Effect of size of TiO₂ nanoparticles embedded into stratum corneum on ultraviolet-A and ultraviolet-B sun-blocking properties of the skin

Recently there has been a strong demand to protect human skin against negative effects of the UV solar light. This problem is interesting due to the increased frequency of human diseases caused by such radiation. We aim to evaluate how the optical properties of the horny layer of skin can be effectively changed by imbedding TiO₂ fine particles to achieve the maximal attenuation of the UV solar radiation. In-depth distribution of TiO ₂ particles embedded into the skin by multiple administration of sunscreens is determined experimentally using the tape-stripping technique. A computer code implementing the Monte Carlo method is developed to simulate photon migration within the 20-μm-thick horny layer filled with nanosized TiO₂ spheres, 25 to 200 nm in diameter. Dependencies of the UV radiation of two wavelengths (310 and 400 nm) absorbed by and totally reflected from, as well as transmitted through the horny layer on the size of TiO ₂ particles are obtained and analyzed. The most attenuating particles are found to be 62 and 122 nm in diameter for 310- and 400-nm light, respectively. The former could be suggested as the main fraction to be used in sunscreens to prevent erythema.