Tribology Transactions

Tribology Transactions, 47: 1-9, 2004 Copyright C

Society of Tribologists and Lubrication Engineers ISSN: 0569-8197 print / 1547-397X online

DOI: 10.1080/05698190490493355

Tribometrology of Skin

NORM GITIS and RAJA SIVAMANI
Center for Tribology, Inc.
Campbell, California

The quantitative assessment of both skin health and skin care products is suggested based on skin tribological properties. Simultaneous multi-sensor measurements of both coefficient of friction and contact electrical impedance allow for fast and quantitative evaluation of skin conditions such as dryness and moisturization, and early diagnosis of skin diseases or of the deterioration in skin functions at a stage that may not be easily discernable visibly. It may be instrumental in developing and testing skin cosmetics and medicine.

KEY WORDS Friction Testing; Biolubrication; Skin

INTRODUCTION

Skin health and beauty is a concern for people of all ages. Physiologically, skin is the first line of defense against any environment, and it is repeatedly subjected to physical and chemical damage. Over the course of time a person’s skin undergoes changes, and to maintain skin health it is important to quantitatively follow these changes. Sometimes these skin changes are visible, such as wrinkles, blemishes, or rashes. In other cases, the changes may not be easily discernable without a quantitative assessment of skin properties. Most people invest in skin care products, and it is important to provide quantitative comparison between them. Some products inhibit water evaporation from the skin, some compounds absorb and directly release hydrating elements into the skin, other com- pounds provide a greasy texture, while others elicit a more sticky texture. Quantification of these properties is crucial in the development of new skin care products. The bulk of dermatological observations are based on qualitative visual or papillatory data. The experience with skin bioengineering proves the sharpness of the biometric focus provided by appropriate instrumentation and execution. Skin tribology has not been widely utilized as appropriate, robust, and facile instrumentation (and validation was not available). The unique multi-sensing technology described in this paper should fill this void in the fields of dermato-physiology, dermato-pharmocology, dermatotoxicology, and cosmetology. Currently, there is no widespread diagnostic technique or method available to quantitatively relate skin properties to skin health. Since skin is a surface, it can be conveniently analyzed and described in tribological terms. Friction and electrical measurements have already been utilized as analytical techniques for skin health research. Utilization of our multi-sensor technology with real-time high-frequency data acquisition allows for dramatic and improvement in data quality of the tribological in vivo and in vitro assessment of human skin.

Knowledge of skin’s tribological properties, namely, surface coefficient of friction and surface contact electrical impedance, provides a quantitative assessment of skin health. This work allows for a fast and quantitative assessment of skin conditions such as dryness and moisturization, and early diagnosis of skin diseases or of the deterioration in skin functions at a stage that may not be easily discernable visibly. It may be instrumental in developing and testing skin cosmetics and medicine.

REVIEW OF FRICTION MEASUREMENTS

When a person feels her or his skin with her or his finger, the resultant perception of the skin property is nothing but friction between the finger and the skin. Thus, friction measurements represent the most straightforward way to exactly mimic the person’s feeling of her or his skin conditions. Friction studies can be conducted non-invasively and can give an invaluable measure of the skin’s health. For example, Naylor (1) showed that moistened skin has elevated friction, El-Shimi (2) demonstrated that drier skin has lower friction. According to Wolfram (3), and Appeldoorn and Barnett (4), friction provides a good quantitative measure of skin assessment. To measure friction, a probe is brought into contact with and is moved relative to the skin. A friction force, which opposes relative movement between the probe and skin, is monitored and then used to calculate the friction coefficient (see Table 1). A perpendicular normal load varies from author to author and is poorly controlled with either static weights or spring. There are two types of designs for the test apparatus that have predominated in prior studies of skin friction, namely a probe moving across the skin either linearly or circularly. For example, Comaish and Bottoms (5) used one of the simplest linear designs: they moved the probe across the skin by attaching it to a pan of weights via pulley. Weights were placed in the pan such that the probe slid over the skin at a constant velocity. The dynamic friction coefficient was calculated by dividing the total weight in the pan by the normal load on the probe. More sophisticated linear designs provided motorized movement of the probe, either unidirectional or reciprocating; the motorization afforded greater control of the constant velocity of the probe. Also, in modern designs, strain gauges measure the friction force as the probe moves along the skin.

