PERMANENT WAVING AND PERM CHEMISTRY 101 demonstrated commercially viability. This presumably has some relationship to the high cost associated with addressing toxicological issues. The main exception to the thiol cat- egory of actives involves the use of sodium sulfi te and bisulfi te, which can be found in weaker, so-called body waves or demi-perms. Although considerable effort has been dedicated to studying the cleavage of cystine disul- fi de bonds, relatively little has been published on their restoration. It is possible that this second step in the transformation is seen as somewhat trivial because bonds can be re- formed by air oxidation. Nonetheless, there is an obvious desire to properly perform this important function, and consequently, treatment with a “neutralizing agent” is prudent. With this said, it is generally not possible to completely rebuild all disulfi de bonds, and consequently, the hair is somewhat depleted in cystine content and therefore left in a compromised state. The variety of shapes and styles that can be created by this process is predominantly related to the accessories used to support the hair during this chemical process. It will be shown that a variety of formulation parameters are available for controlling the strength of these treatments, but tight curls are generally produced by wrapping the hair around rollers with a small diameter, whereas softer, looser curls are formed using larger curling rods. At the time of writing, the permanent wave market has been soft for many years because straight hair styles remain popular, yet this exact same chemistry is commonly used to fl atten curly hair. A survey of the beauty aisle will show several relaxer-type products that use conventional “perm chemistry.” These treatments can be effective on most hair types, although they are generally not strong enough to adequately straighten highly curly African hair. Before the advent of “Brazilian straightening,” the market was already familiar with “Japanese straightening,” which comprises traditional perm chemistry in combination with heat (presumably to drive kinetics). The hullabaloo created by Brazilian straighten- ing products has led to renewed interest in the development of new and improved ap- proaches to straighten hair. This has resulted in some novel product forms reaching the shelves, for example, smoothing creams incorporate conventional perm chemistry into a white opaque, conditioner-like base. In short, there is still considerable interest in this chemistry, even if it does not specifi cally relate to traditional perm products. To this end, it is further recognized that depilatory products often use this exact same chemistry, but in this instance, the intent is to produce more aggressive conditions that completely dis- integrate the hair structure. As with many fundamental aspects of hair structure and chemistry, it is evident that con- siderable learning comes from the related wool industry. Hair and wool possess very similar structures and chemistry, and much can be learned from the literature pertaining to this commercially important relation. Therefore, in certain instances, references from this related fi eld will be used to illustrate points. Figure 1. Structure of some common perm ingredients.

JOURNAL OF COSMETIC SCIENCE 102 AN OVERVIEW OF THE STRUCTURE OF HAIR RELATING TO THE PERM PROCESS A comprehensive review of the complex structure of hair is outside the scope of this article, and accordingly, the reader is referred to excellent texts in this area (3,4). Nonetheless, it is impossible to discuss the chemistry of the perming process without a brief overview. The tensile strength of a hair fi ber is widely believed to be dictated by the inner cortex structure, with no appreciable contribution from the outer protective cuticle. More specifi cally, it is the crystalline, alpha helical keratin protein contained within the inter- mediate fi laments (sometimes called microfi brils) that support the permanent mechanical integrity. Therefore, this region of the hair structure becomes the target for reagents in terms of both diffusion and reaction. More specifi cally, it is the disulfi de cross-links associ- ated with the amino acid cystine within the keratin protein that needs to be broken down and subsequently reformed. To his point, the fi rst part of the process involves incursion through the hair’s outer protective layer, the cuticle. This cumulative structure consists of overlapping tile-like scales which have further substructures as shown in Figure 2. This laminate structure consists of (from bottom to top) an endocuticle, exocuticle, A-layer, and epicuticle, with the degree of cross-linking increasing in this same order. Furthermore, the very outer layer of healthy hair consists of a lipid assembly (the f-layer) that provides an additional hydrophobic outer barrier. In short, water is unlikely to penetrate through the cuticle face in a top-to-bottom manner, except for when occasional advantageous cracks are present. The nature of this penetration pathway represents a topic of debate. Brady (5) advocated a “cell membrane complex diffusion” model where infi ltration occurs through the cell membrane complex “gaps” between the cuticle cells. This same idea has also been promoted by Leeder (6). However, Wortmann et al. (7) question this being the primary pathway because of the low level of this “ex- tracellular” material in the hair. Accordingly, an alternative theory was promoted which involved diffusion through all nonkeratinous components of the fi ber—most pre- dominantly, the endocuticle. This same idea has also been advocated by Swift (8), al- though Gummer (9) suggests that all structures within hair should be considered as penetration routes for the delivery of materials. With all this said, liquid water rapidly penetrates into the hair’s inner structure, and it may therefore be presumed that dissolved species would similarly diffuse readily. How- ever, it is becoming evident that water-soluble materials do take some time to permeate the structure. An analogy is drawn to the principles associated with liquid chromatogra- phy, whereby the mobile phase readily traverses the stationary phase and dissolved species progress at different rates depending on their size and interaction with the column. The reason for belaboring this point relates to the well-known occurrence that hair from Figure 2. Schematic of cuticle substructure.