PERMANENT WAVING AND PERM CHEMISTRY 131 displace. Thioglycolate is a small reducing agent with a relatively high oxidation potential that seemingly diffuses into hair reasonably well. As with all thiols, the activity can be fi ne- tuned through a combination of the solution concentration and pH. In addition, the extent of reaction can seemingly be curbed by the buildup (or presence) of exogenous DTG which limits the chance of overprocessing. On the negative side, a certain amount of structural damage is produced within the hair, although the extent is still considerably less than other shape-shifting alternatives (i.e., caustic relaxers and formaldehyde cross-linking). The two- step nature of the perm process and the odor are considered inconveniences, although the outcome of the process can be somewhat unreliable. The aforementioned summary creates a checklist when considering the resumes of new candidate molecules, but there is still the need for better understanding the role of hair itself. Wortmann and Kure (41) ascribe signifi cance to the cuticle structure and sug- gest it acts as a barrier to penetration, while also providing resistance to fi ber bending. The existence of different cortical cell types is recognized in the wool literature, where markedly different responses in alkali swelling have been reported (42). To date, only a few studies have extended these ideas into the hair literature (43–45). Fina lly, the potential for formulation-related innovations should not be overlooked, wherein there is the possibility for alternative ways of applying the actives to hair and for manipulating penetration through formulation variables, or possibly prewraps there is even the potential for packaging-related innovation to simplify the procedure. To this end, the appearance of the so-called smoothening creams on shelves, essentially represent- ing a conditioner–perm hybrid, is noteworthy. These products are likely to be less aggres- sive than true perms and are intended to produce their effect progressively with repeated treatments, rather than one single application. Hair fashions change over time, but there is a constant desire for those with straight hair to create curls, and those with curly hair to go straight. At the time of writing, straight hair styles have dominated for the past couple of decades, but the cycle that led to “big hair” styles in the 1970s and 80s seems destined to return at some time in the future. In the ab- sence of new technologies, perm chemistry will continue to be how these looks are achieved. ACKN OWLEDGMENTS In u ndertaking this review, it is impossible to not become nostalgic and recall working relationships and friendships that were encountered during employment at Helene Curtis in the early to mid-1990s. I learned so much from the likes of Craig Herb, Kate Martin, Bruce Solka, Bin Chen, Min Liu, Paul Neill, and many others. It was also a delight to work with such skilled and innovative engineers as Steve Dokoupil and Saul Llamas. But most of all, I remember Priscilla Walling who gave me my start in this industry and who contributed immensely to shaping me as an industrial scientist, by providing the perfect mix of encouragement, independence, and sage advice. I al so recognize the contributions of Tom Ventura, Pawel Milczarek and Daniel Kung to the SFTK data generation process. Similarly, Figures 10 and 11 were originally generated by Amy Qualls. I thank TRI-Princeton for access to their library of historical journals and acknowledge many hours of discussion with Randy Wickett on the topic of single- fi ber tensile kinetics. Finally, I thank Rushi Tasker for discussions pertaining to the current status of the perm industry.

