Electrode-tissue Impedance Research Articles

Motion Artifact Reduction Algorithm for Wearable Electrocardiogram Monitoring Systems

An electrocardiogram (ECG) test is usually performed through measurement with Ag/AgCl electrodes. However, this method has disadvantages, such as skin irritation, short shelf life, and conductance variation in gels over time, which limit the usability for long-term monitoring. One of the alternatives is adopting conductive fabric-based electrodes, but this method involves a problem in which a motion artifact (MA) becomes more severe because no gel and adhesive are available to provide a stable interface between the electrodes and the skin. To address this problem, this study presents a conductive fabric-based ECG monitoring system and an MA reduction algorithm. The system can simultaneously measure the ECG and the electrode-tissue impedance (ETI) with the proposed shirt. The MA reduction algorithm aims to reduce MA with ETI information. The MAs in Lead I, II, and III ECG measurement are generated by lifting arms, walking, and jogging. Results of the MA reduction are quantified by the correlation coefficient (CORR) and mean squared error (MSE). The quantitative analysis shows that the MA can be suppressed by the proposed algorithm. Moreover, compared with a reproduced existing approach, the proposed algorithm performs better in most cases.

A CMOS Synchronized Sample-and-Hold Artifact Blanking Analog Front-End Local Field Potential Acquisition Unit With ±3.6-V Stimulation Artifact Tolerance and Monopolar Electrode-Tissue Impedance Measurement Circuit for Closed-Loop Deep Brain Stimulation SoCs

In this paper, an analog front-end (AFE) local-field potential (LFP) acquisition unit with real-time stimulation artifact removal is proposed and verified for closed-loop deep brain stimulation (DBS) applications. The proposed acquisition unit is called the synchronized sample-and-hold stimulation artifact blanking (SSAB) AFE LFP acquisition unit. Both right-leg-driven (RLD) circuit and monopolar electrode-tissue impedance (ETI) measurement circuit associated with the AFE amplifier are also proposed. During closed-loop stimulations, the artifact removal is realized through the SSAB-IPC by blanking the AR-CCIA with a clock synchronized to the stimulation-enable signal and holding the amplifier at its state before stimulation through a sample-and-hold operation. After stimulation, the acquisition unit can quickly recover from the holding state back to the LFP recording state to reduce the discontinuity in LFP recording. The proposed acquisition unit was fabricated in 0.18-um CMOS technology. With the RLD circuit, the measured CMRR is 124 – 145 dB in the signal bandwidth. The fabricated monopolar ETI measurement circuit has a measurement error less than 8.3% with an extra power consumption of 2.65 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula> W. The experimental results have shown that the proposed SSAB AFE LFP acquisition unit is feasible for the integration of SoCs in real-time closed-loop DBS systems for various applications.

3.6 mW Active-Electrode ECG/ETI Sensor System Using Wideband Low-Noise Instrumentation Amplifier and High Impedance Balanced Current Driver.

An active electrode (AE) and back-end (BE) integrated system for enhanced electrocardiogram (ECG)/electrode-tissue impedance (ETI) measurement is proposed. The AE consists of a balanced current driver and a preamplifier. To increase the output impedance, the current driver uses a matched current source and sink, which operates under negative feedback. To increase the linear input range, a new source degeneration method is proposed. The preamplifier is realized using a capacitively-coupled instrumentation amplifier (CCIA) with a ripple-reduction loop (RRL). Compared to the traditional Miller compensation, active frequency feedback compensation (AFFC) achieves bandwidth extension using the reduced size of the compensation capacitor. The BE performs three types of signal sensing: ECG, band power (BP), and impedance (IMP) data. The BP channel is used to detect the Q-, R-, and S-wave (QRS) complex in the ECG signal. The IMP channel measures the resistance and reactance of the electrode-tissue. The integrated circuits for the ECG/ETI system are realized in the 180 nm CMOS process and occupy a 1.26 mm2 area. The measured results show that the current driver supplies a relatively high current (>600 μApp) and achieves a high output impedance (1 MΩ at 500 kHz). The ETI system can detect resistance and capacitance in the ranges of 10 mΩ-3 kΩ and 100 nF-100 μF, respectively. The ECG/ETI system consumes 3.6 mW using a single 1.8 V supply.

