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Laser light and tissue: biophysical aspects of medical laser application

In order to determine the interaction between laser light and biological tissues quantitatively the physical parameters of the biological objects have to be related to the parameters of the laser light. The degree and extent of the effect depend on the one hand on the properties of the tissue which are determined by the structure water content and blood circulation and on the other hand on the geometry of the laser beam and the wavelength. Depending on the duration of the laser irradiation on tissue and on the laser irradiance in surface or volume interaction three types of tissue interactions can be distinguished: photochemical effects photothermal effects and photoionizing effects. With extremely long interaction times and low power densities photo chemical transformation occurs by absorption of light with no primary heating of the tissue. With decreasing interaction time and higher power density the transition to photothermally induced effects begins. The early and main surgical applications for lasers are based on the conversion of laser light into heat. This thermal effect is broadly applied in surgery for tissue removal and tissue coagulation with the sealing of vessels and lymphatics as well as for tissue7weldig. When exceeding a power density of 10 W/cm nonlinear effects result. The high irradiance generates strong electric fields which lead to a dissociation or ionization of the material involved. Thus laser light is converted into kinetic energy. From