TABLE 1—FINDINGS OF IN VIVO SKIN FRICTION STUDIES

Authors

Probe Size, Shape

Probe Material

Probe Motion

Setup of Normal Load

Friction Coefficient

Comaish and Bottoms (5)

Naylor (1)

Prall (6)

El-Shimi (2)

Highley, et al. (7)

Cua, et al. (8)

Asserin, et al. (9)

Johnson, et al. (10)

Elsner, et al. (11)

15-mm ring

8-mm sphere

Disc

12-mm hemisphere

Disc

15-mm disc

3-mm sphere

Lens

15-mm disc

Teflon, nylon, polyethylene, wool

Polyethylene

Glass

Stainless steel (rough), Stainless steel (smooth) Nylon

Teflon

Ruby

Glass

Teflon

Linear

Linear, reciprocating Rotational

Rotational

Rotational Rotational

Linear Linear, reciprocating Rotational

Static weights

Spring load

Static weights

Spring load

Balloon; static weights Static weights

Spring load

0.2 (Teflon)

0.45 (Nylon)

0.3 (Polyethylene)

0.4 (Wool)

0.5–0.6

0.4

0.2–0.4 (Rough)

0.3–0.6 (Smooth)

0.2–0.3

0.34 (Forehead)

0.26 (Volar forearm)

0.21 (Palm)

0.12 (Abdomen)

0.25 (Upper back)

0.7

0.3–0.4 (Dry skin)

0.48 (Forearm)

0.66 (Vulva)

The rotating-probe designs use a rotating wheel or disc pressed onto the surface of the skin with a known normal load. Highley, et al. (7) measured the frictional resistance by determining the angular recoil of the instrument as the wheel contacted the skin, by monitoring the light via a photocell. Comaish, et al. (12) developed a portable device with a torsion spring to measure skin friction.

The major problems of these studies have been data repeatability from test to test and reproducibility from person to person, day to day, and apparatus to apparatus. Indeed, one can hardly expect the same results from probes of very different materials, shapes, and dimensions, performing different motions with different speeds and accelerations. For example, smoother probes showed higher friction for both stainless steel (El-Shimi (2)) and nylon (Comaish and Bottoms (5)) probes, which is related to the larger contact area and adhesion. Even for the same probes and apparatus, a large portion of these problems remains due to the absence of effective real-time closed-loop load control, which is critical on the non-flat and not-so-smooth skin. Monitoring the normal force on the probe is absolutely essential for getting accurate and repeatable friction coefficient measurements. The two methods used to set up the normal load, static weights and springs, had no real-time normal load monitoring; it was just incorrectly assumed that the normal load stays constant during the test. Normal load maintenance is a source of data variation on the non-flat skin, as the probe encounters dips (valleys) and raises (peaks), and so the normal load fluctuates during the test.

Previous studies have focused on correlating the friction with age, gender, anatomical site, and hydration. Friction varies with anatomical site. Cua, et al. (8), (13) found friction coefficients to vary from 0.12 on the abdomen to 0.34 on the forehead. Elsner, et al. (11) measured the vulvar friction coefficient at 0.66 and the forearm friction coefficient at 0.48. Manuskiatti, et al. (14) observed significant differences in skin roughness at various anatomical sites. Differences in environmental influences (i.e., sun exposure) and hydration may contribute to this. Elsner, et al. (11) showed that a more-hydrated vulvar skin had a 35% higher friction than a forearm, which is in agreement with hydration studies that contend that skin has an increased friction under increased hydration.

