Influence of epidermal hydration on the friction of human skin against textiles

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.

Figure 1

Two initial decubitus ulcers according to the classification of the European Pressure Ulcer Advisory Panel (Dealey & Lindholm 2006). (a) Grade 1: non-blanchable erythema of intact skin and (b) grade 2: partial thickness skin loss involving epidermis and/or dermis. The ulcer is superficial and presents clinically as an abrasion, blister or shallow crater (with permission and by courtesy of PAUL HARTMANN AG, Heidenheim, Germany).

Figure 2

Typical viscoelastic behaviour of human skin as a response to a cutometer suction–relaxation cycle (stress time mode). The deformation of the skin on the volar forearm is plotted as a function of time. Following the nomenclature of Agache et al. (1980), the parameters used to describe the deformation and the viscoelastic properties of skin are immediate elastic distension (U_e), delayed distension or viscoelastic creep (U_v), total skin extensibility or deformation (U_f), as well as immediate (U_r) and final (U_a) retraction after removal of the vacuum.

Figure 3

In vivo skin–fabric friction experiments on a force plate. The skin frictional resistance is determined by rubbing the volar forearm in a reciprocating motion against the textile on the force plate. Normal load is controlled by checking needle deflection of a voltage meter.

Figure 4

Typical friction and normal force traces from in vivo skin friction experiments on the force plate. The reciprocating motion between forearm and textile induces compressive and tensile forces to the quartz force plate, which are reflected in bipolar friction force signals. The error crosses denote the variation in the friction force and the time window, in which mean friction forces and the corresponding normal loads were calculated for determining friction coefficients (equation (2.1)).

Figure 5

Determination of the apparent contact area between the inner forearm and the force plate using a pressure-sensitive film. The local pressure distribution and the contact area represented by the number of loaded sensor elements are shown for two subjects. The thenar eminence lay outside of the measuring field and was excluded from the calculation.

Figure 6

Effect of epidermal moisture on the friction of skin against a hospital textile. A high linear correlation between skin hydration and friction coefficient was found for all persons. The influence of moisture on the textile friction was more prominent in women, as indicated by greater slopes obtained from linear fits. For clarity, four typical cases are shown. Subsequent to the 2 min measuring interval of the third immersion, an additional corneometer and friction measurement was performed after a drying time of 2 min at laboratory conditions, explaining five data points in the graph. Diamonds, subject 1 (male); triangles, subject 2 (male); circles, subject 3 (female); squares, subject 4 (female).

Figure 7

Skin moisture content as a function of soaking time into isotonic saline solution. The graph demonstrates the means±1 s.d. for men and women, obtained by corneometry. There is a gradual increase in skin moisture during the three immersion periods. A strong water uptake occurs within the first immersion period (IP1), followed by slow increases in epidermal moisture during IP2 and IP3. The skin seems to become saturated as a consequence of the prolonged soaking procedure. Subsequent to the 2 min measurement interval of IP3, the skin was allowed to recover and dry at laboratory conditions for another 2 min (DT). Skin moisture dropped by 70% (men) and 59% (women) from the maximum value, indicating fast water evaporation and re-establishment of the natural skin condition. Squares, men; circles, women.

Figure 8

Gender-dependent increase in skin moisture content and friction at MSH compared with the natural skin condition. Boxplots for men and women are shown. The box contains the central 50% of the ordered data and stretches between the lower and upper quartile, representing the interquartile range (IQR). The horizontal bar within the box denotes the median. The whiskers indicate the minimum and maximum, or the largest and smallest values that are not outliers. Outliers, i.e. cases/COFs with values greater than 1.5 IQRs (box lengths) from the quartiles, are labelled as open circles. Moisture increased in both genders similarly by approximately 45% (p=0.797). The increase in friction was more distinctive in women and significantly different for the two genders (p=0.016). Filled boxes, male; open boxes, female.

Figure 9

In vivo skin–fabric friction with regard to moisture-related skin types (Heinrich et al. 2003). A gradual increase in friction from very dry to normal skin can be discerned for both genders. Male: very dry, n=2; dry, n=19; normal, n=34. Female: very dry, n=6; dry, n=27; normal, n=22. The skin of women shows greater moisture sensitivity. Filled boxes, male; open boxes, female. Open circles denote outliers, i.e. COFs with values greater than 1.5 box lengths from the quartiles.

Figure 10

Friction of skin in the natural and wet condition. On the wet fabric, the friction was more than twofold higher in both men and women (p=0.974). A statistically significant difference between both genders was found for the friction against the wet fabric (p=0.047), confirming the greater moisture sensitivity of female skin. Filled boxes, male; open boxes, female.