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

Herein, 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.

Electroencephalography (EEG)-based brain−computer interfaces (BCIs) have received ever-growing attention from global researchers and investors recently.Apart from reliable EEG signals, real-world EEG-based BCIs, such as physiological monitoring, [1][2][3] neurofeedback training, 4,5 intelligent control, 6,7 and neuromarketing, 8,9 attach great importance to user-friendliness.With the continuous breakthroughs in microelectronics and signal processing, the development of BCIs, especially noninvasive BCIs, has recently entered a fast lane.In 2019, DARPA launched an ambitious "Next-Generation Nonsurgical Neurotechnology" program with the primary goal of developing high-performance BCI technology comparable to the existing invasive ones. 10As known, EEG signal acquisition is the first crucial procedure for closed-loop noninvasive BCIs, which directly determines the performance of BCIs.Therefore, the development of noninvasive EEG electrodes with high performance and user-friendliness has become a burning question for realworld BCIs.
Currently, wet electrodes are regarded as a gold standard for EEG acquisition because of their minimal potential drift, low-contact impedance, and high signal quality. 11But the electrode setup often involves skin abrasion and conductive gel application with the assistance of a well-trained technician, which is tedious and timeconsuming.Additionally, conductive gels smear the hair and require thorough cleaning after recording, causing discomfort and even skin irrational to participants.It is beyond doubt that wet electrodes are not suitable for realworld BCIs.To improve user-friendliness, researchers are devoted to developing dry electrode technology.In general, dry electrodes direct contact with the skin without any extra electrolyte. 12Therefore, dry electrodes have demonstrated overwhelming advantages of cleanliness, self-application, rapid setup, and wearing comfort.4][15][16][17] Active dry electrodes can partially alleviate the requirement of low electrode−skin impedance by integrating an input ultrahigh impedance preamplifier.These active dry electrodes are less sensitive to environmental noises, but they are still susceptible to motion artifacts. 18,19or these reasons, dry electrodes are not prevalent and are not widely available.
As a compromise solution, semidry electrodes innovatively replace conductive gels with a tiny amount of electrolyte fluids, addressing most of the inherent shortcomings in dry and wet electrodes while maintaining the advantages of both.Typically, the semidry electrodes wet the scalp by releasing trace electrolyte liquid from a built-in reservoir and then maintain a relatively low and stable electrode−skin interface, thereby ensuring reliable and comfortable EEG signal acquisition. 20The semidry electrode concept was proposed by Mota et al. 21in 2013 and has become one of the most competitive electrode solutions ever since.Firstgeneration semidry EEG electrodes deliver trace amounts of the electrolyte into the scalp from electrolyte-retention materials such as microseepage sponge 22 and porous titanium 23 under pressure triggering.However, these semidry electrodes usually suffer from uncontrolled electrolyte release, impedance mismatches, and signal instability. 20,21In addition, they are prone to discomfort due to prolonged pressure on the scalp.5][26][27][28] However, they require regular replenishment of the electrolyte, and the convenience still needs to be improved.The stiff porous wicks or ceramics can also cause discomfort to the participant with prolonged wear.To overcome these problems, our group proposed an innovative semidry electrode based on polyacrylamide/ polyvinyl alcohol superporous hydrogel, enabling an automatically "charge−discharge" electrolyte. 29Like a rechargeable battery, saline was quickly "charged" into superporous hydrogel within few seconds because of its fast-swelling rate and strong absorption capacity.Then, a tiny amount of saline was steadily 'discharged' from superporous hydrogel into the scalp triggered by the capillary force and gravity.The superporous hydrogelbased semidry electrodes exhibited low-contact impedance and could acquire EEG signals highly similar to wet electrodes.However, the superporous hydrogels were often packed into a small cavity with multiple tips due to their poor mechanical performance, leading to a contact issue to the scalp.][32] Therefore, it seems to be a dilemma to achieve both strong mechanical performance and good adhesion in a hydrogel electrode.
To address the issue of mechanical performance, skin adhesion, and conformity, high-strength, and selfadhesive hydrogels have been successfully developed for monitoring electrophysiological signals recently.These hydrogels demonstrated highly adhesive and compliance to the nonhairy skin, resulting in high sensitivity for detecting subtle electrophysiological signals, [33][34][35][36] particularly motion, electrocardiogram (ECG), and electromyogram (EMG).There are also very few studies that have demonstrated the recording of EEG signals in the nonhairy skin-like forehead from only one volunteer. 33,34pparently, the feasibility of real-world BCIs has not been adequately validated in this way, as they do not cover the hairy sites, the main area for EEG signals.As known, the hairy scalp is quite different from the bare skin.To bridge the gap in semidry electrodes for realworld BCIs, it is imperative to realize semidry electrodes with excellent robustness, favorable self-adhesive, and low-contact impedance.
8][39][40]  semidry electrode could capture high-quality EEG signals comparable to conventional wet electrodes but in a more convenient and comfortable manner.Besides, this semidry electrode showed robust P-QRS-T peaks during ECG recording.

