This project has developed an activity-dependent intracortical microstimulation (ICMS) system-on-chip (SoC) fabricated in a 0.35-µm two-poly four-metal CMOS process that converts extracellular neural spikes recorded from one brain region to electrical stimuli delivered to another brain region in real time in vivo. The 10.9-mm2 SoC incorporates two identical 4-channel modules, each comprising an analog recording front-end with total input noise voltage of 3.12 µVrms and noise efficiency factor (NEF) of 2.68, 5.9-µW 10b successive approximation register analog-to-digital converters (SAR ADCs), 12.4-µW digital processor for spike discrimination based on threshold crossing and two user-adjustable time-amplitude windows, and a programmable constant-current microstimulating back-end that delivers up to 94.5 µA with 6b resolution to stimulate the cortical tissue when triggered by neural activity. For autonomous operation, the SoC also integrates biasing and clock generation circuitry, frequency-shift-keyed (FSK) transmitter at 433 MHz, and dc-dc converter that generates a power supply of 5.05 V for the microstimulating back-end from 1.5 V.
The fabricated SoC has been assembled and packaged on a miniature rigid-flex substrate together with a few external components for programming, supply regulation, and wireless operation. The resulting microdevice operates autonomously from a single 1.55-V battery, measures 3.6 cm x 1.3 cm x 0.6 cm, weighs 1.7 g (including the battery), and is capable of stimulating as well as recording the neural response to ICMS in biological experiments with anesthetized laboratory rats. Moreover, it has been interfaced with silicon microelectrodes chronically implanted in the cerebral cortex of an ambulatory rat and successfully delivers electrical stimuli to one cortical region when triggered by neural activity recorded from another distant cortical region with a user-adjustable spike-stimulus time delay. The spike-triggered ICMS is further shown to modulate the neuronal firing rate, indicating that it is physiologically effective.