BACKGROUND/PURPOSE:Sunscreen products today represent a trend of providing not only simple sun protection factor (SPF)/protection of UVA (PFA) but also other additional benefits. For example, as popularized by seasonless use of sunscreens, the special function of water resistance or sand proof is added to sunscreens as well as for leisure. Because a human in vivo test is time consuming and expensive, a screening process has been tried using an accurate in vitro system. In this study, we suggest the development of an in vitro test that can predict the result of in vivo water resistance of sunscreens. METHODS:Water resistance is presented as a comparison of initial SPF and water-exposed SPF by immersion and washing. In order to be comparable with the in vivo test, water immersion and flow were defined as the basic statements. Also, substrate, revolutions per minute (r.p.m.)--rotative velocity--of propeller inducing water flow, and time of immersion were defined as controlled factors. Considering the strength, separation of test material and adhesive texture, a PMMA plate was selected as suitable among commercial substrates: Transpore tape, VITRO SKIN, and PMMA plate. Also, when the PMMA plate was adhered on the wall of a water bath, the water turbulence of the rotational propeller alone was not strong enough to wash off the test material from the substrate. Therefore, PMMA plates were fixed on the axis. In this experiment, the most important thing is whether this in vitro system can predict correctly. Hence, we tried to match the in vitro water resistance following from our control factors and water resistance value of the in vivo test. RESULTS:We found the immersion time and r.p.m. of controlled factors to obtain the target water resistance using design of experiment, MiniTab statistical package. Response optimization yielded the optimal in vitro conditions of 150 r.p.m./60 min. The repeatability and reproducibility of this in vitro system were also good in validation studies. CONCLUSIONS:This study enables to modify an in vivo water resistance test and predict the result of in vivo water resistance by the manufacture of effective equipment and choosing a suitable substrate. Compared with in vivo results, our in vitro system is more time and cost effective, and provides reliable results.