Dry Bioelectrodes Research Articles

Highly Stretchable and Breathable Dry Bioelectrode with Low Impedance for Electrophysiological Monitoring

Highly Stretchable and Breathable Dry Bioelectrode with Low Impedance for Electrophysiological Monitoring

A gold nanowire-integrated soft wearable system for dynamic continuous non-invasive cardiac monitoring

Blood pressure (BP) is a cardiovascular parameter which exhibits significant variability. Whilst continuous BP monitoring would be of significant clinical utility. This is particularly challenging outside the hospital environment. New wearable cuff-based and cuffless BP monitoring technologies provide some capacity, however they have a number of limitations including bulkiness, rigidity and discomfort, poor accuracy and motion artefact. Here, we report on a lightweight, user-friendly, non-invasive wearable cardiac sensing system based on deformation-insensitive conductive gold nanowire foam (G-foam) and pressure-sensitive resistive gold nanowire electronic skin (G-skin). The G-foam could serve as a new soft dry bioelectrode for electrocardiogram (ECG) monitoring; a new soft button-based G-skin design could avoid manual holding for continuous pulse recording. They could be integrated seamlessly with everyday bandage for facile wireless recording of ECG and artery pulses under real-word dynamic environments including walking, running, deep squatting, and jogging. Further machine learning algorithm was developed for estimation of systolic and diastolic BP, showing comparable accuracy to commercial cuff-based sphygmomanometer. The measured dynamic BP changes correlated well with the volunteer’s daily activities, indicating the potential applications of our soft wearable systems for real-time diagnostics of cardiovascular functions in complex dynamic real-world setting.

Fabrication and characteristic of flexible dry bioelectrodes with microstructures inspired by golden margined century plant leaf

To obtain a flexible dry bioelectrode with high performance, this paper proposes a flexible dry bioelectrode with microneedle structures inspired by golden margined century plant leaf. The microneedle structures are fabricated by laser processing method, and then embedded inside the flexible PDMS substrate. In this way, the flexible dry bioelectrode with curved-hook type microneedle structures is obtained. The mechanical and electrical properties of the bioelectrode are characterized. Our results show that the curved-hook type microneedle has a smaller penetration force (0.939 N) but bigger pullout force (0.149 N) compared with triangular microneedles. The curved-hook type microneedle structures allow better anti-destructive and bonding performance. Additionally, compared with traditional Ag/AgCl electrode, the developed flexible dry bioelectrodes exhibit better detection performance of EMG in both time and frequency domain.

Magnetic field assisted laser fabrication and electrical characterizations of metal dry Biolectrode with surface microstructures

Magnetic field assisted laser fabrication is proposed to process metal dry bioelectrode with surface microstructures. The effects of magnetic flux density on the geometrical dimension of surface microstructures of bioelectrode is investigated. The electrode-skin contact impedance is then studied using the two-electrode measurement method. Finally, electromyography (EMG) signal is recorded using bioelectrodes processed in different magnetic flux density. Our results show that the magnetic field has obvious influences on the height and bottom width of microstructure of bioelectrode. When a magnetic field of 100 mT is selected, larger height-width ratio of microstructures is obtained, which provides a stronger ability to penetrate stratum corneum. Consequently, much lower contact impedance is obtained. Signal-noise ratio (SNR) of EMG signal shows a correlation coefficient of 0.9836 with height-width ratio of microstructures on the surface of metal dry bioelectrodes. Raw EMG signals recorded by metal dry bioelectrodes in 100 mT magnetic field show a high SNR up to 27.350, which is slightly higher than that of traditional Ag/AgCl wet bioelectrodes (26.689). By stationary wavelet transform (SWT) de-noising, noise interfused in raw EMG signals is suppressed effectively. Moreover, the de-noised EMG signal recorded using metal dry bioelectrodes processed in 100 mT magnetic field still remains a fairly high SNR.

Impedance and electromyographic properties of metal dry bioelectrode with non-uniform surface microstructure

To develop a bioelectrode with excellent electrical performance, laser micromachining was proposed to fabricate a metal dry bioelectrode with a non-uniform surface microstructure. By controlling the scanning path, scanning times, and output power, metal dry bioelectrodes with different types of surface microstructure distribution (uniform and non-uniform) were obtained. Compared with traditional Ag/AgCl wet electrode, the influence of the surface microstructure distribution on the impedance and electromyographic properties of metal dry bioelectrodes was investigated. Results show that the surface microstructure distribution has little influence on the impedance properties of the bioelectrode within the testing time and frequency domains. However, the electromyographic properties of metal dry bioelectrodes with uniform and non-uniform surface microstructures were better than that of conventional Ag/AgCl wet electrodes. The strongest electromyographic signal was obtained using a metal dry bioelectrode with non-uniform surface microstructure.

