Electrode-skin Interface Research Articles

Time Evolution of the Skin-Electrode Interface Impedance under Different Skin Treatments.

A low and stable impedance at the skin–electrode interface is key to high-fidelity acquisition of biosignals, both acutely and in the long term. However, recording quality is highly variable due to the complex nature of human skin. Here, we present an experimental and modeling framework to investigate the interfacial impedance behavior, and describe how skin interventions affect its stability over time. To illustrate this approach, we report experimental measurements on the skin–electrode impedance using pre-gelled, clinical-grade electrodes in healthy human subjects recorded over 24 h following four skin treatments: (i) mechanical abrasion, (ii) chemical exfoliation, (iii) microporation, and (iv) no treatment. In the immediate post-treatment period, mechanical abrasion yields the lowest initial impedance, whereas the other treatments provide modest improvement compared to untreated skin. After 24 h, however, the impedance becomes more uniform across all groups (<20 kΩ at 10 Hz). The impedance data are fitted with an equivalent circuit model of the complete skin–electrode interface, clearly identifying skin-level versus electrode-level contributions to the overall impedance. Using this model, we systematically investigate how time and treatment affect the impedance response, and show that removal of the superficial epidermal layers is essential to achieving a low, long-term stable interface impedance.

Towards Improving the Quality of Electrophysiological Signal Recordings by Using Microneedle Electrode Arrays.

Electrophysiological signals are recorded generally by metal electrodes placed on body surface. For long term use, the signal quality may decay with the change of interface impedance between electrodes and skin due to the conductive hydrogel dehydration. Besides, electrodes may shift during body movements, which causes unstable signal recordings. To improve the quality of electrophysiological signal recordings on human body surface, a type of microneedle electrode array (MEA) with microneedles around 550 μm in length was fabricated with a magnetization-induce self-assembly method. Compared with the commonly used dry electrode array, the MEA has lower and more stable interface impedance, especially when the electrode-skin interface is under unstable pressures. For electrophysiological signal recording, the MEA can acquire electromyography (EMG) with significantly lower noise energy, higher signal-to-noise ratio, and higher motion-classification accuracy based on the EMG pattern-recognition method. Additionally, high quality electrocardiography (ECG) can be recorded by using the MEA, where more accurate R-peaks are extracted in different scenarios. Besides, there was no report about any discomfort like bleeding or inflammation by all the subjects. This research proved that the microneedles on the MEA can penetrate through the corneum and reach the epidermis of the subjects, which could avoid the influence of corneum and fix the electrode on the body surface for high-quality signal recording especially during body movements. Furthermore, the microneedles would not touch the dermis, enabling a painless signal acquisition. This work is beneficial to the applications of wearable human-machine interface technology.

A Novel Wearable Flexible Dry Electrode Based on Cowhide for ECG Measurement.

The electrocardiogram (ECG) electrode, as a sensor, is an important part of the wearable ECG monitoring device. Natural leather is rarely used as the electrode substrate. In this paper, wearable flexible silver electrodes based on cowhide were prepared by sputtering and brush-painting. A signal generator, oscilloscope, impedance test instrument, and ECG monitor were used to build the test platform evaluating the performance of electrodes with six subjects. The lossless waveform transmission can be achieved with our electrodes. Therefore, the Pearson’s correlation coefficient calculated with input waveform and output waveform of the electrodes based on the top grain layer (GLE) and the split layer (SLE) of cowhide were 0.997 and 0.998 at 0.1 Hz respectively. The skin electrode impedance (Z) was tested, and the parameters of the equivalent circuit model of the skin electrode interface were calculated by a fitting method, indicating that the Z of the prepared electrodes was comparable with the standard gel electrode when the skin is moist enough. The signal-to-noise ratio of the ECG of the GLE and the SLE were 1.148 and 1.205 times that of the standard electrode in the standing posture, which meant the ECG measured by our electrodes was basically consistent with that measured by the standard electrode.

Design and Characterization of a Textile Electrode System for the Detection of High-Density sEMG.

