RHEOLOGY OF STRATUM CORNEUM--I Figure 1. 20 , C Bre•k poinf o /• B D Extension (10-2m) Typical load/extension curve for stratum c'orneum at 30 % rh. 2 x l0 s to 3 x 109N m -•. Such a wide variation seemed unacceptable in terms of dimensional inhomogeneities. The largest possible error arising from the uncertainty in sample thickness would result in a two-fold change in modulus. Another possible source of error is from the holes present in the corneum in the form of hair follicles and sweat ducts, but due to the small area occupied by such appendages any structural non-uniformity arising from this would be expected to be small. It has been mentioned that the dried corneum sheets were variously wrinkled and flatter samples tended to have higher modulus values. In an attempt to standardize the physical state of the corneum a con- ditioning procedure was instituted. If corneum samples which had low modulus values ( • 108 N m -s) at 30•o rh were extended a fixed percentage of their initial length at high rh and allowed to dry out under slight tension (2 g load) in their extended state, the 30•o rh modulus value increased sub- stantially. Samples which had high modulus values to begin with displayed only small increases in modulus when treated in this way. Table I shows the effect of various conditioning extensions on high and low modulus samples. Very little increase in the modulus of either sample is observed for exten- sions greater than 10•o and future samples were conditioned by this amount. After conditioning, the modulus values for all corneum samples fell in the range 1-4 x 109N m -•. High resolution ir spectra were recorded for stratum

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table I Effect of mechanical conditioning on the elastic modulus of two stratum corneum samples conditioning extensions 0 5 10 15 20 25 (N m -s) ELow X 10 -9 0.2 0.8 1.5 1.7 1.8 1.7 EFIigh X 10-9 3.0 3.2 3.6 3.8 3.6 3.8 corneum samples both before and after conditioning and the principal amide band frequencies associated with protein in the c•-helical conformation (9) (amide A = 3285 cm-• amide B = 3064 cm-• amide 1 = 1653 cm-X and amide 11 = 1543 cm -x) were unchanged. This shows that no significant conformer change in the main protein component (keratin) took place due to conditioning extension. A possibility exists that the protein component of the stratum corneurn orientates itself in the direction of stretch, leading to higher modulus values. Examination of the conditioned samples using polarized ir radiation showed that none of the strong band absorptions had acquired any dichroism. From these results it was concluded that the increase in modulus values observed after conditioning was due mainly to 3000 IOOO 30 - Key ..... Error boundaries • "•..•.. Normalized curve \\ x I I I :30 75 I00 5'0 relative humidity • Figure 2. Variation of elastic modules of stratum corneum with relative humidity. - - -, Error boundaries --, normalized curve.