| Preparation of PVA/PAM DNHs
The PVA/PAM DNHs were synthesized by the cyclic freeze−thaw method (Scheme 1).Typically, a certain amount of PVA and DI water was added to a roundbottom flask and heated at 90 °C for 6 h under vigorous stirring to form a uniform PVA solution.A definite amount of the AM monomer was then dissolved and added to the above PVA solution.Afterward, MBA, TEMED, and APS were sequentially added to the above-mixed solution to get a pre-gel solution.As shown in Table S1, four different PVA/ PAM DNHs were synthesized by changing the mass fraction of PVA from 2.5 wt% to 10 wt%.The mass fraction of AM in the mixture was fixed at 17.5 wt% for all hydrogels.The weights of APS, TEMED, and MBA were kept at 0.6%, 0.55%, and 0.06% of the AM monomer, respectively.After the mixture became clear, inert nitrogen gas was purged into the mixture to remove the dissolved oxygen.The pre-gel solution was subsequently injected into a tailor-made mold via a syringe needle and transferred to a 60 °C oven for 1 h to obtain the chemically cross-linked PAM hydrogels.To obtain PVA/PAM DNHs, the asobtained PAM hydrogels were subjected to 12 h of freezing at −20 °C and 6 h of thawing at 25 °C for two cycles.To remove unreacted monomers, the PVA/ PAM DNHs were immersed in absolute ethanol for 48 h, and absolute ethanol was replaced each 12 h.PVA/PAM DNHs were labeled as PVA-x, where x denotes the PVA content.For ease of setup, the electrode cavity (b) is detachably connected to the upper cover (a) by means of a thread.Considering its excellent nonpolarizable property, conductive Ag/AgCl ink (Greentek Pty.Ltd.) was coated onto the inner surface of the upper cover (a) and electrode cavity (b) for efficiently sensing EEG signals.Semidry electrodes were attached to an EEG cap by metal snaps (f) and snap connectors.The tapered PVA/PAM DNH allows the semidry electrode to pass smoothly through the thick hair and make stable contact with the scalp, thereby efficiently lowering the contact impedance.

| Material characterization
The morphologies and microstructures of PVA/PAM hydrogels were measured by scanning electron microscopy (SEM, Zeiss Sigma 300).The hydrogel samples were swollen to equilibrium in DI water at room temperature and then frozen in liquid nitrogen immediately.Before SEM observations, the samples were sputter-coated with a thin layer of gold.The chemical compositions of the PVA/PAM hydrogel were analyzed using an FTIR spectrometer (Thermo Scientific Nicolet iS20) in transmission mode with a wave number range of 4000-400 cm −1 .The hydrogel samples were cut into small pieces and then ground into powder.The hydrogel powders were ground with excess KBr and then compressed to obtain a samplesupported KBr pellet.

| Automatically "charge− discharging" electrolyte behavior
The "charge" electrolyte behavior of the PVA/PAM DNHs was studied by measuring their swelling ratios.First, the samples were dried at 60 °C to a constant weight.Then, the completely dried samples were immersed in DI water and saline (0.9% NaCl solution) at room temperature.Afterward, the samples were taken out and weighed at regular time intervals after gentle surface wiping using napkins.The swelling ratio (Q) is estimated as follows: where W d and W s are the weight of the dried and swollen samples, respectively.In addition, the 'charge' electrolyte behavior of the PVA/PAM DNHs was investigated by monitoring their weight loss.To simulate the scalp contact, the semidry electrodes with fully swollen PVA/PAM DNHs were fixed at the surface of the pigskin at a contact pressure of 200 N/m 2 , which is equal to the contact pressure approximately for EEG applications.The samples were weighted at a predetermined time and then the weight loss of DNHs was estimated.The measurements were performed in triplicate at room temperature.

