646 R. S. Berger, J. A. Mezick and C. M. Papa POLYMERIC FILM AND VEHICLE An acrylate film-forming polymer was selected which could be solubilised by forming the ammoniated salt in an anionic soap emulsion. When the sunscreen preparation dries on the skin, the ammonia evaporates, fixing the insoluble polymer to the skin. Due to this 'fugitive amine' transformation, the film is insoluble in water and resistant to rub-off. It is easily removed, however, by washing with soap and water (Fig. 4). Physical Water Polymer state solubility :!ii'•:•}•"•" ':•:•!iii!ii Raw material Insoluble i '" '" ' '• ................ C•H C•H o%0 o%O o o o,., 0 00• ooos•.screeno Ammoniated Soluble , • • formulation CO0-NH4 CO0-NH 4 CO0-NH 4 Fugitive amine iiii:•:•:•3•:iii$!!i' Skin-dried [ ":{ :::?'""i'" ::::::'::::' i COOH COOH COOH + NH3 Insoluble Soop ond woter removes polymer by emulsification ond/or neutralisafion. For example QøoOoOoOoo ossun•c•noo Soluble COO-No + COO-No + COO-No + Figure 4. Film-former action of polymer (fugitive amine). Scanning electron microscopic examination of the skin following the application of this formulation shows that the film has multiple small holes (Fig. 5). These minute holes account for the findings that the film is non-occlusive and does not interfere with transepidermal water loss through the skin or human eccrine sweat function (Tables III and IV). Table m. Transepidermal water loss (TWL) Film-forming Area No treatment sunscreen product* TWL (Average of six subjects) 0.40 0.35 (rag h -x cm •) * 3.3% octyl dimethyl PABA ANIMAL STUDIES Using a hairless mouse model, the amount of uvl and water immersion were carefully controlled. With no water immersion and uvl exposure, both the film-forming prepara- tion containing 3'3•o octyl dimethyl PABA and a commercial preparation containing 5•o PABA protected the mice from severe skin damage, whereas, the untreated sites were severely damaged. In the group immersed in water for 30 min, followed by uvl exposure, only the film-forming preparation provided significant protection (Table V).

Figure 5. SEM of polymeric film on human skin. Facing p. 646