**5. Conclusion and perspectives**

The possibility of performing *in vivo* studies brings great value to the use of the PA technique in experimental skin research. The potential and relevance of PA measurements in this field have been shown by a large range of experiments being performed by different research groups around the world – actually, the examples here presented must be seen as a sample of what has been done.

PA measurements *in vivo* are able to detect alterations in skin pigmentation. Even in skin regions normally protected from sun exposure, the PA signal level tends to follow clinical classification; actually, the use of the PA technique goes one step ahead, allowing comparative and quantitative research through simple, direct measurements.

Topical drug application has been employed in the treatment of many pathological processes; its efficiency is associated to the efficiency of transdermal drug delivery. PA measurements have been sucessfully employed in transdermal drug delivery studies, allowing a quantitative analysis of the kinetics and effectivity of drug delivery. Different PA experiments point to the fact that gel formulations tend to be more adequate for topical use. Depending on the topically applied product, the form of application can also be determinant in the kinetics of transdermal drug delivery.

Measurements already performed indicate various perspectives for future research, such as: i) the determination of the skin oiliness level; ii) analysis of skin lesions; iii) studies on the photostability of sunscreens and even determination of the sun protection factor (SPF) of sunscreens (through PAS measurements); and iv) further studies on formulation and application form of a wide range of topically applied products.
