Biopotential Signal Acquisition Research Articles

Self-Healing and Shear-Stiffening Electrodes for Wearable Biopotential Sensing and Gesture Recognition.

The achievement of flexible skin electrodes for dynamic monitoring of biopotential is one of the challenging issues in flexible electronics due to the interference of large acceleration and heavy sweat that influence the stability of skin-electrode interfaces. This work presents materials and techniques to achieve self-healing and shear-stiffening electrodes and an associated flexible system that can be used for multichannel biopotential measurement on the skin. The electrode that is based on a composite of silver (Ag) flakes, Ag nanowires, and polyborosiloxane offers an electrical conductivity of 9.71 × 104 S/m and a rheological characteristic that ensures stable and fully conformal contact with skin and easy removal under different shear rates. The electrode can maintain its conductivity even after being stretched by more than 60% and becomes self-healed after mechanical damage. The combination of the electrodes with a screen-printed multichannel flexible sensor allows stable monitoring of both static and dynamic electromyography signals, leading to the acquisition of high-quality multilead biopotential signals that can be readily extracted to yield gesture recognition results with over 97.42% accuracy. The conductive self-healing materials and flexible sensors may be utilized in various daily biopotential sensing applications, allowing highly stable dynamic measurement to facilitate artificial intelligence-enabled health condition diagnosis and human-computer interface.

An Integrated Analog Front-End System on Flexible Substrate for the Acquisition of Bio-Potential Signals.

The application of a versatile, low-temperature thin-film transistor (TFT) technology is presently described as the implementation on a flexible substrate of an analog front-end (AFE) system for the acquisition of bio-potential signals. The technology is based on semiconducting amorphous indium-gallium-zinc oxide (IGZO). The AFE system consists of three monolithically integrated constituent components: a bias-filter circuit with a bio-compatible low cut-off frequency of ≈1Hz, a 4-stage differential amplifier offering a large gain-bandwidth product of ≈955kHz, and an additional notch filter exhibiting over 30dB suppression of the power-line noise. Respectively built using conductive IGZO electrodes with thermally induced donor agents and enhancement-mode fluorinated IGZO TFTs with exceptionally low leakage current, both capacitors and resistors with significantly reduced footprints are realized. Defined as the ratio of the gain-bandwidth product of an AFE system to its area, a record-setting figure-of-merit of ≈86kHzmm-2 is achieved. This is about an order of magnitude larger than the < 10kHzmm-2 of the nearest benchmark. Requiring no supplementary off-substrate signal-conditioning components and occupying an area of ≈11mm2 , the stand-alone AFE system is successfully applied to both electromyographyand electrocardiography (ECG).

Robust, self‐adhesive, and low‐contact impedance polyvinyl alcohol/polyacrylamide dual‐network hydrogel semidry electrode for biopotential signal acquisition

AbstractHerein, we fabricated a flexible semidry electrode with excellent mechanical performance, satisfactory self‐adhesiveness, and low‐contact impedance using physical/chemical crosslinked polyvinyl alcohol/polyacrylamide dual‐network hydrogels (PVA/PAM DNHs) as an efficient saline reservoir. The resultant PVA/PAM DNHs showed admirable adhesive and compliance to the hairy scalp, facilitating the establishment of a robust electrode/skin interface for biopotential signal transmission. Moreover, the PVA/PAM DNHs steadily released trace saline onto the scalp to achieve the minimized potential drift (1.47 ± 0.39 mV/min) and low electrode–scalp impedance (18.2 ± 8.9 kΩ @ 10 Hz). More importantly, the application feasibility of real‐world brain−computer interfaces (BCIs) was preliminarily validated by 10 participants using two classic BCI paradigms. The mean temporal cross‐correlation coefficients between the semidry and wet electrodes in the eyes open/closed and the N200 speller paradigms are 0.919 ± 0.054 and 0.912 ± 0.050, respectively. Both electrodes demonstrate anticipated neuroelectrophysiological responses with similar patterns. This semidry electrode could also effectively capture robust P‐QRS‐T peaks during electrocardiogram recording. Considering their outstanding advantages of fast setup, user‐friendliness, and robust signals, the proposed PVA/PAM DNH‐based electrode is a promising alternative to wet electrodes in biopotential signal acquisition.

