Low-Impedance and Degradable Soft Dual-Mode Hydrogel Sensor for Real-Time Strain and Bioelectric Signal Acquisition PDF Free Download
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Low-Impedance and Degradable Soft Dual-Mode Hydrogel Sensor
for Real-Time Strain and Bioelectric Signal Acquisition
Yang Wang, Jingxi Wang, Chengkuo Lee, and Yuhang Li*
Cite This: ACS Appl. Electron. Mater. 2025, 7, 9678−9691
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sı Supporting Information
ABSTRACT: Wearable bioelectronic systems demand materials
that strike a balance between mechanical softness, reliable electrical
performance, and biocompatibility. Yet, many conventional gels fall
short in practice often showing high interfacial impedance, limited
stretchability, and poor degradability, making them less suitable for
emerging applications in flexible healthcare technologies. Here we
report a degradable, low-impedance Hydrogel developed through a
simple aqueous synthesis using waterborne polyurethane (WPU),
poly(vinyl alcohol) (PVA), and sodium tetraborate (STD). The gel
forms a hybrid cross-linked network based on dynamic borate ester
bonds and hydrogen bonding, which imparts both good
conductivity and mechanical adaptability. Functionally, the materi-
al operates as both a soft strain sensor and a bioelectrical interface.
It can detect joint motion with high sensitivity and simultaneously record clean electrophysiological signals such as
electromyography (EMG), electroencephalography (EEG), and electrocardiography (ECG). When benchmarked against
commercial Ag/AgCl electrodes, the gel exhibits noticeably lower skin−electrode impedance and enhanced signal clarity. The
ability to acquire both strain and EMG signals through a single, integrated interface opens up the possibility of motion artifact
reduction and genuine multimodal sensing, without adding complexity to the system. With its softness, conductivity, and
environmental degradability, this gel presents a versatile and promising platform for future wearable healthcare devices,
neuromuscular diagnostics, and human−machine interaction systems.
KEYWORDS: hydrogel, low impedance bioelectrical monitoring, strain sensing, wearable sensors, multimodal interface
1. INTRODUCTION
In recent years, wearable and flexible biosensors for the
monitoring of bioelectric and human motion signals including
ECG,
1
EMG,
2
EEG,
3
have gained widespread attention for
their applications in clinical diagnostics, motion rehabilitation,
human−machine interaction, and real-time health monitor-
ing.
4−6
Their ability to intimately interface with the skin and
follow complex body deformations makes them ideal for
continuous physiological sensing in dynamic environments.
For human monitoring, muscle activation tracking, brain
computer interfacing, and gait analysis require the simulta-
neous acquisition of bioelectrical signals and mechanical
signals to accurately capture the spatiotemporal dynamics of
neuromuscular activity.
7−10
Collecting only one type of signal
often leads to incomplete or ambiguous interpretations of
complex physiological behavior. Dual-modal sensing thus plays
a vital role in improving the precision, robustness, and
responsiveness of wearable diagnostic and control systems.
However, realizing this dual functionality in a single device
presents significant challenges.
11−15
The sensing interface must
be both electrically sensitive and mechanically compliant,
capable of accurately detecting weak bioelectric potentials
while undergoing large strains without signal degradation. At
the same time, it must maintain tight skin conformity, low
impedance, user comfort, all of which place strict demands on
material design and system integration.
16−20
Traditional electrode materials, including metals, carbon-
based composites, and liquid metals, present inherent
limitations in wearable settings.
21−23
Metallic electrodes,
although highly conductive and commercially mature, are
rigid and poorly matched to soft tissue, leading to high
interfacial impedance and motion artifacts.
24
Carbon-based
electrodes provide improved flexibility, but increasing their
conductivity often compromises softness due to high filler
content. Liquid-metal electrodes oer fluidic deformability but
are prone to leakage, oxidation, and biocompatibility
concerns.
25
Even gel-based electrodes, while oering superior
Received: May 19, 2025
Revised: October 17, 2025
Accepted: October 17, 2025
Published: October 23, 2025
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softness and skin contact, often suer from dehydration,
mechanical fragility, and insucient electrical performance.
26
These challenges highlight the need for new materials that
integrate mechanical compliance with electrical reliability.
27
Polymer-based hydrogel have gained increasing attention as
next-generation materials for soft and skin-conformable
electronics.
28
These gels typically consist of ionically
conductive polymer networks capable of forming robust yet
flexible interfaces with biological tissues.
29
Compared to
traditional conductive materials, hydrogel exhibit lower
modulus, better adhesion, and significantly reduced skin-
electrode impedance. Their ability to collect high-fidelity
signals while maintaining wearer comfort makes them ideal
candidates for flexible electrophysiological interfaces.
30,31
Despite these advances, polymer-based hydrogel still face
limitations, including poor mechanical toughness, limited
conductivity, or low signal stability over time. To overcome
these challenges, researchers have explored various strategies to
enhance the performance of hydrogel. Ye et al. incorporated
cellulose nanofibers (CNFs) into PVA, leveraging the
negatively charged carboxylate groups on CNFs to enhance
ionic mobility.
32
Mu et al. developed polymer hydrogel using
polyurethane, phosphotungstic acid, and ionic liquids, enabling
tunable conductivity and mechanical properties through
electrochemical stimulation, thereby broadening the applica-
tion scenarios for hydrogel.
33
He et al. employed 3D printing
technology to fabricate UV-cured hydrogel with a dual-
continuous nanostructure, simultaneously achieving high
ionic conductivity and excellent stretchability.