| Mechanical performance, fatigue resistance, and adhesion performance
Detailed experimental procedures, including mechanical performance, fatigue resistance, adhesion performance, and semidry electrode performance, are available in Online Supporting Information S1-S3.

| Subjects and ethics
Ten volunteers (six men and four women, 22-30 years) were enrolled to participate in the in vivo trials including electrode-skin impedance measurements and EEG acquisition.All subjects had no history of neurological diseases and normal or corrected visual acuity.The subjects signed an informed consent form before the in vivo trials.All in vivo trials were granted by the local research ethics committee of Hunan University of Technology (No. 2021090601).

| Electrode potential
The open-circuit potentials (OCPs) of the PVA/PAM DNH-based semidry electrodes were recorded for 10 min in 0.9% NaCl solution using a Keithley 2000 digital multimeter (Keithley Instrument, Inc.).The sampling rate for OCP measurements was set at 1 Hz.For comparison, the OCPs of the semidry EEG electrodes in the absence of PVA/PAM DNHs were also recorded.A sintered Ag/AgCl electrode (Greentek Pty.Ltd.) was used as the reference electrode.Three important parameters, equilibrium potential, potential drift, and off-set potential, were calculated referring to our earlier studies.

| Mechanical performance
Figure 2A shows the tensile stress-strain curves of PVA/PAM DNHs with various PVA content.As the PVA content increases from 2.5 wt% to 10.0 wt%, the fracture stress of the DNHs increases from 0.162 MPa to 0.883 MPa, while the fracture strain increases from 500% to 980% (Figure 2B).Correspondingly, their elastic moduli are 34.8,55.7, 72.5, and 84.8 kPa (Figure 2C).The compressive stress-strain curves of PVA/PAM DNHs are illustrated in Figure 2D.Their compressive stress increases from 0.081 MPa to 0.543 MPa as the PVA content increases from 2.5 wt% to 10 wt% (Figure 2E).Their corresponding elastic moduli are 77.6,205, 324, and 556 kPa (Figure 2F).The enhanced mechanical performance is highly associated with the dual-network structure and the crosslinking density of PVA. 42The physical/ chemical dual-network structure can improve its mechanical properties.In addition, hydrogels with more PVA content have higher cross-linking density during freezing, which also contributes to the improvement of mechanical properties.

| Automatically "charging-discharging" electrolyte behavior
For a fast setup, the electrolyte fluids such as saline should be quickly 'charged' into the PVA/PAM DNHs.Therefore, the "charge" electrolyte behavior of the PVA/PAM DNHs was studied by measuring their swelling ratios in DI water and saline.As shown in Figure 3A,B swelling of DNHs in electrolyte liquid gradually slows down, because the swelling rate highly depends on the diffusion; and finally, the swelling of the DNHs is in dynamic equilibrium, with a slight increase in the swelling ratio.The swelling ratios of PVA 2.5 wt%, PVA 5.0 wt%, PVA 7.5 wt%, and PVA 10 wt% in DI water achieve 930%, 750%, 650%, and 550% in 36 h, respectively.As the PVA content increases, the pore size of PVA/PAM DNHs becomes smaller, leading to lower capillarity of pores.Therefore, an increase in the PVA content results in a decrease in the swelling ratio.As shown in Figure 3C, the swelling ratios for those four PVA/PAM DNHs in saline achieve 860%, 700%, 600%, and 500% in 36 h, respectively.The swelling ratios of all PVA/PAM DNHs in saline are a litter lower than those in DI water.It is mainly because saline weakens the hydrogen bonds between the -OH and -NH 2 groups of the polymers with water, resulting in a decrease in the swelling ratio of the hydrogel. 43he "discharge" electrolyte behavior of PVA/PAM DNHs was then investigated by regularly monitoring their weight losses.For all the PVA/PAM DNHs, continuous decreases in weight loss are clearly observed over a period of 22 h (Figure 3D), demonstrating that the saline "charged" into the pores of PVA/PAM DNHs is sustainably discharged into the scalp.It suggests that the PVA/PAM DNHs have excellent "discharge" electrolyte capacity.The weight losses of the four PVA/PAM DNHs with PVA content ranging from 2.5 wt% to 10 wt% are 282.6,260.3, 238.8, and 216.6 mg after 8 h of skin contact.Generally, the "discharge" electrolyte rate highly depends upon the pore size.Therefore, the "discharge" electrolyte capacity of PVA/PAM DNHs decreases as PVA content increases.It is noted that the "discharge" amount of saline for PVA 10 wt% gels is up to 26.1, 123.8, 216.6, and 286.6 mg after 1 h, 4 h, 8 h, and 12 h of skin contact, which are sufficient to maintain a relatively stable electrode-skin interface.To our surprise, the weight decreases almost linearly over 8 h.So, we can safely infer that the semidry electrode can operate stably for at least 8 h.
As discussed in Sections 3.2 and 3.3, the PVA content has the opposite effect on the automatic "charging-discharging" electrolyte capacity and mechanical strength.Among, PVA 7.5 wt% achieves a better balance between "charging-discharging" electrolyte capacity and mechanical strength (Figure S2).Hence, PVA 7.5 wt% gels were chosen as electrolyte-retention materials for semidry EEG electrodes.

