j. Cosmet. Sci., 51, 1-13 (January/February 2000) Film-formation time, skin hydration effects, and physicochemical properties of moisture masks containing different water-soluble chitosans RONG HUEI CHEN and RUEI SHYUN HEH, Department of Food Science, National Taiwan Ocean University, No. 2, Pei-Ning Road, Keelung, Taiwan 202, ROC. Accepted for publication December 15, 1999. Synopsis The effect of the addition of water-soluble chitosans on the skin hydration effects and the physicochemical properties of moisture masks thus prepared were studied. Results show that the apparent viscosity of moisture masks increased with increasing molecular weight and/or with increasing concentration of water- soluble chitosans used in the formula. The film-formation time decreased accordingly. Moisture masks containing 0.5% U3 chitosan showed a similar consistency and film-formation time as those containing 2% methyl cellulose however, the color of the moisture masks containing water-soluble chitosans was slightly orange-red. Moisture masks containing water-soluble chitosans showed superior water-holding capacity over those containing methyl cellulose. The higher the molecular weight of the water-soluble chitosans incor- porated, the better the water-holding capacity of the resultant moisture masks. INTRODUCTION Chitinous materials have a wide range of applications in the food, biomedical, and chemical industries (1,2). In the food industry, chitinous materials can be used as thickening, gelling, foaming, antifreezing, and antimicrobial agents as well as for en- hancing the emulsion stability of proteins, etc. (3). Some of the functional properties mentioned are the same functional properties needed in the processing of cosmetics. However, applications of chitinous materials in cosmetics and biomedicine are limited due to their solvency restrictions (4) because chitin cannot be dissolved in water or in most common organic solvents. Chitin can only be dissolved in concentrated acids such as hydrochloric acid, nitric acid, and sulfuric acid or hexa-fluoro-2 propanol, usually being dissolved in dimethyl acetamide or N-methyl-2-pyrrolidinone and 5% LiCI. Chitosan can be dissolved in diluted hydrochloric acid, nitric acid, 0.5% phosphoric acid, formic acid, acetic acid, and 100% ci•tric acid, but will not dissolve in a neutral aqueous solution (5-7). Many water-soluble chitin derivatives such as N-carboxymethyl chitosan have film-forming ability and thickening properties (8), while succinyl chitosan (9) has water-holding and film-forming properties. Both can be used for cosmetics. An

2 JOURNAL OF COSMETIC SCIENCE o/w skin lotion containing 0.1 g of high-molecular-weight chitosan shows an improving water-binding of skin, indicated by decreasing transepidermal water loss (TEWL) and an increase in the torsional strength of the skin. The skin is perceptibly softer and smoother. Hair care products containing chitosan show superior film-formation ability, combabil- ity, stiffness, and curl retention to synthetic polymer (10-12). Chitosan and microcrys- talline chitin have surface-active properties and can be used to enhance emulsion sta- bility (13,14). Water-soluble chitinous materials can be prepared by a variety of methods: (a) Chemical modification can produce succinyl chitosan (9), carboxymethyl chitosan (8,15), N- sulfofurfuryl chitosan (16), N-trimethyl acetate chitosan (17), and mercapto-chitins (18). (b) Acid hydrolysis using nitric acid can produce a 40% degree of deacetylation chito- sans, or oligomers can be obtained by using nitric acid (19-21). (c) Enzyme hydrolysis uses glycosidase, lysozyme, or chitinase to hydrolyze chitins, or chitosanase to hydrolyze chitosans (22). (d) Mechanical methods such as those reported by Shyur (23) and Li (24) use ultrasonic treatment to prepare water-soluble chitosans of different molecular weights. Chang (25) used shear, ultrasonic, or combined shear and ultrasonic treatments to prepare water-soluble chitosans. It is necessary for skin to maintain a certain level of moisture for normal functioning and vitality. When skin moisture content decreases to a certain level, it loses its normal softness and pliability and becomes hardened and embrittled. The major factor affecting skin moisture content is a natural moisturizing factor (NMF) existing in the stratum corneum. It has moisture absorption properties and is crucial to maintaining a proper moisture level in the skin. Besides, on the surface of the skin, water, water-soluble compounds, and lipids form a protecting film that assists the stratum corneum in minimizing the evaporation of water and alleviating intrinsic and extrinsic detrimental factors such as age, body temperature, humidity, and seasonal changes (26). The loss of the water-holding capacity of the stratum corneum results in dryness of the skin. Of the two types of humectants used in moisture products, occlusive humectants form a hydrophobic film to retard the evaporation of the water from the skin surface. The effect of occlusive humectants on transepidermal water loss (TEWF) can be expressed as the inhibition percentage of TEWF: Inhibition percentage of TEWF = (TEWLch/TEWLco) x 100 Here, TEWLch is the transepidermal water loss after application of a water-holding humectant, and TEWLco is the transepidermal water loss without application of a humectant. The lower the inhibition percentage of TEWL, the better the water-holding properties of the humectant used. Common occlusive humectants are petrolatum, fatty acids, and cholesterols (27). Viscosity, molecular weight, chemical structure, and hy- drophobic and hydrophilic properties of the humectants used can affect the TEWL inhibition percentage. Of the homologous humectants used, the higher the molecular weight, the better the TEWL inhibition percentage. The second type of humectant is an absorbing humectant. These compounds have strong water-absorbing properties and will prevent dryness and cracking of the skin in low-humidity environments. An ideal absorbing humectant has the following properties: (a) Good absorption, (b) consistency in water absorption, (c) lower evaporative loss, (d) water-holding capacity on the skin and in products themselves, and (e) no irritative or burning feeling when added to the