ABSTRACT
Introduction .................................................................................................................................... 373 Cyclodextrins and Photostability ................................................................................................... 374 Transcutol1 .................................................................................................................................... 375 Encapsulation Structures ................................................................................................................ 375 Physical Properties of Organic Particulate UV-Absorbers ............................................................ 377 Inorganic Materials ......................................................................................................................... 377 Penetration Retarders ..................................................................................................................... 378 Vehicle Effects................................................................................................................................. 379 Conclusions .................................................................................................................................... 379 References....................................................................................................................................... 380
There is overwhelming evidence indicating that human skin is damaged in different ways by exposure to sunlight. Of the solar radiation reaching the earth’s surface, the ultraviolet (UV) component (290 to 400 nm) is a major factor leading to skin pathologies that range in severity from inflammatory responses, cutaneous photoageing, dendritic keratitis to skin cancer [1-3]. The expanding knowledge of the deleterious effects of sunlight has promoted the widespread use of topical sunscreen preparations [4, 5], which contain chemicals that absorb, reflect, or scatter UV radiation [6] and are thereby highly effective skin protectants. Organic sunscreen agents are compounds that decrease the intensity of UV light reaching the epidermal strata by absorbing the radiation (typical electron promotion from a lower-to a higher-energy molecular orbital). The activated sunscreen molecule dissipates the excess energy in the form of heat, by fluorescence, phosphorescence, interaction with neighboring molecules or by undergoing photochemical
modifications [7]. Particulate sunscreens present a physical barrier between the incident radiation and the epidermis, scattering or reflecting the radiation. However, to be effective, these agents must remain on or in the outermost layers of the skin, the stratum corneum (SC). One major drawback of current sunscreen formulations is that they are constantly lost from the skin surface by abrasion with clothing, sweating, or swimming; requiring frequent reapplication for continued effectiveness. Moreover, several of the chemical sunscreens currently on the market exhibit intense irritancy and sensitization reactions after absorption into the dermal strata in predisposed individuals, often causing severe immunological problems [8-10]. Even if toxic manifestations are not evident in users, this penetration of the sunscreen represents a loss from the surface with a subsequent reduction in sun protection effectiveness. A significant improvement in sunscreen technology would be the development of a system that retards the penetration of the chemical into the skin and binds the agent in the SC so that minimal loss occurs by diffusion, abrasion, or moisture. The degree of sunscreen penetration depends strongly on the physicochemical properties of the active compound and of the nature of the vehicle in which the sunscreen is applied, that is, polarity of the solvent, particle size, type of vehicle [11]. Therefore, the development of suitable products that prevent penetration of the sunscreen into the skin is a challenge for manufacturers. Some of the vehicular penetration retardation strategies being researched for sunscreens are reviewed below.