Dependence of Skin-Electrode Contact Impedance on Material and Skin Hydration.

Dry electrodes offer an accessible continuous acquisition of biopotential signals as part of current in-home monitoring systems but often face challenges of high-contact impedance that results in poor signal quality. The performance of dry electrodes could be affected by electrode material and skin hydration. Herein, we investigate these dependencies using a circuit skin-electrode interface model, varying material and hydration in controlled benchtop experiments on a biomimetic skin phantom simulating dry and hydrated skin. Results of the model demonstrate the contribution of the individual components in the circuit to total impedance and assist in understanding the role of electrode material in the mechanistic principle of dry electrodes. Validation was performed by conducting in vivo skin-electrode contact impedance measurements across ten normative human subjects. Further, the impact of the electrode on biopotential signal quality was evaluated by demonstrating an ability to capture clinically relevant electrocardiogram signals by using dry electrodes integrated into a toilet seat cardiovascular monitoring system. Titanium electrodes resulted in better signal quality than stainless steel electrodes. Results suggest that relative permittivity of native oxide of electrode material come into contact with the skin contributes to the interface impedance, and can lead to enhancement in the capacitive coupling of biopotential signals, especially in dry skin individuals.

A biomimetic skin phantom for characterizing wearable electrodes in the low-frequency regime

Advances in the integration of wearable devices in our daily life have led to the development of new electrode designs for biopotential monitoring. Historically, the development and testing of wearable electrodes for the acquisition of biopotential signals has been empirical, relying on experiments on human volunteers. However, the lack of explicit control on human variables, the intra-, and inter-subject variability complicates the understanding of the performance of these wearable electrodes. Herein, phantom mimicking the electrical properties of the skin in the low-frequency range (1–1000 Hz), which has the potential to be used as a platform for controlled benchtop experiments for testing electrode functionality, is demonstrated. The fabricated phantom comprises two layers representing the deeper tissues and stratum corneum. The lower layer of the phantom mimicking deeper tissues was realized using polyvinyl alcohol cryogel (PVA-c) prepared with 0.9% W/W saline solution by a freeze-thaw technique. The properties of the upper layer representing the stratum corneum were simulated using a 100 µm thick layer fabricated by spin-coating a mixture of polydimethylsiloxane (PDMS), 2.5% W/W carbon black (CB) for conductance, and 40% W/W barium titanate (BaTiO3) as a dielectric. The hydration of the stratum corneum was modeled in a controlled way by varying porosity of the phantom’s upper layer. Impedance spectroscopy measurements were carried out to investigate the electrical performance of the fabricated phantom and validated against the impedance response obtained across a physiological skin impedance range of five human subjects. The results indicated that the Bode plot depicting the impedance response obtained on the phantom was found to lie in the human skin range. Moreover, it was observed that the change of porosity provides control over the hydration and the phantom can be tuned as per the skin ranges among different individuals. Also, the phantom was able to mimic the impact of dry and hydrated skin on a simulated ECG signal in the time domain. The developed skin phantom is affordable, fairly easy to manufacture, stable over time, and can be used as a platform for benchtop testing of new electrode designs.

Development of fabric electrode for bio-potential signal acquisition in wearable health monitoring and effect of perspiration on signal acquisition

Development of a gel-free bio-potential electrode for the wearable health monitoring applications is a challenging goal. A conductive fabric electrode can replace the traditional conductive gel electrode. This paper describes the development of a conductive fabric electrode with regard to its potential use for electrocardiogram (ECG) acquisition. Since direct contact between the conductive fabric and human skin will be involved, an investigation on the effect of perspiration on the electrical conductivity of fabric is critical. Hence, the developed electrode was treated with alkaline (pH=8.0) and acidic (pH=4.3) perspiration for 3, 8 and 40 h to study the effect of perspiration on the conductivity and surface morphology. The acquired ECG signals were analysed with respect to morphology and frequency distribution. Conductivity tests were carried out on the perspiration-treated test electrodes by two probe method and surface resistivity meter. The ECG signals of volunteers were also recorded. The results showed a slight decrease in conductivity but without affecting the morphology and the quality of ECG signal. Leached silver content in the acid perspiration-treated solution was found to be 0.117 ppm as determined by Atomic absorption spectroscopy. The result shows that soft conducting textile materials can indeed be used as an electrode for ECG acquisition. This is a novel type of gel-free fabric electrode for long term wearable health monitoring applications including space application.

