 2 September 2006; received in revised form 23 November 2006; accepted 25 November 2006 Available online 23 May 2007

Abstract The friction of untreated human skin (finger) against a reference textile was investigated with 12 subjects using a force plate. In touch experiments, in which the subjects assessed the surface roughness of the textile at normal loads of 1.5 ± 0.7 N, the average friction coefficients ranged from 0.27 to 0.71 and varied among individuals due to different states of skin hydration. In experiments, in which the subjects varied the normal load, the friction coefficients were in the same range and showed practically no load dependence, indicating that both adhesion and hysteresis are contributing to the friction behaviour. The results for human skin were compared with apparative friction measurements using different silicone and polyurethane materials as mechanical skin equivalents. A polyurethane coated polyamide fleece with a surface structure similar to that of skin showed the best correspondence with human skin under dry conditions. The friction coefficients of this material increased with the moisture content of the reference textile. A realistic skin model in combination with an objective friction test method would be very useful for the textile industry and allow the efficient development of new textiles with improved and skin-adapted surface and frictional properties for sport and medical applications. © 2007 Elsevier B.V. All rights reserved. Keywords: Tribology; Skin; Textile; Skin model; Friction

1. Introduction The tribology of skin in contact with textiles is important in connection with the comfort of clothing, because the tactile properties of fabrics are closely related to their surface and frictional properties. At the same time, friction at the skin-textile interface is a critical factor for skin injuries in sport and working activities (irritations, abrasions and blisters), which are caused by cyclic mechanical loads if contact pressures and shear forces are high or continue over long enough periods of time. As mechanical contacts can be especially problematic for sensitive, injured or aged skin, the frictional characteristics of textiles are also relevant in the medical area, e.g., in connection with skin diseases, wound healing and the prevention of decubitus. The optimum surface and friction properties of textiles depend on the specific application. In many cases, e.g., for socks and sport T-shirts, textiles with a low coefficient of fric-

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Corresponding author. E-mail address: [email protected] (S. Derler).

0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.11.031

tion against the skin seem to be an appropriate solution. So far, product development has mainly focused on fibre materials and combinations, whereas the surface structure of textiles has hardly been taken into account with respect to friction. Dowson [1] and Sivamani et al. [2] have reviewed the current knowledge on the tribology of human skin, derived mainly from dermatological studies concerning the effects of skin care products. It is generally agreed that skin hydration, lipid films and surface structure are important factors for the frictional properties of skin, whereas conflicting results exist regarding the influence of age and anatomical site; differences in gender or race are considered unimportant. In order to investigate the frictional properties of textiles, different measurement devices using linear or rotational relative movements have been developed in the past [3]. Instead of using arbitrary materials such as steel [4] as the counterpart in contact with textiles, the use of mechanical skin equivalents such as a polymer finger [5] or a polyurethane film [6] seems to be a promising approach. In the present study, tribological experiments were carried out to study the friction of a reference textile against human

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Fig. 1. Measurement of friction coefficients between a finger and a flat textile sample (left) and simulation of touch experiments (middle) using a mechanical skin model (Lorica, right).

skin (finger) as well as in contact with various soft polyurethane (PUR) and silicone materials taken as candidates for mechanical skin equivalents. An appropriate skin model in combination with an objective friction test method would be very useful for the textile industry and allow the efficient development of new textiles with improved surface and frictional properties. 2. Methods and materials 2.1. Friction measurements To measure the friction between skin and textiles, fabric samples were attached to a quartz 3-component dynamometer Kistler 9254 (force plate) and rubbed by 12 subjects (6 females and 6 males with ages between 24 and 61 years) with the index finger (Fig. 1). The normal and the two tangential force components were measured using three charge amplifiers (Kistler, Type 5015) and a transient recorder PC-board (Bakker, Type BE490) for data acquisition. Each subject carried out a series of touch experiments, in which the index finger was rubbed on the textile for 20 s such that the roughness of the fabric surface could be assessed. The sliding movements of the finger ranged from 5 to 10 cm and were parallel to the longitudinal axis of the force plate. The stroke frequencies were 1.0 ± 0.3 Hz and the contact areas between fingertip and fabric were between 1.5 and 3 cm2 . For roughness assessments, the subjects used normal

