352 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS "' 26- • •20- '"" 11]- • 16- '1' I I I 1.00 1.14 1.28 LIMIT STRAIN Figure 11. Average slope of profile, Aq, versus limit strain. skin corresponding to a tension in the skin equal to 60N/re. They also report that the direction of major skin surface markings is aligned perpendicular to the direction of maximum extensibility of the skin. Ferguson and Barbenel have also investigated the effect of stretch on the Ra value. They report a 50 % decrease in the Ra value for unstrained skin at the limit strain. This decrease in Ra value reflects the flattening and reorientation of the surface markings. RV• N1 FOREARM VOLAR ASPECT NORMAL SCAN UNEXTENOEO 800 •m RV2N • FOREARM VOLAR ASPECT NORMAL SCAN EXTENDED Figure 12. Surface profiles obtained,from the volar aspect of the forearm when the arm is in an unextended and extended position.

PROFILOMETRY OF SKIN 353 Results of my investigations corroborate the findings of Ferguson and Barbenel, with regard to the effect of stretch on the skin's surface profile. In addition to stretching the skin with an extensometer, we have looked at the effect of limb position on the surface profiles. Typical surface profiles obtained from two impressions of the forearm are presented in Figure 12. One impression was taken with the arm held in an unextended position and the other was obtained from the identical site, approximately 24 hours later, with the arm fully extended. It appears the subject's position can have a substantial influence on the skin's surface profile. Hoppe (12) has established the relationship between the four roughness parameters, R:•, Rz, Rp and R, (obtained from impressions of the ventral surface of the underarm) and subjects' age. In this investigation, the roughness parameters for 142 subjects ranging in age from 1 to 79 years were compared for age correlation. A significant age correlation was found (Figure 13). The form of the relationship was exponential: i.e., R i =/li e13't, (11) where R i is one of the roughness parameters, R:•, Rz, Rv or R,,/1 i and /3 i are constants for the particular parameter and t is the subjects' age. 250- 200- 150- oo- 50- 10- 0 F•p I I I I i i i 10 20 30 40 50 60 70 AGE(yrs) Figure 13. Roughness parameters, Ra, Rp, Rz and Rr versus subjects' age.