Development of a Synthetic Substrate for the in vitro Performance Testing of Sunscreens

Abstract

Purpose: Polymethylmethacrylate (PMMA) plates generally used for the in vitro testing of sunscreens have failed to yield a satisfactory correlation between the sun protection factor (SPF) in vitro and that in vivo. In the present study, various polymers were investigated as alternative substrates to PMMA plates. Procedures: In total, 14 polymers were tested in terms of ultraviolet (UV) transparency by transmission spectroscopy, and surface properties by contact angle measurement. The polymers that were UV transparent and that showed surface properties similar to human skin were mold-injected in casts with various degrees of roughness to obtain corresponding substrate plates. To assess them, the in vitro SPF of 1 sunscreen was measured on a mold-injected PMMA plate as well as on the proposed polymer plates and compared to the in vivo SPF. Results: Four polymers showed a UV transmittance at 310 nm of more than 50%, as well as a total surface free energy close to the reported value for human skin. Of these, 2 provided an in vitro SPF matching the in vivo SPF contrary to the mold-injected PMMA plate for the tested oil-in-water sunscreen. Conclusion: This work demonstrates the possibilities of using alternative polymers for synthetic substrates for in vitro sunscreen testing.