Iasonas F. Triantis

On cuff imbalance and tripolar ENG amplifier configurations

Electroneurogram (ENG) recording techniques benefit from the use of tripolar cuffs because they assist in reducing interference from sources outside the cuff. However, in practice the performance of ENG amplifier configurations, such as the quasi-tripole and the true-tripole, has been widely reported to be degraded due to the departure of the tripolar cuff from ideal behavior. This paper establishes the presence of cuff imbalance and investigates its relationship to cuff asymmetry, cuff end-effects and interference source proximity. The paper also presents a comparison of the aforementioned amplifier configurations with a new alternative, termed the adaptive-tripole, developed to automatically compensate for cuff imbalance. The output signal-to-interference ratio of the three amplifier configurations were compared in vivo for two interference signals (stimulus artifact and M-wave) superimposed on compound action potentials. The experiments showed (for the first time) that the two interference signals result in different cuff imbalance values. Nevertheless, even with two distinct cuff imbalances present, the adaptive-tripole performed better than the other two systems in 61.9% of the trials.

Design of an adaptive interference reduction system for nerve-cuff electrode recording

This paper describes the design of an adaptive control system for recording neural signals from tripolar cuff electrodes. The control system is based on an adaptive version of the true-tripole amplifier configuration and was developed to compensate for possible errors in the cuff electrode balance by continuously adjusting the gains of the two differential amplifiers. Thus, in the presence of cuff imbalance, the output signal-to-interference ratio is expected to be significantly increased, in turn reducing the requirement for post-filtering to reasonable levels and resulting in a system which is fully implantable. A realization in 0.8-/spl mu/m CMOS technology is described and simulated and preliminary measured results are presented. Gain control is achieved by means of current-mode feedback and many of the system blocks operate in the current-mode domain. The chip has a core area of 0.4 mm/sup 2/ and dissipates 3 mW from /spl plusmn/ 2.5V power supplies. Measurements indicate that the adaptive control system is expected to be capable of compensating for up to /spl plusmn/5% errors in the tripolar cuff electrode balance.

A Simulation Study of the Combined Thermoelectric Extracellular Stimulation of the Sciatic Nerve of the Xenopus Laevis: The Localized Transient Heat Block

The electrical behavior of the Xenopus laevis nerve fibers was studied when combined electrical (cuff electrodes) and optical (infrared laser, low power sub-5 mW) stimulations are applied. Assuming that the main effect of the laser irradiation on the nerve tissue is the localized temperature increase, this paper analyzes and gives new insights into the function of the combined thermoelectric stimulation on both excitation and blocking of the nerve action potentials (AP). The calculations involve a finite-element model (COMSOL) to represent the electrical properties of the nerve and cuff. Electric-field distribution along the nerve was computed for the given stimulation current profile and imported into a NEURON model, which was built to simulate the electrical behavior of myelinated nerve fiber under extracellular stimulation. The main result of this study of combined thermoelectric stimulation showed that local temperature increase, for the given electric field, can create a transient block of both the generation and propagation of the APs. Some preliminary experimental data in support of this conclusion are also shown.

An adaptive ENG amplifier for tripolar cuff electrodes

Electroneurogram (ENG) recording from tripolar cuff electrodes is affected by interference signals, mostly generated by muscles nearby. Interference reduction may be achieved by suitably designed amplifiers such as the true-tripole and quasi-tripole systems. However, in practice their performance is severely degraded by cuff imbalance, resulting in very low output signal-to-interference ratios. Although some improvement may be offered by post filtering, this considerably increases complexity, size and power dissipation, rendering the approach unsuitable for the development of a high-performance ENG recording system which is fully implantable. This paper describes an integrated, fully implantable, adaptive ENG amplifier developed to automatically compensate for cuff imbalance, and thus significantly improve the quality of the recorded ENG. Measured results show that the adaptive ENG amplifier has a yield of 100%, a cuff imbalance correction range of more than /spl plusmn/40%, and an output signal-to-interference ratio of about 2/1 (6 dB) even for /spl plusmn/40% imbalance. The latter should be compared with an input signal-to-interference ratio of 1/500 (-54 dB). The circuit was fabricated in 0.8-/spl mu/m BiCMOS technology, has a core area of 0.68 mm/sup 2/, and dissipates 7.2 mW from /spl plusmn/2.5 V power supplies. The adaptive ENG amplifier advances the state-of-the-art in implantable tripolar nerve cuff electrode recording techniques.