Mechanical properties of surface microstructures of metal dry bioelectrode

The metal dry bioelectrodes have been developed to measure the bioelectrical signals. In this study, the laser micromilling process as an economical and high efficiency way was proposed to fabricate the metal dry bioelectrodes with surface microstructure array. The measuring principle of material hardness was adopted to characterize the mechanical properties of surface microstructures of metal dry bioelectrodes. Moreover, the compressive strength of surface microstructures of metal dry bioelectrodes fabricated with different scan spacings was discussed, and then the bending strength and insertion performance of surface microstructures of metal dry bioelectrodes were further investigated. The results show that surface microstructures of bioelectrode fabricated using three scan spacings including 100 μm, 150 μm and 200 μm exhibited better compressive performance. When the 100 μm scan spacing was selected, the surface microstructures of metal dry bioelectrodes presented larger bending strength, and it could successfully penetrate the silicon film with a small pressure about 1.48 N.

Mechanically robust silver coatings prepared by electroless plating on thermoplastic polyurethane

A simple and low-cost surface functionalization method is proposed to activate a thermoplastic polyurethane (TPU) for the electroless deposition of a silver coating with excellent adhesion and low resistivity. The TPU surface functionalization was performed in solution and consisted in forming a physical interpenetrating network at the TPU surface, involving TPU and polyvinylpyrrolidone (PVP), a polymer displaying a strong affinity for metals. The presence of PVP on the TPU surface and its stability in aqueous solution were assessed by ATR-FTIR and contact angle measurements as a function of the PVP concentration and treatment time. A modified Tollens solution was used to grow a silver film on the TPU substrate, by using the electroless plating method. Compact silver films with an average thickness of 12.5 µm and a resistivity of 8.57 mΩ·cm were obtained for a 24 h plating time. The adhesion strength of the silver film proved to be higher than 8.5 N/cm. The resistance to fatigue of the silver films was studied by performing series of compression/stretching tests (150 cycles). It was concluded that the films kept low resistance values, although displaying a higher sensitivity to compression than to stretching. Furthermore, the films keep a good conductivity for strains up to 400%. The excellent electrical and mechanical properties of the films make them suitable candidates for the coating of multipin dry bioelectrodes. Owing to the high affinity of many metals for PVP, this activation technique has the potential to be extended to the deposition of other metals and other polymers as well, provided a suitable solvent is used.

Electrical impedance performance of metal dry bioelectrode with different surface coatings

To improve the electrical impedance performance of bioelectrodes, a novel metal dry bioelectrodes with different coating layers are developed with laser micromilling and electroplating technology. Based on the analysis of the coating layer on the bioelectrode surface, the effect of different coating layers on the electrical impedance performance of bioelectrodes is investigated. The results show that the silver content increases with electroplating time when the silver layer is coated on the bioelectrode surface. However, the decrease of silver layer weight is observed with much longer electroplating time, and the optimal electroplating time is 20 min. Compared with the uncoated bioelectrode, the bioelectrode coated with silver layer exhibits much lower impedance value and better impedance stability. Especially, when the silver-coated bioelectrode is subsequently coated with silver-silver chloride layer, the lowest impedance value and best impedance stability are obtained.

Highly Elastic Graphene‐Based Electronics Toward Electronic Skin

Epidermal electronics are extensively explored as an important platform for future biomedical engineering. Epidermal devices are typically fabricated using high‐cost methods employing complex vacuum microfabrication processes, limiting their widespread potential in wearable electronics. Here, a low‐cost, solution‐based approach using electroconductive reduced graphene oxide (RGO) sheets on elastic and porous poly(dimethylsiloxane) (PDMS) thin films for multifunctional, high‐performance, graphene‐based epidermal bioelectrodes and strain sensors is presented. These devices are fabricated employing simple coatings and direct patterning without using any complicated microfabrication processes. The graphene bioelectrodes show a superior stretchability (up to 150% strain), with mechanical durability up to 5000 cycles of stretching and releasing, and low sheet resistance (1.5 kΩ per square), and the graphene strain sensors exhibit a high sensitivity (a gauge factor of 7 to 173) with a wide sensing range (up to 40% strain). Fully functional applications of dry bioelectrodes in monitoring human electrophysiological signals (i.e., electrocardiogram, electroencephalography, and electromyogram) and highly sensitive strain sensors for precise detection of large‐scale human motions are demonstrated. It is believed that our unique processing capability and multifunctional device platform based on RGO/porous PDMS will pave the way for low‐cost processing and integration of 2D materials for future wearable electronic skin.

