NeuroPace found a way to implant the RNS in the skull.
The medical advances that determined the NeuroPace device could be implanted in the skull -- rather than the chest with wires snaking up to the brain -- were the most impressive parts of the design, Archer said. However, the electronics design is significant, too.
The RNS captures brain waves on eight electrodes amplified and digitized in four channels at 250 or 500 Hz. Several algorithms monitor activity, waking up a custom processor when they detect pre-seizure patterns to trigger therapy. The stimulation is very configurable but a typical burst may be 100 ms in duration with a pulse rate of 50 Hz with each pulse having a bi-phasic amplitude of 3 or 4 mA, and a width of 160 microseconds per phase.
NeuroPace takes low power operation to rare levels because the device must measure brain waves continuously for the life of its custom lithium battery. "When fully running it consumes 10 microamps which is the leakage spec of many other devices," Archer said.
Heavy use of clock gating, state machines, and custom signal processors help keep consumption low. The system's processor duty cycle is just under one percent.
Most of the electronics are integrated in two mixed-signal ASICs surrounded by a few medical grade off-the-shelf discretes. The 7 x 10 mm chips are made in ancient third or half micron processes for lowest leakage current. One chip mainly handles signal processing and conversion, the other is primarily digital and uses an obscure, licensed eight-bit RISC processor core with SRAM for storing brain-wave recordings.
Archer estimates he has worked on about five completely different ASICs in his 14 years at NeuroPace and several generations of each one.
Looking ahead, "we are evaluating the business cases and needs in a number of indications," Archer said. "One could imagine an implant could download different code and be a different device with a flexible therapy output that could be applied for any number of indications," he said.
In anticipation of a thumbs up from the FDA, NeuroPace is building up manufacturing and other capabilities, he said.
The NeuroPace RNS implant is expected to get FDA approval soon.