With respect to age, friction measurement results are contradictory. Cua, et al. (8) showed no differences in friction with respect to age on the ankle. Elsner, et al. (11) and Asserin, et al. (9) observed no age-related differences in vulvar friction but higher forearm friction in younger subjects; they postulated that the skin exposed to sunlight undergoes photoaging and, thus, forearm skin shows aging, whereas the light-protected vulvar skin does not.

Cua, et al. (13) found no significant skin friction differences between the genders. There are no studies addressing race as pertains to friction, but Manuskiatti, et al. (14) found no differences in skin roughness between black and white skin.

Much of the reviewed research has been devoted to ascertaining how the application of certain ingredients influences the skin surface, is of interest to the cosmetic/moisturizer and lubricant industries. El-Shimi (2) and Comaish and Bottoms (5) showed that friction decreased with the application of talc powder—by 50% for dynamic friction and by 30% for static friction with a polyethylene probe; however, they also found that wetting the talc powder caused an increase in friction. Friction drops after the application of oils and oil-based lubricants but then eventually increases (Comaish and Bottoms, (5)). Prall (6) and Nacht et al. (15) found that friction rises with the addition of emollients and creams in a similar fashion to water; however, the cream effects lasted for hours whereas the water effects lasted for minutes. Hills, et al. (16) observe that at an elevated temperature of 45^◦C most emollients lowered friction to a greater degree than at a room temperature of 18◦C.

TABLE 2—FINDINGS IN SKIN CAPACITANCE STUDIES

Comparative Study

Authors

Capacitance Findings

Age

Gender

Anatomical site

Clinical skin dryness

Moisturizer application

Water hydration

Cua, et al. (8)

Elsner, et al. (11)

Frodin, et al. (18)

Cua, et al. (13)

Elsner, et al. (11)

Blichmann, et al. (19)

Tagami (20)

Lod ´en, et al. (21)

Tagami (20)

Hashimoto-Kymasaka, et al. (22) Blichmann, et al. (19)

Frodin, et al. (18)

Blichmann, et al (19)

Tagami (20)

Difference on the palm

No difference

Differences among various anatomical sites

Differences between vulva and forearm

Differences between palm and forearm

Differences among various anatomical sites

Decrease with increased skin dryness

Decreased with increased skin dryness

Decreased in psoriatic lesions, same in suction blisters

Significant increase in skin treated with cream

Significant increase 2 hours after cream application; significant decrease on the day after stopping treatment

Increase, then return to pre-hydration values after 7 min

Increase, then returned to pre-hydration values after 3 min

REVIEW OF ELECTRICAL MEASUREMENTS

Another measurement technique used to assess the skin is based on its electrical properties: capacitance, conductance, and impedance. The dry stratum corneum is a dielectric medium. Addition of water makes the stratum corneum responsive to an electrical field (Leveque and De Rigal (17)). The electrical methods are mostly used for assessment of skin hydration.

Capacitance measurements involve two oppositely charged plates held in close proximity. An electric field is formed between them, and the maximum charge on each plate is known as the capacitance. When dielectric materials are introduced into the gap between the two plates, they increase capacitance; for example, water increases capacitance by a factor of 81 compared to vacuum. Thus, capacitance measurements are convenient for monitoring a hydration level. A number of capacitance studies of the effects of age, gender, anatomical site, clinical skin dryness, and moisturizer applications are outlined in Table 2.

Electrical resistance of skin to an electrical current is measured at low currents (a few microamperes) so that the skin is not harmed in the measurement process. Studies in skin resistance and conductance are summarized in Table 3.

