In silico Investigation of Light Intensity and Temperature Distribution in Tissue for Vaccine Adjuvant Using NIR...

We simulated the effective reaction area and thermal safety using ray tracing and heat conduction for the newly proposed laser adjuvant for vaccines. Vaccines are considered the most economical and efficient method for preventing infectious diseases. However, the localized distribution of vaccine antigens reduces the effectiveness of immune induction. Adjuvants, combined with vaccines to enhance the immune response, carry the risk of side effects and autoimmune diseases, necessitating safer and more effective technologies. We propose a novel, non-invasive, and effective method that employs lasers as adjuvants. Since the immunostimulatory effect of near-infrared laser irradiation has been demonstrated in intradermal vaccines, we aim to develop new laser adjuvants for various types of vaccines. In a previous study, laser irradiation of mouse skin at a wavelength of 1270 nm induced adjuvant effects, including producing reactive oxygen species and ATP. In this study, we developed numerical models of mouse and human skin models to validate their accuracy and evaluate the effects of laser adjuvants. The skin models comprised three layers: epidermis, dermis, and connective tissue. The light irradiation conditions were as follows: wavelength of 1270 nm, irradiance 1.0 and 2.0 W/cm², and irradiation time of 180 s. The center of irradiation on the skin surface was where the maximum temperature was measured. At an irradiance of 2.0 W/cm2-a condition under which adjuvant effects were confirmed in in vivo experiments on mice-the light successfully reached the mast cells, and the maximum temperature was 38.6°C, proving its safety. However, the light did not adequately penetrate the mast cells under the same conditions in human skin. These findings highlight the need to explore optimized laser irradiation conditions and techniques for effective adjuvant use based on the skin models developed in this study.