An Integrated Analog Readout for Multi-Frequency Bioimpedance Measurements

Bioimpedance spectroscopy is used in a wide range of biomedical applications. This paper presents an integrated analog readout, which employs synchronous detection to perform galvanostatic multi-channel, multi-frequency bioimpedance measurements. The circuit was fabricated in a 0.35-μm CMOS technology and occupies an area of 1.52 mm2. The effect of random dc offsets is investigated, along with the use of chopping to minimize them. Impedance measurements of a known RC load and skin (using commercially available electrodes) demonstrate the operation of the system over a frequency range up to 1 MHz. The circuit operates from a ±2.5 V power supply and has a power consumption of 3.4-mW per channel.

High-Power CMOS Current Driver With Accurate Transconductance for Electrical Impedance Tomography

Current drivers are fundamental circuits in bioimpedance measurements including electrical impedance tomography (EIT). In the case of EIT, the current driver is required to have a large output impedance to guarantee high current accuracy over a wide range of load impedance values. This paper presents an integrated current driver which meets these requirements and is capable of delivering large sinusoidal currents to the load. The current driver employs a differential architecture and negative feedback, the latter allowing the output current to be accurately set by the ratio of the input voltage to a resistor value. The circuit was fabricated in a 0.6-μm high-voltage CMOS process technology and its core occupies a silicon area of 0.64 mm2. It operates from a ± 9 V power supply and can deliver output currents up to 5 mA p-p. The accuracy of the maximum output current is within 0.41% up to 500 kHz, reducing to 0.47% at 1 MHz with a total harmonic distortion of 0.69%. The output impedance is 665 kΩ at 100 kHz and 372 kΩ at 500 kHz.

An improved, very long time-constant CMOS integrator for use in implantable neuroprosthetic devices

This paper describes a CMOS integrator for use with an adaptive neural amplifier. The device can also be used standalone. It utilizes transconductance cancellation to achieve a large time-constant, which is tunable between 1-17 s. The design presented here features reduced complexity compared to previous published work for that application. Measured results demonstrate integration with no output drift, tunable time-constant and low total harmonic distortion. The device can also be operated as a low-pass filter.

A novel front-end for impedance spectroscopy

A novel method for calculating unknown impedances is presented. In contrast to synchronous detection (SD), which provides the real and imaginary components, the magnitude and phase of the impedance are calculated. Non-accurate generation of the required in-phase and quadrature signals required in SD and mismatches between the channels generate large errors. The proposed method does not require such signals. The measured potential across the sensor is rectified and then low-pass filtered to obtain the magnitude. The phase is calculated by two comparators followed by an XOR gate. A prototype of the system was built using discrete components and proof of concept measurements were carried out using resistors and capacitors of known values.

A high output impedance CMOS current driver for bioimpedance measurements

Advances in bioimpedance measurement applications need current drivers with high output impedance in the frequencies ranging from DC to a number of MHz. In particular, it needs greater than 10 MΩ output impedance at low frequencies, e.g., 10 kHz. This paper presents an current driver based on operational transconductance amplifiers (OTAs) using a feedback loop to provide over 10 MΩ output impedance ranging from DC to 10 kHz and 1 MΩ at 1 MHz. In addition, a stable performance over a wide frequency range (<4 MHz) is simulated. The circuit has been designed in a 0.6-μm CMOS process using ±2.5 V power supplies. Simulated results are presented.

Towards a reconfigurable sense-and-stimulate neural interface generating biphasic interleaved stimulus

This work presents an eight channel neural stimulator interface based on an address event communication protocol. The system developed decodes, integrates and interleaves the spike patterns using a continuous interleave sampling (CIS) strategy, which drives a complex biphasic current waveform to a bipolar electrode configuration. This paper describes the basic concept, circuit design, simulation results and hardware implementation in a standard 0.35mum CMOS technology