Muscle activity monitoring in dynamic conditions is a crucial need in different scenarios, ranging from sport to rehabilitation science and applied physiology. The acquisition of surface electromyographic (sEMG) signals by means of grids of electrodes (High-Density sEMG, HD-sEMG) allows obtaining relevant information on muscle function and recruitment strategies. During dynamic conditions, this possibility demands both a wearable and miniaturized acquisition system and a system of electrodes easy to wear, assuring a stable electrode-skin interface. While recent advancements have been made on the former issue, detection systems specifically designed for dynamic conditions are at best incipient. The aim of this work is to design, characterize, and test a wearable, HD-sEMG detection system based on textile technology. A 32-electrodes, 15 mm inter-electrode distance textile grid was designed and prototyped. The electrical properties of the material constituting the detection system and of the electrode-skin interface were characterized. The quality of sEMG signals was assessed in both static and dynamic contractions. The performance of the textile detection system was comparable to that of conventional systems in terms of stability of the traces, properties of the electrode-skin interface and quality of the collected sEMG signals during quasi-isometric and highly dynamic tasks.

An In-Laboratory Comparison of FocusBand EEG Device and Textile Electrodes Against a Medical-Grade System and Wet Gel Electrodes

In this study, FocusBand’s textile electrodes were tested against medical-grade wet electrodes to investigate the possible differences in skin-electrode interface behavior and EEG signals’ quality. In vivo electrical impedance spectroscopy and simultaneous forehead EEG measurements were performed with ten healthy subjects. In addition, the FocusBand device was tested in a stand-alone manner against a medical-grade reference system using similar test measurements. Compared to wet electrodes, textile electrodes had higher median absolute skin-electrode impedances five minutes after the attachment, but this difference decreased to 50–55 % of the initial during the first 60 minutes on all measured frequencies (1–1000 Hz). Textile and wet electrodes produced highly consistent EEG signals at forehead Fp1 and Fp2 locations during a resting stage and focusing measurements. From those, Fp1 signals were more consistent in terms of normalized cross-correlations and agreement of the relative spectral powers. A stand-alone comparison showed that the FocusBand device can be used to record forehead biopotential signals, but the quality was not as consistent as with a medical device. Based on the results of impedance characteristics, a recording made by FocusBand’s textile electrodes may be more susceptible to artifacts than that made by the medical-grade wet electrodes. However, FocusBand’s textile electrodes can be used, after a short stabilization period, to reliably record forehead EEG signals with a quality almost equal to that of medical-grade wet electrodes.

Body-Area Powering With Human Body-Coupled Power Transmission and Energy Harvesting ICs.

This paper presents the body-coupled power transmission and ambient energy harvesting ICs. The ICs utilize human body-coupling to deliver power to the entire body, and at the same time, harvest energy from ambient EM waves coupled through the body. The ICs improve the recovered power level by adapting to the varying skin-electrode interface parasitic impedance at both the TX and RX. To maximize the power output from the TX, the dynamic impedance matching is performed amidst environment-induced variations. At the RX, the Detuned Impedance Booster (DIB) and the Bulk Adaptation Rectifier (BAR) are proposed to improve the power recovery and extend the power coverage further. In order to ensure the maximum power extraction despite the loading variations, the Dual-Mode Buck-Boost Converter (DM-BBC) is proposed. The ICs fabricated in 40 nm 1P8M CMOS recover up to 100 μW from the body-coupled power transmission and 2.5 μW from the ambient body-coupled energy harvesting. The ICs achieve the full-body area power delivery, with the power harvested from the ambiance via the body-coupling mechanism independent of placements on the body. Both approaches show power sustainability for wearable electronics all around the human body.

Fabrication of Bendable Microneedle-Array Electrode by Magnetorheological Drawing Lithography for Electroencephalogram Recording

Continuous electroencephalogram (EEG) recording is vital for brain science research, monitoring pathological and physiological conditions, and the diagnosis of brain disease. Conventional wet electrodes require skin preparation and a gel electrolyte to reduce electrode–skin interface impedance (EII), which limits its further application in long-term EEG signal recording. Microneedle-array electrodes (MAEs) have been employed in EEG recording, as they can be used directly without skin preparation or the gel electrolyte. However, most microneedles used in MAE are rigid and have a high aspect ratio; thus, they can break easily. Hence, we developed a novel bendable MAE for EEG recording. It is made of a flexible polydimethylsiloxane (PDMS) pillar array with rigid microneedle tips. The PDMS pillar array could help the MAE pass through hair and enable the microneedle tips to penetrate a stratum corneum layer into a scalp, thus reducing the EII. Moreover, the PDMS pillar array is bendable, which prevents the microneedles tips from breaking. The MAE can record EEG signals well in a BIOPAC system and an Emotiv EPOC+ device. The bendable MAE may be an alternative electrode for long-term EEG recording.

