693 MOISTURE IN THE CUTICLE SHEATH Cuticle cell delamination was also observed to occur at higher temperatures between the range of 150 and 220°C and with levels of elongation less than 5%. These temperature and elongation ranges are typical during hot ironing processes. The characteristics of thermal lifting produced under these high temperature conditions were similar to those already described for low temperatures. However, in many cases, a new form of delamination was observed. In this new form of thermal delamination, the cuticle cells did not lift forming parabolic shapes but stayed attached to the hair surface, forming rather blister patterns (see Fig. 6a). Furthermore, blister formation was not detected when the hair had been equilibrated at low moisture conditions, thus indicating the need of moisture for their occurrence. When these blistered cuticle cells were broken by friction, they left visible endocuticular remnants on the surface (see Fig. 6b), indicating that lifting occurred by breakage at the junction between exocuticle and endocuticle. Figure 5. Optical microscopy images of the same hair fiber, immediately after being blow-dried at 80°C for 50 s while subjected to 5% elongation (Figure 5a) and after the same surface was slightly rubbed with a comb (Figure 5b). Note the broken patterns of cuticle cells in Figure 5b. Figure 6. Optical microscopy images of cuticle cell blister patterns, immediately after hot ironing (Fig. 6a) and after the cuticle cell blisters were broken by friction (Figure 6b) the blisters were produced by gently applying two cycles of hot ironing at 220°C to the fiber.

694 JOURNAL OF COSMETIC SCIENCE Since in blister formation, the portion of cuticle cells that is lifted consists of the epicuticle and exocuticle joined together, leaving behind endocuticular remnants, we will refer to this type of lifting as exocuticular delamination. This contrasts with what was observed before for endocuticular lifting, in which the solid portion of cuticle cell that was lifted consisted of the epicuticle, exocuticle, and endocuticle glued together. Its mechanism of formation can again be explained by changes occurring in the interlayer moduli as the exocuticle, endocuticle, and CMC lose moisture. When the hot iron surface enters transiently in contact with the hair surface, it rapidly dehydrates and hardens the epicuticle and exocuticle, while contracting the endocuticle. The contracting and hardening of these layers cause stress concentrations across the exocuticle and endocuticle causing their junction to break with a slight lifting leading to blister formation. SHAPE RECOVERY OF LIFTED OR BUCKLED CUTICLE CELLS BY HYDRATION It is well known that water acts as the main plasticizer of keratin fibers (7), and water plasticization effects on hair have been mainly ascribed to hydration effects in the amorphous proteins of the cortex. However, moisturizing video experiments carried out with hair fibers presenting lifted and buckled cuticle cells showed that the cuticle cells also sense changes of moisture in the environment. In these experiments, the cuticle cells were seen to respond almost immediately, particularly when exposed to high levels of moisture. This rapid response occurs as long as their endocuticle is not damaged and therefore indicates that the cuticle cells undergo plasticization as they absorb moisture from the environment. The video experiments showed that the light interference patterns (LIPs) corresponding to lifted cuticle cells disappear almost instantaneously as the hair is exposed to small puffs of air containing high levels of moisture (i.e., 90% RH). The LIPs disappear when the endocuticle of the lifted or buckled cuticle cells is plasticized by water and allows them to recover their normal shape. This recovery process closes the air gap between lifted cuticle cells and those immediately below. Furthermore, when puffs of moisturized air containing lower levels of moisture (i.e., 70% RH) were applied to hair there was no plasticization and shape recovery. The above experiments indicate that for cuticle cell shape recovery to occur, the endocuticle requires high levels of water absorption. This contrasts with other observations showing that thermal delamination only occurs when the endocuticle loses high levels of moisture, strongly suggesting that the endocuticular protein structure has the characteristics of a hydrogel in its native state. Such structure will allow it to absorb high levels of water when exposed to the environment. Furthermore, the experiments suggest that not only the endocuticle but also other cuticle cell layers exchange moisture with the environment. Unfortunately, currently, there is no available technique by which one can obtain the thermal water absorption isotherm of the cuticle sheath separately from the cortex. Thus, the inference is that moisturization effects on the cuticle sheath must rely on indirect observations such as the one described in this paper. BUCKLING AND LIFTING CUTICLE CELLS BY IMMERSION IN ISOPROPYL ALCOHOL As it was shown in the previous section, lifted and buckle-shaped cuticle cells with failure at the CMC can recover their normal shape through rehydration. In fact, the recovery was so efficient that when the hair was analyzed again by microscopy, the previously lifted