Bioheat Transfer During Infrared Laser Irradiation of Tissue Phantoms for Replacing Traditional Moxibustion

Traditional moxibustion generates heat stimulation that expands blood vessels and promotes blood circulation. We developed a novel noncontact-type thermal therapeutic system using an infrared laser diode. The device allows direct interaction of laser light with the skin, thereby rendering a temperature distribution on both on the skin's surface and deep under the skin. Optical and thermal properties--the absorption coefficient, specific heat, and thermal conductivity--are the three most important parameters for a tissue phantom used as a substitute for real skin. We found that these parameters could be manipulated using the concentration of agar gel, and we fabricated multi-layer tissue phantoms using combinations of agar gels with different concentrations. The temperature distribution inside the tissue phantom during laser irradiation was measured using inserted thermocouples and thermal imaging. The temperature increased with agar gel concentration, and could reach a maximum value under the surface of the tissue phantom. Further analysis of the temperature distribution yielded controllable parameters for laser irradiation (output power, spot size, wavelength, and pulse width) to induce a similar thermal effect with moxibustion therapy. With known compositions and the opto-thermal properties of a patient's skin tissue, we manipulated the temperature distribution inside the tissue by optimizing the laser parameters, which will ultimately enable patient-specific thermal therapy.