The Active Electrode in the Living Brain: The Response of the Brain Parenchyma to Chronically Implanted Deep Brain Stimulation Electrodes.

Deep brain stimulation is an established symptomatic surgical therapy for Parkinson disease, essential tremor, and a number of other movement and neuropsychiatric disorders. The well-established foreign body response around implanted electrodes is marked by gliosis, neuroinflammation, and neurodegeneration. However, how this response changes with the application of chronic stimulation is less well-understood. To integrate the most recent evidence from basic science, patient, and postmortem studies on the effect of such an "active" electrode on the parenchyma of the living brain. A thorough and in-part systematic literature review identified 49 papers. Increased electrode-tissue impedance is consistently observed in the weeks following electrode implantation, stabilizing at approximately 3 to 6 mo. Lower impedance values are observed around stimulated implanted electrodes when compared with unstimulated electrodes. A temporary reduction in impedance has also been observed in response to stimulation in nonhuman primates. Postmortem studies from patients confirm the presence of a fibrous sheath, astrocytosis, neuronal loss, and neuroinflammation in the immediate vicinity of the electrode. When comparing stimulated and unstimulated electrodes directly, there is some evidence across animal and patient studies of altered neurodegeneration and neuroinflammation around stimulated electrodes. Establishing how stimulation influences the electrical and histological properties of the surrounding tissue is critical in understanding how these factors contribute to DBS efficacy, and in controlling symptoms and side effects. Understanding these complex issues will aid in the development of future neuromodulation systems that are optimized for the tissue environment and required stimulation protocols.

Chronic neural interfacing with cerebral cortex using single-walled carbon nanotube-polymer grids

Objective. The development of electrode arrays able to reliably record brain electrical activity is a critical issue in brain machine interface (BMI) technology. In the present study we undertook a comprehensive physico-chemical, physiological, histological and immunohistochemical characterization of new single-walled carbon nanotubes (SWCNT)-based electrode arrays grafted onto medium-density polyethylene (MD-PE) films. Approach. The long-term electrical stability, flexibility, and biocompatibility of the SWCNT arrays were investigated in vivo in laboratory rats by two-months recording and analysis of subdural electrocorticogram (ECoG). Ex-vivo characterization of a thin flexible and single probe SWCNT/polymer electrode is also provided. Main results. The SWCNT arrays were able to capture high quality and very stable ECoG signals across 8 weeks. The histological and immunohistochemical analyses demonstrated that SWCNT arrays show promising biocompatibility properties and may be used in chronic conditions. The SWCNT-based arrays are flexible and stretchable, providing low electrode-tissue impedance, and, therefore, high compliance with the irregular topography of the cortical surface. Finally, reliable evoked synaptic local field potentials in rat brain slices were recorded using a special SWCNT-polymer-based flexible electrode. Significance. The results demonstrate that the SWCNT arrays grafted in MD-PE are suitable for manufacturing flexible devices for subdural ECoG recording and might represent promising candidates for long-term neural implants for epilepsy monitoring or neuroprosthetic BMI.

Designing and Implementing a Novel Transcranial Electrostimulation System for Neuroplastic Applications: A Preliminary Study.