loads of 1.5 ± 0.7 N which resulted in typical contact pressures between 3 and 10 kPa. In order to study the influence of contact pressure on measured friction coefficients, each subject carried out a series of additional experiments in which the normal load of the finger on the fabric was varied over a range of up to 15 N. The friction between finger and textile was simulated apparatively using a previously developed device, with which friction coefficients of textiles against a mechanical skin model can be measured under a variety of test conditions (Fig. 1). The contact area of a fabric sample with the skin model is a circle of 28.5 mm in diameter (6.4 cm2 ). During a friction test, the fabric remains stationary while the skin model on its metallic support is submitted to a reciprocating movement (500 cycles). The friction force between the skin model and a fabric sample is measured by a quartz force sensor (Kistler, Type 9203) combined with a charge amplifier (Kistler, Type 5011B) and sampled using a transient recorder PC-board (Bakker, Type BE490). Friction coefficients are determined for all sliding friction cycles (Fig. 2). Based on the observation of human touch assessments, the following conditions were defined for friction measurements: a normal load of 3 N, resulting in a contact pressure of 4.7 kPa, and a stroke of 20 mm at a frequency of 1.25 Hz. All experiments took place at a temperature of (20 ± 1) ◦ C and a relative humidity of (65 ± 2)%. The subjects were acclimatized for at least 10 min in the laboratory, but their skin was neither cleaned nor treated before the friction measurements.

Fig. 2. Friction coefficients (tangential force divided by the normal force) measured in a touch experiment (left) compared to apparative measurement results using the skin model Lorica (right). Points indicate individual dynamic friction coefficients that are determined over periods of stationary sliding between the directional changes.

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Table 1 Characteristics of materials investigated as skin models Material

Surface roughness Ra (␮m)

Hardness shore A

Lorica® Soft (0.9 mm), skin-like surface PUR sheet material (1 mm), smooth Silicone sheet material (1.5 mm), smooth Vinylpolysiloxane, smooth Vinylpolysiloxane, matt Vinylpolysiloxane, rough Vinylpolysiloxane, replica of Lorica

14.93 ± 1.73 0.211 ± 0.022 1.060 ± 0.120 0.006 ± 0.001 1.166 ± 0.052 11.94 ± 1.38 Not measured

42.5 ± 1.8 70.2 ± 0.4 51.2 ± 1.0 48.4 ± 0.7 48.4 ± 0.7 48.4 ± 0.7 48.4 ± 0.7

To determine surface roughness parameters, a laser profilometer (Altisurf 500, Cotec) was used in the case of Lorica and a surface profilometer (Perthometer M1, Mahr) for the other materials. Hardness shore A was measured based on DIN 53505(2000), using multi-layer samples with a total thickness of about 9 mm.

2.2. Materials A wool fabric as specified in the Martindale abrasion test [ASTM D4966-98(2004)] served as reference textile throughout this study. Seven materials were investigated as mechanical skin models (Table 1). The artificial leather Lorica® Soft, which is used for testing the slip resistance of floor coverings in barefoot areas [7], consists of a polyamide fleece with a PUR coating. It is characterized by a surface structure similar to that of skin (Fig. 1) along with surface roughness parameters in the same range [8,9]. The other materials investigated were two relatively smooth sheet materials made of PUR and silicone, respectively, negative vinylpolysiloxane replicas of three different glass surfaces as well as positive replicas of Lorica. 3. Results Mean friction coefficients between skin and the reference textile measured in touch experiments ranged from 0.27 to 0.71, showing considerable differences among the individual subjects (overall mean value ± S.D.: 0.415 ± 0.124). The skin of all subjects was investigated in the untreated condition, so that the variations in the results reflect a certain spectrum of skin hydration states as well as differences in the skin lipid concentrations. Around 15 full friction cycles with two sliding movements in opposite directions were analysed for each touch experiment (Fig. 2). The standard deviations of the corresponding friction coefficients were between 0.011 and 0.087, indicating variations due to differences between individual sliding cycles and, in some cases also between the two sliding directions. For both measurements shown in Fig. 2, the contact pressures were about 5 kPa and the resulting friction coefficients slightly below 0.3. As expected, the friction coefficient signal of the apparative measurement is more regular and smoother than that of the touch experiment. In the touch experiments, in which the normal loads were varied between about 0.2 and 15 N, practically constant coefficients of friction were found for most subjects. Beside typical results for a subject with dry skin, Fig. 3 also illustrates the case of a subject with moist skin, in which the friction coefficients decreased with increasing normal load. In touch experiments

