Raja K. Sivamani1,2, Gabriel Wu2, Norm V. Gitis1 and Howard I. Maibach2

Background/purpose: Few studies have focused on the simultaneous measurement of the friction and electrical properties of skin. This work investigates the feasibility of using these measurements to differentiate between the effects of chemicals commonly applied to the skin. In addition, this study also compares the condition of the skin and its response to application of chemicals across gender, ethnicity, and age at the volar forearm.

Method: Friction and electrical tests were performed on 59 healthy volunteers with the UMT Series Micro-Tribometer (UMT). A 13-mm-diameter copper cylindrical friction/electrical probe was pressed onto the skin with a weight of 20 g and moved across the skin at a constant velocity of 0.4mm/s. Each volunteer served as his or her own control. The friction and electrical impedance measurements were performed for polyvinylidene chloride occlusion and for the application of glycerin and petrolatum.

Results: No differences were found across age, gender, or ethnicity at the volar forearm. Polyvinylidene chloride (PVDC) occlusion showed a small increase in the friction and a small decrease in the electrical impedance; petrolatum increased the friction by a greater amount but its effect on the impedance was comparable to PVDC occlusion; glycerin increased the friction by an amount comparable to petrolatum, but it decreased the impedance to a much greater degree than petrolatum or the PVDC occlusion. An amplitude/mean measurement of the friction curves of glycerin and petrolatum showed that glycerin has a significantly higher amplitude/mean than petrolatum.

Conclusion: The properties of the volar forearm appear to be independent of age, gender, and ethnicity. Also, the simultaneous measurement of friction and electrical impedance was useful in differentiating between compounds administered to the skin.

Key words: age – ethnicity – friction – impedance – skin

Blackwell Munksgaard 2003

Accepted for publication

SKIN HEALTH is a major concern for people of all ages, gender, and ethnicity. As a result, most people will invest in skin products and it will be important to provide quantitative comparisons between them. Some compounds like petrolatum (Vaseline) increase skin hydration by inhibiting water evaporation from the skin surface; other compounds will absorb into the skin to carry and directly release hydrative elements into the skin.

Also, some compounds provide a greasy texture while others elicit a more sticky texture. Quantification of these properties will be important in the development of new skin care. The most convenient testing apparatus will be

one that is noninvasive for simplicity of measurement. Previous noninvasive tests have focused on friction (1) and electrical measurements such as capacitance (2–8), conductance (2, 4, 5, 7, 8), and impedance (9, 10). Several tests have focused on using an individual test to measure the influence of different skin care products (5–7, 11–19). However, differentiation between products is much harder when only using one assay. This study ascertains if using several measurements simultaneously could be of advantage in tracking changes in the skin after product application. The assays include evaluation of friction coefficient, electrical impedance, and an amplitude/mean calculation, explained later, of the friction curves (14).

Age groups were split into the following categories: young (18–40 years old), middle (41–59 years old), and old (greater than 60 years of age). The tests were performed in a controlled room with constant temperature (21–26 1C) and relative humidity (50–70%). Volunteers were asked to refrain from using creams prior to coming for the test and were asked to rest for 30min after arriving at the clinic. The test sites on their arms were cleaned with isopropyl alcohol prior to testing.

Anatomical site and applied interventions

Tests were conducted on the right and left volar forearms. Any visible hairs were gently clipped to prevent their influence on the friction and electrical measurements. The forearm was chosen for measurement due to a decreased amount of hair found there and the relative ease in using the measurement apparatus. Four sites along the right and left forearm were measured as outlined in Fig. 1.

Different treatments were administered at each site of the forearm and these included occlusion, glycerin, and petrolatum. Occlusion was achieved by wrapping the arm in polyvinylidene chloride (PVDC) (saran wrap) for 30 min to prevent water loss. The USP grade glycerin solution was mixed as suggested by the manufacturer of the glycerin from the local pharmacy (one part glycerin to two parts water). The glycerin solution was applied at 3mg/cm2. Petrolatum was also acquired from the local pharmacy, and was applied at 0.5mg/cm2. Measurements of the petrolatum- and glycerin-treated sites were taken after leaving the treatments on for 1 min and dabbing the skin with a paper towel to remove the excess. Glycerin was applied to site 3 in 50% of the volunteers and to site 4 in 50% of the volunteers (the same was performed for petrolatum) so that the effects of glycerin and petrolatum would not be affected by the choice of anatomical site along the volar forearm.

