In-vivo Test: In-vivo animal experiment in a rat is performed to verify the efficacy of the proposed stimulator for seizure suppression application. Fig. 8(a) shows the experimental environment for the test. The PC updates the stimulation parameters to the FPGA through the UART interface, followed by the SPI interface from FPGA to the stimulation IC. While stimulation pulses are applied to the rat, the commercial RHS recording platform (from intan technologies) records the ECoG signals, which is transmitted to the PC. As shown in Fig. 8(b), a micro-fabricated graphene electrode, which shows the impedance characteristics of Fig. 9, is directly attached to the somatosensory and/or motor cortical area of the brain for the experiment. Bicuculline is used to mimic seizure and is injected into the brain’s somatosensory and motor cortex at a concentration of 15mM. Fig. 10(a) shows the entire waveform of bicuculline-induced seizure, which is suppressed through stimulation. Stimulation is performed with a frequency of 5 Hz, which is the most effective for seizure inhibition. Stimulation is continued for 40 seconds with the amplitude of 50µA and the pulse width of 10µs. Fig. 10(b-e) shows the waveforms when epilepsy, stimulation, inter-period of seizure to stop, and suppression occurs, respectively. After 40 seconds stimulation, it takes 14 seconds to suppress the seizure.
A performance summary and comparison with other neural stimulators are presented in Table I. This work achieves high compliance per supply voltage through a low-cost standard CMOS technology. The 64-channel stimulator can be independently configured with a high resolution of 8 bits. In addition, for the feasibility verification, an animal test is peformed.
Conclusions: We present a 64-channel implantable current mode stimulator IC with a passive charge balancing scheme for seizure suppression. The proposed stimulator uses passive CB with a bootstrapped switch, demonstrating sub-nC charge-balanced operation. The regulated cascode driver satisfies a high compliance voltage ratio of 0.98 per supply voltage in 0.18 mm standard CMOS technology. The amplitude, pulse width, and activated channel of the stimulation waveform are variably adjusted with the SPI interface. In-vivo test shows the feasibility of seizure suppression through a 5Hz stimulation frequency with 50µA current amplitude and 10µs pulse width.