j. Soc. Cosmet. Chem., 45, 347-352 (November/December 1994) Mechanism of hair straightening MICHAEL WONG, GABRIELA WIS-SUREL, and JOSEPH EPPS, Clairol Inc,, 2 Blachley Road, Stamford, CT 06922. Received November 22, 1993. Presented at the Annual Meeting of the Society of Cosmetic Chemists, New York, December 1993. Synopsis The practice and theory of permanent hair straightening are discussed. Hair straightening as a cosmetic process is the reverse of hair waving. It removes hair curliness and makes wavy hair straight. While it is generally accepted that permanent hair straightening can be achieved only at very high pH with strong alkali, we have observed that it can also be achieved at neutral pH with lithium salt, or even at acidic pH with resorcinol. Also, while some of these treatments do result in considerable loss of cystine, others leave the cystine entirely intact. Lanthionine is produced in some cases, but is absent in others. It appears that the prime requirement for an effective hair straightening treatment is to be able to induce supercontraction of the hair fiber. Based on these observations, a model is proposed to explain permanent hair straightening in terms of certain molecular events such as chain folding and alpha-beta transformation of the polypeptide in the hair keratin. INTRODUCTION Several processes are applicable to hair straightening, which has become one of the more common hair styling practices. There are the hot comb and press iron, which were popular years ago. There are the "Jeri-curl" products, and there are the chemical relaxers. They all are able to straighten hair to some extent, but not with the same degree of permanency. The hot comb process, for example, which relies on the actions of heat and moisture, can only achieve hair straightening that is temporary. The hair will revert to its original curly state even on exposure to high humidity. The Jeri-curl process, which is based on the "thioglycolate waving system," can produce hair straight- ening that is more resistant to humidity, but the hair will still revert on washing. The chemical relaxers, which are typically formulated with strong alkali at pH in excess of 13, are the only ones that are able to achieve permanent hair straightening that will survive washing. The general belief is that hair straightening is analogous to hair waving. When hair is treated with a chemical relaxer, hair cystine is cleaved, and a new crosslink, the lanthionine, is subsequently formed to help stabilize the hair in the straight configuration. The theory is reasonable in view of the fact that lanthionine is a major reaction product between alkali and cystine (1-3). Also, published data have shown that hair treated with chemical relaxers indeed contain, a substantial amount of lanthionine. However, we have observed several aspects of hair straightening that are 347

348 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS unique in themselves and have led us to believe that permanent hair straightening is governed by a different molecular process. EXPERIMENTAL HAIR SAMPLES Two types of hair samples were used in the experiments. Curly hair samples, obtained from Afro-Americans, were used to assess the efficacy of hair straightening. Typically, these hair samples consisted of coils of about 0.5 cm in diameter. To study the fiber swelling and supercontraction characteristics, Caucasian dark brown hair was used. All hair samples were cleaned with a solution of 12.5% sodium lauryl sulfate prior to treatments. EVALUATION OF HAIR STRAIGHTENING EFFICACY Temporary hair straightening is typically characterized by a ready reversion of the hair fiber to its original curliness after even one washing. A criterion that we used in this study to determine the permanency of hair straightening was how well the hair fiber remained in a straight configuration after the fiber was immersed in water (at room temperature) for five minutes and dried. The degree of hair straightness was determined visually: the hair fiber was considered straight when there was no evidence of curliness. EVALUATION OF HAIR SWELLING AND HAIR SUPERCONTRACTION To determine the degree of hair swelling, the diameter of a wet hair fiber was measured prior to treatment using a stereoscopic microscope equipped with an eyepiece that has a micrometer scale. The diameter of the same hair fiber was again measured after exposure to a treatment reagent. To determine the extent of fiber supercontraction, the length of straight Caucasian hair fiber was immersed in water and measured in a microcapillary before and after the treatment. ANALYSIS OF CYSTINE AND LANTHIONINE CONTENT Hair samples were first hydrolyzed by heating with 6 N HCI in a sealed tube for 24 hours at 110øC. After removal of the acid in a rotary evaporator at 30øC and dilution with 0.2 N sodium citrate buffer (pH 2.2), the amino acid content was determined with a Beckman 120-C amino acid analyzer. RESULTS AND DISCUSSION HAIR STRAIGHTENING WITH THIOGLYCOLATE/HYDROGEN PEROXIDE SYSTEM One of the unusual aspects of hair straightening is the fact that conventional hair waving products are not particularly effective in achieving permanent hair straightening. When a straight hair fiber is set in a curly configuration (wound on a rod, for example) and