Simulation and verification electrical properties of liquid metal flexible bioelectrodes

Flexible electronics have grown rapidly due to their potential for portable physiological monitoring and treatment in recent years, such traditional flexible electronic devices with flexible features are usually fabricated on a very thin substrate, which do not directly contact the skin. This may generate large coupling impedance with the skin and a high signal noise for a sensitive detection of weak physiological signals. In this article, we presented a new conceptual flexible bioelectrode based on liquid metal and built an electrical model to illustrate the electrode–skin interface and tested the model on electrode–skin conductivity. It shows that the liquid metal flexible electrode closely attached to the curved surface can bring uniform potential and electric field distribution under skin, and the interface has the smallest impedance, The present work revealed the potential of such flexible bioelectrode in many aspects of biomedical use, such as electronic skin, sensitive electrocardiogram monitoring, acute ventricular defibrillation and so on.

Electrode–skin impedance imbalance measured in the frequency domain

Objective:To measure the magnitude and phase imbalance of the electrode–skin interface for silver/silver chloride (Ag/AgCl) and silver (Ag) electrode pairs, highlighting the need to balance both the magnitude and phase of the electrode–skin interface. Methods:The electrode–skin impedance imbalance between two electrodes placed on ten healthy subjects was recorded over a frequency range of 1Hz–100kHz, using three electrode configurations: Ag/AgCl electrodes without skin preparation (Ag/AgClNSP), Ag/AgCl electrodes with skin preparation (Ag/AgClSP), and Ag electrodes with skin preparation (Ag-SP). A compensation network was developed to simulate impedance imbalance reduction using the experimental data. Results:The mean electrode–skin impedance imbalance at 50Hz was (37.6±47.1)kΩ and (15.0±18.3) degrees using the Ag/AgClNSP electrode configuration; (4.52±7.65)kΩ and (4.6±6.9) degrees using the Ag/AgClSP electrode configuration; and (36.2±45.1)kΩ and (3.4±3.6) degrees using the Ag-SP electrode configuration. The compensation network resulted in a mean reduction in impedance imbalance over the bioelectrical signal range (1Hz–500Hz) of 284.3kΩ and 11.9degrees; 4.6kΩ and 6.2degrees; 86.7kΩ and 2.5degrees for the Ag/AgClNSP, Ag/AgClSP and Ag-SP electrode configurations respectively. Conclusion:This study confirmed that the electrode–skin impedance imbalance can be large, and varies between subjects. Although abrasive skin preparation reduces the electrode–skin impedance imbalance, it does not guarantee a balanced electrode–skin interface, therefore, balancing the electrode–skin impedance using a compensation network has the potential to decrease bioelectrical signal interference. Significance:As bioelectrical signal interference is a function of electrode–skin impedance imbalance, to improve noise immunity of bioelectrical signal recordings, added compensatory impedance is required to balance the electrode–skin interface.

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 10kHz to 100kHz 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 143μW in sleep mode estimated a battery life of 90 days with a 250mAh 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.

Invasive electrical impedance myography at different levels of contraction of gastrocnemius muscle of rat.

Electrical impedance myography (EIM) is as an experimental technique that associates muscle impedance with muscular activity. Changes in muscle impedance during contraction occur mainly due to changes in the morphological and physiological characteristics of muscles that lead to different impeditivities in comparison with the resting condition. There is no consensus on the details of muscular impedance during muscle activity. EIM measurements on humans are also influenced by factors such as the electrode-skin interface, layers of skin and fat, and the connective tissue that can generate undesirable effects in the impedance signal. These effects can be avoided if EIM measurements are carried out directly on the muscle by using the models of animals. This study investigates changes in the EIM signal in the gastrocnemius muscles of Wistar rats during different levels of muscular contraction. In vivo experiments were conducted on 19 male rats. The muscle was exposed, fixed on a load cell, and electrically stimulated to evoke different levels of muscle contraction. Signals of the components of impedance were analyzed against the muscular force signal. The results show moderate correlations (p < 0.05) among the impedance-related parameters of resistance (r = -0.76), reactance (r = 0.57), and phase (r = 0.53). In addition to providing an experimental protocol for the invasive collection of data on electrical impedance to minimize problems associated with surface electrodes, this study shows that of the components of impedance, resistance is most affected by the intensity of muscular contractions and that morphological changes influence impedance mainly at low intensities.

