A Fully-Passive Wireless Microsystem for Recording of Neuropotentials using RF Backscattering Methods.

The ability to safely monitor neuropotentials is essential in establishing methods to study the brain. Current research focuses on the wireless telemetry aspect of implantable sensors in order to make these devices ubiquitous and safe. Chronic implants necessitate superior reliability and durability of the integrated electronics. The power consumption of implanted electronics must also be limited to within several milliwatts to microwatts to minimize heat trauma in the human body. In order to address these severe requirements, we developed an entirely passive and wireless microsystem for recording neuropotentials. An external interrogator supplies a fundamental microwave carrier to the microsystem. The microsystem comprises varactors that perform nonlinear mixing of neuropotential and fundamental carrier signals. The varactors generate third-order mixing products that are wirelessly backscattered to the external interrogator where the original neuropotential signals are recovered. Performance of the neuro-recording microsystem was demonstrated by wireless recording of emulated and in vivo neuropotentials. The obtained results were wireless recovery of neuropotentials as low as approximately 500 microvolts peak-to-peak (µV(pp)) with a bandwidth of 10 Hz to 3 kHz (for emulated signals) and with 128 epoch signal averaging of repetitive signals (for in vivo signals).

A miniature fully-passive microwave back-scattering device for short-range telemetry of neural potentials.

This paper describes a fully passive telemetry technique based on microwave backscattering. In this technique, a subharmonically-pumped passive mixer is coupled to a bio-probe and one or two miniature antennas. When interrogated by an RF excitation, this device generates an amplitude modulated RF backscattering component centered at twice the frequency of excitation. An external sensitive receiver can be used to demodulate the backscattering component and recover the bio-potential. A simple prototype based on solid state diodes has been fabricated and tested for 2.4/4.8 GHz and has the dimensions of 11.5x4.6 mm2 and thickness of approximately 1 mm. Experiments with this very simple device show that low-frequency signals (fm<1 kHz) as low as 1 mV can results in double-sideband levels of greater than -126 dBm for an incident RF power of less than 1 mW/cm2. The proposed device is intended to be coated with an insulating bio-compatible coating and serve as a telemetry chip for chronic implantation inside the body.