34
These
advances have made significant progress in reducing
impedance, enhancing mechanical strength, and extending
the lifespan of hydrogel. However, developing new material
systems with superior integrated properties combining low
cost, optimized fabrication processes, low modulus, low
impedance, and broad applicability in bioelectronic monitoring
remains an important research direction.
35
In this work, we report a soft, degradable, and low-
impedance water-based hydrogel constructed from WPU,
PVA, and STD. By leveraging hydrogen bonding interactions
from WPU and dynamic borate ester cross-linking of PVA, we
design a hybrid physically cross-linked network that exhibits
excellent mechanical adaptability and good conductivity. This
material functions as a dual-mode sensor capable of
simultaneously detecting strain and bioelectrical signals from
a single interface, enabling multimodal data acquisition with
minimal hardware complexity.
2. EXPERIMENTAL SECTION
2.1. Materials. PVA1788 was purchased from Aladdin Reagent
Co., Ltd. and prepared as an aqueous solution by heating and stirring
in a water bath at 90 °C for 2 h. The WPU used was an anionic
dispersion (38 wt %) obtained from Rigid Bio Co., Ltd. STD was
supplied by Sinopharm Group and used as a 5 wt % aqueous solution.
In the formulation described in this work, a component ratio of 1:1:1
refers to 1 part of PVA solution (prepared by dissolving 1 g of PVA in
9 g of water to obtain a 10 wt % solution), 1 part of WPU dispersion
(2.7 g of WPU dispersion, containing 1 g of solid polyurethane), and
1 part of 5 wt % STD (1 g of solution, corresponding to 0.05 g of
borax). Other ratios mentioned in the study follow the same
calculation principle.
2.2. Micro-/Nanostructural Characterization. The micro-
structural morphology of the gel samples was observed using a
desktop scanning electron microscope (EM-30+) operated at an
accelerating voltage of 15 kV with a magnification of 2000×. Prior to
imaging, the gel samples were pretreated by freeze-drying for 24 h,
followed by gold sputter coating to enhance conductivity. The
morphological features were then examined under the SEM.
2.3. Thermal Characterization. Thermal characterization was
performed using a comprehensive thermal analyzer (HCT-4, Hengjiu
Instrument Co., Ltd., Beijing, China). Approximately 25 mg of freeze-
dried gel sample was used for each test. The temperature range was
set from 25 to 110 °C. Thermogravimetric (TG), dierential thermal
analysis (DTA), and temperature variation data were simultaneously
recorded during the measurement.
2.4. Tensile and Compression Tests. Tensile and compression
tests were conducted using a universal testing machine (UTM5000,
SUNS, China). For the tensile tests, gel samples were prepared with
dimensions of 5 mm ×25 mm ×10 mm. For compression tests, cubic
specimens with dimensions of 1 cm ×1 cm ×1 cm were used. Both
tests were performed at a constant loading rate of 5 mm/s.
2.5. Rheological Measurements. Rheological properties of the
gels were measured using an Anton Paar MCR 302e rheometer. Both
amplitude sweep and frequency sweep tests were performed. The
angular frequency ranged from 0.1 to 100 rad/s, and the shear strain
in the amplitude sweep varied from 0.0001 to 1.
2.6. Degradation and Dehydration Tests. For the degradation
test, 2 g of gel was immersed in 1000 mL of deionized water at room
temperature. The weight of the gel was measured at predefined
intervals to monitor mass loss over time. For the dehydration test, 2 g
of gel was placed in an oven at 60 °C, and its weight was recorded at
regular time intervals to evaluate the water loss behavior.
2.7. Strain Sensing Test. A 1 g gel sample was attached to
various body joints, including finger joints, wrist, throat, and knee.
The two ends of the gel were connected to a digital multimeter
(Keithley 2001, Tektronix, USA) via conductive wires. The
multimeter was interfaced with a computer to continuously record
the resistance variation during joint movements.
2.8. Bioelectrical Signal Monitoring. A multidimensional
bioelectrical signal acquisition module connected to an Arduino
microcontroller was used for data collection. For EMG measurements,
three electrodes were placed on the inner forearm. EEG electrodes
were placed on the left and right forehead, with a reference electrode
positioned behind the ear. ECG measurements, two electrodes were
attached to the left chest (working and reference), and one electrode
to the right chest. The acquired signals were processed using fast
Fourier transform (FFT) filtering to remove noise and extract relevant
features.
2.9. Electrical Resistance Measurement. All electrical resist-
ance measurements were performed on fresh, fully hydrated gel
samples under ambient laboratory conditions (25 ±2°C, 40−50%
RH), which accurately reflect the operating state of the material in
practical bioelectronic applications. To evaluate the strain-dependent
resistance change, gel samples were cut into rectangular strips with
dimensions of 40 mm ×5 mm ×5 mm (length ×width ×thickness).
The resistance was measured using a digital multimeter under
dierent tensile strains by incrementally stretching the gel to
predefined lengths. At each strain level, measurements were repeated
three times to ensure reproducibility. The relative resistance change
(ΔR/R0) was calculated, where R0is the initial resistance at rest.
2.10. Electrochemical Impedance Spectroscopy Measure-
ments. EIS was carried out using a potentiostat/galvanostat (Gamry
Reference 12070, Gamry Instruments, USA) to characterize the skin−
electrode interfacial impedance. A modified configuration commonly
adopted for skin−electrode studies was employed. A commercial Ag/
AgCl electrode or the fabricated gel electrode was attached to the skin
surface and used as the working electrode. Both the counter and
reference electrodes were Ag/AgCl electrodes placed adjacently on
the skin to provide current return and potential reference. This
pseudothree-electrode configuration is functionally equivalent to a
two-electrode measurement and allows reliable assessment of
electrode−skin impedance under practical conditions. The impedance
spectra were recorded over a frequency range of 0.2−100,000 Hz with
an AC perturbation amplitude of 10 mV. The data were analyzed
using equivalent circuit modeling, and curve fitting was performed via
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Python scripts (based on the impedance.py library and custom-
written routines). Candidate models included resistive, capacitive
(constant phase element, CPE), inductive (L), and diusion
(Warburg, W) components. The optimal equivalent circuit for each
electrode was determined according to the minimum mean squared
error (MSE) criterion.