Characterization of impedance properties of metal dry bioelectrodes with surface microstructure arrays

A metal dry bioelectrode with surface microstructure arrays is fabricated using the laser direct micromilling technology with advantages of high efficiency and low cost. The impedance properties of the metal dry bioelectrode are then analyzed using the two-electrode measurement method. The equivalent circuit of the electrode-skin contact model has been developed, and the influences of current frequency, electrode size, laser power and contact pressure on the impedance properties are investigated. The results show that the impedance of the bioelectrode decreases with increasing electrode diameter in the range of 6–10mm. With increasing laser power, the impedance of the bioelectrode tends to decrease gradually. When a contact pressure is applied on the bioelectrode surface, its impedance drastically decreases. Compared with the traditional silver/silver chloride (Ag/AgCl) electrode, the impedance of the bioelectrode shows good stability. Due to the advantages of convenient, repeatable use, without the removal of the stratum corneum (the outermost layer of the skin), the developed metal dry bioelectrode has broad prospects in the collection and application of bioelectrical signals.

Experimental investigation on surface wettability of copper-based dry bioelectrodes

Laser micromilling technology was used to fabricate a dry bioelectrode with a surface microstructure array, which was then treated with stearic acid ethanol solution. A superhydrophobic dry bioelectrode was obtained. Based on the scanning electron microscopy results, the formation of the dry bioelectrode with a surface microstructure array had been discussed. Later, the contact angle on the bioelectrode surface was analyzed by changing the parameters such as laser processing parameters, spacing of microstructure, and the working media. The results show that the contact angle of the bioelectrode surface treated with stearic acid ethanol solution was approximately 151°, and this value was compared with the result for bioelectrodes that were not treated or placed in air. The laser power and number of scans had significant influences on the contact angle, while the scanning speed had only a slight influence. When the spacing of microstructure was in the range of 0.1–0.5mm, the contact angle increased with a decrease in the spacing of microstructure. In addition, the contact angle on the bioelectrode with deionized water or saline water as the working media was greater than 150°. Our developed dry bioelectrode presented good hydrophobic properties to help to realize a stable measurement system over a long time, which has many applications in the detection and measurement of biological signals.

Laser direct micromilling of copper-based bioelectrode with surface microstructure array

The laser direct micromilling is proposed to fabricate the microstructure arrays on the surface of dry bioelectrodes using red copper material. Based on the principle of laser machining and SEM results, the forming process of microstructure arrays on the surface of copper-based bioelectrodes is discussed. When the process parameters are varied, the effect of process spacing, laser output power, scanning speed and number of scan on the morphology and geometrical dimension of microstructure array of bioelectrode is investigated. The results show that the cone surface microstructure can be fabricated when process spacing is set to 0.1mm. Surface roughness of microstructure is greatly changed with different scanning speeds. The height of surface microstructure and recast layer is greatly increased with increasing laser output power. When smaller laser output power or less number of scan are selected, surface microstructure array is difficult to be fabricated. However, it is easy to generate the damage of surface microstructure when the larger output power or excessive scanning times are selected. Moreover, our developed copper-based bioelectrode shows a hydrophobic property when the spacings are selected in the range of 0.1–0.3mm. Eventually, the optimized process parameters are obtained to fabricate the bioelectrode with cone microstructure array.

Electrochemical and structural characterization of nanocomposite Agy:TiNx thin films for dry bioelectrodes: the effect of the N/Ti ratio and Ag content