The friction and electrical measurements utilized the UMT Series Micro-Tribometer (UMT) (11), a computer-controlled bench-top instrument adapted to measure tribological parameters on the skin (Fig. 2).

The UMT provides precision translational, rotational, and linear motions to a variety of friction pair specimens, with speeds ranging from 0.1 to 10 m/s. A normal load is applied by a closed-loop servomechanism in the instrument’s upper stage, and can be kept constant or linearly increasing. The range capacity for the applied normal force is from 0.5mN to 200 N. The UMT measures and records the friction force and normal load at a total sampling rate of 20 kHz. The measurement probe chosen consisted of a 13-mm-diameter copper cylindrical friction/electrical probe (Fig. 2) attached to a suspension system. The suspension system is in turn attached to a strain gauge-based force sensor. For each measurement, the probe was pressed onto the skin of the volar forearm with 20 g, maintained constant by the servo feedback loop. Then, the probe was moved proximally on the forearm across the skin linearly for 10mm at a constant speed of 0.4 mm/s. Electrical measurements were performed by applying an alternating current of 10 mA, at a frequency of 10 kHz. Next, the probe was lifted off the skin and the measurement was repeated thrice. Parameters calculated were the dynamic coefficient of friction and the electrical impedance. An additional parameter calculated from the friction coefficient measurement curves was the amplitude/mean for untreated skin and the various interventions (Fig. 3). This measurement is representative of the stickiness of the skin surface. As the amplitude/mean number increases, the skin is thought to be stickier and have more surface roughness (11, 14).

No significant differences were found for the friction coefficient or the electrical impedance of volar forearm between age, gender, or ethnicity (data shown for age in Fig. 4). However, there was a significant difference between the distal volar forearm (sites 1 and 3 in Fig. 1) and the proximal volar forearm (sites 2 and 4 in Fig. 1). The friction coefficient was elevated and the electrical impedance was decreased for the proximal volar forearm (Fig. 5). This result suggests that there is variation in hydration and perceived smoothness of the skin along the volar forearm. Amplitude/meanmeasurements showed that there were no significant variations among the anatomical sites (Fig. 6).

All interventions showed no significant differences between gender or age. Differences in measurements arose as a result of the different interventions applied.

Occluded skin showed an increase in friction coefficient and a decrease in electrical impedance (Figs 7 and 8). As the PVDC covers the skin, it prevents the evaporation of water from the skin surface and the water remains trapped. As a result, the skin becomes more hydrated and the presence of water decreases the electrical impedance as observed. Also, with increased hydration, the skin surface is more ‘sticky’ due to water adhesion between the probe and the skin, and the contact area will increase. Both these effects result in an increased friction coeffi-cient over untreated skin measurements. The amplitude/mean measurement for occluded skin was higher than for untreated skin (Fig. 9).

Petrolatum (applied to site 3 in half the volunteers and site 4 for the other half)

When applied, the petrolatum will coat the skin surface and inhibit loss of water through this barrier. Similar to the PVDC, it is reported to increase skin hydration primarily through this occlusive effect (14). The electrical impedance measurement showed that petrolatum application lowered the skin impedance by a similar amount as occlusion (Fig. 8). This similarity may be due to the fact that petrolatum is thought to hydrate through occlusion like the PVDC. However, the friction coefficient for the petrolatumtreated skin was much higher than for PVDC occlusion (Fig. 7), indicating that the petrolatum may have been absorbed into the skin, unlike the PVDC. The amplitude/mean measurement showed that the petrolatum acted to lower the stickiness of the surface (Fig. 9). This result is quantitative support for the qualitative perception that petrolatum is greasy and tends to make the surface more slippery.