3. RESULTS AND DISCUSSION
3.1. Synthesis and Characterization of PVA-WPU Gel.
A highly stretchable and ionically conductive hydrogel was
fabricated by a straightforward aqueous-phase protocol
involving PVA, WPU, and STD. As illustrated in Figure 1a, a
10 wt % PVA aqueous solution and a WPU dispersion were
first mixed at a 1:1 volume ratio to form a homogeneous
precursor solution. Subsequently, 1 mL of 5 wt % STD
aqueous solution was rapidly added under continuous stirring.
Gelation occurred within 1 min at room temperature, yielding
a self-supporting and translucent white hydrogel.
The gelation mechanism is primarily governed by the
dynamic formation of borate ester bonds between the hydroxyl
groups on PVA chains and the boron species derived from
STD. In parallel, additional coordination interactions and
hydrogen bonding occur between functional groups within
WPU and the PVA−borate network, contributing to the
formation of a robust and flexible three-dimensional network
structure. STD undergoes hydrolysis to generate tetrahydrox-
yborate ions (B(OH)4
−) in water, which can reversibly react
with vicinal diol groups on the PVA backbone to form dynamic
borate ester linkages PVA−B(OH)−PVA. These reversible
covalent interactions create a chemically cross-linked network,
endowing the gel with mechanical stability and inherent self-
healing potential. Nevertheless, the binary PVA−STD system
alone tends to yield a brittle gel due to the rigidity imposed by
the borate cross-links.
Figure 1. Synthesis of the hydrogel. (a) Schematic illustration of the gelation process involving PVA, WPU, and STD. (b) Photographs showing the
gel formation process. (c) FTIR spectra of WPU, PVA, and the synthesized hydrogel. (d) FTIR dierence spectra.
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The incorporation of WPU markedly enhances the
mechanical resilience and toughness of the gel. As shown in
the predicted network model (Figure 1a), this improvement
can be attributed to multiple noncovalent interactions.
Hydrogen bonds are likely formed between the urethane
groups (−NH−COO−) of WPU and the carboxyl or hydroxyl
functionalities of PVA and STD. In parallel, weak coordination
may occur between amine or carboxylate groups in WPU and
borate species. Furthermore, the amphiphilic structure of WPU
likely contributes to improved interfacial compatibility and
molecular entanglement within the network.
The critical role of multicomponent synergy was further
elucidated through control experiments, as shown in Figure 1b.
Individually, both the 10 wt % PVA solution and the WPU
dispersion remained transparent liquids, and their 1:1 mixture
also showed no signs of gelation, indicating no significant
interaction between PVA and WPU alone. Upon addition of 1
mL of 5 wt % STD solution to this binary mixture, however,
rapid gelation occurred, resulting in the formation of a
uniform, white, and self-supporting hydrogel that firmly
adhered to the vial wall. This visual transition suggests that
the simultaneous presence of PVA, WPU, and STD is
necessary to initiate the formation of a robust three-
dimensional network structure, highlighting the necessity of
all three components for gel formation and mechanical
integrity.
FTIR spectra of WPU−STD, PVA−STD, and PVA−WPU−
STD are shown in Figure 1c. A distinct band at 1330−1360
cm−1is assigned to the asymmetric B−O stretching of borate
ester linkages, confirming the formation of dynamic covalent
cross-links between PVA hydroxyl groups and borate species
from STD.
36
The broad absorption between 3200−3600 cm−1,
corresponding to O−H and N−H stretching, becomes more
intense and broadened in the PVA-containing systems,
indicating an enhanced hydrogen-bonding network.
37
A very
weak shoulder in the 2700−2800 cm−1region is observed in
PVA-based gels, generally associated with aldehydic C−H
overtones or Fermi resonance bands reported in partially
hydrolyzed or oxidized PVA.
38
In addition, a weak band in the
1700−1800 cm−1range is consistently present in all samples,
and is more conservatively attributed to residual carbonyl
moieties, strong hydrogen-bonded species, or water-related
contributions.
39
FTIR dierence spectra were plotted using WPU as the
baseline (Figure 1d). After subtracting the WPU background, a
broad positive band remains in the high-wavenumber region,
highlighting hydroxyl-related contributions from PVA/STD
and indicating strengthened hydrogen-bonding networks.
40
A
Figure 2. Thermal behavior, mechanical adaptability, degradability, and microstructure of the gel. (a−c) Thermogravimetric analysis (TGA) and
dierential scanning calorimetry (DSC) curves of WPU, PVA, and the synthesized hydrogel. (d) Demonstration of the gel’s deformability, self-
healing ability, and ultrahigh stretchability. (e) Degradation behavior of the gel in deionized water. (f) Water loss of the gel under heating at 60 °C.
(g−i) SEM images showing the microstructures of WPU, PVA gel, and the hydrogel, respectively.
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derivative-like positive−negative feature is observed around the
carbonyl region, which is characteristic of a band position or
intensity shift upon hybridization, consistent with interfacial
interactions between WPU and PVA/STD. In the fingerprint
region, several residual bands attributable to C−O/C−O−C
vibrations from PVA and B−O related vibrations from STD
persist, further evidencing the participation of PVA/STD in
the hybrid network once the WPU background is removed.
