Photoacoustic Technique Applied to Skin Research: Characterization of Tissue, Topically Applied Products and Transdermal Drug Delivery

Abstract

The photoacoustic (PA) effect basically consists in the production of acoustic waves due to the absorption of modulated (or pulsed) radiation by a sample. Graham Bell discovered the PA effect in 1880, when he noticed that the incidence of modulated light on a diaphragm connected to a tube produced sound. Thereafter, Bell studied the PA effect in liquids and gases, showing that the intensity of the acoustic signal observed depended on the absorption of light by the material. In the nineteenth century, it was known that the heating of a gas in a closed chamber produced pressure and volume changes in this gas. However, there were many different theories to explain the PA effect. Rayleigh said that the effect was due to the movement of the solid diaphragm. Bell believed that the incidence of light on a porous sample expanded its particles, producing a cycle of air expulsion and reabsorption in the sample pores. Both were contested by Preece, who pointed the expansion/contraction of the gas layer inside the photoacoustic cell as cause of the phenomenon. Mercadier explained the effect conceiving what we call today thermal diffusion mechanism: the periodic heating of the sample is transferred to the surrounding gas layer, generating pressure oscillations. The lack of a suitable detector for the PA signal made the interest in this area decline until the invention of the microphone. Even then, research in this field was restricted to applications in gas analysis up to 1973, when Rosencwaig started to use the PA technique in spectroscopic studies of solids and, together with Gersho, developed a mathematical model for the generation of the PA signal in solid samples – the Rosencwaig-Gersho (RG) Model (Rosencwaig & Gersho, 1976). In condensed matter samples, one of the most important mechanisms for PA signal generation is the thermal diffusion, classically described by the RG model. According to this model, the (modulated) radiation absorbed by condensed matter samples is converted into heat, causing temperature modulation in the surrounding atmosphere. This eventually produces the mechanical effect of periodic expansion and contraction originating sound waves that can be detected by a microphone. Since the publishing of the RG model and, soon after that, of the generalized theory for the PA effect by McDonald and Wetsel (1978), the PA technique has already proved its