Adaptive analogue calibration technique to compensate electrode motion artefacts in biopotential recording

This study presents an adaptive analogue calibration method to compensate electrode–skin impedance mismatch during biopotential signal (electrocardiogram, electroencephalogram and electromyogram) acquisition with enhanced immunity to electromagnetic interference (EMI). The method continuously measures the variation of the impedance mismatch between the electrode–skin interfaces arising primarily due to motion artefacts, and thereafter compensate for the resulting distortions at the output. The compensation is done with the help a proportional–integral–derivative controller in the feedback loop, together with the acquisition of the biopotential signals. A common mode shunt feedback at the output attenuates the common mode EMI and reduces the common mode deviation. As compared to previously reported techniques, the proposed technique refutes any need for offline manual calibration and shows a significant improvement in EMI attenuation without interrupting the main system's operation. This study explains the proposed technique with the comprising blocks, illustrates the theoretical models and analyses, and evaluates the superior performances of the proposed method by comparing the responses with those obtained from the other EMI-Immune electrode mismatch compensating front-ends existing in the literature.

Wearable and Flexible Textile Electrodes for Biopotential Signal Monitoring: A review

Wearable electronics is a rapidly growing field that recently started to introduce successful commercial products into the consumer electronics market. Employment of biopotential signals in wearable systems as either biofeedbacks or control commands are expected to revolutionize many technologies including point of care health monitoring systems, rehabilitation devices, human–computer/machine interfaces (HCI/HMIs), and brain–computer interfaces (BCIs). Since electrodes are regarded as a decisive part of such products, they have been studied for almost a decade now, resulting in the emergence of textile electrodes. This study presents a systematic review of wearable textile electrodes in physiological signal monitoring, with discussions on the manufacturing of conductive textiles, metrics to assess their performance as electrodes, and an investigation of their application in the acquisition of critical biopotential signals for routine monitoring, assessment, and exploitation of cardiac (electrocardiography, ECG), neural (electroencephalography, EEG), muscular (electromyography, EMG), and ocular (electrooculography, EOG) functions.

A Chopper Stabilized Current-Feedback Instrumentation Amplifier for EEG Acquisition Applications

A low-noise, low-power, chopper-stabilized, current-feedback instrumentation amplifier (CFIA) for bio-potential signal acquisition applications is presented. The proposed design includes an ac-coupled chopper-stabilized CFIA to reduce 1/f noise. It also has a switched-capacitor integrator to reduce the input offset and provide a high-pass filter. The proposed amplifier is designed using a Samsung 0.13-μm CMOS process. The CFIA shows an rms input referred noise voltage of 0.75 μV within a bandwidth of 0.01-100 Hz, a common-mode rejection ratio of 125 dB, and total power dissipation of 2.3 μW at a power supply voltage of 1 V.

A Chopper-Stabilized Amplifier With a Tunable Bandwidth for EEG Acquisition Applications

A chopper-stabilized amplifier for bio-potential signal acquisition applications is presented. The proposed design includes an AC-coupled chopper-stabilized amplifier to reduce the 1/f noise and a tunable MOS-capacitor and a tunable pseudo-resistor to control the bandwidth. It employs a positive feedback input impedance boosting loop (IBL) to increase the input impedance and common-mode rejection ratio (CMRR). The proposed amplifier is designed and fabricated using a Magnachip/SK Hynix $0.18~\mu \text{m}$ 1 poly-6 metal CMOS process and tested with a DC supply voltage of 1.5 V ( $V_{\mathrm {DD}}$ of 0.75 V and $V_{\mathrm {SS}}$ of −0.75 V). The proposed amplifier achieves a midband gain of 47.6 dB, a CMRR of 105.6 dB, and an input-referred noise power spectral density (PSD) of $120~\text{nV}/\surd\text{Hz}$ at 100 Hz with a total power dissipation of 855 nW. The noise-efficiency-factor (NEF) of the proposed amplifier is 2.91. The low-pass corner frequency can be tuned from 200 to 500 Hz. The chip area of the proposed amplifier is 0.065 mm2.