These qualitative features support a picture in which PVA/
STD provide the functional groups responsible for chemical
cross-linking in the hybrid gel, while WPU mainly modulates
the local environment through hydrogen bonding and physical
entanglement.
The gel’s improved stretchability and flexibility are likely
attributed to the synergistic interplay between dynamic borate
ester cross-linking, extensive hydrogen bonding, and physical
entanglement mediated by the soft and hard segments of
WPU. This combination appears to promote a well-balanced
integration of mechanical robustness and flexibility within the
hydrogel network. Such a design may also facilitate rapid
gelation at room temperature, making the material a promising
candidate for multifunctional wearable electronics and
biosensing applications.
3.2. Thermal Analysis of Gels. To further investigate the
properties of the materials, thermogravimetric and DSC
analyses were conducted. The samples analyzed included
18.1 mg of WPU solution, 23.1 mg of PVA gel, and 19.7 mg of
ion gel. Given the water content in the materials, the
temperature range was set from room temperature to 100
°C, with a total analysis time of 50 min. The experimental
results are presented in Figure 2a−c, showing the weight
change (blue curve), dierential thermal signal (red curve),
and temperature (black curve).
In Figure 2a, corresponding to the WPU solution, the weight
gradually decreases with increasing temperature, stabilizing at
around 23 min with a total weight loss of 69.1%. The DSC
curve decreases continuously during the first 15 min, indicating
an endothermic process primarily attributed to water
evaporation. Between 15 and 23 min, a distinct exothermic
peak is observed, followed by stabilization at around 26 min.
This exothermic peak suggests a polymerization reaction of the
WPU under heating. Figure 2b illustrates the behavior of the
PVA gel. As the temperature increases, the weight decreases
and stabilizes at approximately 25 min, with a total weight loss
of 90.4%. The DSC curve predominantly reflects an
endothermic process due to water evaporation, and no
significant chemical reactions were observed during the
analysis. In Figure 2c, the thermal behavior of the ion gel,
the focus of this study, is presented. The weight loss trend is
similar to the previous two samples, with a gradual decrease
stabilizing at around 88.1% weight loss. However, the DSC
curve deviates significantly after 18 min. While the initial trend
matches that of the WPU solution and PVA gel, the dierential
thermal signal exhibits a slower increase between 20 and 30
min, and the stabilization time is notably longer. This suggests
that the ion gel undergoes a combined eect of water
evaporation and polymerization. This behavior also indicates
that the WPU in the ion gel does not undergo chemical
reactions but instead forms hydrogen bonds with the boron-
PVA network. These hydrogen bonds significantly enhance the
water retention capability of the material, underscoring the
unique structural characteristics of the ion gel.
3.3. Properties and Degradation of the Gel. The
hydrogel exhibits remarkable plasticity, stretchability, self-
healing, and biodegradability, making it highly promising for
flexible and transient electronic applications. As shown in
Figure 2d, various patterns including square and star were
fabricated using a printed model, illustrating the excellent
plastic deformability of the gel. The abundant hydrogen
bonding interactions within the gel matrix allow rapid
reconfiguration and shape retention after molding.
The gel also demonstrates excellent self-healing behavior.
When two broken gel segments are brought into contact, they
rapidly readhere, forming a unified structure. An observation
visually highlighted by bonding gels of dierent colors. This
property is particularly advantageous for applications in self-
healing circuits and wearable systems. To provide quantitative
validation, we conducted tensile tests before and after a 1 h
self-healing process at room temperature. As shown in Figure
S3, the hydrogel retained most of its stretchability, confirming
the eective reformation of its supramolecular network via
reversible boronate ester bonds between PVA and sodium
tetraborate.
Regarding extensibility, the hydrogel exhibits remarkable
capacity for large-area deformation. It can be stretched from a
compact initial shape into a substantially enlarged and ultrathin
film. Figure S4 displays the morphological change during
stretching, while Figure S5 shows the highly extended and
thinned gel placed in front of a human face, providing intuitive
visual confirmation of the extreme planar deformation and
scale.
The biodegradability of the gel was evaluated by immersing
2 g of the hydrogel in 1000 mL of deionized water at room
temperature. As shown in Figure 2e, gels with dierent
compositions exhibited similar degradation behavior, following
a nearly linear mass-loss profile with a rate of approximately
0.25 g/h. Once the remaining mass fell below 50% of the
original, the gel lost its structural integrity and could no longer
maintain its shape.
The water loss behavior of the gel was characterized at an
elevated temperature of 60 °C (Figure 2f). Rapid evaporation
occurred during the first 5 h, followed by a slower phase and
eventual stabilization around 10 h; in the extended measure-
ments between 10 and 12 h, the weights of gels with dierent
formulations remained essentially unchanged, indicating that
the system had reached a stable state. To assess environmental
stability under ambient conditions (25 °C, ∼50% RH), we
monitored the gel’s electrical resistance over 3 days. As shown
in Figure S6, the resistance increased by only ∼8% within the
first 12 h, indicating stable short-term electrical performance.
However, a marked increase was observed beyond 24 h, with
resistance rising by ∼84% at 24 h and exceeding three times
the initial value by 36 h due to progressive water evaporation.
These results suggest that the gel is well-suited for short-
duration or encapsulated use, while longer-term applications
may require additional packaging strategies to retain perform-
ance.
