ABSTRACT
The impact of sunscreen formulations on the barrier properties of human skin are often overlooked leading to formulations with components whose effects on barrier mechanical integrity are poorly understood. The aim of this study is to demonstrate the relevance of carrier selection and sunscreen photostability when designing sunscreen formulations to protect the biomechanical barrier properties of human stratum corneum (SC) from solar ultraviolet (UV) damage. Biomechanical properties of SC samples were assayed after accelerated UVB damage through measurements of the SC's mechanical stress profile and corneocyte cohesion. A narrowband UVB (305-315â¯nm) lamp was used to expose SC samples to 5, 30, 125, and 265â¯Jâ¯cm-2 in order to magnify damage to the mechanical properties of the tissue and characterize the UV degradation dose response such that effects from smaller UV dosages can be extrapolated. Stresses in the SC decreased when treated with sunscreen components, highlighting their effect on the skin prior to UV exposure. Stresses increased with UVB exposure and in specimens treated with different sunscreens stresses varied dramatically at high UVB dosages. Specimens treated with sunscreen components without UVB exposure exhibited altered corneocyte cohesion. Both sunscreens studied prevented alteration of corneocyte cohesion by low UVB dosages, but differences in protection were observed at higher UVB dosages indicating UV degradation of one sunscreen. These results indicate the protection of individual sunscreen components vary over a range of UVB dosages, and components can even cause alteration of the biomechanical barrier properties of human SC before UV exposure. Therefore, detailed characterization of sunscreen formulation components is required to design robust protection from UV damage.
ABSTRACT
The excited states of 4-tert-butyl-4'-methoxydibenzoylmethane (BM-DBM) and 4-tert-butyl-4'-methoxydibenzoylpropane (BM-DBP), the model of the keto form of BM-DBM, have been studied through measurements of UV absorption, fluorescence, phosphorescence, and electron paramagnetic resonance spectra in ethanol at 77 K. The energy levels and lifetimes of the lowest excited triplet (T(1)) states of BM-DBP, dibenzoylpropane (DBP), the model of the keto form of dibenzoylmethane (DBM), and the keto and enol forms of BM-DBM and DBM were determined. The energy level of the T(1) state of the keto form is much higher than that of the enol form in BM-DBM. The effect of tautomerization on the T(1) lifetime is small in DBM but large in BM-DBM. The methoxy and tert-butyl groups play an important role in lengthening the T(1) lifetimes of BM-DBP and the keto form of BM-DBM. The nature of the T(1) states of the keto and enol forms of BM-DBM can be explained in terms of the mixing of the (3)npi* and (3)pipi* states. The observed energy levels and the lifetimes of the T(1) states of the UV absorbers provide a useful suggestion for designing more photostable UV absorbers.