Fig. 3. Dependence of friction coefficients on the applied normal load for moist and dry skin (according to skin surface hydration measurements using a Corneometer CM 825, Courage + Khazaka electronic) in comparison with the skin equivalent Lorica measured against the reference fabric on the force plate.

using a finger covered with the skin model Lorica, its friction behaviour was found to be comparable with that of dry skin. Fig. 4 shows a comparison among different mechanical skin equivalents, used for measuring friction coefficients against the reference fabric. The results of Lorica represented the case of dry skin in touch experiments, whereas a smooth silicone material showed rather low and a smooth PUR material very high friction coefficients. Replicas made of vinylpolysiloxane generally gave much higher friction coefficients as observed in touch experiments with subjects. Two smooth versions of replicas showed significantly higher friction coefficients than the two replicas with a rough surface and the surface structure of Lorica, respectively. In order to study the potential of the skin equivalent Lorica to represent not only dry, but also moist and wet skin conditions, the influence of moisture on its frictional properties was investigated in additional experiments, in which defined amounts of water were applied in the interface between the skin model and the reference fabric. First results show (Fig. 5), that friction coefficients increase with moisture content before stabilizing at a certain level.

Fig. 4. Friction coefficients (mean value ± S.D.) of the reference fabric against seven different skin models, measured on the friction test device. The horizontal lines represent the mean value (0.415) ± 1 S.D. (0.124) of the friction coefficients found in touch experiments with 12 subjects.

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Fig. 5. Influence of moisture on the friction of the reference fabric against the skin model Lorica. The data points represent mean values after 500 friction cycles, the error bars correspond to ±1 S.D. (results for four to six measurements per amount of water).

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coefficients, as would be expected for an increased effective contact area and adhesion. It is known that the friction coefficients of skin increase with hydration, but can be relatively low in the completely wet condition [1]. In friction experiments with wool fabric samples which were moistened by different amounts of water, the skin model Lorica behaved in qualitative agreement with real skin (Fig. 5). However, further investigations with human subjects are needed to study the influence of skin hydration on friction in detail and to define standard skin conditions, for which specific mechanical skin models can be developed for textile testing. For measuring skin hydration levels, instruments detecting the electrical capacitance of the stratum corneum such as the Corneometer CM 825 [14] are applicable. A systematic study on the relationship between friction and hydration of the human skin requires a large number of subjects. On this basis, experimental skin-simulating materials can be reliably validated for different physiological skin conditions.

4. Discussion

5. Conclusions

For wool sliding on skin, Comaish and Bottoms [10] reported a friction coefficient of 0.4. Kenins [11] who investigated the friction of different wool fabrics on the forearm and the finger of subjects, found friction coefficients between 0.32 and 0.48 for light contacts on dry skin and values between 0.48 and 1.23 for wet skin. Friction coefficients in the same order of magnitude were found in the touch experiments presented here (0.27–0.71), but comparisons seem problematic because of different test conditions. Due to the viscoelastic material properties, the friction behaviour of human skin depends on load, effective contact area and elastic modulus [1]. The friction of a viscoelastic material sliding on a rigid surface is mainly determined by adhesion due to molecular bonding of surface atoms in both contacting materials as well as hysteresis, resulting from the delayed recovery of the viscoelastic material after deformation by the surface asperities of the rigid material [12]. According to Wolfram [13] the friction of skin is primarily determined by adhesion, implying that friction coefficients increase with decreasing load and as the modulus decreases or the skin becomes softer, e.g., due to hydration. The hysteresis component of friction is normally considered unimportant for skin, but should increase as the load is increased and the skin becomes softer [1]. In the touch experiments described here, no general relationship was found between friction coefficients of skin and load. In most cases, the coefficients of friction were practically constant, indicating that not only adhesion, but also deformation of both the skin and the reference textile might play a role. In the case of one subject with moist skin, however, the friction coefficient increased with decreasing load as expected for friction due to adhesion (Fig. 3). Among the investigated skin models, Lorica showed the best correspondence with the friction behaviour of human skin under dry conditions. The results for vinylpolysiloxane illustrated a measurable influence of the surface roughness and structure on friction coefficients. The smooth surfaces showed higher friction