| Self-adhesive property
The excellent adhesion property of the PVA/PAM DNHs is essential for the stable electrode-skin interface. 44Therefore, the adhesion strength of the PVA/ PAM DNHs was also investigated.As shown in Figure 4A, the PVA 7.5 wt% exhibits strong adhesion on various solid/soft surfaces (including silicone rubber, ceramic, iron, plastic, glass, nickel, rubber, PET, and wood) without any surface modification.In addition, the PVA/PAM DNHs adhere firmly to the forearm without falling off when the arm swings.The PVA/PAM DNHs can also be easily peeled off from the arm without any residues (Figure 4B).Moreover, the arm also did not feel any pain or damage during the peeling process.The PVA/PAM DNH also adheres conformably to the knuckles and does not fall off even when the fingers are flexed.Furthermore, 90°peel tests were also performed to quantify their adhesive strength (Figure 4C).The peel forces in the histogram indicate that the PVA/PAM DNHs have strong adhesion to hydrophilic substrates such as aluminum and steel.Interestingly, the PVA 7.5 wt% DNHs also showed good adhesion to pigskin with an interfacial toughness of 197 N/m, comparable to hydrophilic substrates.It can be inferred confidently that the PVA/PAM DNHs can be attached firmly to human skin, due to their very similar characteristic to human skin.However, the PVA/PAM DNHs showed poor adhesion strength on the hydrophobic PTFE.No hydrogel fragments were left on the substrate surface after peel tests.These results suggest that PVA/PAM DNH with good adhesion is one of the most promising candidates for human health monitoring.

| Fatigue resistance
Flexible semidry electrodes are often exposed to large and repetitive external stimuli, which can lead to deformation and damage to the hydrogel structure.So, the development of hydrogels with fatigue resistance is therefore important to extend their life.To gain insight into the fatigue resistance of PVA 7.5 wt% DNHs in response to tension, continuous loading-unloading tests were performed at 500% constant strain.The PVA/PAM DNHs were tested through six loading-unloading cycles without dwell time.As illustrated in Figure 5A, a large hysteresis loop with a high energy dissipation and peak stress (≈100 kJ/m 3 , 180 kPa) occurs in the first loading-unloading cycle, which may be due to the breakage of physical hydrogen bond cross-linking in the network. 45After the first cycle, the hysteresis loops almost overlap in the subsequent loading-unloading cycles, with almost constant dissipation energy and peak stress (Figure 5B).After six loading-unloading tests, the corresponding peak stress still retains 94.2% of the initial level (Figure 5C).These results signify that the hydrogels possess an excellent fatigue resistance to tensile deformation.
To assess the fatigue resistance to compression, consecutive compression loading-unloading tests were performed at 60% constant strain without dwell time.The hysteresis loops of the PVA/PAM DNHs almost overlap during the six loading-unloading cycles (Figure 5D), with negligible change in dissipation energy and peak stress (Figure 5E).The dissipation energy and peak stress still maintain at 68.5% and 98.1% of the initial levels, respectively (Figure 5F), further demonstrating the excellent fatigue resistance in response to compressive deformation.The PVA/PAM DNHs are compressed under a certain pressure during EEG recording, so the excellent fatigue resistance to compression is very critical for practical applications.Moreover, the elastomer-like antifatigue and superior energy dissipation capacity can significantly alleviate the external forces transmitted to human skin, thereby preventing the skin from damage.