TABLE 3—FINDINGS IN SKIN RESISTANCE STUDIES

Comparative Study

Authors

Conductance Findings

Gender

Anatomical site

Clinical skin dryness

Moisturizer application

Water hydration

Cua, et al. (13)

Blichmann, et al. (19)

Tagami, et al. (23)

Hashimoto-Kymasaka, et al. (22)

Tagami, et al. (23)

Blichmann, et al. (19)

Tagami, et al. (23)

No significant differences

Differences between palm and forearm

Differences among various anatomical sites

Decrease with increased dryness

Decreased in psoriatic lesions and increased in suction blisters

Increase in moisturized skin

13-fold increase, dropped to a 2-fold increase after 15 min

Increase, then return to pre-hydration levels after 3 min

Increase, then return to pre-hydration levels after 4 min

Most of the electrical assessment has involved measurements of either capacitance or resistance (conductance), but a few studies have looked at a parameter that combines them both: electrical impedance. Nicander, et al. (24) investigated impedance differences of skin across anatomical locations, age, and gender and found the impedance to vary by anatomical site, but not by gender. In a subsequent study, Nicander and Ollmar (25) observed seasonal variations in the electrical impedance on all of the tested anatomical sites except for the neck.

For reasonable repeatability of electrical data, ambient humidity has to be controlled to control the amount of water in the stratum corneum; relative humidity above 60% should be avoided. The apparatus must be placed on hair-free skin; too much coarse hair or an inclined position reduces the accuracy of results. Skin should be exposed to the ambient conditions for 5 min before taking measurements. There must be at least 10-s waiting time between measurements, because repeated measurements on the same skin location may change skin conditions.

STATE-OF-THE ART TRIBO-METROLOGY

The multi-sensing technology has been implemented on a commercially available, portable Skin Micro-Tribometer model UMT, a photo of which is shown in Fig. 1. It provides comprehensive tribological measurements for different types of samples in a variety of biomedical applications. For example, it is successfully utilized for testing bathroom tissues on skin, soap on skin, surgical staples and sutures, medical needles, shaving blades, aftershave lotions, toothpastes, and toothbrushes.


Fig. 1—Skin Micro-Tribometer model UMT.		Fig. 2—Skin Micro-Tribometer measures forearm in vivo.

The Skin Micro-Tribometer has significant technological and design advantages over previous devices used to measure skin friction or electrical properties. In performing its test functions, the UMT is capable of providing precision linear, rotational, and reciprocating motions with programmable speeds in the range of 0.1 µm/s to 10 m/s. A normal load is tightly controlled with a closed-loop servomechanism and can be programmed to be either constant or changing gradually or by steps, with user-defined tolerances, in the total range from 0.1 mN to 1 kN. A number of tribological parameters, namely friction force and coefficient, normal load, electrical contact resistance, capacitance or impedance, skin deformation (elastic, plastic, creep) or wear depth, temperature, and contact acoustic emission can all be measured and recorded simultaneously, with a sampling rate of 20 kHz. Digital video with magnifying optics is also readily available.

Fig. 3—Skin Micro-Tribometer measures finger skin in vivo.

The UMT is utilized for both in vitro testing on the artificial or cut-off skin (laid on the sample table seen in Fig. 1) and in vivo testing on people’s arms (see photo in Fig. 2), fingers (see photo in Fig. 3), and other body parts (not shown). Comfortable hand and arm supports (see Figs. 2 and 3) allow for in vivo skin tests of these limb sites on the bench-top tester configuration. Special handheld adapters are used for measurements on the face, back, legs, and other anatomical sites.

The measurement probe chosen for most of the measurements in this work consisted of a 12-mm copper cylindrical fric-tion/electrical probe attached to a proprietary suspension system. In some measurements, nylon, Teflon, and stainless steel balls were used. In each test, the probe was pressed onto the skin with a constant load of 0.2 N so that the contact pressure was enough to maintain a constant contact with the skin but not too high in order to avoid skin macro-deformations and focus the measurements on the skin surface only. For studies involving under-skin layers and tissues, higher loads were used.

The probe was moved across the skin in a straight line at the speed of 1 mm/s for 10 mm. The slow motion typically produces the most repeatable results and so seems to be the most useful for dermatological applications. Some tests were performed at higher speeds of 1 cm/s, and even 5 cm/s on longer traces, to better simulate the motion of a person’s finger when checking her or his skin, which may be important for characterization and marketing of cosmetic products.

