Influence of nonhomogeneous distribution of topically applied UV filters on sun protection factors

Abstract

The aim of the present study is the development of a method to determine quantitatively in vivo the influence of homogeneity of the distribution of sunscreen containing UV filters on the sun protection factor (SPF). The SPF of a sunscreen applied either topically or inside an optical cell (pure or in a solvent) fixed above the skin is determined in vivo. In both cases, in vivo measurements using the erythema formation are carried out. Identical optical parameters of the skin are realized in both experiments. In addition, both in vitro (using tape stripping) and in vivo microscopic measurements are performed to analyze the homogeneity of distribution of the topically applied substances. An SPF of 8 is measured in the experiment applying the UV filters topically, whereas this value increases by a factor of 10 if the same amount of filter substances is distributed homogeneously in solution inside the optical cell. Tape strips removed from skin treated with the sunscreen reflect the inhomogeneous distribution of the topically applied substances on the skin. The direct correlation of homogeneity of distribution with the SPF opens up the possibility to increase the SPF by optimizing the formulation.

Secondary Plant Metabolites for Sun Protective Cosmetics: From Pre-Selection to Product Formulation

“…Organic (synthetic) and mineral UV filters pose relevant challenges related to their photo-stability in vitro and in vivo under UV irradiation [81,82], their homogeneous distribution onto the skin [83][84][85], and their capability to reach the viable layers of epidermis and obtain sufficient accumulation in the stratum corneum [86] while also avoiding their systemic penetration due to the widespread concerns on their toxicity [87]. A major role is played by vehicles, which undergo a continuous technological evolution in the highly competitive cosmetic world, moving from the traditional vaseline or oil-in-water (o/w) emulsion delivery systems.…”

Gain or Loss? Sunscreen Efficiency after Cosmetic Pretreatment of the Skin

“…Within certain time limits the distribution of substances on tape strips represents the local in vivo conditions [11]. Therefore, information on horizontal and vertical distribution (homogeneity and penetration depth) of applied substances can be obtained.…”

“…Ambitious research work is being undertaken, in order to optimize the product-defining characteristics of sunscreen filters and sunscreen formulations. Under real-life conditions, however, the effectiveness of sunscreens is often limited due to an insufficient quantity being applied [8,9,10], inhomogeneous filter distribution [11,12,13] and filter abrasion [14,15,16]. Whereas application habits such as quantity and frequency of sunscreen use can be changed intentionally, limitations in product efficacy, due to skin morphology and condition, are more difficult to be overcome.…”

“…This approved, minimally invasive method enables the penetration, secretion and reservoir properties of endogenous or topically applied substances in the stratum corneum of the skin to be studied under in vivo conditions. Inter alia, this method has been used previously to investigate UV filter characteristics and external influences on sunscreen behavior and efficiency [11,18,19,20,21,22]. …”

“…Underlying this consideration is the knowledge that the uneven surface profile of the skin, marked by furrows and wrinkles, dominantly limits the homogeneity of UV filter distribution, which thus reduces the achievable sun protection by a factor of up to 10 [11]. Cosmetic products such as creams or lotions claim to smoothen the human skin.…”