| Electrode potential
Excellent nonpolarizable property of the PVA/PAM NDH-based semidry electrodes is a prerequisite for acquiring high-quality biopotential signals.The electrode assemblies for OCP measurements of semidry electrodes are depicted in Figure 6A.Essentially, the OCPs of Ag/ AgCl coating electrodes were measured when the PVA/ PAM DNHs were absent.The OCP curves for two types of electrodes are shown in Figures 6B,C.The equilibrium potential of PVA/PAM DNH-based semidry electrodes (1.271 ± 0.170 mV, n = 5) is greater than that of Ag/AgCl coating electrodes (0.352 ± 0.178 mV, n = 5), probably due to the relatively slow diffusion of saline in the PVA/PAM DNHs.However, the offset potential of semidry electrodes is 0.456 mV, comparable to that of the Ag/AgCl coating electrode (0.479 mV), indicating highly reproducible electrode potential for semidry electrodes.More importantly, the average potential drift of semidry electrodes (1.47 ± 0.39 μV/min) is comparable to that of Ag/AgCl coating electrodes (2.71 ± 1.87 μV/min).The potential drifts are below 10 μV/min for both typical electrodes, demonstrating that the semidry electrodes can offer a stable and smooth baseline for biopotential signal recording, particularly DC-mode EEG acquisition.All these results also suggest that the nonpolarizable properties of semidry electrodes, in terms of potential drift and off-set potential, are not influenced by the PVA/PAM DNHs.

| Electrode-scalp impedance
Electrode-scalp impedance is a critical parameter for capturing high-quality biopotential signals.At 100 kHz, the grand average impedance of semidry electrodes across 10 volunteers and nine locations is 0.69 ± 0.12 kΩ, reflecting that the PVA/PAM DNHs possess excellent ionic conductivity of the PVA/PAM DNHs.After immersing in saline, the conductivity of PVA/PAM DNHs increases significantly from 0.026 to 0.91 S/m, due to the incorporation of NaCl during swelling.As an ionic conductor in a circuit, the full swollen PVA/PAM DNH can light the LED bulb with bright light (Figure S3).At 10 Hz, the grandaverage impedance for the semidry electrode-scalp interface is 36.4 ± 16.8 kΩ, suggesting that the electrode-scalp impedance is dominated by skin impedance and contact impedance.A single semidry electrode-scalp impedance at 10 Hz is approximately estimated as 18.2 ± 8.9 kΩ.7][48] Due to the ability to automatically "charge and discharge" the electrolyte, the saline 'charged' in the PVA/PAM DNH rapidly "discharges" to the scalp surface, which greatly lowers the skin impedance and contact impedance, ultimately resulting in a relatively low electrode-scalp impedance.As shown in Figure 6D, the semidry electrode-scalp impedance is highly subject-dependent, because the individual characteristics of the skin and hair are quite different. 49,50The semidry electrode-scalp impedance ranges from 5.5 ± 1.2 kΩ (Subject 9, a man with short and thin hair) to 34.7 ± 3.6 kΩ (Subject 1, a woman with long and thick hair).The semidry electrode-scalp impedance for Subject 8 is close to the average impedance over all subjects, so Subject 8 was chosen as the representative subject to investigate the electrode stability.It was reported that no obvious signal degradation occurred in the standard EEG frequency bands when the electrode-skin impedance is below 40 kΩ. 51Therefore, it is worth emphasizing that the highest semidry electrode-scalp impedance (10 Hz) from subject 9 is less than 40 kΩ, which is acceptable for a modern EEG amplifier.As shown in Figure 6E, the impedance values (10 Hz) for semidry electrodes at different locations are very similar for each participant, with the average impedance from 15.8 ± 6.3 kΩ (Oz) to 20.2 ± 8.9 kΩ (Cz).To our surprise, the impedance variations for each participant between all eight locations are within 5 kΩ, which minimizes the impedance mismatch and thus improves the common-mode rejection ratio.
The stability of the PVA/PAM DNH-based semidry electrodes was also evaluated by monitoring the change in electrode-scalp impedance (10 Hz) over time (Figure 6F).After 1 h, the electrode-scalp impedance for a pair of semidry electrodes decreases from the initial value of 34.3 ± 8.6 kΩ to 25.4 ± 3.6 kΩ, which was mainly due to the continuous accumulation of the saline "discharged" from PVA/PAM DNHs.Subsequently, the impedance increased slowly with time, increased to the initial value after 6 h, and further increased to 38.2 ± 5.6 kΩ at 8 h.Overall, the electrode-scalp impedance of the semidry electrodes levels off with a slight increase, mainly due to the sustained release of saline from the PVA/PAM DNHs.This strongly suggests that the semidry electrodes can acquire biopotential signals reliably and stably for at least 8 h.