Effective Biopotential Signal Acquisition: Comparison of Different Shielded Drive Technologies

Biopotential signals are mainly characterized by low amplitude and thus often distorted by extraneous interferences, such as power line interference in the recording environment and movement artifacts during the acquisition process. With the presence of such large-amplitude interferences, subsequent processing and analysis of the acquired signals becomes quite a challenging task that has been reported by many previous studies. A number of software-based filtering techniques have been proposed, with most of them being able to minimize the interferences but at the expense of distorting the useful components of the target signal. Therefore, this study proposes a hardware-based method that utilizes a shielded drive circuit to eliminate extraneous interferences on biopotential signal recordings, while also preserving all useful components of the target signal. The performance of the proposed method was evaluated by comparing the results with conventional hardware and software filtering methods in three different biopotential signal recording experiments (electrocardiogram (ECG), electro-oculogram (EOG), and electromyography (EMG)) on an ADS1299EEG-FE platform. The results showed that the proposed method could effectively suppress power line interference as well as its harmonic components, and it could also significantly eliminate the influence of unwanted electrode lead jitter interference. Findings from this study suggest that the proposed method may provide potential insight into high quality acquisition of different biopotential signals to greatly ease subsequent processing in various biomedical applications.

A 18.5 nW 12-bit 1-kS/s Reset-Energy Saving SAR ADC for Bio-Signal Acquisition in 0.18-μm CMOS

This paper presents three low-power design techniques for successive approximation registers (SAR) analog-to-digital converter (ADC) for bio-potential signal acquisition: skip-reset, delta ( $\Delta$ ) readout with MSB-rounding, and tri-level split monotonic switching. The skip-reset scheme reduces not only reference energy but also digital switching energy for the ADC reset. The $\Delta$ -readout process with the proposed MSB-rounding technique shifts the location of the resolvable range using the previous digital code to increase the hit-rate. Finally, the tri-level split monotonic switching scheme minimizes the CDAC switching activity in predictive residue generation for the $\Delta$ -readout process. A prototype ADC was fabricated in a 0.18- $\mu \text{m}$ CMOS technology and occupies an active area of 0.17 mm2. At a 1.5-V supply voltage and a 1-kS/s sampling-rate with the electrocardiogram signal input, the ADC power consumption could be reduced to 18.5 nW, corresponding to 71% power saving, and owing to the proposed techniques from a conventional SAR ADC consuming 63.5 nW.

Synchronous acquisition of multi-channel signals by single-channel ADC based on square wave modulation.

Synchronous acquisition of multi-channel biopotential signals, such as electrocardiograph (ECG) and electroencephalograph, has vital significance in health care and clinical diagnosis. In this paper, we proposed a new method which is using single channel ADC to acquire multi-channel biopotential signals modulated by square waves synchronously. In this method, a specific modulate and demodulate method has been investigated without complex signal processing schemes. For each channel, the sampling rate would not decline with the increase of the number of signal channels. More specifically, the signal-to-noise ratio of each channel is n times of the time-division method or an improvement of 3.01×log2n dB, where n represents the number of the signal channels. A numerical simulation shows the feasibility and validity of this method. Besides, a newly developed 8-lead ECG based on the new method has been introduced. These experiments illustrate that the method is practicable and thus is potential for low-cost medical monitors.

A 15-Channel Digital Active Electrode System for Multi-Parameter Biopotential Measurement

This paper presents a digital active electrode (DAE) system for multi-parameter biopotential signal acquisition in portable and wearable devices. It is built around an IC that performs analog signal processing and digitization with the help of on-chip instrumentation amplifiers, a 12 bit ADC and a digital interface. Via a standard ${\rm I}^{{2}}{\rm C}$ bus, up to 16 digital active electrodes (15-channels) can be connected to a commercially available microcontroller, thus significantly reducing system complexity and cost. In addition, the DAE utilizes an innovative functionally DC-coupled amplifier to preserve input DC signal, while still achieving state-of-the-art performance: 60 nV/sqrt(Hz) input-referred noise and ${\pm} $ 350 mV electrode-offset tolerance. A common-mode feedforward scheme improves the CMRR of an AE pair from 40 dB to maximum 102 dB.

