Influence of electrode impedance changes on the common-mode rejection ratio in bioimpedance measurements

Methods of poly(3,4)-ethylenedioxithiophene (PEDOT) electrodeposition on metal electrodes for neural stimulation and recording

Conductive polymers are of great interest in the field of neural electrodes because of their potential to improve the interfacial properties of electrodes. In particular, the conductive polymer poly (3,4)-ethylenedioxithiophene (PEDOT) has been widely studied for neural applications.Objective:This review compares methods for electrodeposition of PEDOT on metal neural electrodes, and analyses the effects of deposition methods on morphology and electrochemical performance.Approach:Electrochemical performances were analysed against several deposition method choices, including deposition charge density and co-ion, and correlations were explained to morphological and structural arguments as well as characterisation methods choices.Main results:Coating thickness and charge storage capacity are positively correlated with PEDOT electrodeposition charge density. We also show that PEDOT coated electrode impedance at 1 kHz, the only consistently reported impedance quantity, is strongly dependent upon electrode radius across a wide range of studies, because PEDOT coatings reduces the reactance of the complex impedance, conferring a more resistive behaviour to electrodes (at 1 kHz) dominated by the solution resistance and electrode geometry. This review also summarises how PEDOT co-ion choice affects coating structure and morphology and shows that co-ions notably influence the charge injection limit but have a limited influence on charge storage capacity and impedance. Finally we discuss the possible influence of characterisation methods to assess the robustness of comparisons between published results using different methods of characterisation.Significance:This review aims to serve as a common basis for researchers working with PEDOT by showing the effects of deposition methods on electrochemical performance, and aims to set a standard for accurate and uniform reporting of methods.

Wireless, Artefact Aware Impedance Sensor Node for Continuous Bio-Impedance Monitoring

Body bio-impedance is a unique parameter to monitor changes in body composition non-invasively. Continuous measurement of bio-impedance can track changes in body fluid content and cell mass and has widespread applications for physiological monitoring. State-of-the-art implementation of bio-impedance sensor devices is still limited for continuous use, in part, due to artefacts arising at the skin-electrode (SE) interface. Artefacts at the SE interface may arise due to various factors such as motion, applied pressure on the electrode surface, changes in ambient conditions or gradual drying of electrodes. This paper presents a novel bio-impedance sensor node that includes an artefact aware method for bio-impedance measurement. The sensor node enables autonomous and continuous measurement of bio-impedance and SE contact impedance at ten frequencies between 10 kHz to 100 kHz to detect artefacts at the SE interface. Experimental evaluation with SE contact impedance models using passive 2R1C electronic circuits and also with non-invasive in vivo measurements of SE contact impedance demonstrated high accuracy (with maximum error less than 1.5%) and precision of 0.6 Ω. The ability to detect artefacts caused by motion, vertically applied pressure and skin temperature changes was analysed in proof of concept experiments. Low power sensor node design achieved with 50mW in active mode and only 143 μW in sleep mode estimated a battery life of 90 days with a 250 mAh battery and duty-cycling impedance measurements every 60 seconds. Our method for artefact aware bio-impedance sensing is a step towards autonomous and unobtrusive continuous bio-impedance measurement for health monitoring at-home or in clinical environments.

Multifrequency right-side, localized and segmental BIA obtained with different bioimpedance analysers

The aim of this study is to compare two commercial bioimpedance analysers, BioparHom Z-Métrix and Impedimed SFB7, measuring the impedance of three different body segments. The segments measured were 'right-side' (or 'whole-body'), 'segmental right-lower limb' and 'localized longitudinal right-quadriceps'. The comparison was made on electrical models of each segment, including electrode-skin impedance, and in vivo on nine healthy volunteers. Both devices are designed to measure right-side impedances and, in the present study, as the length of the segment investigated decreased, the accuracy of the impedance measured was found to decrease. The accuracy of the devices was calculated via measurements performed on RC networks of known values. It was found that adding electrode-skin contact impedances in the electrical model affected the accuracy by both devices.

An instrumentation amplifier as a front-end for a four-electrode bioimpedance measurement

The performance of a monolithic instrumentation amplifier used as an interface for a four-electrode bioimpedance measurement is examined with a commercially available impedance meter based on an auto-balancing bridge. The errors due to particularities in the input stage of the impedance meter, when used without a front-end, were several orders of magnitude higher than the measured quantity. The analysis was performed on an electrical circuit model of the skin and electrodes over a frequency range of 20 Hz to 1 MHz. The achieved accuracy with balanced electrode impedances for the frequencies up to 100 kHz can be below 0.2% for impedance magnitude and 0.1 degrees for impedance phase, which is within the specified basic accuracy range of the LCR-meter used for the measurements. At frequencies above 100 kHz the errors are increasing and are higher than the LCR-meter's basic accuracy. This study indicates that use of an instrumentation amplifier as a front-end with the particular LCR-meter can significantly improve the measurement accuracy of the four-electrode bioimpedance measurement at low frequencies.

Evolution in impedance at the electrode-skin interface of two types of surface EMG electrodes during long-term recordings

The evolution in impedance at the electrode-skin interface of Beckman and Red Dot electrodes was assessed during long-term recordings. Impedance was measured between each pair of electrodes, arranged in a bipolar configuration on tibialis anterior (n=13). A waveform constructed of sinusoids of known frequencies, evenly distributed on a log scale from 1-16,384 Hz, was applied through the electrodes, and the result recorded by a DAQ system. SEMG signals were recorded at 1000 Hz during isometric dorsiflexion contractions of 30 s, performed every 15 min for 2 h. Impedance data were acquired at 65,536 Hz immediately before and after SEMG recordings. Large individual differences in impedance levels were observed at low frequencies. At high frequencies, impedance values depended only on the electrode type. Impedance decreased steadily with time for Beckman electrodes (p < 0.05), but did not decrease significantly for Red Dot electrodes. The magnitude of the reduction over time varied widely between individuals, and was related to the initial impedance values. The impedance-bandwidth product remained constant for each electrode type (95% confidence intervals 146.2-148.2 and 126.1-127.8 for Beckman and Red Dot electrodes respectively). When skin impedance is electrically modelled with a simple network containing a resistor and a capacitor, the capacitance varies with the properties of the electrode used, whereas resistance is dependent on the subject. Furthermore, the EMG spectrum is unaffected by impedance provided skin preparation is sufficient to reduce the impedance below 55 komega.