Infrared nerve stimulation: modelling of photon transport and heat conduction

Infrared neural stimulation (INS) is a novel technique for stimulating neurons with infrared light, rather than the traditional electrical means. There has been significant discussion in the literature on the mechanisms behind INS, while recent work has shown that infrared light stimulates neurons by causing a reversible change in their membrane capacitance. Nevertheless, the effect of different laser parameters on neuronal responses is still not well understood. To better understand this and to assist in designing light delivery systems, modelling of spatial and temporal characteristics of light delivery during INS has been performed. Monte Carlo modelling of photon transport in tissue allows the spatial characteristics of light to be determined during INS and allows comparisons of varying geometries and fibre designs. Finite element analysis of heat conduction can then be used to reveal the behavior of different pulse durations and the resulting temperature decay. The combination of the two methods allows for further insights into the mechanisms of INS and assists in understanding different mechanisms which promote INS. The model suggests there may be two regimes of INS, namely temperature limited for pulses under 100 μs and temperature gradient limited for longer pulses. this is compatible with previously published data, but requires further experimentation for confirmation. The model also provides a tool for optimising the design of emitters and implants.