A silver/silver chloride woven electrode with convex based on electrical impedance tomography

Advanced wearable medical devices are widely used for health monitoring, i.e. recording and analyzing signals from human organs. However, the signal quality is influenced significantly by medical electrodes, which became the bottleneck for health monitoring due to unstable impedance under long-term daily usage. Here, a novel monitoring electrode was designed with uniform micro-convex shape to increase the impedance reliability of electrode-skin interface in long-term monitoring and under dynamic conditions, and we set up the electrode-skin interface model to describe the electrode-skin interface and impedance distribution clearly. Furthermore, we investigated how the electrodeposition factors affect the electrode deposition uniformity and electrode resistance. In this study, we compared the monitoring performance among two sets of electrodes: 1) Ag electrodes and Ag/AgCl electrodes, 2) plain Ag/AgCl electrodes, jacquard Ag/AgCl electrodes, and commercial wet electrodes. Specifically, the monitoring performance was quantified by the electrode-skin interface impedance during long-term monitoring, under different dynamic conditions. The optimal electrodeposition process was a ‘small step’ multi-potential method with the 1-1.5-2 V mode. The jacquard Ag/AgCl electrodes presented the best monitoring performance under all three measurement scenarios across four types of electrodes. The results indicated that the proposed electrodes could be used for long-term medical monitoring under daily life activities based on electrical impedance tomography.

Electrode Placement Strategies for the Measurement of Radial Artery Bioimpedance: Simulations and Experiments

Continuous cardiac output is a significant matter that must be considered in monitoring the functioning of cardiovascular system. A noninvasive sensing system that is based on measuring electrical bioimpedance variations of radial artery is proposed to evaluate the physical condition of a patient. To optimize the signal acquisition, different electrode placement strategies are compared with the goal of finding the most suitable ones. The finite-element method (FEM) simulation of sensitivity distribution and experimental measurements on radial artery are performed for experimenting circular and distal electrode placements. Importantly, also the modified electrode placement strategies, guided by the idea of focused impedance method (FIM), are investigated. To reduce the number of electrodes in measurement setup, its novel version is proposed: a five-electrode FIM strategy. The efficiency of the proposed strategy can be seen in the reduced amount of required number of electrodes: the smaller area of electrode-skin contact interface results in smaller influence of uncertainties. Moreover, the reduced number of electrodes contributes to benefit through developing simpler instrumentation having lower energy consumption and cheaper manufacturing costs. The results show the advances of the proposed novel five-electrode FIM strategy for measuring the pulsating volume of blood in radial artery with the goal of determining the central aortic pressure (CAP) of blood.

Optimization of Reduced GO-Based Cotton Electrodes for Wearable Electrocardiography

The quality of Electrocardiography (ECG) signal is dependent on the electrode's performance. Comfort and long-term monitoring are the main benefits of a dry and flexible electrode compared to conventional silver/silver chloride (Ag/AgCl) electrode. The main objective of this study is to develop high performance textile-based electrode by optimising fabrication method and electrode design. Cotton fabric was dipped into graphene oxide (GO), followed by reduction process to form reduced graphene oxide-cotton (rGOC), where L-ascorbic acid (C6H8O6) was used as the reducing agent. Conductivity and skin-electrode interface impedance of the fabricated cotton were characterized using Four-point probe (Van der Pauw) and Potentiostat, respectively. This study focuses on the investigation of electrode design that includes fabrication methods, electrode sizes and shapes. The performance of the reduced GO-based cotton (rGOC) electrode in terms of ECG signal quality was compared to conventional Ag/AgCl electrode and metal clamp under static and dynamic wearable conditions. Results from the conducted experiments show that the fabricated electrode's performance is influenced by dipping time and electrode design, with circle-shape electrode shows the highest conductivity (up to 9k S/m at 1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area) compared to square- and rectangular-shape electrodes (<; 8k S/m and 14.55 S/m, respectively, at 1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area). The circle-shape rGOC electrode's performed better (SNR 14.85±0.22 dB) than Ag/AgCl electrode (SNR 11.26±0.18 dB) and metal clamp (SNR 12.28±0.72 dB) in capturing static ECG signal. A wearable circle rGOC electrode with 1.7 cm radius performed also similarly under static (SNR 32.60±0.72 dB) and dynamic (SNR 30.27±1.37 dB) ECG monitoring, respectively.