JOURNAL OF COSMETIC SCIENCE 132 REFE R ENCES (1) E. G. McDonough, The development of machineless permanent waving, J. Soc. Cosmet. Chem., 1(3), 183–189 (1948). (2) C. R. Robbins, “Chemical composition of different hair types,” in Chemical and Physical Behavior of Human Hair, 5th Ed. (Springer-Verlag, New York, 2012). (3) J. A. Swift, “The structure and chemistry of human hair,” in Practical Modern Hair Science (Allured Books, Carol Stream, IL, 2012). (4) C. R. Robbins, “Morphological, macromolecular structure and hair growth,” in Chemical and Physical Behavior of Human Hair, 5th Ed. (Springer, 2012). (5) P. R. Brady, Diffusion of dyes into natural fi bers, Rev. Progr. Color., 22, 58–78, 1992. (6) J. D. Leeder, Comments on “pathways for aqueous diffusion in keratin fi bers”, Tex. Res. J., 69, 229 (1999). (7) F. J. Wortmann, G. Wortmann, and H. Zahn, Pathways for dye diffusion in wool fi bers, Tex. Res. J., 67, 720–724 (1997). (8) J. A. Swift, Further comments on “pathways for aqueous diffusion in keratin fi bers”, Tex. Res. J., 70, 277–278 (2000). (9) C. Gummer, Elucidating penetration pathways into the hair fi ber using novel microscopic techniques, J. Cosmet. Sci., 52, 265–280 (2001). (10 ) L. Salce, J. J. Cincotta, S. Barlow, A. Rubinstein, and E. J. Klemm, Reduction of hair in the presence of exogenous disulfi de, J. Soc. Cosmet. Chem., 38, 99–107 (1987). (11 ) J. B. Speakman, The reactivity of the sulphur linkage in animal fi bers – part 1. The chemical mecha- nism of permanent set, J. Soc. Dyers Col., 52, 335–346 (1936). (12 ) J. P. E. Human and H. Lindley, The reactivity of the disulfi de bonds of stretched wool fi bers, Tex. Res. J., 27, 917 (1957). (13 ) L. J. Wolfram, Reactivity of disulphide bonds in strained keratin, Nature, 206, 304–305 (1965). (14 ) C. E. Reese and H. Eyring, Mechanical properties and the structure of hair, Tex. Res. J., 20, 743–750 (1950). (15 ) E. T. Kubu, The stress relaxation of fi brous materials. I. instrumentation and preliminary results, Tex. Res. J., 22, 765–777 (1952). (16 ) E. T. Kubu and D. J. Montgomery, II Kinetics of the reduction of wool keratin by cysteine, Tex. Res. J., 22, 778–782 (1952). (17 ) R. R. Wickett, Kinetic studies of hair reduction using a single fi ber techniques, J. Soc. Cosmet. Chem., 34, 301–316 (1983). (18 ) R. R. Wickett and B. G. Barman, Factors affecting the kinetics of disulphide reduction in hair, J. Soc. Cosmet. Chem., 36, 75–86 (1985). (19 ) R. R. Wickett and R. Mermelstein, Single fi ber stress decay studies of hair reduction and depilation, J. Soc. Cosmet. Chem., 37, 461–473 (1986). (20 ) R. R. Wickett, Disulfi de bond reduction in permanent waving, Cosmet. Toilet., 106, 37–47 (1991). (21 ) M. A. Manuszak, E. T. Borish, and R. R. Wickett, The kinetics of disulfi de bond reduction in hair by ammonium thioglycolate and dithioglycolic acid, J. Soc. Cosmet. Chem., 47, 49–58 (1996). (22 ) M. A. Manuszak, E. T. Borish, and R. R. Wickett, Reduction of human hair by cysteamine and am- monium thioglycolate: a correlation of amino acid analysis and single fi ber tensile kinetic data, J. Soc. Cosmet. Chem., 47, 213–227 (1996). (23 ) E. G. Bendit, There is no Hookean region in the stress-strain curve of keratin, J. Macromol. Sci. Phys., B17(1), 129–140 (1980). (24 ) T. A. Evans, T. N. Ventura, and A. B. Wayne, The kinetics of hair reduction, J. Soc. Cosmet. Chem., 45, 279–298 (1994). (25 ) C. M. Bamford and C. F. H. Tipper. Eds., Comprehensive Chemical Kinetics, Vol. 22 (Elsevier, Amster- dam, Oxford, New York, NY, 1980). (26 ) M. J. Suter, Chemistry in permanent waving – past, present and future, J. Soc. Cosmet. Chem., 1(2), 103–108 (1948). (27 ) T. Toy’oka and I. Kazuhiro, New fl uorgenic reagent having halogenbenzofurazan structure for thiols: 4-(aminosulfonyl)-7-fl uoro-2,1,3-benzoxadiazole, Anal. Chem., 56, 2461–2464 (1984). (28 ) D. J. Evans, A method for determining the penetration of reducing agents into wool using fl uorescent microscopy, Tex. Res. J., 59, 569–576 (1989). (29 ) J. H. Sharp, C. W. Brindley, and B. N. N. Achar, Numerical data for some commonly used solid state reaction equations, J. Am. Ceram. Soc., 49, 379–382 (1966).