Figure 2g−i depict the microscopic morphologies of WPU,
PVA gel, and WPU-PVA gel observed through SEM. In Figure
2g, irregular layered structures and pores are evident, which are
likely attributed to the sublimation of ice crystals during the
freeze-drying process. This structure reflects the micro-
structural changes caused by solvent migration and phase
separation during the drying of WPU. Figure 2h reveals well-
ordered stripe-like patterns, which are presumed to result from
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the network structure formed by the dynamic covalent bonds
between PVA and STD, demonstrating the uniformity and
dynamic cross-linking properties of the PVA gel. Figure 2i
displays a prominent three-dimensional network structure in
the WPU-PVA gel, likely stemming from the chemical cross-
linking between PVA and borax, as well as the dynamic
hydrogen bonding interactions between borax and WPU. This
well-defined network structure imparts excellent toughness and
stretchability to the gel.
3.4. Tensile and Compression Properties of the Gel.
The mechanical properties of hydrogels are a key focus in their
research and application. To investigate their behavior, tensile,
compression, and rheological tests were conducted on particle
gels with varying compositions. Figure 3a−c present the tensile
strain−stress profiles of gels with dierent concentrations of
STD and WPU. The insets in each figure provide detailed
views of the stress−strain behavior near the peak. The results
indicate that stress increases rapidly at the beginning of
stretching, followed by a gradual decrease. Across all tested
formulations, the maximum stress reached only 3300 kPa,
demonstrating the gels’ remarkable deformability under tensile
stress. Furthermore, the gels could stretch up to 2000% of their
original length without breaking. Despite undergoing plastic
deformation, the exceptional toughness of these materials
makes them well-suited for applications requiring large
deformations.
Notably, as the WPU content increased, the peak stress
decreased significantly, indicating that the material became
softer (Figure 3d). Additionally, it was observed that excessive
additions of borax or WPU significantly degraded the
material’s performance when the PVA content remained
constant. Based on the results, an optimal ratio of WPU to
borax is recommended to be between 1 and 3. To further
evaluate the eects of borax and WPU content, Figure 3e
summarizes the peak stress and elongation at break for nine
dierent formulations. With increasing borax content, the peak
stress consistently rose, and the corresponding strain shifted to
higher values, suggesting that higher borax concentrations
enhance the tensile strength of the gels while making them
more brittle. This trend was observed consistently across all
WPU concentrations.
The compressive strain-distance curves for gels with varying
compositions are presented in Figure S1. As the gels are
compressed, the stress gradually increases, reaching a
maximum when the compression strain reaches the preset
50%. Among the tested formulations, gels with ratios of 2:1:3
and 3:1:3 demonstrated superior resistance to compressive
deformation. Upon releasing the compression, the measured
stress gradually decreased to near zero over approximately 30 s.
On the other hand, increasing the WPU content enhances the
toughness of the gels, although its influence on the overall
compressive stress trends is less pronounced. These findings in
Figure 3f suggest that higher borax concentrations compromise
the gel’s compressive resistance, whereas WPU contributes to
improved material flexibility and toughness.
To further evaluate the feasibility of the gel for strain sensing
applications, we conducted a 100-cycle tensile test at 50%
strain and monitored the resistance variation. As shown in
Figure S7, the electrical signal exhibited highly repeatable
fluctuations with relatively small drift throughout the cycles.
These results confirm that the sensor maintains excellent signal
Figure 3. Mechanical properties of hydrogels with varying WPU and borax content. (a−c) Tensile stress−strain curves of hydrogels with dierent
borax concentrations (intragroup) and WPU contents (intergroup). Insets show zoomed-in views near the peak stress. (d) Eect of increasing
WPU content on tensile strength and material softness. (e) Summary of peak stress and elongation at break for nine gel formulations with varying
WPU-to-borax ratios. (f) Compressive stress under 50% strain for gels with dierent compositions, illustrating the influence of borax and WPU on
compressive resistance and toughness.
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Figure 4. Rheological properties of hydrogels under varying frequencies, strains, and component ratios. (a) Storage modulus (G′, solid lines) and
loss modulus (G″, dashed lines) of hydrogels with varying WPU contents as a function of frequency. Dierent colors represent dierent WPU
concentrations. (b) Loss factor (tan δ) of hydrogels with varying WPU contents as a function of frequency. (c) G′(solid lines) and G″(dashed
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integrity under repeated deformation, supporting its potential
use in wearable strain-sensing scenarios.
3.5. The Rheological Properties of the Gel. To
investigate the impact of the ratio of WPU and STD on the
gel’s rheological properties, we examined the rheological
behavior under varying proportions of WPU and STD. The
experimental results are shown in Figure 4.
Figure 4a shows how the storage modulus (G′, solid lines)
and loss modulus (G″, dashed lines) of gels change with
frequency under dierent WPU contents. G′tends to increase
as frequency rises, which suggests that the material behaves
more elastically at higher frequencies. This might be due to the
restricted motion of polymer chains in the dual-network
structure�consisting of PVA chemically cross-linked by STD
and WPU interacting through hydrogen bonding�which
makes it harder for the system to dissipate energy at high
frequencies.
In comparison, G″first increases and then gradually
decreases with frequency, peaking around ω= 6.31 rad/s.
This pattern probably reflects the dynamic bond breaking and
reconstruction in the gel network. At lower frequencies, the
network has enough time to respond and dissipate energy,
while at higher frequencies, relaxation becomes limited and
Figure 4. continued
lines) of hydrogels with varying STD concentrations under dierent frequency. Dierent colors represent dierent STD contents. (d) Tan δunder
dierent STD contents. (e) G′(solid lines) and G″(dashed lines) of hydrogels with varying WPU contents as a function of strain amplitude.
Dierent colors represent dierent WPU concentrations. (f) Tan δof hydrogels with varying WPU contents as a function of strain amplitude. (g)
G′(solid lines) and G″(dashed lines) of hydrogels with dierent STD concentrations under varying strain amplitudes. (h) Tan δwith varying STD
concentrations as a function of strain amplitude.
