j. Cosmet. sci., 51,239-252 (July/August 2000) Preparation of a new soft capsule for cosmetics K. MIYAZAWA, I. YAJIMA, I. KANEDA, and T. YANAKI, Basic Research Center, Shiseido Co. Ltd., 1050 Nippa-cho, Kohoku-ku, Yokohama, Kanagawa, 223-8553Japan. Accepted for publication July 10, 2000. Presented in part at the Annual Meeting of The Pharmaceutical Society of Japan, Gifu, Gifu Pref., Japan, March 29, 2000. Synopsis A novel microcapsule for cosmetics was studied. The microcapsule was prepared by using an O/W/O emulsification technique and showed high thermostable properties (70øC). Agar was employed to gel the water phase of the emulsion. The size of the microcapsules was controlled by the process temperature and the stirring speed: the higher the temperature or the stirring speed, the smaller the diameter of the microcapsule produced. The strength of the agar gel phase was investigated, and the effect of the ratio of the internal oil to the gel (water) phase was observed. Young's modulus decreased with increase of the internal oil ratio. For cosmetic applications of the microcapsules, the all trans-retinol palmirate or ethyl linoleate used as a model ingredient was stabilized by microencapsulation. The remaining percentages of all trans-retinol palmirate and ethyl linoleate in the microcapsule after the four-week experiments at 50øC were 87% and 95%, respectively. On the other hand, those in oil solution were 72% and 91%, respectively. This micro- capsule shows suitable properties for not only cosmetic use, but also for applications in foods and other products. INTRODUCTION Microcapsules are used in food industry (1,2), medical treatment (3-6), printing (7), cosmetics (8-10), etc. The methods of preparation are roughly divided into three types: (a) chemical (polymerization) (11-13), (b) physiochemical (coacervation) (14-18), and (c) physical (spray-drying and employing double-cylinder equipment) (19,20). The main purposes of microencapsulation are the protection of core substances from the surround- ing environment and the control of internal agent release (21). The biological compat- ibility and the excellent texture of the capsule are also the most essential factors for its application to cosmetics. The current microcapsules for cosmetic use are mainly prepared by a coacervation method using gelatin (22-24) or various other polymers (25-27). However, there is a disadvantage to employing the coacervation method. Because the membrane of microcapsules prepared by the coacervation method is vulnerable, glutar- aldehyde treatment is essential to cross-link gelatin for enhancing the strength of the capsule membrane. As a result, a complicated washing process has to be introduced to 239

240 JOURNAL OF COSMETIC SCIENCE remove the unreacted glutaraldehyde, which may have an adverse effect on the human body. To improve on the complex procedures for preparing the microcapsules, an easy and useful method employing agar as a gelling agent was investigated. Agar is a polysaccharide extracted from seaweed (for example, Ge/idium, Graci/aria, Pteroc/adia, and Ahnfeltia) and mainly consists of agarose (28) and agaropectin (29). The former can give a high gel-forming property and the latter gives a very weak or no gel-forming property. The ratio ofagarose to agaropectin depends upon the species and the habitation area of the original seaweed or the extracting process. The molecular weight is also controlled by the process conditions described above, and it is generally from 1,000 to 100,000. The precise gel formation mechanism of agar is discussed by Arnott et al. (30). Agar molecules exist as random coils when they are dissolved in hot water (85øC). At low temperature (30øC), they form double helices, which further cross-link each other to make a gel network. This gel-forming process is thermoreversible (31), and the gel shows large hysteresis against the change of temperature. A typical aqueous agar solution (1.5%) forms a gel at approximately 30øC, and it keeps the gel structure until above 80øC. This property was successfully used for preparing a novel microcapsule. EXPERIMENTAL MATERIALS Agar (M-7 ©, Ina Food, Ina, Nagano, Japan) was employed for preparing the microcap- sules. Polyoxyethylene hydrogenated caster oil (HCO-60 ©, 60 mol of ethylene oxide lEO] units per molecule, Nikko Chemicals, Tokyo, Japan) and polyoxyethylene grafted poly(dimethylsiloxane) (SC9450N ©, Shin-Etsu Chemical, Tokyo) were used as hydro- philic and lipophilic surfactants, respectively. Distilled water was used throughout this study. Squalane (FITODERM ©, Hispano Quimica, Barcelona, Spain), liquid paraffin (Matsumura Chemical, Tokyo), and cetyl 2-ethylhexanate (Nikkol CIO ©, Nikko Chemi- cals) were used as internal oil phase. Decamethylcyclopentasiloxane (Shin-Etsu Chemi- cal) and poly(dimethylsiloxane) (KF-96-A6T ©, Shin-Etsu Chemical) were used as outer oil phase. All trans-retinol palmirate (1.7 million IU/g) was a gift from Nihon Roche, Tokyo. Ethyl linoleate was purchased from Nakarai Tesque, Kyoto. All other chemicals were of the highest purity or high-performance liquid chromatography (HPLC) grade. METHODS Procedure of microencapsulation. Microcapsules were prepared by a procedure shown in Figure 1. A primary O/W emulsion was prepared with a hydrophilic nonionic surfactant (HCO-60) using a homogenizer (T. K. Homo Mixer, Tokusyu Kika, Tokyo) (32,33). Agar was dissolved in water at 90øC and mixed with the O/W emulsion at 50øC. Then the dispersion was added to an outer oil phase that contained nonionic surfactant (SC9450N) under agitation and allowed to cool until below 30øC. A mechanical stirrer (Speed Control Motor USC-H8A25SPTS319, Yokogawa Sertec, Tokyo) and the homog- enizer were employed for agitating the mixture of O/W emulsion and agar in the outer oil phase with stirring speeds of 100-400 rpm and 1000-5000 rpm, respectively. The droplet structure of microcapsules was observed by a light microscope (BX-60, Olympus, Tokyo) and a cryo-scanning electron microscope (cryo-SEM, S-4200, Hitachi,