Determination of sun protection factors. Correlation between in vivo human studies and an in vitro skin cast method

Abstract

Synopsis Sun protection factors were determined by both an in vitro method which used resin casts taken from replicas of human skin and by an in vivo SPF method. Thirty-eight product development samples were tested for the level of sun protection using both methods and the results were compared. The values obtained showed a positive relationship which was closely approximated by a log-linear regression of in vivo data on in vitro data (regression coefficient, r(2)= 0.86). It is concluded that the cast technique is quick, convenient, inexpensive and in its present form useful for screening sunscreen products prior to in vivo SPF testing.

Validation of an in vitro sun protection factor (SPF) method in blinded ring‐testing

“…It has long been a desire to develop an in vitro SPF test method, recognizing the potential advantages of such methodology, including (i) the use of a non‐human model, (ii) the significant improvements in speed and cost, (iii) the improved repeatability and reproducibility, (iv) the elimination of technically challenging procedures (e.g. MED determination) and (v) the use of a method which is significantly more amenable to continuous improvement . Importantly, there is also a current mandate from the European Commission to develop and deploy in vitro sun protection test methods (EC Recommendation 2006/647/EC ), which states ‘…preference should be given to in vitro testing methods delivering equivalent results, as in vivo methods raise ethical concerns.…”

Problems of in vitro SPF Measurements Brought about by Viscous Fingering Generated during Sunscreen Applications

“…This method requires the process of irradiation of UV light on a human's back as a test subject. Challenges thus have been made to develop an in vitro SPF measurement method [2,3,4,5,6,7,8,9], since it gives results more quickly, is less expensive and is more ethical. For an in vitro SPF measurement, the recommended amount of sunscreens is applied on a standard substrate to prepare the sample.…”

Shedding light on sunscreens

“…75,[80][81][82][83] Many other in vitro methods have been developed. 81,82,[84][85][86] None of the methods described so far takes into account the water resistance of the sunscreen tested, otherwise known as its substantivity, and several clinical methods for the assessment of this have now been developed, such as exercise-induced sweating, 87,88 sauna-induced sweating, 89 water-streaming, 87,90 bathing 89,91-93 and swimming. 88,92,93 These all demonstrate that there may be a significant drop in the sunscreen SPF value after skin immersion and that such reductions vary according to the formulation used.…”