During probe sliding, the dynamic friction coefficient was measured with a proprietary strain-gauge sensor, monitoring simultaneously and independently both normal load and friction force, with the resolution of 0.2 mN. In situ electrical measurements were performed by applying an alternating current of a small constant amplitude of 10 µA, with frequency of 10 kHz, measuring the voltage across the probe, and calculating the electrical impedance. Next, the probe was lifted off the skin and the measurement was repeated twice for a total of three data points that were then averaged for each test.

Fig. 4—Calculation of the amplitude-to-mean ratio.

Fig. 5—Layout of test sites on right and left volar forearms.

An additional parameter obtained from the friction curves was the friction variation coefficient, calculated as its amplitude-to-mean ratio (Fig. 4). As the friction variation coefficient increases, the skin is expected to be stickier or rougher.

IN VIVO MEASUREMENTS OF SKIN INTERVENTIONS

Sixty healthy (by history) adult volunteers were tested. A volunteer was self-classified to a certain race when all four grandparents similarly identified. Age groups were split into the following: young (18-40 years old), middle (41-59 years old), and old (greater than 60 years of age). The tests were carried out in a controlled room with constant temperature and humidity. Volunteers were asked to refrain from wearing creams prior to coming to the test and were asked to rest for 30 min upon arriving at the clinic. The test sites on their arms were cleaned with isopropyl alcohol prior to testing. The tests were conducted on the right and left volar forearms. Any visible hair was gently clipped to prevent its influence on the measurements. The forearm was chosen for the tests due to the low amount of hair there, relative ease in using the measurement apparatus, and consistent test results. Four sites along the right and left forearm were measured as outlined in Fig. 5. Different treatments were administered at each site of the forearm, including no treatment, occlusion by wrapping the arm in polyvinylidene chloride or PVDC (saran wrap) for 30 min to prevent water loss, glycerin applied at 3 mg/cm^2,and petrolatum applied at 0.5 mg/cm2 (interchanged between the sites). 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 excess. Results are shown in Figs. 6 and 7 as percent variation from the values recorded for untreated skin.

Fig. 6—Percent increase in coefficient of friction for three skin treatments, compared to untreated skin.

Fig. 7—Percent decrease in electrical impedance for three skin treatments, compared to untreated skin.

No significant differences were found in either friction or electrical parameters of volar forearms among age, gender, or ethnicity. The untreated skin on the volar forearm produces similar frictional and electrical characteristics across gender, age, and ethnicity and so can be used as a site for comparative testing of skin care products. Because there is some variation with anatomical site on the volar forearm, comparisons among chemicals on the same volunteer can be done by comparing similar anatomical sites on the right and left volar forearm. The similarity between young and old volar forearm skin may be related to the suggestion that the volar forearm is partly protected from the sun. In fact, it corresponds to the previous studies that skin hidden from sun exposure, like that on the volar forearm, showed no significant differences between young and old people (Elsner, et al. (11); Cua, et al. (13); Nicander, et al. (24). Thus, the volar forearm may be a good anatomical site for testing skin care products.

There were substantial differences between the distal (sites 1 and 3 in Fig. 5) and proximal (sites 2 and 4) volar forearm sites, with the friction coefficient higher by about 30% and the electrical impedance lower by about 15% for the proximal sites compared to the distal sites. This difference may be due to both hydration and smoothness of the skin along the volar forearm. Thus, researchers have to be very careful in choosing the forearm site for skin testing.

All of the skin treatments showed no significant differences among gender, age, and ethnicity but quite dramatic and well-expected differences as a result of the different interventions.

Occluded skin showed an increase in friction and a decrease in impedance (Figs. 6 and 7). As the PVDC covers the skin, it prevents water evaporation, and so the trapped water decreases the electrical impedance. The increased hydration makes the skin stickier due to water-mediated adhesion between the probe and the skin, which results in increased friction and friction variation coefficient for the occluded skin (Fig. 8).

