**1. Introduction**

286 Acoustic Waves – From Microdevices to Helioseismology

Seidel, H., Heide R. (1986). Long-term effects of whole-body vibration: a critical survey of

Tasaki, I., Davis, H. & Legouix J.P. (1952). The space-time pattern of the cochlear

Thurlow, W.R. (1943). Studies in auditory theory:II The distortion of distortion in the inner

Wadsten C.J., Bertilsson C.A., Sieradzki H. & Edström S. (1985). A randomized clinical trial

Wever, E.G. & Bray, C.W. (1930). Auditory nerve impulses, *Science*, Vol.71, No.1834, p.215,

Vollrath, M., Schreiner, Chr.(1982). Influence of argon laser stapedotomy on cochlear

Ziarani, A.K. & Konrad A. (2004). A novel metod of estimation of DPOAE signals. *IEEE Transactions on biomedical engineering*, Vol.51, No.5, pp.864-868, ISSN 0018 9294 Ziemski, Z. (1970). Ototoxity of selected organic solvents of industrial plastics in

*Otorhinolaryngol* Vol. 242, No. 2, pp. 135-139. ISSN Print 0302-9530

Vol. 24, No.5, pp. 502 – 519, ISSN Print 0001-4966

0131

0096- 3445

128

ISSN Print 0036 8075

pp.1-31, ISSN Print 365-5237

the literature. *Int Arch Occup Environ Health*, Vol.58, No.1, pp. 1-26, ISSN Print 0340-

microphonics (guinea pig) as recorded by differential electrodes. *J*. *Acoust*. *Soc. Am.*,

ear. *Journal of Experimental Physology: General*, Vol.32, No.4, pp. 344 – 350, ISSN

of two topical preparations (framycitin/gramicidin and oxytetracycline /hydrocortisone with polymyxin B) in the treatment of external otitis. *Arch* 

potentials I: Alteration of cochlear microphonics (CM). *Acta Otolaryngol*, suppl **385**,

experimental animals. *Papers of Medical University in Wroclaw*, Vol.15, No.1, pp.59-

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

Photoacoustic Technique Applied to Skin Research:

PA signal.

2011).

section 4 of the present chapter.

(Gao et al., 2005; Qiu et al., 2008).

conventional optical spectroscopy).

light by the relation

**2.3 Studies on the modulation frequency: depth profile** 

characterization of multilayer systems (as skin itself).

Characterization of Tissue, Topically Applied Products and Transdermal Drug Delivery 289

composition or structure (as it occurs during the polimerization of a dental resin, for instance), the propagation of heat inside the sample is also modified, thereby altering the

We must also mention the possibility of performing photosynthesis studies using PA measurements as a function of time (W.J. Silva et al., 1995). When PA measurements are performed in photosynthesizing samples as plant leaves, the PA signal presents, in addition to the photothermal component, a photobaric component, resulting from the gas exchanges associated to the photosynthesis process (Acosta-Avalos et al., 1996). This allows the study of the so-called photosynthetic induction, that is, the increase of the net photosynthetic rate that occurs when a plant is shifted from darkness to light (Sui et al.,

As stated by Bodzenta et al. (2002) in their work on PA detection of drug diffusion into a membrane, PA measurements give the possibility for investigations in relatively long time periods. This makes the PA technique suitable for the monitoring of dehydration processes (Lopez et al., 2005) and of changes occurring in time in biological tissues such as skin. It is possible to study, for example, the kinetics of transdermal drug delivery through the analysis of PA measurements as a function of time. One example will be presented at the

In thermally thick samples (as skin tissue), only the light absorbed within the first thermal diffusion length (μT) of the sample/tissue contributes to the PA signal (Rosencwaig, 1980). As the thermal diffusion length depends on the modulation frequency (f) of the incident

> *<sup>T</sup>* <sup>=</sup> *<sup>f</sup>* α

where α is the thermal diffusivity of the sample, it is possible to perform depth-profile studies, with the evaluation of the penetration depth of a product (or even a microorganism) in tissue. The possibility of performing depth-profile studies is particularly interesting in the

The frequency dependence analysis of the PA signal can also be employed in the determination of the thermal properties (thermal diffusivity, thermal effusivity) of a sample or material (Balderas-Lopez & Mandelis, 2001), including biological tissues as porcine skin

**2.4 Measurements as a function of the wavelength: Photoacoustic spectroscopy**  Photoacoustic spectroscopy (PAS) is already incorporated to the roll of useful photothermal techniques since the 1980s (Rosencwaig, 1980; Vargas & Miranda, 1988). Besides the possibility of rendering depth-profile analysis in multi-layered samples, PAS presents at least two additional advantages over other spectroscopy techniques: i) as transmitted and reflected light do not interfere in PAS measurements, it is a "more direct" technique, representing a direct measurement of the light absorption by the sample; ii) it allows the study of optically opaque and highly scattering samples (which could not be analyzed by

π

(1)

μ

relevance in a large number of very different fields, from the polymerization of dental resins (Balderas-Lopez et al*.*, 1999) to photosynthesis studies (Malkin & Puchenkov, 1997; Herbert et al., 2000).
