**3. Photoacoustic measurements and the characterization of skin**

Biological materials are sometimes difficult to study employing conventional techniques that require previous preparation of the samples, because these materials can have its properties significantly altered by preparation processes as solubilization, for example. The PA technique does not require previous preparation; it can be described as a non-invasive technique that allows even *in vivo* measurements.

In general, biological tissues can be characterized as highly scattering samples; however, this is not a problem for PA mesurements, in which the signal is based in the direct absorption of radiation. As pointed by Cahen and co-workers (1980), "the relative insensitivity to scattered light of the PA signal makes such measurement an attractive way to measure biological samples *in vivo*". These features explain the potential of the PA technique in the study of opaque materials and complex biological systems such as skin. PA measurements can be employed to determine the absorption characteristics of the skin itself or topically applied products, as well as kinetic changes related to transdermal drug delivery.

Skin diseases can also be studied through PA measurements. In 2010, Swearingen et al. developed a PA methodology to determine the nature of skin lesions (pigmented and vascular) *in vivo*, which is important because misdiagnosis may even lead to cancerous lesions not receiving proper medical care. These authors irradiated skin with two laser wavelengths (422 and 530nm), with the relative response at these two wavelengths (422nm/530nm) indicating whether the lesion is pigmented or vascular, due to the distinct absorption spectrum of melanin and hemoglobin (Swearingen et al., 2010).
