| Optogenetics is a fast growing neuromodulation technique, which can remotely manipulate the specific activities of genetically-targeted neural cells and associated biological behaviors with millisecond temporal precision through light illumination. Application of optogenetics in neuroscience studies has created an increased need for the development of light sources and the instruments for light delivery. Micro-Light-emitting diodes (mu-LEDs) offer a great option as the light source for optogenetics since they are power-efficient, low-cost and suitable for integration with wireless electronics. Furthermore, arrays of individually addressable mu-LEDs have been developed to accomplish multisite in-vivo stimulation. However, a critical challenge of using mu-LEDs as the light source for optogenetics is their intrinsically low out-coupling efficiency and wide irradiation angles due to the Lambertian emission pattern, which results in a big loss of the radiation. Consequently, mu-LEDs must be driven with high power in order to reach the required light intensity of 1 mW/mm2 and 7 mW/mm2 for effective activation of excitatory and inhibitory opsins at the target site, respectively. However, this is not suitable for wireless operation, and could induce potential thermal interference or damage to tissues due to Joule heating effect. In this thesis, an implantable, micro-lens-coupled LED stimulator has been proposed to be applied as the light source for neural stimulation. A reflector and a microlens were coupled with the ?-LED chip for light collection and collimation, giving rise to a significantly improved light irradiance. A novel microfabrication method, vapor-induced dewetting, was developed to make self-organized SU-8 microlens arrays. It was later involved in the fabrication and integration process of micro-lens-coupled LED stimulators. An optimization on the device structure was carried out using optical simulation in order to attain optimal penetration capabilities of the light. The optimized micro-lens-coupled LED stimulator was microfabricated and measured both in air and in tissue experimentally. Significant improvement of >60% in light intensity was achieved, validating its functionality and potential as the light source for optogenetic neuromodulation in dep cortical layers. |
