[Paper Review] StimDust: A mm-scale implantable wireless precision neural stimulator with ultrasonic power and communication

David K. Piech, Benjamin C. Johnson+8 more|arXiv (Cornell University)|Jul 19, 2018

Neuroscience and Neural Engineering92 references4 citations

TL;DR

StimDust presents a 1.7 mm³, batteryless, wireless neural stimulator powered and communicated with ultrasound, enabling precise, implantable neuromodulation. It achieves real-time, low-latency stimulation control via ultrasonic backscatter communication and operates safely below FDA diagnostic ultrasound limits, demonstrating reliable in vivo performance in rat sciatic nerve stimulation.

ABSTRACT

Neural stimulation is a powerful technique for modulating physiological functions and for writing information into the nervous system as part of brain-machine interfaces. Current clinically approved neural stimulators require batteries and are many cubic centimetres in size -- typically much larger than their intended targets. We present a complete wireless neural stimulation system consisting of a 1.7 mm3 wireless, batteryless, leadless implantable stimulator (the "mote"), an ultrasonic wireless link for power and bi-directional communication, and a hand-held external transceiver. The mote consists of a piezoceramic transducer, an energy storage capacitor, and a stimulator integrated circuit (IC). The IC harvests ultrasonic power with high efficiency, decodes stimulation parameter downlink data, and generates current-controlled stimulation pulses. Stimulation parameters are time-encoded on the fly through the wireless link rather than being programmed and stored on the mote, reducing power consumption and on-chip memory requirements and enabling complex stimulation protocols with high-temporal resolution and low-latency feedback for use in closed-loop stimulation. Uplink data indicates whether the mote is currently stimulating; it is encoded by the mote via backscatter modulation and is demodulated at the external transceiver. We show that the mote operates at an acoustic intensity that is 7.8% of the FDA limit for diagnostic ultrasound and characterize the acoustic wireless link's robustness to expected real-world misalignment. We demonstrate the in vivo performance of the system with motes acutely implanted with a cuff on the sciatic nerve of anesthetized rats and show highly repeatable stimulation across a wide range of physiological responses.

Motivation & Objective

• To develop a minimally invasive, implantable neural stimulator that eliminates the need for onboard batteries and leads.
• To enable high-precision, closed-loop neural stimulation with low-latency feedback using ultrasonic power and communication.
• To reduce on-chip memory and power consumption by time-encoding stimulation parameters via wireless downlink instead of on-chip storage.
• To ensure safe operation under clinical ultrasound exposure limits while maintaining robust wireless link performance despite misalignment.
• To validate the system's reliability and repeatability in vivo using acute implantation in anesthetized rats.

Proposed method

• The stimulator, or 'mote', integrates a piezoceramic transducer for ultrasonic energy harvesting and bidirectional communication.
• An on-chip integrated circuit (IC) decodes downlink data for stimulation parameters and generates current-controlled pulses.
• Stimulation parameters are encoded in time-domain modulation on the ultrasonic carrier, minimizing memory and power use.
• Uplink feedback indicating active stimulation is transmitted via backscatter modulation of the ultrasonic signal.
• The system uses a hand-held external transceiver to transmit power and receive status feedback through the same ultrasonic link.
• The mote's energy is stored in a capacitor, enabling pulsed stimulation without continuous power draw.

Experimental results

Research questions

• RQ1 Can a mm-scale, batteryless neural stimulator be powered and controlled wirelessly using ultrasound?
• RQ2 Can ultrasonic communication support real-time, low-latency downlink of complex stimulation parameters without on-chip storage?
• RQ3 Can the system achieve reliable uplink feedback via backscatter modulation under realistic misalignment conditions?
• RQ4 Is the acoustic intensity used within safe limits for diagnostic ultrasound as defined by the FDA?
• RQ5 Can the system deliver highly repeatable and controllable neural stimulation in vivo?

Key findings

• The StimDust mote operates at an acoustic intensity of 7.8% of the FDA limit for diagnostic ultrasound, ensuring safety.
• The ultrasonic wireless link demonstrated robust performance under realistic misalignment conditions, maintaining reliable communication.
• In vivo experiments in anesthetized rats showed highly repeatable physiological responses across multiple stimulation trials.
• The system successfully delivered current-controlled stimulation pulses with low-latency feedback via backscatter uplink.
• The absence of onboard batteries and leads enables a 1.7 mm³ implantable form factor, significantly smaller than current clinical devices.
• Time-encoding of stimulation parameters reduced on-chip memory and power consumption, enabling complex, high-temporal-resolution protocols.