Optical method plays an important role in clinical diagnosis and treatment, but suffers from limited penetration depth of light in turbid tissue. The optical clearing technique can improve the light delivery significantly through immersion of tissues into Optical Clearing Agents (OCAs). However, the barrier function of stratum corneum makes it difficult for optical clearing of skin by topical application of OCAs. Addition of penetration enhancers to OCAs can improve the skin clearing efficacy, but most investigations were performed on in vitro skin. Here, to evaluate the efficacy of this method on in vivo skin, direct observation and measurement of diffuse reflectance spectra were performed after topical application of different mixtures. One OCA, PEG-400, and three penetration enhancers (PEs), Thiazone, Azone and Propylene Glycol (PG), were used. The results indicated that the addition of penetration enhancers could improve the optical clearing efficacy of rat skin in vivo significantly, the dermal blood vessels could be observed directly with PEs.Among the three penetration enhancers, Thiazone induced the largest enhancement of clearing efficacy, and the enhancement induced by PG is the least. This study is very helpful for in vivo application of OCAs to enhance skin optical clearing non-invasively.

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