Heating and optoacoustic temperature determination of cell cultures

This paper describes the further investigation into the capabilities of the already established noncontact optoacoustic method to measure temperature profiles in cell cultures during controlled heating. The technic is scalable in spatial and temporal resolution. The intra and extracellular medium is heated by a thulium laser (wavelength 1.94 mu m; power up to 25W). With a second Q-switched thulium laser (2.01 mu m; up to 3 mJ) the sample medium temperature is simultaneously probed in the dish (20 mm diameter) via the photoacoustic effect. The pressure waves emitted due to the thermoelastic expansion of water are measured with an ultrasonic hydrophone at the side of the dish. The amplitudes of the waves are temperature dependent and are used to calculate the temperature/time course at 10 locations. Temperatures of up to 70 degrees C with a heating power of up to 25 W after 5 s were measured, as well as lateral temperature profiles over time. Measurements in water show temperature fluctuations likely due to thermal convection and water circulation. Since measurements in agar do not show similar temperature fluctuations, this theory seems to be confirmed. In conclusion optoacoustics can serve as a real-time non-contact technique to determine temperature changes in cell and organ cultures as well as in vivo and during hyperthermia based therapies.