4.7 Article

An Energy-Efficient Wirelessly Powered Millimeter-Scale Neurostimulator Implant Based on Systematic Codesign of an Inductive Loop Antenna and a Custom Rectifier

Journal

IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS
Volume 12, Issue 5, Pages 1131-1143

Publisher

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
DOI: 10.1109/TBCAS.2018.2852680

Keywords

Biomedical implant; energy harvesting; inductive coupling; neurostimulation; rectifier; switched-capacitor based stimulation; wireless power transfer

Funding

  1. National Science Foundation
  2. Division Of Behavioral and Cognitive Sci
  3. Direct For Social, Behav & Economic Scie [1533688] Funding Source: National Science Foundation

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In this work, a switched-capacitor-based stimulator circuit that enables efficient energy harvesting for neurostimulation applications is presented, followed by the discussion on the optimization of the inductive coupling front-end through a codesign approach. The stimulator salvages input energy and stores it in a storage capacitor, and, when the voltage reaches a threshold, releases the energy as an output stimulus. The dynamics of the circuit are automatically enabled by a positive feedback, eliminating any stimulation control circuit blocks. The IC is fabricated in a 180 nm CMOS process and achieves a quiescent current consumption of 1.8 mu A. The inductive coupling front-end is optimized as a loaded resonator, in which the input impedance of the custom rectifier directly loads the inductive loop antenna. The loaded quality factor and the rectifier's efficiency determine the reception sensitivity of the stimulator, while a balance should he achieved for the robustness of the system against dielectric medium variations by increasing the reception bandwidth. The finalized stimulator adopts a 4.9 mm x 4.9 mm inductive loop antenna and achieves an overall assembly dimension of 5 mm x 7.5 mm. Operating at the resonant frequency of 198 MHz, the stimulator works at a 14 cm distance from the transmitter in the air. An animal experiment was performed, in which a fully implanted stimulator excited the sciatic nerve of a rat that consequently triggered the movement of the limb.

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