Recently, a specific repetitive transcranial magnetic stimulation (rTMS) waveform, namely, the theta burst stimulation (TBS) protocol, has been proposed for more efficiently inducing neuroplasticity for various clinic rehabilitation purposes. However, few studies have explored the feasibility of using the TBS combined with direct current (dc) waveform for brain neuromodulation; this waveform is transcranially delivered using electrical current power rather than magnetic power. This study implemented a prototype of a novel transcranial electrostimulation device that can flexibly output a waveform that combined dc and the TBS-like protocol and assessed the effects of the novel combinational waveform on neuroplasticity. An in vivo experiment was conducted first to validate the accuracy of the stimulator's current output at various impedance loads. Using this transcranial stimulator, a series of transcranial stimulation experiments was conducted on the brain cortex of rats, in which electrode-tissue impedance and motor evoked potentials (MEPs) were measured. These experiments were designed to assess the feasibility and efficacy of the new combinational waveforms for brain neuroplasticity. Our results indicated that the transcranial electrostimulation system exhibited satisfactory performance, as evidenced by the error percentage of less than 5% for current output. In the animal experiment, the dc combined with intermittent TBS-like protocol exerted a stronger neuroplastic effect than the conventional dc protocol. These results demonstrated that the combination of electrical dc and TBS-like protocols in our system can produce a new feasible therapeutic waveform for transcranially inducing a promising neuromodulatory effect on various diseases of the central nervous system.

Wireless EEG patch sensor on forehead using on-demand stretchable electrode sheet and electrode-tissue impedance scanner.

A wireless electroencephalogram (EEG) sensor using a stretchable electrode sheet and electrode-tissue impedance measurement module is presented herein. The sensor can be attached to the forehead using biocompatible gel with the electrode sheet. The sensor is compactly designed for 3 cm × 9 cm × 6 mm with weight of 12 g. Impedance scanning circuit is also proposed to evaluate the skin surface condition before EEG measurements. We developed the impedance scanning board for 3 cm × 5 cm × 3 mm, with weight of 5.6 g. Results show that the proposed system demonstrates a promising performance in diagnosing the Alzheimer's disease using frequency domain analysis.

A 176-Channel 0.5cm3 0.7g Wireless Implant for Motor Function Recovery after Spinal Cord Injury.

Epidural spinal stimulation has shown effectiveness in recovering the motor function of spinal cord transected rats by modulating neural networks in lumbosacral spinal segments [1, 2]. The state-of-the-art neuromodulation implant [3] reports a 4-channel stimulator with wireless data and power links for small animal experiments, yet weighs 6g and has a volume of 3cm3. It is preferable that the implant package has a comparable size to its bioelectronics and a high-density stimulator to support stimulation with high spatial resolution. Furthermore, the epidural electrode should be soft and flexible because a mechanical mismatch exists at the tissue-electrode interface [1]. Unlike other implant/SoCs that stimulate with pre-loaded patterns [4–5], the implant for motor function recovery should be capable of adaptively adjusting its stimulation patterns at run time in response to the subject's varying physiological states [2]. Measuring the electrode-tissue impedance is also critical to ensure safe stimulation. Deriving the equivalent circuit model of the electrode-tissue interface determines the safe stimulation boundary (i.e. pulse width and intensity) to ensure the electrode overpotential is within the water window [6]. However, an SoC implementation of this function has not been reported.

Comparison of Twitch Responses During Current‐ or Voltage‐Controlled Transcutaneous Neuromuscular Electrical Stimulation

Neuromuscular electrical stimulation (NMES) is an established method for functional restoration of muscle function, rehabilitation, and diagnostics. In this work, NMES was applied with surface electrodes placed on the anterior thigh to identify the main differences between current-controlled (CC) and voltage-controlled (VC) modes. Measurements of the evoked knee extension force and the myoelectric signal of quadriceps and hamstrings were taken during stimulation with different amplitudes, pulse widths, and stimulation techniques. The stimulation pulses were rectangular and symmetric biphasic for both stimulation modes. The electrode-tissue impedance influences the differences between CC and VC stimulation. The main difference is that for CC stimulation, variation of pulse width and amplitude influences the amount of nerve depolarization, whereas VC stimulation is only dependent on amplitude variations for pulse widths longer than 150 μs. An important remark is that these findings are strongly dependent on the characteristics of the electrode-skin interface. In our case, we used large stimulation electrodes placed on the anterior thigh, which cause higher capacitive effects. The controllability, voltage compliance, and charge characteristics of each stimulation technique should be considered during the stimulators design. For applications that require the activation of a large amount of nerve fibers, VC is a more suitable option. In contrast, if the application requires a high controllability, then CC should be chosen prior to VC.