Agy:TiNx nanocomposite thin films sputtered with different N/Ti atomic ratios and Ag atomic contents were characterized from the structural and morphological points of view. Their electrochemical behaviour was studied in a synthetic sweat solution, aiming at selecting a suitable material for biolectrode applications. An increase of the N/Ti atomic ratio, which is accompanied by an increase of the Ag atomic content, leads to a substantial increase of the roughness and porosity of the samples, especially for N/Ti ratios >0.2. For N/Ti atomic ratios up to 0.3 (15 at.% Ag) no metallic Ag segregation is visible in the TiNx matrix. Hence, the possible formation of TiAg and Ti2Ag intermetallics or even a Ag/TiAg/Ti2Ag phase mixture, although not demonstrated, should not be disregarded. As for the N/Ti atomic ratio = 0.7 (32 at.% Ag) sample, the Ag phases are predominantly concentrated near the interface with the substrate. The amount of Ag phases at the surface of the films remains somewhat low for all TiN under-stoichiometric films, even for Ag atomic contents up to 32 at.%. When the TiNx matrix reaches the stoichiometric condition (sample with N/Ti atomic ratio = 1 and 20 at.% Ag), Ag segregation occurs and metallic Ag aggregates are visible at the surface of the film, leading to a substantially different electrochemical behaviour. The impedance of the Agy:TiNx films in synthetic sweat solution is mainly ruled by the roughness/porosity variation, thus the higher the N/Ti atomic ratio, the lower the impedance. The interfacial film/sweat electrochemical noise and drift were similar for all films and comparable to the results obtained for commercial Ag/AgCl electrodes (except for the N/Ti atomic ratio = 1 and 20 at.% Ag film). In view of the results, it may be concluded that the samples with N/Ti atomic ratios = 0.3 (15 at.% Ag) and 0.7 (32 at.% Ag) are the most appropriate for further bioelectrode development.

Liquid silicone rubber (LSR)-based dry bioelectrodes: The effect of surface micropillar structuring and silver coating on contact impedance

Bioelectrode–skin contact impedances need to be minimized to enable high quality recordings of biopotentials. Low contact impedances can be achieved with large, effective bioelectrode–skin contact areas. In this study, novel dry microstructured electrodes based on electrically conductive liquid silicone rubber (LSR) were developed and characterized with electrical impedance spectroscopy (EIS) measurements. Two different micropillar structures with inter-pillar distances of 20μm and 100μm, respectively, were compared with smooth LSR electrodes, as such or coated with silver (Ag). A gelatin gel based skin model was used in EIS measurements to overcome the problems associated with real skin measurements. Both microstructurings were found to significantly decrease the contact impedance modulus. In addition, the Ag coating caused highly significant (p<0.01) decrease in impedance modulus for each surface topography, e.g. for smooth electrodes the impedance drop was roughly from 140±30kΩ to 50±10kΩ at 1Hz. The Ag coating combined with microstructuring expectedly revealed the most superior electrical characteristics as the contact impedance declined even as low as 10±2kΩ at 1Hz. Instead, no statistically significant differences in impedance values were detected between the two microstructurings in the uncoated or Ag-coated electrodes. However, more sparse microstructuring (100) seemed to offer lower variation, i.e. more reproducible results. In conclusion, present microstructured electrodes that do not pierce any layer of skin but improve the contact to skin might be a good alternative to conventional wet electrodes.

Fabrication and impedance measurement of novel metal dry bioelectrode

To obtain the bioelectrodes with better stability, a novel construction of metal dry bioelectrode was designed to meet the specific application requirements of electrode performance for bioelectric impedance technique. The laser micromachining process was then employed to fabricate the microstructure arrays on the surface of dry bioelectrodes. Based on the material removal process of laser micromachining, the forming mechanism of surface microstructure arrays on the metal dry bioelectrode was further discussed. Using the two-electrode measurement method, the impedance values of electrode-skin for three types of bioelectrodes were recorded by measuring the left forearm position. Comparing to the conventional silver/silver chloride (Ag/AgCl) ECG electrode and metal planar bioelectrode, the metal dry bioelectrode with surface microstructure arrays presented less than 50Ω variations of contact impedance and better stability because of significant improvement of contact interface between electrode and skin, suggesting that it is an outstanding candidate for long-term clinical applications.

Novel dry electrodes for ECG monitoring

The development, fabrication and characterization of two novel dry bioelectrodes—conductive and capacitive ones—for biopotential monitoring are presented. The new electrodes have the potential to improve the applicability of dry electrodes in ambulant recording of ECG by reducing motion artifacts as well as the contact impedance to the skin. Furthermore, a passive filter network is integrated into the new electrodes to suppress slow offset fluctuation of the ECG signal caused e.g. by motions like breathing or changes in the electrode–skin interface properties. Compared to standard gel electrodes these new electrodes exhibit equivalent and superior contact impedances and biosignals. The integrated filter network effectively suppresses fluctuating offset potentials.