ABSTRACT
156For many centuries ointments and creams have been used to improve the healing of wounds, to treat skin diseases in empirical ways, and, moreover, to retard the aging process of the skin and to preserve its natural beauty. Modem cosmetics and dermato-logical preparations are identified as semisolids [1] due to their unique property of being in the solid state under ambient conditions and being transformed to the liquid state when mechanically stressed during application on the skin. This property allows the systems to spread easily on the surface of the skin. This semisolid state is also the main difference between fluid liquid emulsions (in cosmetics often named milks) and creams, although in modem cosmetic preparations the transition from a cream to an emulsion may be gradual. This semisolid state is attributed to particular structural elements, namely crystalline—and in some cases liquid crystalline gel—stmctures of colloidal dimensions that form a three-dimensional network within the system. They are responsible for the consistency and stability of the creams, for their application properties such as proper feel, spreadability, and cooling effects and their possible interactions with skin lipids. Furthermore, their formation during the manufacturing process requires special attention and mixing speed, and shear stresses have to be adapted to not interfere adversely in the crystallisation process of these crystalline stmctures. Additionally, the colloidal gel stmctures are primarily responsible for the physical aging of topical preparations. As a consequence these colloidal gel stmctures form the inherent networks of semisolid preparations, whereas they are absent in liquid emulsions. Because these colloidal gel structures form additional phases in semisolid systems, they may be defined as multiphase emulsions.