| Preliminary validation of application feasibility for real-world BCIs
To validate the application feasibility for real-life BCIs, EEG signals in two classic BCI paradigms were simultaneously recorded by the semidry electrodes and wet electrodes at adjacent sites.Figure 8  To quantify the signal similarities between the semidry and wet electrodes, the temporal correlations of the EEG signals were also analyzed.In the eye open/ closed paradigm, the mean temporal cross-correlation coefficient between these two typical electrodes over 10 participants is 0.919 ± 0.054, with the highest value at P3 (0.964 ± 0.014) and the lowest value at C3 (0.842 ± 0.027) (Figure 10A).In the N200 speller paradigm, the mean temporal cross-correlation coefficient between these two typical electrodes is 0.912 ± 0.050, with the highest value at Pz (0.963 ± 0.012) and the lowest value at C4 (0.843 ± 0.025) (Figure 10B).The lowest correlation coefficients occur at positions C3 or C4, possibly related to the relatively poor contact at these positions due to the imperfectly matched cap.Nevertheless, the EEG signals for the semidry/wet electrode pairs are highly similar, and the temporal cross-correlation coefficient matches or even exceeds the previous ones. 21,25,27,29,47,48,52,53The results demonstrate that the PVA/PAM DNH-based semidry electrodes are as reliable as wet electrodes in EEG acquisition.It was noted that the results only demonstrate the correlations of the ERPs.Next, the BCI performance of the semidry electrodes will be further validated by four classic BCI paradigms.
Unlike superporous hydrogel, 29 the PVA/PAM DNHs can achieve a good balance between "charging-discharging" electrolyte capacity and mechanical strength.The fracture of the hydrogel is effectively avoided due to the improved mechanical properties, which prolongs the service time of the PVA/PAM DNH-based semidry electrodes.In addition, the PVA/PAM DNHs cannot be packed into the small cavity with multiple tips even under excessive pressure, ensuring reliable contact with the scalp.The excellent adhesion property of the PVA/PAM DNHs also improves the stability of the electrode-skin interface.Together with their selfapplication, rapid setup, and user-friendliness, the semidry electrodes will be a feasible alternative to the wet and dry electrodes in BCI applications, especially for real-life scenarios.

