Radiofrequency impedance spectroscopy of biological tissues under heating by homogeneous laser radiation

We have developed an original method to measure the temperature dependence of the biological tissue electrical properties based on its heating by homogeneous optical radiation.The conventional approach in hyperthermia research involves heating due to a thermal conductivity through the surface of the sample. The novel technique is based on the emission of heat sources in the sample volume caused by the absorption of optical radiation.The method was verified using chicken liver and aloe parenchyma samples, which were uniformly irradiated in a special chamber with an optically scattering inner coating. The electrical impedance of the samples was measured using a 4-electrode technique in the frequency range 100 Hz-1 MHz. In order to approximate and analyze the electrical impedance module, an equivalent electrical circuit based on the Cole-Cole function was used and the dependences of the approximation parameters on time and temperature were obtained. Applying the Arrhenius formulation to the kinetics of low-frequency resistance, we obtained the parameters of the kinetics of degradation of the biological tissues (critical temperatureTcrand activation energyEa):Ea=(16±4)·105J·mol^-1,Tcr=63±1°C for the aloe parenchyma andEa=(4.5±2)·105J·mol^-1,Tcr=83±1°Cfor the chicken liver.