Petrolatum coated the skin and served as a barrier for the water loss. Similarly to the PVDC, it is reported to increase skin hydration primarily through this occlusive effect. Indeed, petrolatum application lowered the skin impedance by an amount similar to that for the occlusion (Fig. 7). Friction for the petrolatum-treated skin, however, was higher than after the PVDC occlusion (Fig. 6), indicating that the petrolatum may have been absorbed into the skin, unlike the PVDC. The friction variation coefficient indicated that the petrolatum lowered the skin stickiness (Fig. 8). These results give a solid quantitative support for the qualitative perception that petrolatum is greasy and tends to make skin more slippery.

Glycerin increased friction similarly to that of petrolatum. However, it dropped impedance to a much greater degree than either petrolatum or PVDC occlusion. This may be reflective of the higher rate and amount of glycerin absorbed directly into the skin, which may allow for higher skin hydration as measured by the lower skin impedance. The highest friction variation coefficient showed that glycerin increased the skin stickiness the most (Fig. 8).

Fig. 8—Amplitude-to-mean ratio for three skin treatments.

So, three surface interventions were quantitatively differentiated with the simultaneous use of electrical and friction measurements. The electrical impedance provided a measure of the water levels under the skin surface and revealed the treatment ability to absorb into the skin, whereas the friction measurement revealed the treatment ability to affect the exposed surface of the stratum corneum. For example, the PVDC wrap caused the smallest decrease in skin impedance (Fig. 7) and the smallest increase in friction (Fig. 6) compared to the other interventions, which suggests that the occlusion was not effectively absorbed into the skin (impedance) and so affects the surface moderately (friction). Both glycerin and petrolatum raised friction by a similar amount (Fig. 6), but the petrolatum did not absorb as readily into the skin as glycerin, evidenced by its lesser effect on the electrical impedance (Fig. 7); also, its substantially lower friction variation coefficient shows that petrolatum makes the skin greasier than with glycerin (Fig. 8).

IN VIVO MEASUREMENTS OF SKIN MOISTURIZERS

Hydration is a complex phenomenon influenced by intrinsic (i.e., age, anatomical site) and extrinsic (i.e., ambient humidity, chemical exposure) factors. Earlier studies have revealed that drier skin had lowered friction, whereas hydrated skin had increased friction (Table 1). Indeed, water increases adhesive forces between skin and a probe, as well as softens the skin, which in turn increases contact area and friction between a probe and the skin. Thus, there is an increased frictional resistance between the hydrated skin and a probe. Since water evaporates within minutes, the skin returns to its pre-hydration state in a few minutes; a dried skin becomes less supple and allows the probe to glide more easily over it, which results in a lower friction (Comaish and Bottoms (5); Blichmann, et al. (19); Tagami (20); Lod´

en, et al. (21); Blichmann and Serup (26); Courage (27); Denda (28)).

The aforementioned studies were focused on an intermediate level of hydration—when the skin is moistened without an appreciable "slippery" layer of water on its surface. In general, a skin response to water is much more complex, because the very wet skin also has low friction due to the hydrodynamic effects, whereas the very dry (clinically dry) skin becomes rough and increases a mechanical component of friction.

We performed an experimental comparison of three different moisturizing creams: a common older formulation, an advanced daytime cream, and the most advanced highest-performance nighttime cream. Ten healthy adult volunteers were tested on the same locations of both their right and left volar forearms. Each test included three sequential unidirectional runs, done before cream application (to establish the reference levels of the test parameters), then every 5 min for 1 hour, then after 5 hours, and finally after 24 hours.

The frictional and electrical data were very consistent across all the volunteers and are summarized in Fig. 9. One can see that all three moisturizers had the same qualitative effect of increasing friction and decreasing electrical impedance, but quantitatively differed substantially. The common low-performance cream lasted for less than an hour; the advanced cream lasted for several hours, and the effect of the high-performance cream was still measurable a day later. The repeatability and reproducibility of these results was within 10%, which is sufficient for dermatological studies. These tests provide a scientific justification to the pricing of these creams, which quantifies their functional quality.

