A Sub-millimeter, Inductively Powered Neural Stimulator

Abstract

Wireless neural stimulators are being developed to address problems associated with traditional lead-based implants. However, designing wireless stimulators on the sub-millimeter scale (<1 mm3) is challenging. As device size shrinks, it becomes difficult to deliver sufficient wireless power to operate the device. Here, we present a sub-millimeter, inductively powered neural stimulator consisting only of a coil to receive power, a capacitor to tune the resonant frequency of the receiver, and a diode to rectify the radio-frequency signal to produce neural excitation. By replacing any complex receiver circuitry with a simple rectifier, we have reduced the required voltage levels that are needed to operate the device from 0.5 to 1 V (e.g., for CMOS) to ~0.25–0.5 V. This reduced voltage allows the use of smaller receive antennas for power, resulting in a device volume of 0.3–0.5 mm3. The device was encapsulated in epoxy, and successfully passed accelerated lifetime tests in 80°C saline for 2 weeks. We demonstrate a basic proof-of-concept using stimulation with tens of microamps of current delivered to the sciatic nerve in rat to produce a motor response.

Magnetic Strategies for Nervous System Control

“…It has approximate dimensions of 2×0.5×0.5 mm, and operates by rectifying induced voltages at a predetermined resonance frequency (e.g. 10 MHz) to produce a DC electric field capable of depolarizing adjacent neurons (Freeman et al 2017). Like TMS, this device relies on external application of magnetic fields, but in this case the field is used solely as a source of wireless power for an electrical device.…”

Magnetoelectric materials for miniature, wireless neural stimulation at therapeutic frequencies

“…1, Methods). In experiments using ME films and Pt electrodes we found that high-frequency biphasic stimulation at the ME resonance frequency (typically 20-150 kHz) was not effective to stimulate APs in cultured HEKs, as predicted by the low-pass filtering properties of the cell membrane (Freeman et al 2017). To create an effective monophasic stimulus waveform, we used a Schottky diode to rectify the voltage to create entirely positive or negative voltage waveforms depending on the diode direction.…”

“…Thus, when the receiver coils are miniaturized, the output power reduces and becomes more sensitive to perturbations in the distance or angle between the transmitter and receiver (Fotopoulou & Flynn 2011). For example, Freeman et al demonstrated that small inductive coils less than 1 mm in diameter can power stimulators for the sciatic nerve in anesthetized rats (Freeman et al 2017); however, the stimulation frequencies have been limited to less than 50 Hz and stable performance has been difficult to achieve in freely moving animals due to the reduced power coupling efficiency that accompanies changes in the angle and distance between the receiver and transmitter during behavior (Maeng et al 2019). Near-field inductive coupling has also been used to power optogenetic stimulators in freely moving mice (Shin et al 2017).…”

A Fully Integrated RF-Powered Energy-Replenishing Current-Controlled Stimulator

“…Conventional constant-current stimulators can be implemented in a small area using integrated circuit technology, yet dissipate substantial power across the current source itself when powered by a DC supply voltage. Recently developed millimeter-sized stimulation systems [25], [33], [34] were implemented with off-the-shelf discrete components so that their power efficiencies were compromised with simplicity in implementation. On the contrary, adiabatic stimulators, which slowly ramp the supply voltage up and down to minimize the voltage drop across the current source and recycle charge from electrode and tissue capacitance as described in Section II-C, can be more energy efficient, yet typically require large off-chip passives to synthesize the adiabatic voltage waveforms [28], [35].…”

“…This is typically accomplished in wirelessly-powered systems via a two-step rectification/boosting (and/or regulation) process that introduces cascaded losses and further degrades efficiency. As nextgeneration neural stimulation devices continue to shrink in size [25], [33], [34], [36], in some cases via full on-chip integration of all necessary neuroinstrumentation functionality [37], [38], inclusion of energy-efficient adiabatic stimulation functionality is necessary in a small, fully-integrated form factor. This paper presents an adiabatic current-controlled stimulator architecture, depicted in Fig.…”