26 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS example, account should be taken not only of the surface activity of a product but also that of the skin and its secretions. The skin is, in fact, often regarded as being bathed in an emulsion of sebum, sweat and epider- mal remnants. The properties of this substrate will vary from the normal, for example, if a cosmetic is generally applied soon after the skin has been washed with soap and water or if it is used immediately after shaving. As already indicated, a clear example of substantivity is the relative ease with which cationic materials can be deposited on body tissues and retained there. There are many anionic sites available on the substrate with numerous carboxy groups available for interaction. A recent publication, by Janet Woodard (6) of Dow Chemical, describes the substantive properties of the polyethylenimines. These behave as cationics in an aqueous environment, owing to the many amino-nitrogens which become positively charged, especially at a pH of about 4. The addition of a polyethylenimine to a shampoo is claimed to result in uptake on the hair--especially on damaged hair--and to improve combability and manageability. It is of interest to consider also the behaviour of antiperspirants. In order to work at all, they must presumably have a substantive effect some- where in the eccrine sweat gland or duct. Kligman (7) has shown that dimethyl-sulphoxide (DMSO), a well-known agent for enhancing skin penetration, increases the antiperspirant effect of the customary aluminium salts. This finding is of theoretical interest although DMSO is probably too toxic for inclusion in an antiperspirant formulation in practice. In another paper (8), Kligrnan maintains that the aluminium salts inhibit sweating by converting the eccrine sweat duct into a so-called 'leaking hose' this prevents the sweat from reaching the skin surface. Others have tried to explain antiperspirant activity in terms of inhibiting the secretory portion of the eccrine gland, or blocking the opening of the eccrine duct. So, the fundamental nature of antiperspirant activity is still open to question. The lack of theoretical clarity makes it difficult to understand what kind of substantive action is really needed to improve antiperspirant performance, but obviously a long-term effect is desirable. As mentioned already, anticaries action may well depend on the principle of substantivity. Fluoride in the diet or in the drinking water is principally of benefit to infants and children up to about 8 years of age. By this time the enamel of the second dentition is almost fully formed, even though some of the permanent teeth have not yet erupted. Fluoride provided systemically dur- ing the course of enamel formation is built into the enamel as fluorapatite. Topically-applied fluoride, on the other hand, only affects the enamel surface.

SUBSTANTIVITY OF COSMETIC INGREDIENTS 27 Studies on fluoride uptake by Weatherell (9) and his colleagues at Leeds, suggest that soluble fluorides at the levels included in typical fluoride toothpastes only give a negligible uptake into sound enamel. Much greater incorporation seemingly occurs if the enamel has been partially demineral- ized in an acidic buffer. The effectiveness of fluoride toothpastes has been confirmed in numerous clinical trials. Weatherell's findings indicate that they may function mainly by arresting the progress of 'incipient' carious lesions that is, these toothpastes greatly increase the fluoride content of enamel if it has been partly demineralized by acid from the dental plaque. This line of reasoning also suggests that artificial means might be employed to 'condition' the enamel surface, so that its fluoride uptake is enhanced. Katz, Muhler and Beck (10), in fact, have indicated two main ways to attempt this, (a) by formulating a dentifrice with a relatively low pH or (b) by treating the enamel with a sequestering agent for calcium ions. Some doubt is cast on the idea of conditioning the enamel by a recent paper from Aasenden, De Paola and Brudevold (11). In a clinical study on 8-11-year- olds, they observed equal protection against caries by daily rinsing with either a neutral sodium fluoride solution or an acidulated phosphate-fluoride preparation with a similar fluorine content. Enamel biopsies showed a greater uptake of fluoride from the acidified rinse but evidently the difference was not enough to improve the anticaries performance. So it seems rather unlikely that the anticaries effect of a fluoride toothpaste could be greatly improved by formulating an acidic or calcium-chelating preparation. In any event, the deliberate promotion of enamel demineralization in order to enhance fluoride uptake must obviously be employed with discretion, to avoid causing serious damage. Another long-standing attempt to achieve substantivity--namely, with the silicone oils--is aimed at blocking the effects of other substances on the skin or elsewhere. For example, silicones are used in hand creams to counter- act the degreasing effect of detergents. Cosmetic formulators greeted the silicones with high hopes when they first appeared. The results have often been disappointing but the fault may have been due to unsuitable formula- tion, rather than to failings in the silicones as such. The silicones have featured in a number of attempts to deal with the dental calculus problem. The aim here is not so much water-repellancy as the prevention of crystallization on the tooth surface. The two concepts may well be interconnected. However, the tooth surface is always exposed to an aqueous medium, the saliva, which has excellent wetting properties. A bacterial plaque is rapidly formed on the enamel. Extracellular dextrans