CONCLUSIONS

Our research confirms earlier studies that differences in skin due to aging, health, hydration, and other factors are reflected in skin frictional and electrical characteristics. The novel technology and instrumentation for simultaneous measurements of friction coefficient and electrical impedance is an effective tool of skin assessment. It can serve as a quantitative method of evaluating skin health in dermatology on various body parts of different people. It is also useful for quantitative functional evaluation of skin care products like creams, lotions, soaps, and so forth.

The experimental data suggest that there is little variation in volar forearm skin parameters across gender, age, and ethnicity, which makes this anatomical site promising for testing cosmetics.

ACKNOWLEDGMENTS

We would like to thank Dr. Howard Maibach with the University of California in San Francisco for his dermatological guidance and supply of most of the volunteers, Ms. Peggy Farranghoh at the San Francisco Ocean Avenue Dermatology Clinic for organizing the volunteers, Mr. Gabriel Wu with the University of California in San Francisco for participation in some of the measurements, and Mr. Bill Sloan and Mr. Alex Belov of the Center for Tribology for making the special probes.

REFERENCES

(1) Naylor, P. F. D. (1955), "The Skin Surface and Friction," Bri. J. Dermatol., 67, pp 239-248.

(2) El-Shimi, A. F. (1977), "In Vivo Skin Friction Measurements," Jour. Soc. Cosmetic Chemists, 28, pp 37-51.

(3) Wolfram, L. J. (1983), "Friction of Skin," J. Soc. Cosmet. Chem., 34, pp 65476.

(4) Appeldoorn, J. and Barnett, G. (1963),"Frictional Aspects of Emollience," Proc. Sci. Sect. Toilet Goods Association, 40, pp 28-35.

(5) Comaish, S. and Bottoms, E. (1971), "The Skin and Friction: Deviations from Amonton’s Laws, and the Effects of Hydration and Lubrication," Brit. J. Dermatol., 84, pp 37-43.

(6) Prall, J. K. (1973), "Instrumental Evaluation of the Effects of Cosmetic Products on Skin Surfaces with Particular Reference to Smoothness,"

J. Soc. Cosmet. Chem., 24, pp 693-707.

(7) Highley, D. R., Coomey, M., DenBeste, M. and Wolfram, L. J. (1977),"Fric-tional Properties of Skin," J. Invest. Dermatol., 69, pp 303-305.

(8) Cua, A. B., Wilheim, K. P. and Maibach, H. I. (1990), "Friction Properties of Human Skin: Relation to Age, Sex and Anatomical Region, Stratum Corneum Hydration and Transepidermal Water Loss," Brit. J. Dermatol., 123, pp 473-479.

(9) Asserin, J., Zahouani, H., Humbert, Ph., Couturaud, V. and Mougin, D. (2000), "Measurement of the Friction Coefficient of the Human Skin In Vivo. Quantification of the Cutaneous Smoothness," Colloids Surfaces B: Biointerfaces, 19, pp 1-12.

(10) Johnson, S. A., Gorman, D. M., Adams, M. J. and Briscoe, B. J. (1993), "The Friction and Lubrication of Human Stratum Corneum," Thin Films in Tribology, Eds. Dowson, D. et al., Proc. 19th Leeds-Lyon Symp. on Trib., Elsevier Science Publishers, B. V., pp 663-672.

(11) Elsner, P., Wilhelm, D. and Maibach, H. I., (1990), "Friction Properties of Human Forearm and Vulvar Skin: Influence of Age and Correlation with Transepidermal Water Loss and Capacitance."Dermatologica, 181,pp88-91.

(12) Comaish, J. S., Harborow, P. R. H. and Hofman, D. A. (1973),"A Hand-Held Friction Mete," Brit. J. Dermatol., 89, pp 33-35.

