GaN-on-silicon MicroLEDs for neural interfaces

This chapter reviews the new application of microLEDs for implantable neural probes that will enable the challenging task—functional mapping of local circuit connectivity in the brain. An array of microLEDs can be utilized as an opto-electrophysiology tool that can modulate target neurons with high spatial, temporal resolution with the help of genetic engineering, called optogenetics. Optogenetic methods utilize genetic modification techniques to induce the target neurons to express light-sensitive ion channels or opsins, allowing for cell-type specific neuronal modulation at specific wavelengths. For example, Channelrhodopsin-2 (ChR2) excites neurons at blue light (470 nm), while halorhodopsin (NpHR) rapidly and reversibly silences spontaneous activity at green light (530 nm). By integrating microLEDs with recording electrodes, high-density optoelectrodes can be developed for study of neuronal circuits from co-localizing optical stimulation sites and electrical monitoring electrodes in the same neuronal population. After introducing a brief history of in vivo opto-electrophysiology tools, this chapter reviews the design and fabrication of the implantable silicon (Si) electrode arrays with monolithically-integrated gallium nitride/indium gallium nitride (GaN/InGaN) multi-quantum-well (MQW) LEDs, having a 460-nm emission wavelength. With techniques such as modification of the doping density of the silicon substrate, introduction of a shielding layer, and shaping the stimulation pulses, the stimulation artifact can be eliminated. The high spatial-resolution of optical stimulation provided by the microLED optoelectrode has been demonstrated by finely adjusting the radiant fluxes generated from two adjacent microLEDs and observing the change in activities of neurons in the vicinity of 50 μm in separation.