Helen N. Schwerdt

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 neurorecording 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 (μVpp) 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).

Analysis of Electromagnetic Fields Induced in Operation of a Wireless Fully Passive Backscattering Neurorecording Microsystem in Emulated Human Head Tissue

This paper reports on a fully passive microsystem that wirelessly records and transmits neuropotentials exclusively by means of electromagnetic backscattering techniques, affording substantially simpler circuitry and potentially safer and more reliable approach for implantable wireless neurorecording. A fundamental practical barrier for wireless brain-implantable microsystems includes heat dissipation by on-chip circuitry, which may cause permanent brain damage. Hence, measurement of thermal profiles of surrounding tissue induced by operation of wireless implants is imperative in assessing the safety of these devices. Evaluation of specific absorption rate (SAR) is especially relevant for wireless electromagnetic transmission schemes operating at microwave frequencies and directly relates to the heat generated within biological tissue media. In this study, computational and empirical methods are used to measure SAR within a human-head-equivalent phantom during operation of the embedded fully passive wireless neurorecording microsystem. The maximum average SAR, coinciding with the worst case scenario, measured within 1 g of brain tissue is

Wireless multichannel acquisition of neuropotentials

<\hbox{0.45}\pm \hbox{0.11 W/kg}

Wireless performance of a fully passive neurorecording microsystem embedded in dispersive human head phantom

, complying with the U.S. FCC threshold (1.6 W/kg).

A Fully Passive Wireless Backscattering Neurorecording Microsystem Embedded in Dispersive Human-Head Phantom Medium

Implantable brain-machine interfaces for disease diagnosis and motor prostheses control require low-power acquisition of neuropotentials spanning a wide range of amplitudes and frequencies. Here, we present a 16-channel VLSI neuropotential acquisition system with tunable gain and bandwidth, and variable rate digital transmission over an inductive link which further supplies power. The neuropotential interface chip is composed of an amplifier, incremental ADC and bit-serial readout circuitry. The front-end amplifier has a midband gain of 40 dB and offers NEF of less than 3 for all bandwidth settings. It also features adjustable low-frequency cut-off from 0.2 to 94 Hz, and independent high-frequency cut-off from 140 Hz to 8.2 kHz. The Gm-C incremental DeltaSigma ADC offers digital gain up to 4096 and 8-12 bits resolution. The interface circuit is powered by a telemetry chip which harvests power through inductive coupling from a 4 MHz link, provides a 1 MHz clock for ADC operation and transmits the bit-serial data of the neurpotential interface across 4 cm at up to 32 kbps with a BER less than 10-5. Experimental EEG recordings using the neuropotential interface and wireless module are presented.

A color detection glove with haptic feedback for the visually disabled

This paper reports the wireless performance of a biocompatible fully passive microsystem implanted in phantom media simulating the dispersive dielectric properties of the human head, for potential application in recording cortical neuropotentials. Fully passive wireless operation is achieved by means of backscattering electromagnetic (EM) waves carrying 3rd order harmonic mixing products (2f0±fm=4.4-4.9 GHz) containing targeted neuropotential signals (fm≈1-1000 Hz). The microsystem is enclosed in 4 μm thick parylene-C for biocompatibility and has a footprint of 4 mm × 12 mm × 500 μm. Preliminary testing of the microsystem implanted in the lossy biological simulating media results in signal-to-noise ratios (SNR) near 22 (SNR≈38 in free space) for millivolt level neuropotentials, demonstrating the potential for fully passive wireless microsystems in implantable medical applications.

Passive, on chip and in situ detection of neuropotentials

This letter reports a microfabricated fully passive circuit for extracting and transmitting targeted neuropotentials wirelessly via the backscattering effect without any internal power source or harvester. Radiating electromagnetic waves experience attenuation, phase and wavelength alteration, and random scattering effects when propagating through dispersive biological media (i.e., human head), and these effects are augmented at microwave frequencies required for practical miniaturization of the integrated microsystem antenna. The authors examine the fully passive microsystem for wireless recording of emulated neuropotentials as implanted in a phantom mimicking the human head. The wireless measurements of emulated neuropotentials acquired by the microsystem demonstrate its promising capabilities for neurological applications.

Miniaturized Passive Hydrogel Check Valve for Hydrocephalus Treatment

The following paper describes the design and preliminary results of a compact color detecting and feedback system. The device is intended for use by those with vision-loss or disabilities who may benefit by a means of perceiving color. The device consists of a glove incorporating optical color sensors along with tactile switches affixed to the fingertips, a haptic feedback interface wrapped around the fore arm, and a microprocessor unit to control communication between sensing and feedback. The color data and finger selection is encoded to spatial and temporal parameters on tactors. The study extends on earlier investigations by Tapson et. al. that have successfully demonstrated the capability of accurately identifying colors through haptic feedback. To further extend the field, the present research attempts to characterize color information in a temporally and spatially continuous representation to more realistically map the features of color space, and allow higher resolution of color perception.

Preliminary thermal characterization of a fully-passive wireless backscattering neuro-recording microsystem

A fully-passive, on chip wireless system is proposed for in situ detection of neuropotentials. The system aims to replace conventional wired brain-sensing technologies, thus enhancing patient mobility and preserving safety. It consists of an implanted neurosensor that comprises an implanted antenna and mixer, and an exterior RF interrogator. Link budget issues are discussed, highlighting the challenges of detecting human brain neuropotentials as small as 10s of μVpp.

Hydrogel check valve with non-zero cracking pressure for use as a potential alternative hydrocephalus treatment method

Improvements in cerebrospinal fluid (CSF) draining techniques for treatment of hydrocephalus are urgently sought after to substitute for current CSF shunts that are plagued by high failure rates. The passive check valve aims to restore near natural CSF draining operations while mitigating possible failure mechanisms caused by finite leakage or low resilience that frequently constrain practical implementation of miniaturized valves. A simple hydrogel diaphragm structures core passive valve operations and enforce valve sealing properties to substantially lower reverse flow leakage. Experimental measurements demonstrate realization of targeted cracking pressures (PT ≈ 20-110 mmH2O) and operation at -800 <; ΔP <; 600 mmH2O without observable degradation or leakage.