Figure 5. Electrical and electrophysiological signal acquisition using the hydrogel. (a) Resistance−strain curves of gels with varying STD contents.
(b) Frequency-dependent impedance spectra of the gel electrode and a commercial Ag/AgCl electrode. (c) ECG signals recorded using the gel
electrode and a commercial Ag/AgCl electrode. (d) EEG signals recorded using both the gel and commercial electrodes. (e) EEG waveforms
recorded from a subject at rest and during gameplay. (f) Frequency power spectra of EEG signals under resting and gaming states. (g) Time−
frequency spectrogram of EEG signals during gameplay. (h) Time−frequency spectrogram of EEG signals during rest. (i) EMG signals recorded
from the forearm during four handgrip cycles.
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energy dissipation drops o. This general trend did not vary
much across dierent WPU ratios.
It is also worth noting that in the gels without WPU, both G′
and G″are significantly higher, which indicates a stier and
more rigid network. The introduction of WPU softens the gel,
as expected.
Figure 4b plots the loss factor (tan δ), which decreases with
increasing frequency. That aligns with a more elastic response
at high frequencies and less viscous energy loss.
Figure 4c,d look at how dierent STD contents aect G′and
G″. As STD concentration increases, both moduli go up. That
is likely because more cross-linking increases the network
density, making the structure more resistant to deformation
and able to store more energy. Although G″follows a similar
frequency-dependent curve, tan δdoes not show a very clear
trend across the dierent STD concentrations�probably
because the eects of cross-linking and molecular friction are
competing.
The strain-amplitude-dependent results are shown in Figure
4e. For gels containing WPU, the linear viscoelastic region is
very narrow�G′stays roughly constant only below 0.02 strain.
After that, both G′and G″drop quickly, which suggests the
network starts to break down. Compared with the PVA-only
gels, the WPU-containing ones are clearly more deformable,
with lower modulus throughout.
In Figure 4f, we plotted tan δversus strain amplitude. At
small strains, values are fairly stable. As strain increases, tan δ
also increases, especially in the PVA-only gels where it goes
above 1. This seems to show that WPU helps to stabilize the
network dynamically, since those gels showed slower increases
in tan δ.
Figure 4g,h further show the impact of STD content under
varying strain amplitudes. G′and G″both increase with higher
cross-linker concentrations, and tan δalso rises with strain,
pointing to more internal friction and energy dissipation in
tightly cross-linked networks.
3.6. Electrical and Electrophysiological Performance
of the Hydrogel. The electrical and electrophysiological
performance of the WPU−PVA hydrogel was systematically
evaluated, as shown in Figure 5. The electrical resistance of the
gel under dierent strains is presented in Figure 5a. As the gel
is stretched, the resistance increases gradually and monotoni-
cally, which is typical of ionically conductive gels and reflects a
predictable deformation-dependent response. This regular
trend indicates that the conductive network within the gel
remains structurally stable during strain.
The observed low interfacial impedance and enhanced
conductivity stem from the gel’s hybrid cross-linked network
composed of hydrogen bonding, borate ester formation, and
electrostatic interactions. In addition, to provide a basic
estimate of ionic conductivity, the bulk resistance of the gel
was measured using a digital multimeter under ambient
hydrated conditions. For a rectangular sample with dimensions
of 40 mm ×5 mm ×5 mm, and an average resistance of
approximately 30 kΩ, the resulting estimated conductivity is
5.3 ×10−4S·cm−1, which is within the expected range for
water-based hydrogels.
The introduction of STD into the PVA matrix establishes
dynamic borate ester bonds (PVA−B−PVA), forming
continuous ionic pathways that facilitate ecient ion
migration.
41
Meanwhile, the incorporation of WPU introduces
polar domains such as urethane and carbonyl groups, which
enhance water retention and facilitate ion solvation. Together
with the borate cross-links, this contributes to a highly
percolated and adaptive conduction network.
40,42
This
synergistic network structure lowers the ionic transport barrier,
enabling fast ion conduction across the gel bulk and at the gel−
skin interface.
Compared to conventional electronic conductors, where the
mismatch between electron conduction and biological ionic
environments often leads to high contact resistance, the ionic
conduction nature of the gel matches the intrinsic ion
transport properties of biological tissues. This matching
mechanism, combined with the gel’s soft and conformal
contact at the skin interface, substantially reduces interfacial
impedance. Electrochemical impedance spectroscopy (EIS)
further corroborates this interpretation. As shown in the
Nyquist plots (Figure S8), the gel electrode exhibits a
compressed semicircular arc with a markedly reduced diameter
compared to commercial Ag/AgCl electrodes, followed by a
sloped line in the low-frequency region. This feature indicates
capacitive-dominated behavior accompanied by ecient ionic
conduction at the gel−skin interface. In addition, the Bode and
Nyquist analyses collectively confirm that the gel electrode
maintains significantly lower interfacial impedance across a
broad frequency range, thereby highlighting its superior ionic
transport properties and reduced electrode−skin polarization
resistance. These results clearly demonstrate the gel’s
advantage in minimizing interfacial impedance and enhancing
electrode performance relative to conventional Ag/AgCl
electrodes.
The mechanical flexibility and electrical stability of the gel
under strain are attributed to the dynamic hybrid network
formed by PVA−STD covalent bonds and the flexible WPU
matrix. The elastic segments of WPU chains allow for
segmental relaxation, maintaining the continuity of ionic
pathways even during large deformations. Meanwhile, the
dynamic borate ester cross-links enable reorganization of the
polymer network under mechanical stress, preventing crack
propagation and preserving ionic conductivity.
