NAIL PERMEATION 377 The cross-linked keratin network comprising the thumbnails seemed to have a tighter structure than that present in toenails, causing thumbnails to be effective permeation barriers despite their reduced thickness. Thus, location of the nails is an important factor in determining the basic nature of the nail permeability barrier. It is unclear whether thumbnails differ in structure from other fingernails however, our results suggest that structural differences may exist. While pH did not affect the intrinsic permeability properties of the human nail sig- nificantly, increasing temperature statistically enhanced water permeation. The effects of both pH and temperature change were reversible. REFERENCES (10) (11) (1) G. V. Gupchup and J. L. Zatz, Structural characteristics and permeability properties of the human nail: A review, J. Cosmet. Sci., 50(6), 363-385 (1999). (2) H. P. Baden and J. C. Kvedar, "Biology of Nails," in Dermatology in General Medicine, T. B. Fitzpatrick, A.Z. Eisen, K. Wolff, and K. F. Austin, Eds. (McGraw-Hill, Inc., New York, 1993), Vol. 1, pp. 294-297. (3) U. Runne and C. E. Orfanos, The human nail: Structure, growth and pathological changes, Curt. Probl. DermatoL, 9, 102-149 (1981). (4) K. A. Walters, G. L. Flynn, and J. R. Marvel, Physicochemical characterization of the human nail: I. Pressure sealed apparatus for measuring nail plate permeabilities. J. Invest. Dermatol., 76(2), 76-79 (1981). (5) M.H. Soong, Transport properties of drugs and model compounds across the human nail, Ph.D. dissertation, University of Minnesota (1991). (6) K.A. Walters, G. L. Flynn, and J. R. Marvel, Penetration of the human nail plate: The effects of vehicle pH on the permeation of miconazole, J. Pharm. Pharmacol., 37, 498499 (1985). (7) D. Mertin and B. C. Lippold, In-vitro permeability of the human nail and of a keratin membrane from bovine hooves: Influence of the partition coefficient octanol/water and the water solubility of drugs on their permeability and maximum flux, J. Pharm. Pharmacol., 49, 30-34 (1997). (8) K. Diem and C. Lenmer, Scientific Tables (Documenta Geigy), 7th ed. (Geigy Pharmaceuticals, New York, 1970), pp. 278-283. (9) E. Tolgyesi and F. Fang, "Action of Nucleophilic Reagents on Hair Keratin," in Hair Research: Status and Future Aspects, C. E. Orfanos, W. Montagna, and G. Stuttgen, Eds. (Springer-Verlag, New York, 1981), pp. 116-122. A. Martin, Physical Pharmacy, Physical Chemical Principles in the Pharmaceutical Sciences, 4th ed. (Waverly Co., Baltimore, 1993), pp. 324-361. G. L. Flynn, E. E. Linn, T. Kurihara-Bergstrom, S. K. Govil, and S. Y. Y. Hou, "Parameters of Skin Condition and Function," in Transdermal Delivery of Drugs, A. F. Kydonieus and B. Berner, Eds. (CRC Press, Boca Raton, 1987), Vol. II, pp. 3-17. (12) R.J. Scheuplein, Mechanism of percutaneous absorption. I. Routes of penetration and the influence of solubility, J. Invest. Dermatol., 45,334 (1965).

j. Cosmet. Sci., 51, 379-399 (November/December 2000) A curing method for permanent hair straightening using thioglycolic and dithiodiglycolic acids S. OGAWA, K. FUJII, K. KANEYAMA, K. ARAI, and K. JOKO, Research and Development Center, Milbon Co. Ltd., 2-I 7-2 Akagawa, Osaka 535-0005, Japan (S.O., K.F., K.K., K.A.), and Kyoto Women's University, Kitahiyoshi, Kyoto 605-850I, Japan (K.J. ). Accepted for publication September 2I, 2000. Presented in part at the 4th Scientific Conj•rence of the Asian Societies of Cosmetic Scientists, Bali, I999, and in Proceedings of SCAS as "A practical method for permanent hair straightening and the permanency related to supercontraction induced by microstructure change,"pp. 348-36 I. Synopsis A successful process of permanent hair straightening was achieved by applying a hot iron press technique to hair treated with an alkaline solution containing thioglycolic acid (TGA) and dithiodiglycolic acid (DTDG). Supercontraction of the hair was determined on the basis of fiber length. The effectiveness of straightening was evaluated as a function of supercontraction, which was dependent on the concentration ratio of DTDG to TGA, pH, time, and heat-treatment temperature. A range of around 5% to 8% supercontraction was found to be necessary to achieve successful permanent hair straightening, while at relatively higher contraction levels the smoothness of the fiber surface tended to be lost. About 89% of the initial content of cystine was retained in straightened hairs, and about 11% was converted to cysteic acid. X-ray diffraction and high-pressure differential scanning calorimetry were used to estimate the amount of o•-crystallites. The relationship between the melting enthalpy and the degree of supercontraction was discussed in relation to the stability in the straight configuration. Important conclusions include the observations that supercontraction of less than 10% is caused by randomization of the o•-helix and that permanency related to hair straightening is a result of the irreversible transformation of the o•-crystal into amorphous phase. INTRODUCTION Several hair-straightening processes have been applied to practical uses. A thioglycolate system has mostly been used as the reverse process of permanent waving, which includes an oxidation process after the reduction step under a straight configuration forced on kinky and curly hairs. In recent years, a hot iron press technique has become common and has been applied in hair straightening. Although all these processes are able to straighten hair to some extent, the hair will revert to its original or near original curly 379