Optical and thermal modeling of an optrode microdevice for infrared neural stimulation

Infrared neural stimulation is a promising medical technique using pulsed infrared light for generating temperature-controlled firing of neurons. A combined optical and thermal model of a stimulating microtool-or so-called optrode-has been developed to investigate the amount, the spatial distribution, and the temporal behavior of the thermal excitation. Ray tracing and Fourier optics were used to describe the propagation and scattering of light in the optrode, and the finite element method was applied to model heat transfer. The scattered intensity distribution profiles were calculated based on measured surface roughness of the device and were integrated into the ray optics model. As a validation of the optical model, the simulated and measured values of the light efficiency of the microoptical system are compared. The temperature rise of the brain tissue during the infrared stimulation was estimated using the combined model. Using 30 mW total power and a single 100 ms pulse, the excitation resulted in a temperature rise of 3°C of the brain tissue. The spatial and temporal distributions of the tissue temperature are discussed in the paper. The proposed combined model is an efficient tool for the investigation and optimization of the stimulation process and for further development of the optrode configuration.