Flexible and robust dry electrodes based on electroconductive polymer spray-coated 3D porous graphene for long-term electrocardiogram signal monitoring system

In this study, highly flexible and conductive dry electrodes were developed using a biocompatible polymer (poly (ethylene dioxythiophene): poly (styrene sulfonate), (PEDOT:PSS)) loaded mechanically robust laser-induced graphene (LIG). The circular-shaped LIG electrode with an area of 0.7854 cm2 was directly patterned using a CO2 laser. The PEDOT:PSS was spray-coated on the LIG to enhance the electrical conductivity (164.2 Scm−1) and electrode robustness. The prepared electrodes were characterized by the Field Emission Scanning Electron Microscope (FESEM), X-ray photoelectron spectroscopy (XPS), X-Ray Diffraction (XRD) and Raman Spectra. Experimental results showed that the surface resistivity and skin contact impedance of the electrode were 17.4 Ω/sq and 385 kΩ at 10 Hz. The impedance at the electrode–skin interface was relatively stable, thereby the SNR value of the electrocardiogram (ECG) signal obtained from the fabricated electrodes (12.9 dB) was comparable to Ag/AgCl electrodes (13.3 dB). The electrodes could be effectively and symmetrically attached to the skin since it is thin and flexible. The fabricated electrodes were successfully operated when the participant is resting, and fewer motion artifacts are observed during movement. Finally, ECG signals were demonstrated in a mobile application for real-time and long-term monitoring by attaching participants’ fingers with optimized dry electrodes.

On-skin graphene electrodes for large area electrophysiological monitoring and human-machine interfaces

On-skin sensors capable of capturing human electrophysiological signals are valuable for a variety of areas, such as wearable health monitoring, prosthesis, human machine interfaces (HMI). Extensive research efforts have been devoted to developing on-skin electrodes and they offer potential advantages over conventional rigid electrodes. However, those approaches usually have limitations as they require sophisticated and costly fabrication processes to produce only a small number of electrodes. Here we report the reduced graphene oxide (rGO) electrodes prepared by simultaneous room temperature reduction and patterning process for scalable fabrication of a large number of on-skin electrodes. Due to the low thickness and adhesive substrate adopted, the rGO electrodes can form conformal contact with skin even during deformation, leading to reduced impedance in skin-electrode interface. The rGO electrodes have been successfully adopted for the measurements of electromyography (EMG), electrooculogram (EOG)and electroencephalogram (EEG) signals with high fidelity. And the implementation of a HMI task is demonstrated through the recognition of five typical human hand grasping gestures via a multi-channel rGO electrode array for the control of a dexterous robotic hand to mimic the gestures. The rGO electrodes presented in this study are promising for practical applications in healthcare and HMI.

Augmented Wire-Embedded Silicon-Based Dry-Contact Sensors for Electroencephalography Signal Measurements

The aim of this study was to develop a novel dry electroencephalography (EEG) sensor with a soft, pliable pad that conforms to the contours of the skin and skull, providing a suitable surface contact area for collecting electrical potential signals and ensuring a reliable connection. In this study, based on our experience in developing flexible silver/silicon-based dry-contact sensors (SBDSs) for biosignal measurements, we proposed a new, augmented wire-embedded silicon-based dry-contact sensor (WSBDS) with a long lifespan and better performance in EEG measurements. The following two augmentation concepts were proposed in this design and implemented in fabrication: 1) the addition of a metal stud and 2) the embedding of copper wires into the fingers of an acicular SBDS. The forehead sensor is suitable for forehead EEG measurements, and the acicular sensor is designed for application to hair-covered sites, where it can overcome hair interference to achieve satisfactory scalp contact while maintaining low impedance at the skin-electrode interface. Finally, this augmented WSBDS performed well in human EEG recording in a designed brain-computer interface (BCI) experiment and is feasible for practical applications.