The chronic challenge-new vistas on long-term multisite contacts to the central nervous system.

The chronic challenge-new vistas on long-term multisite contacts to the central nervous system.

A 345 µW Multi-Sensor Biomedical SoC With Bio-Impedance, 3-Channel ECG, Motion Artifact Reduction, and Integrated DSP

This paper presents a MUlti-SEnsor biomedical IC (MUSEIC). It features a high-performance, low-power analog front-end (AFE) and fully integrated DSP. The AFE has three biopotential readouts, one bio-impedance readout, and support for general-purpose analog sensors The biopotential readout channels can handle large differential electrode offsets ( ${\pm} $ 400 mV), achieve high input impedance ( ${>}$ 500 M $\Omega$ ), low noise ( ${ 620 nVrms in 150 Hz), and large CMRR ( ${>}$ 110 dB) without relying on trimming while consuming only 31 $\mu$ W/channel. In addition, fully integrated real-time motion artifact reduction, based on simultaneous electrode-tissue impedance measurement, with feedback to the analog domain is supported. The bio-impedance readout with pseudo-sine current generator achieves a resolution of 9.8 m $\Omega$ / $\surd$ Hz while consuming just 58 $\mu$ W/channel. The DSP has a general purpose ARM Cortex M0 processor and an HW accelerator optimized for energy-efficient execution of various biomedical signal processing algorithms achieving 10 $\times$ or more energy savings in vector multiply-accumulate executions.

The impact of head movements on EEG and contact impedance: an adaptive filtering solution for motion artifact reduction.

Designing and developing a comfortable and convenient EEG system for daily usage that can provide reliable and robust EEG signal, encompasses a number of challenges. Among them, the most ambitious is the reduction of artifacts due to body movements. This paper studies the effect of head movement artifacts on the EEG signal and on the dry electrode-tissue impedance (ETI), monitored continuously using the imec's wireless EEG headset. We have shown that motion artifacts have huge impact on the EEG spectral content in the frequency range lower than 20 Hz. Coherence and spectral analysis revealed that ETI is not capable of describing disturbances at very low frequencies (below 2 Hz). Therefore, we devised a motion artifact reduction (MAR) method that uses a combination of a band-pass filtering and multi-channel adaptive filtering (AF), suitable for real-time MAR. This method was capable of substantially reducing artifacts produced by head movements.

Cortical activation following chronic passive implantation of a wide-field suprachoroidal retinal prosthesis

Objective. The research goal is to develop a wide-field retinal stimulating array for prosthetic vision. This study aimed at evaluating the efficacy of a suprachoroidal electrode array in evoking visual cortex activity after long term implantation. Approach. A planar silicone based electrode array (8 mm × 19 mm) was implanted into the suprachoroidal space in cats (ntotal = 10). It consisted of 20 platinum stimulating electrodes (600 μm diameter) and a trans-scleral cable terminated in a subcutaneous connector. Three months after implantation (nchronic = 6), or immediately after implantation (nacute = 4), an electrophysiological study was performed. Electrode total impedance was measured from voltage transients using 500 μs, 1 mA pulses. Electrically evoked potentials (EEPs) and multi-unit activity were recorded from the visual cortex in response to monopolar retinal stimulation. Dynamic range and cortical activation spread were calculated from the multi-unit recordings. Main results. The mean electrode total impedance in vivo following 3 months was 12.5 ± 0.3 kΩ. EEPs were recorded for 98% of the electrodes. The median evoked potential threshold was 150 nC (charge density 53 μC cm−2). The lowest stimulation thresholds were found proximal to the area centralis. Mean thresholds from multiunit activity were lower for chronic (181 ± 14 nC) compared to acute (322 ± 20 nC) electrodes (P < 0.001), but there was no difference in dynamic range or cortical activation spread. Significance. Suprachoroidal stimulation threshold was lower in chronic than acute implantation and was within safe charge limits for platinum. Electrode-tissue impedance following chronic implantation was higher, indicating the need for sufficient compliance voltage (e.g. 12.8 V for mean impedance, threshold and dynamic range). The wide-field suprachoroidal array reliably activated the retina after chronic implantation.