| CONCLUSIONS
In this study, a robust, self-adhesive, and low-contact impedance PVA/PAM DNH-based semidry electrode was fabricated for real-world BCIs.The mechanical performance and "charge-discharge" saline behavior were tailored by regulating the PVA content.The PVA 7.5 wt% gels were chosen as the saline reservoir due to their satisfactory compromise between mechanical performance and "charge-discharge" saline behavior.The resultant PVA/PAM DNHs showed excellent adhesive and compliant properties to the hairy scalp, promoting the establishment of a robust electrode/skin interface for biopotential signal transmission.Moreover, the PVA/PAM DNHs steadily released a tiny amount of saline onto the scalp to achieve negligible potential drift and low electrode-skin impedance.The PVA/PAM DNH-based semidry electrodes displayed low and stable impedance (18.2 ± 8.9 kΩ), and their electrode potential was very stable with the potential drift as low as 1.47 ± 0.39 mV/min.The semidry electrode could effectively capture EEG signals in the open/closed eye and N200 speller paradigms, and the EEG signals for these two typical electrodes are highly similar.Besides, this semidry electrode could also capture distinct P-QRS-T peaks during ECG measurements.More importantly, this semidry electrode could acquire both EEG and ECG signals in a more convenient and comfortable manner, and the service time of the semidry electrode is up to 8 h for a single charge of saline.Overall, the PVA/PAM DNH-based semidry electrodes demonstrated outstanding advantages such as high durability, excellent selfadhesive, automatic 'charge-discharge' saline capacity as well as low-contact impedance, robust signals, and satisfactory user-friendliness, which are very suitable for real-world biopotential signal acquisition.This study offers a unique perspective for developing nextgeneration wearable healthcare devices.
By teaching a new trick to a very old dog, robust, self-adhesive, and low impedance physical/chemical crosslinked PVA/PAM DNH-based semidry electrodes were fabricated to record EEG signals at the hairy scalp for biopotential signal acquisition (Scheme 1).By regulating the PVA content, the PVA/PAM DNHs achieved the enhancement in mechanical strength while maintaining good automatic "charge−discharge" saline.The optimal PVA/PAM DNHs showed admirable adhesive and compliance to the hairy scalp, facilitating the establishment of a robust electrode/skin interface for electrophysiological signal transmission.Moreover, as an efficient electrolyte reservoir, PVA/PAM DNHs steadily released a tiny amount of saline onto the scalp to reduce the potential drift and electrode−scalp impedance.More importantly, the application feasibility of real-world BCIs was adequately validated by 10 participants using two classic BCI paradigms, and the proposed PVA/PAM DNH-based S C H E M E 1 Scheme illustration of the fabrication of PVA/PAM DNH-based semidry electrodes for biopotential signal acquisition.

2. 3 |
Fabrication of PVA/PAM DNHbased semidry EEG electrodes As shown in Scheme S1, the PVA/PAM DNH-based semidry EEG electrode includes an upper cover (a), an electrode cavity (b), an Ag/AgCl coating (c), a PVA/PAM DNH (d), and a metal snap (e).The upper cover (a) and electrode cavity (b) were made of plastic via 3D printing.

3 |
RESULTS AND DISCUSSION 3.1 | Material characterization

Figure
Figure 1A-D shows the SEM images of four PVA/PAM DNHs with different PVA content.All the hydrogels exhibit abundant open porous structures, which facilitate automatically 'charging-discharging' electrolyte capacity.Remarkably, the PVA content has a significant influence on the microstructures of PVA/PAM DNHs.As the PVA content increases from 2.5 wt% to 10 wt%, the pore size of the PVA/PAM DNHs decreases from 15 ± 4 μm to 1.0 ± 0.3 μm (Figure 1E).As shown in Scheme 1, the physical-chemical PVA/PAM DNHs were synthesized via a two-step method.The covalently crosslinked networks of PAM interpenetrated with PVA chains were formed as the first network.Then, physically cross-linked networks of PVA were formed as the , the swelling of the PVA/PAM DNHs is roughly divided into three stages.First, the samples swell quickly within the first few hours due to the hydrophilic surface and capillary action.Then, the F I G U R E 2 Mechanical performance of the PVA/PAM DNHs with various PVA content.(A) Tensile stress-strain curves of PVA/PAM DNHs with different PVA content; the corresponding peak stress (B) and elastic modulus (C) for tensile tests; (D) compressive tensile stressstrain curves of these PVA/PAM DNHs; the corresponding tensile strength (E) and elastic modulus (F) for compression tests.Elastic modulus estimated in the initial linear range from the stress-strain curves.PVA/PAM DNH, physical/chemical crosslinked polyvinyl alcohol/polyacrylamide dual-network hydrogel.

F
I G U R E 3 Automatically "charging-discharging" electrolyte behavior of PVA/PAM DNHs with various PVA content."Charge" electrolyte behavior of four PVA/PAM DNHs by monitoring their swelling behavior in DI water (A) and saline (B).(C) Swelling ratios of four PVA/PAM DNHs in DI water and saline.(D) Weight losses of four PVA/PAM DNHs in contact with pigskin.PVA/PAM DNH, physical/chemical crosslinked polyvinyl alcohol/polyacrylamide dual-network hydrogel.