This mechanical−electrical coupling ensures that impedance
and signal amplitude exhibit minimal fluctuations under cyclic
strain. The uniform distribution of mechanical stress across the
gel prevents localized mechanical failure, thus maintaining
stable acquisition of bioelectrical signals during dynamic
motions such as joint bending and muscle contraction. As a
result, the gel sensor demonstrates high reliability for
simultaneous strain and electrophysiological monitoring, even
during large, repetitive deformations.
3.7. Validation of Electrophysiological Signal Acquis-
ition. To verify the functionality of the gel electrodes in
electrophysiological signal acquisition, ECG, EEG, and EMG
signals were recorded. The ECG signals acquired using the gel
electrodes and commercial Ag/AgCl electrodes are shown in
Figure 5c. No significant dierences in signal intensity or
amplitude were observed between the two types of electrodes.
Similarly, Figure 5d compares EEG signals obtained with the
commercial electrodes and the gel electrodes developed in this
work. The EEG waveforms and signal intensities are nearly
identical. Figure 5e shows the measurement of brain waves of
volunteers at rest and when playing moba games with the gel
electrode in this work. It can be seen that the amplitude of
brain waves in the blue line (playing games) is significantly
higher than that in the rest state. Figure 5f presents the EEG
frequency power plots for rest and playing, and it can be seen
that the EEG power of playing at 0−8 Hz is significantly higher
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than that of the rest state. Figure 5g,h further demonstrate The
time-frequency patterns during this process. The spectrograms
reveal significant dierences in the EEG power distribution
between the two states. In the playing state (Figure 5g), the
power in the low-frequency range (0−8 Hz) is significantly
higher than in the resting state (Figure 5h). Specifically, the
power in the delta (0−4 Hz) and theta (4−8 Hz) bands is
noticeably enhanced, particularly around the 6−10 s mark.
This increased low-frequency activity is commonly associated
with cognitive engagement, attention, and decision-making,
which are crucial during gaming. In contrast, the resting state
EEG (Figure 5h) exhibits markedly lower power in the 0−8 Hz
range, except for a short burst around 2 s, likely due to
spontaneous neural fluctuations. The overall power distribu-
tion in the resting state is more homogeneous, reflecting a
relaxed and less cognitively demanding condition.
In Figure 5i, an EMG signal set is displayed, recorded by
placing the gel electrode on the anterior forearm and
generating signals through a handgrip motion. The signal
patterns for four consecutive handgrips are consistent,
Figure 6. Strain sensing performance of the hydrogel electrode on dierent body regions. (a) Photograph of finger bending. (b) Photograph of
knee bending. (c) Strain signal recorded during finger motion. (d) Strain signal recorded during knee motion. (e) Photograph of wrist bending. (f)
Photograph of throat deformation during swallowing. (g) Strain signal recorded during wrist motion. (h) Strain signal recorded during swallowing.
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demonstrating the reliability and repeatability of the gel
electrodes for EMG signal acquisition.
3.8. For Strain Monitoring in Humans. Figure 6
demonstrates the strain-sensing capabilities of the proposed
Figure 7. Simultaneous acquisition of EMG and strain signals using a single gel electrode. (a) Raw and filtered EMG signals recorded during
handgrip motion. (b) Short-time Fourier transform (STFT) magnitude plot of the EMG signal. (c) RMS amplitude of the filtered EMG signal with
detected grasping events. (d) Strain signal directly measured from the gel during handgrip cycles. (e) Strain signal indirectly extracted from the
EMG channel via signal decomposition.
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polymer hydrogel electrodes in monitoring human motion
across dierent body regions, including the fingers, knees,
wrists, and throat. In the finger bending test (Figure 6a,c), the
gel electrode exhibited high sensitivity in capturing strain
variations associated with finger flexion and extension. The
recorded strain signals (Figure 6c) displayed distinct
fluctuations corresponding to the bending amplitude and
speed, indicating the electrode’s ability to detect subtle finger
movements with excellent dynamic response characteristics.
For knee motion monitoring (Figure 6b,d), the electrode was
applied to the skin above the knee while the subject performed
repeated bending and extension. The strain signals (Figure 6d)
exhibited a well-defined periodic waveform, with peaks and
troughs aligning with the knee joint movement cycle. This
result highlights the electrode’s capability in accurately tracking
large-scale joint deformations and its potential for wearable
biomechanical monitoring applications.
The wrist bending experiment (Figure 6e,g) further
validated the electrode’s performance in capturing strain
signals from more complex hand motions. The strain response
(Figure 6g) demonstrated a consistent and periodic pattern
corresponding to wrist flexion and extension, indicating stable
electrical ouWPUt even under large strain conditions. This
suggests that the gel electrode is well-suited for applications
requiring high flexibility and reliable signal acquisition.
Additionally, the throat swallowing experiment (Figure 6f,h)
demonstrated the electrode’s potential for bioelectronic
applications. During swallowing, the electrode eectively
detected minute deformations in the throat muscles, as evident
in the strain signal variations shown in Figure 6h. The distinct
signal changes indicate the feasibility of using the gel electrode
for real-time monitoring of swallowing activity, which could be
beneficial for speech rehabilitation and dysphagia diagnosis.
Overall, the developed soft polymer hydrogel electrode exhibits
excellent mechanical adaptability, high strain sensitivity, and
stable electrical performance, making it a promising candidate
for human motion monitoring and wearable biomedical
applications.
3.9. The Same Gel Used for the Simultaneous
Monitoring of EMG and Strain. The same gel sensor was
utilized to simultaneously monitor both electromyography
(EMG) signals and strain variations, exploiting its dual
sensitivity to bioelectrical and mechanical stimuli. In this
experiment, the subject’s hand performed periodic grasp-and-
release actions at a steady rhythm, mimicking repetitive motor
tasks commonly seen in rehabilitation or gesture recognition
scenarios.