Integrated Circuits and Electrode Interfaces for Noninvasive Physiological Monitoring

This paper presents an overview of the fundamentals and state of the-art in noninvasive physiological monitoring instrumentation with a focus on electrode and optrode interfaces to the body, and micropower-integrated circuit design for unobtrusive wearable applications. Since the electrode/optrode-body interface is a performance limiting factor in noninvasive monitoring systems, practical interface configurations are offered for biopotential acquisition, electrode-tissue impedance measurement, and optical biosignal sensing. A systematic approach to instrumentation amplifier (IA) design using CMOS transistors operating in weak inversion is shown to offer high energy and noise efficiency. Practical methodologies to obviate 1/f noise, counteract electrode offset drift, improve common-mode rejection ratio, and obtain subhertz high-pass cutoff are illustrated with a survey of the state-of-the-art IAs. Furthermore, fundamental principles and state-of-the-art technologies for electrode-tissue impedance measurement, photoplethysmography, functional near-infrared spectroscopy, and signal coding and quantization are reviewed, with additional guidelines for overall power management including wireless transmission. Examples are presented of practical dry-contact and noncontact cardiac, respiratory, muscle and brain monitoring systems, and their clinical applications.

A configurable and low-power mixed signal SoC for portable ECG monitoring applications.

This paper describes a mixed-signal ECG System-on-Chip (SoC) that is capable of implementing configurable functionality with low-power consumption for portable ECG monitoring applications. A low-voltage and high performance analog front-end extracts 3-channel ECG signals and single channel electrode-tissue-impedance (ETI) measurement with high signal quality. This can be used to evaluate the quality of the ECG measurement and to filter motion artifacts. A custom digital signal processor consisting of 4-way SIMD processor provides the configurability and advanced functionality like motion artifact removal and R peak detection. A built-in 12-bit analog-to-digital converter (ADC) is capable of adaptive sampling achieving a compression ratio of up to 7, and loop buffer integration reduces the power consumption for on-chip memory access. The SoC is implemented in 0.18 μm CMOS process and consumes 32 μ W from a 1.2 V while heart beat detection application is running, and integrated in a wireless ECG monitoring system with Bluetooth protocol. Thanks to the ECG SoC, the overall system power consumption can be reduced significantly.

Correlation Between Electrode-Tissue Impedance and Motion Artifact in Biopotential Recordings

Ambulatory monitoring of the electrocardiogram (ECG) is a highly relevant topic in personal healthcare. A key technical challenge is overcoming artifacts from motion in order to produce ECG signals capable of being used in clinical diagnosis by a cardiologist. An electrode-tissue impedance is a signal of significant interest in reducing the motion artifact in ECG recordings on the go. A wireless system containing an ultralow-power analog front-end ECG signal acquisition, as well as the electrode-tissue impedance, is used in a validation study on multiple subjects. The goal of this paper is to study the correlation between motion artifacts and skin electrode impedance for a variety of motion types and electrodes. We have found that the correlation of the electrode-tissue impedance with the motion artifact is highly dependent on the electrode design the impedance signal (real, imaginary, absolute impedance), and artifact types (e.g., push or pull electrodes). With the chosen electrodes, we found that the highest correlation was obtained for local electrode artifacts (push, pull, electrode) followed by local skin (stretch, twist, skin) and global artifacts (walk, jog, jump). The results show that the electrode-tissue impedance can correlate with the motion artifacts for local disturbance of the electrodes and that the impedance signals can be used in motion artifact removal techniques such as adaptive filtering.

