Neurograin Project

The Neurograin project targets the development of next generation neurotechnology for very large-scale recording and stimulation of distributed brain areas through a wirelessly networked ensemble of completely implantable microdevices (Figure 1).

We have developed sub-millimeter, hermetically sealed implants (implant volume <0.01 mm³) integrating microchiplets with capabilities for neural recording and electrical microstimulation, wireless power delivery and bidirectional networked telemetry. We envision the deployment of these devices across the brain in numbers ranging up to thousands. Active areas of research and development in this neurotechnology arena are:

1. Low-power, Low-noise custom microelectronic circuit design: We are using advanced CMOS process technologies for development of high-performance, low-noise, ultra-low-power integrated circuit solutions for a completely wireless neural interface implementation Current prototype devices measure 500 µm x 500 µm, and enable 500 Hz ECoG band measurement as well as biphasic current-mode electrical microstimulation up to 25 µA. Additionally, each individually addressable microchiplet integrates RF circuitry for high-efficiency energy harvesting and networked telecommunications (Figure 2).

**Figure 2:** Neurograin microchiplets, designed in 65 nm RF-CMOS. Each device is wirelessly powered through inductive coupling at ~ 1GHz. Chiplets are individually addressable, and communicate using synchronized time-domain multiple access networking (s-TDMA) at 10 Mbps for up to 1000-node network. Broadband ECoG acquisition (500 Hz bandwidth) and programmable current-mode stimulation ( up to 25 µA) have been validated in in-vitro, ex-vivo and in-vivo(ongoing) tests.

2. Wireless Energy Transfer and Telecommunications: We utilize near-field RF inductive coupling for transcutaneous wireless power delivery to the implanted microdevices; our current approach offers an order of magnitude improvement in energy harvesting capabilities over standard approaches. We use a single-channel approach for power delivery and bidirectional communication relying on a combination of carrier modulation (ASK-PWM) and synchronized time-domain multiple access (s-TDMA) RF backscatter and are working to develop an optimized electromagnetic framework for energy delivery and synchronized, bidirectional, networked wireless telecommunications with high-efficiency data links in the 10s of Mbps range.

**Figure 3:** Multi-coil stack enhancing uniform distribution of magnetic flux for improved electromagnetic coupling efficiency across ~4 cm² area ( to power up to 1000 Neurograin chiplets). The chiplets use RF backscatter at ~ 1GHz to transmit BPSK-encoded data at 10 Mbps. Data is recovered by an external RF transceiver with bit-error-rates (BER) < 1e^-5 .The wireless network perates on a synchronized time-domain schedule with a nominal cycle time of 100 ms (accommodating 1000 ECoG chiplets communicating at 10 Mbps).

3. Portable Embedded System solutions for real-time Neurocomputational processing: We are implementing a system architecture enabling real-time coordination and management of the implanted Neurograin devices by a wearable external telemetry hub leveraging Software-Defined-Radio (SDR) and Ultra-High-Performance Field Programmable Gate Array (FPGA) technologies. The latter is also anticipated to relay data to high-performance neural processors in-situ and/or on the cloud for neural decoding/encoding facilitating closed-loop brain-computer interface control.

4. High-throughput hermetic packaging of microdevices: We have developed a stacked thin-film (~ 100 nm) conformal coating approach to hermetically encapsulate Neurograin microdevices. The microchiplets are batch processing using Atomic Layer Deposition (ALD) techniques enabling the production of a biocompatible implants with less than 1% increment in overall device volume between unpackaged and packaged devices ( Figure 4).