CRACKING OF HUMAN HAIR CUTICLES 145 2o Figure 2. Continued. cut them in half. One half was used as a control, and the other half was subjected to cycles of blow-drying and wetting at different temperatures and for different intervals of time. The number of cracks already present in the control were counted and compared with those appearing in the thermally cycled fibers. A total of ten fibers per each trial were analyzed. The average number of cracks per millimeter found in the control samples was 6 + 2.1. After several trials it was found that short thermal cycles of ten seconds of blow-drying at 75øC combined with ten seconds of wetting at 25øC were able to reproduce and increase dramatically the number of short longitudinal cuticle cracks. In Figures 2a, 2b, and 2c are shown typical images of hair fibers subjected to 20 and 60 of these cycles. A total of 21 cracks can be counted in the 76 x 52 micrometer section of hair represented in Figure 2b. In Figure 2c it can be seen that the total number of visible cracks reaches a value of 26 the surface hair dimensions captured by this micrograph are approximately 67 x 42 microns. The number of cracks shown in Figures 2b and 2c correspond to an average of 472 and 562 cracks per millimeter of hair, respectively. Such high concen- trations of cuticle cracks are rarely found in people's hair and can only be produced in the laboratory. Figures 3a, 3b, and 3c show higher magnifications of these cracks, while Figure 4 shows the average number of cuticle cracks found in hair samples as a function of thermal cycles in relation to the control. In this last figure it can be seen that the number of cracks per unit area on the hair surface increases up to a limit and then levels off with the number of thermal cycles. A close examination of Figures 1, 2, and 3 reveals that, in all cases, the applied thermal cycles resulted in the production of cracks similar to those already observed in the panel

146 JOURNAL OF COSMETIC SCIENCE x2 4k " 3a 3b 3c. x2.4k Figure 3. Magnified views of cracks shown in Figure 2 as follows: 3a after 20 thermal cycles 3b and 3c after 60 thermal cycles. Each cycle consisted of ten seconds of blow-drying at 75øC followed by ten seconds of wetting at 25øC. Number of short longitudinal cuticle cracks per mm of hair 700 (a) Average number of • . -- , 600 - cracksinha, I I L ' 500 - 4OO 3O0 200 - / ' (b) Average number . /•' ofcra•s in controls • '100 - f (SD=2.1) '• 0 5 10 15 20 25 30 35 40 45 50 55 60 Number of applied thermal cycles Figure 4. Variations in the average number of cuticle cracks found in the laboratory as follows: (a) in thermally cycled fibers as the number of thermal cycles increases, and (b) in their corresponding half snippet used as a control (non-exposed to thermal cycles). Each point represents average of cracks found in ten hair samples. Each cycle consisted of ten seconds of blow-drying at 75øC followed by ten seconds of water immersion at 25øC. Error bars represent one standard deviation about the mean. analysis. This observation clearly shows that the cuticle cracks found in hair subjects from the panel arise mainly as a consequence of subjecting hair to thermal stresses during blow-drying. An analysis of the cracks shown in Figures 1, 2, and 3 indicates that their