Infrared nerve stimulation: modelling of photon transport and heat conduction

Abstract

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 behaviour 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.

A three-dimensional thermal model of the human cochlea for magnetic cochlear implant surgery

“…Energy balances that represent these effects for the magnet, perilymph, and EA are presented in Eqs. ( 4) to (6). For the dimensional analysis, it is assumed that the thermophysical properties of the perilymph, magnet, and EA are independent of temperature.…”

“…Heat transfer within the cochlea has been studied in applications such as infrared neural stimulation [6,7,8,9,10] and therapeutic hypothermia [11,12,13]. As an initial attempt to evaluate the thermal impact of the magnet detachment process on cochlear tissues, a simplified uncoiled model of the cochlea with an inserted implant EA has been developed [14].…”

Experimental validation of a three-dimensional heat transfer model within the scala tympani with application to magnetic cochlear implant surgery

“…The blood perfusion rate in the cochlea is less than 1 ml min•g [8]. In infrared neural stimulation, the energy dissipated by perfusion in the cochlea is negligible compared to the magnitude of the heat source [7], [9], [18], [19], [8]. For example, Thompson et al [7], [8] discussed all possible biological cooling methods in the cochlea (perfusion, cochlear fluid flow, evaporation of water from tissues), and concluded that these mechanisms are much lower than the magnitude of the laser heat source power (6.25 mW) used for infrared neural stimulation implants.…”

“…In infrared neural stimulation, the energy dissipated by perfusion in the cochlea is negligible compared to the magnitude of the heat source [7], [9], [18], [19], [8]. For example, Thompson et al [7], [8] discussed all possible biological cooling methods in the cochlea (perfusion, cochlear fluid flow, evaporation of water from tissues), and concluded that these mechanisms are much lower than the magnitude of the laser heat source power (6.25 mW) used for infrared neural stimulation implants. Here, an input power of 21 mW applied for 10 s is necessary to detach the magnet by assuming that the magnet is attached to the implant electrode array by 1 mm 3 of paraffin having a melting point of 43 • C. This is more than three times larger than the power reported by Thompson et al [7], [8].…”

“…For example, Thompson et al [7], [8] discussed all possible biological cooling methods in the cochlea (perfusion, cochlear fluid flow, evaporation of water from tissues), and concluded that these mechanisms are much lower than the magnitude of the laser heat source power (6.25 mW) used for infrared neural stimulation implants. Here, an input power of 21 mW applied for 10 s is necessary to detach the magnet by assuming that the magnet is attached to the implant electrode array by 1 mm 3 of paraffin having a melting point of 43 • C. This is more than three times larger than the power reported by Thompson et al [7], [8]. The conclusions of Thompson et al [7], [8] may be applicable for magnetic cochlear implant surgery, but heat dissipation by blood perfusion should also be compared to other heat dissipation mechanisms in the scala tympani, and not solely to the external input power.…”