A wireless implantable multichannel microstimulating system-on-a-chip with modular architecture

Abstract—A 64-site wireless current microstimulator chip (Interestim-2B) and a prototype implant based on the same chip have been developed for neural prosthetic applications. Modular standalone architecture allows up to 32 chips to be individually addressed and operated in parallel to drive up to 2048 stimulating sites. The only off-chip components are a receiver inductive-capac-itive (LC) tank, a capacitive low-pass filter for ripple rejection, and arrays of microelectrodes for interfacing with the neural tissue. The implant receives inductive power up to 50 mW and data at 2.5 Mb/s from a frequency shift keyed (FSK) 5/10 MHZ carrier to generate up to 65 800 stimulus pulses/s. Each Interestim-2B chip contains 16 current drivers with 270 A full-scale current, 5-bit (32-steps) digital-to-analog converter (DAC) resolution, 100 M output impedance, and a voltage compliance that extends within 150 and 250 mV of the 5 V supply and ground rails, respectively. It can generate any arbitrary current waveform and supports a variety of monopolar and bipolar stimulation protocols. A common analog line provides access to each site potential, and exhausts residual stimulus charges for charge balancing. The chip has site potential measurement and in situ site impedance measurement capabilities, which help its users indicate defective sites or characteristic shifts in chronic stimulations. Interestim-2B chip is fabricated in the AMI 1.5 m standard complemen-tary metal–oxide–semiconductor (CMOS) process and measures 4 6 4 6 0 5 mm. The prototype implant size including test connectors is 19 14 6 mm, which can be shrunk down to 0.5 CC. This paper also summarizes some of the in vitro and in vivo experiments performed using the Interestim-2B prototype implant. Index Terms—Charge balancing, frequency shift keying, im-plant, microstimulation, modular architecture, neural prostheses, stimulation strategy, system-on-a-chip (SoP), wireless link. I