Passive Impedance Matching for Implanted Brain–Electrode Interfaces

Abstract

We propose a new technique for matching the high impedance of sub-cranial electrodes to wireless brain implants that is passive, highly tolerant of the dc offset voltage caused by the electrochemical reaction in the recording electrode, and complemented by an improved external interrogator design that exhibits reduced phase noise. As compared to previous wireless and batteryless brain implants, the proposed approach offers a remarkable improvement in sensitivity by 25 times. The proposed system consists of an external interrogator and a neuro-recorder implanted under the scalp. For operation, the interrogator sends a 2.4-GHz carrier signal to turn on the implant. This carrier self-biases a PNP Bipolar Junction Transistor (BJT) that enables matching to the recording electrode at frequency $f_{neuro}$ in a batteryless manner. Concurrently, the recorded neuropotentials (at frequency $f_{neuro}$ ) pass through a Schottky diode that allows them to mix with the carrier and generate a 4.8 $\text{GHz}\pm f_{neuro}$ modulated signal. The latter is then transmitted back to the interrogator for demodulation. To verify the implant's operation, in-vitro measurements are presented. Measurement results demonstrate that emulated neuropotentials as low as 200 $\mu V_{pp}$ can be detected at a 33- $\text{k}\Omega$ electrode impedance. As such, the proposed system presents a game-changing capability for a wide range of applications.