Amplitude independent versus amplitude dependent muscle activity detection algorithms: a comparative study

&lt;span&gt;The amplitude dependent muscle activity detection algorithms of the surface electromyography (sEMG) signals are very sensitive to the changes in the background noise levels and the performance of these amplitude-based methods is highly deteriorated when the Signal to Noise ratio (SNR) of the sEMG signal is low. sEMG signals of deep and small muscles as well as sEMG signals recorded from patients that have neuromuscular diseases may not meet this SNR requirement which motivates the need for amplitude independent algorithms that can detect weak muscle activities. &lt;/span&gt;&lt;span lang="EN-GB"&gt;Moreover, the sEMG signal amplitude is not constant during the recording time due to the variation in the characteristics of the electrode-skin interface and due to the changes in the ground reference level&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;span lang="EN-GB"&gt; Therefore, the performance of the muscle activity detection algorithms should not be affected by the involuntary amplitude variations of the sEMG signal in order to achieve reliable control of robotic devices intended for disabled people.&lt;/span&gt;&lt;span lang="EN-GB"&gt;To accentuate the importance of the amplitude independent muscle activity detection methods over the amplitude dependent detection methods, a comparative study has been conducted in this paper between the performance of an amplitude independent muscle activity detection algorithm (FLA-MSE algorithm) and three amplitude dependent algorithms with respect to the detection capability of weak muscle activities and with respect to the immunity against false alarms. The results have showed that the performance of the amplitude independent algorithm outperformed the performance of the amplitude dependent algorithms for detecting weak muscle activities and for robustness against false alarms.&lt;/span&gt;

Comparison of the Carbon Nanofiber-/Fiber- and Silicone-Based Electrodes for Bioimpedance Measurements

Electrodes for bioimpedance measurements remain a challenge. In practice, commercially available nonpolarizable silver/silver chloride (Ag/AgCl) gel electrodes prove to be the best option for bioimpedance-based testing. The hydrogel layer reduces the problematic electrode–skin interface impedance, which results in a highly reliable signal. Our workgroup is developing a wearable device that estimates aortic blood pressure from the bioimpedance taken from the wrist area. Understandably, wet electrodes do not suit for a wearable, and there is a need for dry, soft, reusable, and stable electrodes. This study proposes stretchable carbon nanofiber/carbon fiber silicone electrodes as an alternative for cardiac signal measurements with bioimpedance. Five different electrode materials are tested and analyzed: Ag/AgCl gel, Ag/AgCl dry, two carbon and silicone composite materials, and carbon textile. To compare the electrodes’ characteristics, the current through the tissue under the changing pressure, frequency, contact duration, and skin preparation is registered, and the electrode–skin impedance is calculated. The soft and stretchable carbon fiber silicone electrodes proved to have similar response as rigid nonpolarizable Ag/AgCl dry electrodes. Selected methods and proposed instrumentation ensure acceptable reproducibility of the base value and the modulation depth of the measured impedance despite the different electrode materials, large variation of the electrode–skin impedance, and the actual measurement current.

Noise Optimization of CMOS Front-End Amplifier for Embedded Biomedical Recording

This paper deals with the noise optimization of a low-power front-end amplifier (FEA) integrating a band-pass filter for EEG signal recording. The AC-coupled and capacitive feedback structures have been adopted in order to reject the large DC offset generated at the electrode–skin interface. A high open-loop gain with acceptable phase margin was obtained with the proposed operational transconductance amplifier architecture. The methodology to reach a low noise-to-power trade-off is based on selecting the suitable operation region of each transistor, especially the input differential pair by considering the gm/ID parameter. The proposed amplifier was implemented in both 0.13 μm and 0.18 μm CMOS processes with a supply voltage set to ± 1 V. For both technologies, the obtained gain and bandwidth are practically similar, while the phase margin obtained in the first process (0.13 μm) is higher than the one obtained in the second process (0.18 μm); however, both values insure system stability. A lower total input-referred noise of 1.7 µVrms with a noise efficiency factor of 3.4 was obtained in 0.18 µm CMOS process. The designed FEA has been found suitable for ECG biopotential recording.