CRACKING OF HUMAN HAIR CUTICLES 143 tion of these individuals could, however, be divided into two main groups, namely one with a high number of cuticle cracks and the other with a very low number. The first group, representing about 40% of the panel, showed an average of 184 + 15.5 cracks per mm of hair (cpmh), while the other 60% showed an average of about 7 + 1.3 cpmh. Figures la and lb show typical images of these cracks as found in the panel analysis. Incidentally, the higher number of cuticle cracks corresponds to the hair of people who manifested to blow-dry their hair on a daily basis. It was precisely this observation that suggested the possibility of reproducing the cracks in the laboratory by cycles of wetting and blow-drying. In Figure l it can be seen that the particularity of these cracks is their position on the cuticles, i.e., they always appear aligned parallel to the longitudinal axis of the hair fiber, and their length is no longer than a single cuticle size. The cracks were found to occur more frequently at the discontinuities formed by the cuticle edges. They seemed to be initiated near the middle section of the cuticle at that end close to the cortex. Their direction of propagation also seemed to be towards the outer edges of the cuticles. Figure lb shows, for instance, a long crack whose width is larger at the cuticle end close to the cortex and very sharp at its tip towards the outer edge of the cuticle this crack has not yet been able to propagate all the way throughout the outer cuticle edge. CRACK REPRODUCTION IN THE LABORATORY After observing that the short longitudinal cracks were consistently found in a large portion of people from the panel, a way to reproduce them in the laboratory was researched. First, single hair fibers were subjected systematically to cyclical tension, torsion, and bending stresses, and it was found that none of these conditions could reproduce the cracks. The cuticle patterns of damage resulting from these stresses have been reported elsewhere (10-12). The next step was to take fibers three inches long and ' x2.4k x4.0k la ='" lb Figure 1. Typical cuticle cracks found in hair from individuals who blow-dry their hair. la, x2.4k lb, x4.0k.

144 JOURNAL OF COSMETIC SCIENCE I . ! litk x 13k v 2?3 -- -- 7. I .3tlkx 8kv 3•'9 2b Figure 2. Cuticle cracks reproduced in the laboratory as follows: 2a and 2b after 20 thermal cycles 2c after 60 thermal cycles.