Computational thermal analysis of the photothermal effect of thermoplasmonic optical fiber for localized neural stimulation in vivo

Optical neuromodulation is a versatile neural stimulation technology that enables highly localized excitatory or inhibitory stimulation of neuronal activities. Photothermal neural stimulation using thermoplasmonic metallic nanoparticles for light to heat conversion has been suggested as an optical neural stimulation technology without genetic modification. Optical fibers implementing the thermoplasmonic effect were recently developed for localized neural stimulation, and the successful demonstration of localized neural stimulation in vitro was reported. However, before photothermal neural stimulation is further applied in the brains of live animals and ultimately in human trials, a safety analysis must carefully be performed for the thermal effect of stimulation in vivo. With the complexity of the physical structure and different thermal properties of the brain and surrounding body, the resulting thermal effect could vary despite the same power of light delivered to the optical fiber. In addition, dynamic thermal properties of the brain such as the daily blood perfusion rate change or metabolic heat generation must also be carefully considered for the precise implementation of photothermal neural stimulation. In this work, an in-depth computational analysis was conducted of the photothermal effects using a thermoplasmonic optical fiber for in vivo neural stimulation. The effects of the experimental design and stimulation protocols on the thermal effect in the brain were analyzed. We believe that the results provide a good experimental guideline for safely conducting photothermal neural stimulation using the thermoplasmonic optical fiber technology.