(13) Cua, A. B., Wilhelm, K. P. and Maibach, H. I. (1995), "Skin Surface Lipid and Skin Friction: Relation to Age, Sex, and Anatomical Region," Skin Pharmacy, 8, pp 246-251.

(14) Manuskiatti, W., Schwindt, D. A. and Maibach, H. I. (1998), "Influence of Age, Anatomic Site and Race on Skin Roughness and Scaliness," Dermatology, 196, pp 401-407.

(15) Nacht, S., Close, J., Yeung, D. and Gans, E. H. (1981), "Skin Friction Coefficient: Changes Induced by Skin Hydration and Emollient Application and Correlation with Perceived Skin Feel," J. Soc. Cosmetic Chem., 32, pp 55-65.

(16) Hills, R. J., Unsworth, A. and Ive, F. A. (1994), "A Comparative Study of the Frictional Properties of Emollient Bath Additives Using Porcine Skin, Brit. J. Dermatol., 130, pp 37-41.

(17) Leveque, J. L. and De Rigal, J. (1983), "Impedance Methods for Studying Skin Moisturization," J. Soc. Cosmet. Chem., 34, pp 419-428.

(18) Frodin, T., Helander, P., Molin, L. and Skogh, M. (1988), Hydration of Human Stratum Corneum Studied In Vivo by Optothermal Infrared Spectroscopy, Electrical Capacitance Measurement, and Evaporimetry," Acta Derm. Venereol., 68, pp 461-467.

(19) Blichmann, C. W., Serup, J. and Winther, A. (1989), "Effects of Single Application of a Moisturizer: Evaporation of Emulsion Water, Skin Surface Temperature, Electrical Conductance, Electrical Capacitance, and Skin Surface (Emulsion) Lipids," Acta Derm. Venereol., 69, pp 327330.

(20) Tagami, T. (1994), Bioengineering of the Skin: Water and the Stratum Corneum, Eds. Elsner, P., Berardesca, E. and Maibach, H. I., CRC Press, Boca Raton, FL, pp 171-175.

(21) Loden, M., Olsson, H., Axell, T. and Linde, Y. W. (1992), "Friction, Capacitance and Transepidermal Water Loss in Dry Atopic and Normal Skin, Bri. J. Dermatol., 126, pp 137-141.

(22) Hashimoto-Kumasaka, K., Takahashi, K. and Tagami, H. (1993), "Electrical Measurement of the Water Content of the Stratum Corneum In Vivo and In Vitro under Various Conditions: Comparison between Skin Surface Hygrometer and Corneometer in Evaluation of the Skin Surface Hydration State," Acta Derm. Venereol., 73, pp 335-336.

(23) Tagami, H., Ohi, M., Iwatsuki, K., Kanamaru, Y., Yamada, M. and Ichijo,

B. (1980), "Evaluation of the Skin Surface Hydration in Vivo by Electrical Measurements," J. Invest. Dermatol., 75, pp 500-507.

(24) Nicander, I., Nyren, M., Emtestam, L. and Ollmar, S. (1997), "Baseline

´Electrical Impedance Measurements at Various Skin Sites—Related to Age and Gender," Skin Res. Technol., 3, pp 252-258.

(25) Nicander, I. and Ollmar, S. (2000), "Electrical Impedance Measurements at Different Skin Sites Related to Seasonal Variations," Skin Res. Technol., 6, pp 81-86.

(26) Blichmann, C. W. and Serup, J. (1988),"Assessment of Skin Moisture," Acta Derm. Venereol., 68, pp 284-290.

(27) Courage, W. (1994), Bioengineering of the Skin: Water and the Stratum Corneum, Eds. Elsner, P., Berardesca, E. and Maibach, H. I., CRC Press, Boca Raton, pp 171-175.

(28) Denda, M. (2000), Dry Skin and Moisturizers: Chemistry and Function, Eds. Loden, M. and Maibach, H., CRC Press, Boca Raton, FL, pp 147-153.