Friction coefficients measured between human skin and a reference textile were found to vary considerably among individuals. The observed variations were mainly attributed to differences in skin hydration and variations in the lipid content of the skin surface. In order to investigate the frictional properties of textiles by means of an objective test method, human skin has to be simulated by an appropriate skin model. In friction measurements using different polyurethane and silicone materials, a polyurethane coated polyamide fleece with a surface structure similar to that of skin showed the best correspondence with human skin under dry conditions. Similar to real skin, the friction coefficients of this material increased with the moisture content of the reference textile. Further research on the influence of moisture on the frictional properties of human skin is necessary to define standards for dry, moist and wet skin conditions. On this basis, refined skin models, e.g., with surface modifications using liquid films, could then be developed and specified for the realistic testing of textiles regarding friction against the skin. References [1] D. Dowson, Tribology and the skin surface, in: K.-P. Wilhelm, P. Elsner, E. Berardesca, H.I. Maibach (Eds.), Bioengineering of the Skin: Skin Surface Imaging and Analysis, CRC Press, Boca Raton, 1997, pp. 159–179. [2] R.K. Sivamani, J. Goodman, N.V. Gitis, H.I. Maibach, Coefficient of friction: tribological studies in man—an overview, Skin Res. Technol. 9 (2003) 227–234. [3] N. Gitis, R. Sivamani, Tribometrology of skin, Tribol. Trans. 47 (2004) 461–469. [4] M.-A. Bueno, B. Lamy, M. Renner, P. Viallier-Raynard, Tribological investigation of textile fabrics, Wear 195 (1996) 192–200. [5] S.S. Ramkumar, D.J. Wood, K. Fox, S.C. Harlock, Developing a polymeric human finger sensor to study the frictional properties of textiles. Part I: Artificial finger development, Textile Res. J. 73 (2003) 469–473. [6] B. Bhushan, G. Wei, P. Haddad, Friction and wear studies of human hair and skin, Wear 259 (2005) 1012–1021. [7] Bfu regulation R9729, Pr¨ufreglement f¨ur Bodenbel¨age mit erh¨ohter Gleitfestigkeit, Bern, 2001.

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[8] W. Manuskiatti, D.A. Schwindt, H.I. Maibach, Influence of age, anatomic site and race on skin roughness and scaliness, Dermatology 196 (1998) 401–417. [9] J. Moon, G. Yi, C. Oh, M. Lee, Y. Lee, M. Kim, A new technique for three-dimensional measurements of skin surface contours: evaluation of skin surface contours according to the ageing process using a stereo image optical topometer, Physiol. Meas. 23 (2002) 247–259. [10] S. Comaish, E. Bottoms, The skin and friction: deviations from Amonton’s laws, and the effects of hydration and lubrication, Br. J. Dermatol. 84 (1971) 37–43.

[11] P. Kenins, Influence of fiber-type and moisture on measured fabric-to-skin friction, Textile Res. J. 64 (1994) 722–728. [12] D.F. Moore, The Friction and Lubrication of Elastomers, Pergamon Press, Oxford, 1972. [13] L.J. Wolfram, Friction of skin, J. Soc. Cosmetic Chem. 34 (1983) 465– 476. [14] G. Khazaka, Assessment of stratum corneum hydration: Corneometer CM 825, in: J. Fluhr, P. Elsner, E. Berardesca, H.I. Maibach (Eds.), Bioengineering of the Skin, CRC Press, Boca Raton, 2005, pp. 249–261.