2008 TRI/PRINCETON CONFERENCE 87 count of gaps and micro-cavities increases substantially when the hair fi bers are subjected to any of the following treatments, namely: (i) Cycles of mechanical stresses, (ii) cycles of thermal stresses, or (iii) immersion in solvents (see Figure 1). It should be mentioned here that the average number of gaps and cavities found along the various hair fi bers was seen to vary strongly from subject to subject. The shape and size of defects giving rise to the appearance of patterns of light interference was observed to be dependent on the type of stresses applied to the hair fi bers. Cycles of mechanical extension and retraction usually led to the formation of extended gaps or large open cavities between the two top cuticle cells (see Figure 2). In most cases the top cuticle cells appeared deformed, buckled, and separated from the second one that remained ce- mented. A few cuticle cells, however, showed de-cementation and gap formation but did not appear deformed or buckled. Furthermore, it was noted that gaps also formed after the second, third, or fourth cuticle cell. In these instances stacks of two or three cuticle cells appeared cemented but separated from the underlying adjacent cuticle cell. Most gaps and openings created by mechanical extension were seen to extend from the tips of the cuticle cells towards the cortex and in most cases they were fi lled with air as indicated by the appearance of patterns of light interference after stress application. It is worth mentioning that in certain cases cuticle cells presenting gaps and microcavi- ties though always apparent by light interference analysis were not often detected by scanning electron microscopy (SEM). The main reason for this is that light interference analysis is sensitive to the presence of gaps and cavities deep underneath the cuticle sheath surface regardless whether the cuticle cells are buckled or not. Previous research has al- ready shown that these types of extended gaps involving cuticle cell decementation are formed by Poisson contraction stresses acting on the cuticle sheath as the hair fi bers are subjected to cycles of elongation and retraction (6). The cyclic radial contraction and extension of the hair fi bers during elongation produces cuticle cell cement breakage by fatigue, and also induces viscoelastic deformations on the whole cuticle cell, therefore, causing buckling and creating extended gaps or cavities fi lled with air. Figure 1. Average count of gaps and microcavities (15% SD) in a section of 50×50 μm as a function of hair length obtained from four groups of ten hair fi bers each after various treatments as follows: (a) After 20 cycles of 15% extension and retraction. (b) After 20 cycles of 10-s blow-drying at 65°C followed by 10 s of immer- sion in water. (c) After immersion in IPA for 3 min. (d) With no treatment.

JOURNAL OF COSMETIC SCIENCE 88 The shape and size of gaps and microcavities created by thermal stresses varied depending on the source of high temperature and number of cycles, and on whether the fi bers were immersed in water after each temperature cycle. Figure 3a,b shows SEM and optical micrographs of elongated cavities forming channel-like patterns ~3 to 4 um long pro- truding away from the cuticle cell surface with and without cracks. Crack formation by thermal stresses has already been reported in the past (7), however, the results described here indicate that channel like deformations may form before cracking occurs. Further- more, the fact that these protrusions produce patterns of light interference indicates that they consist of hollow cavities fi lled with air. It is worth to note that these types of cavities are not caused by decementation and deformation of the whole cuticle cell as previously seen in the case of extended cavities produced by mechanical stresses. The thermal cavi- ties appear rather to be produced by more localized deformations. Yet other types of localized defects appear at the cuticle cells sub-surface when the hair fi - bers are exposed to cyclic contact with hot surfaces, i.e. curling or fl at irons. These types of Figure 2. Micrographs showing typical gaps and cavities formed by buckled cuticle cells after 20 cycles of 15% extension and retraction obtained as follows: (2a) by SEM, and (2b) by optical microscopy using light interference analysis. Figure 3. Micrographs (250×) of hair showing thermal cracks and narrow channel-like deformations after exposure to 15 cycles of 30-s blow-drying at 75°C followed by 30 s of immersion in water. Micrograph 3a was obtained by SEM. Micrograph 3b, displaying patterns of light interference, was obtained by optical microscopy.