Laser photo-induced effects: a focus on the photothermal interaction.

Laser therapies are based on the principle of light absorption by tissues and rely on three types of interactions: photochemical, photothermal and photomechanical interactions. In the first case, the photon energy is employed to trigger photochemical reactions, in the second the energy is converted into heat, while in the latter case mechanical effects are elicited. The prevalence of one or the other effect can be studied by considering the “chart of photo-induced effects” as detailed below. Generally speaking, laser interaction with tissue is governed by the following parameters: (i) laser wavelength, irradiance and light delivery mode for the laser source; (ii) the absorption and scattering properties (i. e. absorption and scattering coefficients) for the tissue. Irradiance, defined as the amount of light energy impinging on the tissue per unit of time and area, determines the intensity of the laser beam interacting with the target tissue. After introducing a classification of the photo-induced effects, we will concentrate on the photothermal ones, analyzing their role in both therapeutic effects and safety issues.