A 160 &lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$\mu{\rm A}$&lt;/tex&gt;&lt;/formula&gt; Biopotential Acquisition IC With Fully Integrated IA and Motion Artifact Suppression

This paper proposes a 3-channel biopotential monitoring ASIC with simultaneous electrode-tissue impedance measurements which allows real-time estimation of motion artifacts on each channel using an an external μC. The ASIC features a high performance instrumentation amplifier with fully integrated sub-Hz HPF rejecting rail-to-rail electrode-offset voltages. Each readout channel further has a programmable gain amplifier and programmable 4th order low-pass filter. Time-multiplexed 12 b SAR-ADCs are used to convert all the analog data to digital. The ASIC achieves >; 115 dB of CMRR (at 50/60 Hz), a high input impedance of >; 1 GΩ and low noise (1.3 μVrms in 100 Hz). Unlike traditional methods, the ASIC is capable of actual motion artifact suppression in the analog domain before final amplification. The complete ASIC core operates from 1.2 V with 2 V digital IOs and consumes 200 μW when all 3 channels are active.

Pulse Energy as a Reliable Reference for Twitch Forces Induced by Transcutaneous Neuromuscular Electrical Stimulation

Voltage-controlled neuromuscular electrical stimulation has been considered to be safer in noninvasive applications notwithstanding the fact that voltage-controlled devices purportedly generate forces less predictable than their current-controlled equivalents. This prompted us to evaluate relevant electrical parameters to determine whether forces induced by voltage-controlled stimuli were able to match to those induced by current-controlled ones, which tend to evoke forces that were more predictable. Force magnitudes corresponding to current- and voltage-controlled stimuli were aligned with respect to electric charge (equivalent to average current intensity) and electrical energy (equivalent to average power) of the same stimulation pulse to determine which provided a better coherence. Consistency of forces evaluated with energy was significantly (p < 0.001) better than that evaluated with electric charges, suggesting that electrically stimulated forces can be reliably predicted by monitoring the energy parameter of stimulation pulses. The above results appear to show that electrode-tissue impedance, a factor that makes charge and energy evaluations different, redefined the actual effects of current intensities in generating favorable results. Accordingly, novel schemes that track the energy (or average power) of a stimulation pulse may be used as a reliable benchmark to associate mechanical (force) and electrical (stimulation pulse) characteristics in transcutaneous applications of electrical stimulation.

The Role of Materials Biocompatibility for Functional Electrical Stimulation Applications

The biocompatibility of all metallic, polymeric, or ceramic materials used for functional electrical stimulation is governed by the inevitable inflammatory tissue response, but possibly also by immunological reactions to the bulk material or released constituents. Besides chemical, physical, and corrosion properties of the conductive electrode materials, increased surface area and roughness of the electrode can influence tissue contact and signal delivery, and can also affect electrode-tissue impedance due to increased connective tissue encapsulation. The polymeric materials used for electrode insulation and those for leads and stimulator packaging seem more or less compatible in an aggressive biological environment. For the long-term performance of electrodes and leads, the relative motion in the various implant bed situations also has to be considered.

A 30 $\mu$W Analog Signal Processor ASIC for Portable Biopotential Signal Monitoring

This paper presents the design and implementation of an analog signal processor (ASP) ASIC for portable ECG monitoring systems. The ASP ASIC performs four major functionalities: 1) ECG signal extraction with high resolution, 2) ECG signal feature extraction, 3) adaptive sampling ADC for the compression of ECG signals, 4) continuous-time electrode-tissue impedance monitoring for signal integrity monitoring. These functionalities enable the development of wireless ECG monitoring systems that have significantly lower power consumption yet that are more capable than their predecessors. The ASP has been implemented in 0.5 μm CMOS process and consumes 30 μW from a 2 V supply. The noise density of the ECG readout channel is 85 nV/√Hz and the CMRR is better that 105 dB. The adaptive sampling ADC is capable of compressing the ECG data by a factor of 7 and the heterodyne chopper readout extracts the features of the ECG signals. Combination of these two features leads to a factor 4 reduction in the power consumption of a wireless ECG monitoring system. Furthermore, the proposed continuous-time impedance monitoring circuit enables the monitoring of the signal integrity.