Glycerin (applied to site 3 in half the volunteers and site 4 for the other half)

The friction coefficient decreased by about 70% after glycerin application, and this decrease was similar to the decrease in friction coefficient seen with petrolatum application. However, the drop in electrical impedance was much greater for the glycerin application as opposed to the petrolatum or the PVDC occlusion. This may be a reflection of the speed and the amount of glycerin that is absorbed directly into the skin. This may allow for the skin to hydrate to a greater degree as measured by the lower skin impedance. The amplitude/mean measurement showed that glycerin application increased the skin stickiness above both untreated skin and occluded skin    (Fig. 9).

Noninvasive methods to measure the skin function can greatly ease the process of testing skin and products. Friction and electrical measurements have been explored as avenues to measure skin health, but few studies have explored differentiation between different skin effects with different chemicals through the simultaneous use of friction and electrical measurements. This study suggests that the untreated measurements of the skin on the volar forearm are consistent across gender, age, and ethnicity and can be used as a site for testing skin chemicals against one another. Since there is some variation with anatomical site on the volar forearm (20), comparisons among chemicals on the same volunteer can be made by comparing similar anatomical sites on the right and left volar forearm. The properties of the skin on the volar forearm do not seem to change across gender, age, or ethnicity (data shown for age in Fig. 4). The lack of differences between young and old volar forearm skin was surprising since collagen breakdown and crosslinking would lead to changes in the properties of the skin. The results from this study suggest that the volar forearm is somehow protected, perhaps from the sun. In fact, it has been shown in previous studies that skin that is hidden from sun exposure shows no significant differences between young and old people (3). Another study of friction properties also found that there were no age-related differences on the volar forearm (21). Egawaet al. (21) and this study suggest that the volar forearm may be an ideal anatomical site for studying the effects of skin products, since other factors will be minimized at this location.

Three interventions were quantitatively differentiated with using the electrical and friction measurements. The electrical impedance provided a measure of the water levels under the skin surface and revealed the intervention’s ability to ‘absorb’ into the skin. The friction measurement revealed the intervention’s ability to affect the exposed surface of the stratum corneum. In the case of the PVDC occlusion, skin impedance did not greatly decrease (Fig. 8), the friction coefficient increased slightly compared to the other interventions (Fig. 7), and the amplitude/mean measurement showed that the change in skin stickiness was not as drastic as the other interventions (Fig. 9). This suggests that occlusion is not effectively ‘absorbed’ into the skin (impedance), it affects the surface by a moderate amount (friction coefficient), and it tends to make the surface stickier with the water build-up (amplitude/mean). Similar analysis can be extended to glycerin and petrolatum to differentiate between the two. Friction measurements show that both chemicals have increased the friction coefficient and it would seem that each chemical’s effect is similar. However, comparisons along the change in skinimpedance and the amplitude/mean measurement show that one can differentiate between the two compounds’ skin effects. Petrolatum does not absorb as readily into the skin as glycerin and coats the exposed surface of the skin, evidenced by its lesser effect on the impedance (Fig. 8). The lowered amplitude/mean measurement for petrolatum shows that it makes the skin greasier than glycerin (Fig. 9).

Conclusion

These data suggest that there is little variation in volar forearm skin across gender, age, and ethnicity and this site would be effective for the testing of skin and cosmetic products. Tribological measurements, namely friction and electrical measurements, can be used in conjunction to gather differentiable data for various skin compounds. The data showed that PVDC occlusion, glycerin solution, and petrolatum were different when comparing across friction coefficient, electrical impedance, and amplitude/mean measurements.

We do not wish to overgeneralize on the basis of sample size (59) examined; larger cohorts may show subtle differences not ascertained here.

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Address:

Professor H. I. Maibach

Department of Dermatology

School of Medicine

University of California at San Francisco

Box 0989, Surge 110

San Francisco, CA 94143-0989

USA

Tel: 415.476.2468

Fax: 415.753.5304

e-mail: himjlm@itsa.ucsf.edu