Brain is one of the most temperature sensitive organs. Besides the fundamental role of temperature in cellular metabolism, thermal response of neuronal populations is also significant during the evolution of various neurodegenerative diseases. For such critical environmental factor, thorough mapping of cellular response to variations in temperature is desired in the living brain. So far, limited efforts have been made to create complex devices that are able to modulate temperature, and concurrently record multiple features of the stimulated region. In our work, the in vivo application of a multimodal photonic neural probe is demonstrated. Optical, thermal, and electrophysiological functions are monolithically integrated in a single device. The system facilitates spatial and temporal control of temperature distribution at high precision in the deep brain tissue through an embedded infrared waveguide, while it provides recording of the artefact-free electrical response of individual cells at multiple locations along the probe shaft. Spatial distribution of the optically induced temperature changes is evaluated through in vitro measurements and a validated multi-physical model. The operation of the multimodal microdevice is demonstrated in the rat neocortex and in the hippocampus to increase or suppress firing rate of stimulated neurons in a reversible manner using continuous wave infrared light (λ = 1550 nm). Our approach is envisioned to be a promising candidate as an advanced experimental toolset to reveal thermally evoked responses in the deep neural tissue.

No 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.

Infrared light is a promising candidate for the treatment of neurodegenerative diseases. Optimizing the device parameters to achieve the best optical and mechanical performance is essential for reliable in vivo operation. In this work, mechanical strength simulations and coupled optical and thermal model were used to determine optimal design parameters for maximizing overall device efficiency. Our analysis reveals that minimizing the number of integrated optical elements and optimizing of the optical path leads to 33% relative in-coupling efficiency improvement at equal mechanical robustness. Using a symmetric optrode tip with an angle of 15°, the efficiency showed further 17% relative improvement due to the enhancement of out-coupling at the tip. To investigate the temperature rise of the brain tissue during the infrared stimulation in the case of the optimized device, a thermal simulation with pulsed infrared excitation was developed. Our results show that the optimized device provides a temperature rise of 4.42°C as opposed to 3°C for the original setup.

No abstract