J. Soc. Cosmet. Chem. 25, 519-534 (1974) ¸ 1974 Society of Cosmetic Chemists of Great Britain Development of a skin cream designed to reduce dry and flaky skin J. D. MIDDLETON*? Synopsis--Dry and flaky SKIN can result from a low extensibility in the STRATUM COR- NEUM. Because the EXTENSIBILITY of corneum depends upon its water content, HUMEC- TANTS are often added to skin products to increase the corneum water content. Measurements of extensibility and water holding capacity in isolated animal comeurn showed that conventional humectants such as glycerol, sorbitol or sodium lactate can be effective but that the effect is lost on rinsing the comeurn in water. It was found that isolated animal comeum adsorbed LACTIC ACID and that the resulting increase in extensibility was retained after rinsing in water. In consumer tests, hand skin dryness and flaking was assessed by trained observers. Two weeks' use of HAND LOTIONS, containing SODIUM LACTATE or lactic acid adjusted to pH 4, resulted in less hand skin dryness and flaking than the use of control lotions. A lactic acid lotion also resulted in less dryness and flaking than a sodium lactate lotion. INTRODUCTION The function of the surface layer of skin, the stratum corneum, is to provide a protective layer over the surface of the body. This layer prevents excessive evaporative water loss and protects against physical and chemical * Environmental Safety Division, Unilever Research Laboratory Colworth/Welwyn, Colworth House, Shambrook, Bedford. ? Present address: Inveresk Research International, Inveresk Gate, Musselburgh, Midlothian, EH21 7UB, Scotland. 519

520 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS insults in the environment. Because much of the body surface is continu- ously stretching and flexing the stratum corneum must be flexible and extensible in order to conform without cracking to changes in body posture. The extensibility of corneum depends upon its water content (1, 2) and upon its temperature (3, 4). When the skin surface is exposed to cold or dry conditions, there must be a gradient of decreasing water content and tem- perature from the base of the corneum to the surface. This means that the corneum surface tends to be less extensible than the deeper layers so that surface cracks and flaking occur more frequently than deep cracks or chapping. One objective of skin cream and lotions is, therefore, to maintain a high corneum water content and extensibility so that skin cracking and flaking is less likely to occur. Several authors (2, 5-7) have shown that the water content of corneum depends upon the presence within the corneum of hygroscopic substances which can hold water in atmospheres of normal humidity. These hygro- scopic substances are contained within the corneum cells by the cell walls which are permeable to water but not to electrolytes and they cannot be extracted unless the cell wall is damaged (2). The water held by the hygro- scopic materials maintains the extensibility of the corneum (2). The cell wall can be damaged by physical disruption, by extracting its lipids with solvents or by prolonged treatment with detergents which also extract lipids (8). The loss of hygroscopic substances resulting from this damage reduces the water content and extensibility of corneum (2). The knowledge of the way in which corneum normally holds water and maintains its extensibility enables one to postulate that extensibility of damaged corneum could be increased by adding hygroscopic substances and a number of skin preparations contain humectants such as glycerol and various mixtures simulating the natural hygroscopic substances. However, in intact corneum the natural hygroscopic substances are kept in by the cell walls. When hygroscopic materials are added to solvent- or detergent- damaged corneum they may penetrate into the corneum cells but are un- likely to be retained and will be washed out when the skin is exposed to water. Their effect on extensibility is therefore likely to be only temporary. The objective of the work reported in this paper was to find a hygro- scopic material which could increase the extensibility of solvent-damaged corneum and which would be retained by the corneum so that the increased extensibility would survive immersing the corneum in water. Solvent- rather than detergent-damaged corneum was selected because the damage is