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

Abstract

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.

Optimization of an optrode microdevice for infrared neural stimulation

“…The excitation wavelength was 1310 nm. According to our previous simulations the main losses occurred due to the reflections and due to the scattering on the material boundaries of the in-coupling and out-coupling interfaces of the device [5]. The in-coupling loss can be reduced using index matching glue between the fiber end and the optrode.…”

“…The optimization of the device is based on our previous work [5] and is supplemented with mechanical modeling to investigate the mechanical strength of the device. We have calculated the von Mises stress in the device using a finite element model (FEM) developed in COMSOL ® Multiphysics -Solid Mechanics module [12] (see Fig.2).…”

“…The output of the optical model was the overall device efficiency values in air as well as the absorbed intensity distribution in the brain tissue. To specify the heating effect during NIR stimulation a finite element thermal model was developed [5]. The model uses the absorbed light intensity calculated by the ray tracing model as heat source modulated by a periodic top-hat pulse with pulse-width of 100 ms and period of 500 ms.…”

“…The excited volume of neural tissue should be precisely determined for any stimulation device, therefore a reliable and validated numerical model is an efficient tool to predict the effect of design parameters on the device. In our latest work, we introduced the design and fabrication of an implantable optrode microdevice [4], and developed its coupled optical and thermal model validated by optical measurements [5]. The model incorporated ray tracing to simulate the optical propagation in the device, Fourier scattering [6,7] to describe the scattering on the optrode boundary and a calculation of the temperature distribution of the excited brain tissue as a consequence of the absorbed light intensity.…”

“…The electrical signal is transferred by a printed circuit board (d) and an electrical connector (c).z According to the simulations the overall efficiency of the device (defined as the ratio of the power delivered to the brain tissue and the output power from the fiber) was 30.5±2.5%, which was in good agreement with the measurements. Using an excitation of 30 mW total power and 100 ms pulse width, the size of the excited volume was about 1 mm 3 and the value of the maximal temperature rise of about 3 °C occurred within 0.1-0.2 mm from the tip [5]. In this paper, the optimization procedure of the optrode device is discussed.…”