Figure 7a presents both the raw and filtered EMG signals.
The raw signal exhibits high-frequency noise and baseline drift
due to motion artifacts and environmental interference. After
preprocessing�including bandpass filtering�the filtered
EMG signal clearly captures the burst-like activation patterns
corresponding to each muscle contraction during hand closure.
These periodic activations suggest that the gel-based EMG
interface has sucient fidelity to resolve transient neuro-
muscular events. To further characterize the temporal and
spectral properties of the signal, Figure 7b shows the Short-
Time Fourier Transform magnitude plot. Distinct bursts of
power in the 20−60 Hz frequency range align with muscle
activation intervals. The spectral energy is modulated over
time, reinforcing the observation of cyclical muscle engage-
ment. This also validates the sensor’s capability to track
dynamic muscular eort in both time and frequency domains.
In Figure 7c, RMS (root-mean-square) amplitude of the
filtered EMG signal is computed as a feature for quantifying
signal intensity over time. Local maxima in the RMS curve
correspond to distinct grasping events, which are automatically
detected and marked as red dots. This approach not only
provides a straightforward way to quantify repetitions but also
enables event-based analysis of muscle activation, which is
critical for fatigue assessment, rehabilitation monitoring, and
gesture classification. In parallel, the strain signal measured
directly from the same gel sensor is shown in Figure 7d. The
strain waveform exhibits clear periodic patterns corresponding
to hand closure and release, validating the mechanical
deformation of the sensor under joint movement. The high
signal-to-noise ratio and consistent waveform profile highlight
the gel’s mechanical responsiveness and its potential use in
strain sensing.
Figure 7e shows the strain signal extracted indirectly from
the EMG channel via signal decomposition and artifact
separation techniques. This demonstrates that even when
strain and EMG signals are mixed in a single channel due to
simultaneous mechanical and electrical activation, advanced
filtering methods (e.g., adaptive filtering, wavelet decom-
position) can eectively isolate the mechanical component.
This cross-validation between direct and indirect strain signals
reinforces the robustness of the sensor’s multimodal perform-
ance.
This study illustrates the feasibility of using a single gel
interface to concurrently and independently capture EMG and
strain data. Such a configuration simplifies device architecture
while maintaining multimodal functionality. More importantly,
it opens up promising avenues for artifact removal by using
strain-derived signals to regress out motion-induced distortions
in EMG, for future multisensor fusion strategies in wearable
bioelectronics and human−machine interaction systems.
4. CONCLUSION
In summary, we developed a degradable hydrogel by
integrating PVA, WPU, and STD into a dynamic hybrid
network. This gel exhibits mechanical softness, stretchability,
and moderate ionic conductivity, enabling the acquisition of
both strain and bioelectrical signals from the human body.
Compared to conventional gel electrodes, the material shows
lower interfacial impedance and improved conformability
under hydrated conditions, making it suitable for short-term
skin-interfaced applications. Through experimental validation,
we demonstrated that the gel is capable of monitoring various
types of human motion and electrophysiological signals,
including the simultaneous acquisition of EMG and strain
signals from a single sensor. This dual-modal sensing capability
may contribute to simplified wearable system design through
signal decomposition and motion artifact removal. Moreover,
the gel’s water-based composition and its disintegration
behavior in aqueous environments suggest potential for
environmental and biomedical degradability, oering sustain-
ability advantages for disposable or short-term biomedical
devices. Overall, this work presents a practical and adaptable
material platform for integrated multimodal biosensing, with
promising potential in personalized health monitoring and
human−machine interaction systems.
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■ASSOCIATED CONTENT
*
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsaelm.5c01031.
Schematic illustration of the network structure of the
hydrogel (Figure S1); compressive strain−time curve of
the hydrogel and the influence of STD concentration
(intragroup) and WPU content (intergroup) on
compressive strain response (Figure S2a−d); stress
curves of normal and self-healing gels (Figure S3);
demonstration of the stretchability of the hydrogel
(Figure S4); photographic evidence of the gel’s excep-
tional stretchability under large deformation (Figure
S5); time-dependent resistance variation of the hydrogel
under room temperature conditions (Figure S6);
repeated resistance variation data (Figure S7); Nyquist
plot and equivalent circuit model (Figure S8); photo-
graph of a single gel electrode (Figure S9) (PDF)
■AUTHOR INFORMATION
Corresponding Author
Yuhang Li −Institute of Solid Mechanics, School of Aeronautic
Science and Engineering, Beihang University, Beijing 100191,
China; Ningbo Innovation Research Institute of Beihang
University, Ningbo, Zhejiang 315800, China; Liaoning
Academy of Materials, Shenyang 110004, China;
Tianmushan Laboratory, Hangzhou, Zhejiang 311115,
China; orcid.org/0000-0001-9865-5221;
Email: liyuhang@buaa.edu.cn
Authors
Yang Wang −Institute of Solid Mechanics, School of
Aeronautic Science and Engineering, Beihang University,
Beijing 100191, China
Jingxi Wang −School of Biological Science and Medical
Engineering, Beihang University, Beijing 100191, China
Chengkuo Lee −Department of Electrical and Computer
Engineering, National University of Singapore, Singapore
117583, Singapore; orcid.org/0000-0002-8886-3649
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsaelm.5c01031
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
This research was funded by the National Natural Science
Foundation of China (Grant No. U23A20111), “111 Center”
(Grant No. B18002) and the Ningbo International Sci-tech
Cooperation Projects (2024H009).
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