CUTICLE DECEMENTATION AND BUCKLING 215 •. ??kx 2. •Skx 8k Figure 1. Typical patterns (la and lb) of cuticle lifting and buckling found in hair from a panel of 100 individuals. not show the presence of endocuticular debris at their internal surfaces, indicating, thus, the absence of endocuticular rupture. In these micrographs it can be seen that the damaging stresses not only produce failure of the cuticular cement but also force the cuticle cells to bend and distort, forming a buckling effect. As will be discussed in the following paragraphs, this type of cuticle lifting was found to increase with the number of applied tensile cycles and was observed to be very sensitive to moisture. EFFECTS OF RELATIVE HUMIDITY AND NUMBER OF STRAIN CYCLES In general, it was observed that the lower the moisture content in the testing chamber the lower was the inception strain level needed to produce cuticle lifting and buckling. Furthermore, at constant moisture the number of lifted and buckled cuticle cells in- creased with the number of strain cycles and also with the strain level. For instance, in Figures 2a, 2b, and 2c are shown surfaces of hair fibers subjected to 50, 100, and 200 strain cycles, respectively, under a constant strain level at 10% extension and a relative humidity of 10%. In this figure it can be seen that the number of cuticle edges lifted and buckled increases with the number of stressing cycles. It should be mentioned here that although the number of decemented and lifted cuticle edges was seen to increase with the number of strain cycles, the size of the cuticle decemented area was observed to increase only with the strain level. In Figure 3 is shown the surface of a hair fiber subjected to 200 cycles but at a higher extension (25%) and at 10% RH. As the moisture content in the chamber housing the tensile tester increased, the cuticle decemented area decreased and the number of lifted and buckled cuticle cells decreased considerably. This was also true even when the fiber was strained beyond 30%. For instance, in Figures 4a and 4b are shown the surfaces of hair fibers subjected to 200

216 JOURNAL OF COSMETIC SCIENCE 2a 2b " x • 2½ e. ?&kx : . ??kx 8 77 x Figure 2. Surfaces of hair fibers after cyclical extension of 10% at 10% RH with different numbers of strain cycles as follows: la, 50 cycles lb, 100 cycles lc, 200 cycles. strain cycles under a constant extension value of 15% but at two different moisture contents, namely 65% and 80% RH, while in Figure 4c is presented the surface of a hair fiber subjected to 200 strain cycles at an extension of 30% and 100% RH. In these figures it can be clearly seen that when a higher amount of moisture is absorbed by the hair, little to no damage is observed on any of the individual cuticle cells instead, the damage takes place on the whole cuticular envelope. In Figures 5a and 5b it can be seen in more detail that after stressing a hair fiber at 100% RH and 30% extension, the cuticle envelope has been detached from the cortex and cut transversally by deep cracks. EFFECTS OF SOLVENT SWELLING In order to investigate whether the absorption of other solvents had an effect similar to that of water and to determine whether swelling is a phenomenon related to the dis- appearance of cuticle lifting and buckling, several hair samples were strain-cycled during immersion while swelling in the following solvents: DMSO, ethanol, and isopropanol. The experiments showed that when hair fibers are strain-cycled at 15% extensions, 10 or 15 minutes after their immersion in any of these solvents, the cuticle still decements and buckles in a manner similar to that of fibers tested under the most extremely dry conditions. However, if longer swelling time periods are allowed to elapse, the degree of cuticle lifting and buckling decreases considerably, until it disappears totally after 36 hours of hair swelling. The fibers that were tested fully swollen presented deep trans- versal cracks identical to those produced at high moisture contents. The fact that the cuticle is seen to be lifted soon after solvent immersion indicates that plasticization of the cuticle cement by the solvent plays only a limited role in elimi- nating cuticle lifting and buckling. It seems, rather, that when a non-swollen or dehy-