F
I G U R E 4 Self-adhesive property of PVA/PAM DNHs with 7.5 wt% PVA.(A) Images of PVA/PAM DNHs directly adhere to different solid surfaces; (B) PVA/PAM DNHs adhered to human skin at the forearm and finger and then peeled off from the human skin.(C) Peel force of the PVA/PAM DNHs at four different substrates including aluminum, steel, pigskin, and PTFE, at a peel rate of 100 mm/min.PVA/ PAM DNH, physical/chemical crosslinked polyvinyl alcohol/polyacrylamide dual-network hydrogel.

F I G U R E 5
Fatigue resistance of the PVA/PAM DNHs with 7.5 wt% PVA.(A) Six consecutive tensile load-unload tests for PVA 7.5 wt% at 500% constant strain.(B, C) Corresponding change in dissipation energy and peak stress.(D) Six consecutive compression load-unload tests at 60% constant strain.(E, F) Corresponding change in dissipation energy and peak stress.PVA/PAM DNH, physical/chemical crosslinked polyvinyl alcohol/polyacrylamide dual-network hydrogel.

F I G U R E 6
Electrochemical performance of the PVA/PAM DNH-based semidry electrodes.(A) Schematic illustration of the setup for OCP measurements; OCPs of the semidry electrodes in the presence (B) and absence (C) of the PVA/PAM DNH; electrode/scalp impedance (100 kHz and 10 Hz) for a pair of the semidry electrodes recorded at various subjects (D) and electrode sites (E); and (F) stability of the semidry electrodes monitored by the 10 Hz impedance within 8 h (a pair of electrodes).PVA/PAM DNH, physical/chemical crosslinked polyvinyl alcohol/polyacrylamide dual-network hydrogel.
Considering its low electrode-skin impedance, superior mechanical compliance, and excellent selfadhesiveness, the PVA/PAM DNHs show promising prospects as an electrolyte-retention element for acquiring subtle ECG signals.The photos of the PVA/ PAM DNH-based semidry and commercial gel-based electrodes are shown in Figure 7A,B.The PVA/PAM DNHs were adhered to on the electrodes to improve the bonding to the skin, substantially reducing the imperfect contact caused by human activity.The semidry electrodes were attached to the chest and forearm of a representative participant (Figure 7C,D), and commercial ECG electrodes were also attached to the same locations for comparison.Distinguished and minimized fluctuated P, QRS, and T waves are clearly identified in the ECG waveforms for both typical electrodes.In addition, the ECG signals from both types of electrodes are very similar.More importantly, the PVA/PAM DNH-based semidry electrodes can acquire high-quality ECG signals after 24 h of continuous use, indicating high feasibility in long-term healthcare monitoring.PVA/PAM DNH semidry electrodes combined with portable and mobile ECG equipment can acquire high-quality ECG signals for remote diagnosis of cardiovascular problems.
shows the representative EEG signals for the semidry and wet electrodes in the eye open/closed paradigm.Both the semidry and wet electrodes exhibit distinct alpha rhythms during eye closure.Moreover, the EEG signals of the two types of electrodes at adjacent sites are highly F I G U R E 7 Results of ECG signal recording.Photos of PVA/PAM DNHbased semidry (A) and commercial gelbased electrodes (B); ECG signals recorded at the chest (C) and forearm (D) from Subject #8 using these two typical electrodes.PVA/PAM DNH, physical/ chemical crosslinked polyvinyl alcohol/ polyacrylamide dual-network hydrogel.similar, preliminarily indicating high-quality EEG signals for the semidry electrodes.In the eye open/closed paradigm, the representative spectra of these two typical electrodes at electrode location Oz are plotted in Figure 9A,B.For both the semidry and wet electrodes, distinct and similar alpha waves are observed at 8-12 Hz during eye closure, while alpha waves are significantly suppressed during eye open.The representative spectra for the N200 speller paradigm are shown in Figure 9C,D.Similarly, these two typical electrodes exhibit apparent N200 responses at 200 ms for the target stimulus.

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I G U R E 8 Representative EEG signals from subject #8 in the eyes open/closed paradigm.F I G U R E 9 (A and B) Representative spectra for the eye open/closed paradigm at electrode Oz from Subject 9. (C and D) Representative spectra for the N200 speller paradigm at electrode Pz from Subject 6.