A reconfigurable medically cohesive biomedical front-end with ΣΔ ADC in 0.18µm CMOS.

This paper presents a generic programmable analog front-end (AFE) for acquisition and digitization of various biopotential signals. This includes a lead-off detection circuit, an ultra-low current capacitively coupled signal conditioning stage with programmable gain and bandwidth, a new mixed signal automatic gain control (AGC) mechanism and a medically cohesive reconfigurable ΣΔ ADC. The full system is designed in UMC 0.18μm CMOS. The AFE achieves an overall linearity of more 10 bits with 0.47μW power consumption. The ADC provides 2(nd) order noise-shaping while using single integrator and an ENOB of ~11 bits with 5μW power consumption. The system was successfully verified for various ECG signals from PTB database. This system is intended for portable batteryless u-Healthcare devices.

A Versatile High Swing Current Mode Instrumentation Amplifier with an Integrated Band-Pass Filter for Bio-Potential Signal Acquisition

A Versatile High Swing Current Mode Instrumentation Amplifier with an Integrated Band-Pass Filter for Bio-Potential Signal Acquisition

A $160~\mu {\rm W}$ 8-Channel Active Electrode System for EEG Monitoring

This paper presents an active electrode system for gel-free biopotential EEG signal acquisition. The system consists of front-end chopper amplifiers and a back-end common-mode feedback (CMFB) circuit. The front-end AC-coupled chopper amplifier employs input impedance boosting and digitally-assisted offset trimming. The former increases the input impedance of the active electrode to 2 GΩ at 1 Hz and the latter limits the chopping induced output ripple and residual offset to 2 mV and 20 mV, respectively. Thanks to chopper stabilization, the active electrode achieves 0.8 μVrms (0.5-100 Hz) input referred noise. The use of a back-end CMFB circuit further improves the CMRR of the active electrode readout to 82 dB at 50 Hz. Both front-end and back-end circuits are implemented in a 0.18 μm CMOS process and the total current consumption of an 8-channel readout system is 88 μA from 1.8 V supply. EEG measurements using the proposed active electrode system demonstrate its benefits compared to passive electrode systems, namely reduced sensitivity to cable motion artifacts and mains interference.

Electrochemical Modification of Silver Coated Multifilament for Wearable ECG Monitoring Electrodes

This paper presents a method to fabricate textile structural electrodes from material preparation to electrode structure design and testing. Silver/Silver Chloride (Ag/AgCl) was assumed to be the best electrode material system for acquisition of biopotential signals. A AgCl coating has been grown on silver (Ag) coated multilament yarn to form Ag/AgCl combination using constant voltage electrolytic deposition in 0.9% wt sodium chloride bath. The AgCl thickness could be controlled by varying processing time (t) and voltage (V). Surface morphology of the treated fibres were studied by scanning electron microscopy (SEM) which revealed that AgCl grain size became bigger and denser as increased processing time and voltage. The impedance of the treated fibre was analyzed by electrochemical impedance spectroscopy (EIS) analysis from 0.1H z to 1000H z which shown that impedance also increased with processing time and voltage. The prepared Ag/AgCl multilament yarn was fabricated into wearable electrode using embroidery technique. ECG testing confirmed that the electrodes made from treated fibre can acquire high quality signal.

Ultra-low-power biopotential interfaces and their applications in wearable and implantable systems

Traditionally the monitoring of the biopotential signals are only limited to clinical applications. On the other hand, there is a growing demand for these biopotential signals to be used in non-clinical applications in order to improve the quality of life and enable the interaction between humans and machines. However, such non-clinical applications of biopotential signal monitoring requires various improvements not only in terms of size and comfort of the biopotential acquisition systems, but also in terms of their power dissipation. An important building block of the biopotential acquisition systems is the front-end circuitry that defines the quality of the extracted signals and unfortunately consumes unacceptable power, when the currently available circuitry is considered. Therefore, this paper focuses on the advances in low-power and high-performance readout circuit design for the acquisition of biopotential signals. In addition, several application examples will be demonstrated, which proves that the realization of high-performance and low-power readout circuits can actually enable the implementation of miniaturized and comfortable biopotential acquisition systems extending the usage of such systems towards non-clinical applications.