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Page 4 of 28 Zhang et al. Soft Sci 2024;4:39 https://dx.doi.org/10.20517/ss.2024.34

PROPERTY OF HYDROGELS FOR BIOELECTRONICS
Mechanical properties of hydrogels
The mechanical property of hydrogels is a crucial factor in brain-machine interfaces, as it will significantly
influence the biocompatibility of the electrodes and affect performance and functionality. In the context of
non-invasive devices, our group designed a unique polymer composite hydrogel, which exhibits stretchable,
self-healing, and degradable properties at room temperature , as illustrated by Figure 2A-C. The hydrogel
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material can form patterned electrodes through a self-vaporization process with the optimized mass ratio of
glycerol and hydroxyethyl cellulose (HEC) aqueous solution. At room temperature of 25 °C and humidity of
40% relative humidity (RH), the glycerol-doped hydroxyethyl cellulose gel-based material (GHECs)
demonstrates excellent extensibility (~304%) and rapid self-healing capability (~9 min). Furthermore,
GHECs exhibit controllable degradation behavior due to their solubility in water. Based on these properties,
we designed and fabricated a range of flexible and stretchable hydrogel devices, including self-healing
electrodes, transient electronics, and robotic tactile sensors, to show the applications of the hydrogel.

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Figure 2D and E shows the combability of hydrogel material with biological tissues . Therefore, hydrogels
are widely applied in the fields of biomedicine and bioelectronics, including wearable hydrogel electronics,
hydrogel coatings, and hydrogel soft robots. However, due to the presence of surface moisture, it is still a
challenge to achieve strong adhesion between materials due to the hetero-interface. In 2015, Yuk et al. at the
Massachusetts Institute of Technology (MIT) proposed the adhesion principle of tough hydrogel, which is
attributed to the long-chain polymer network anchoring and internal energy dissipation within tough
hydrogels . Compared to physical interactions, chemical bond anchoring offers higher intrinsic adhesion
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strength and absorbs a significant amount of energy during the separation process, which results in an
interface adhesion exceeding 1,000 J/m .
2

The electrophysiological signals often include artifacts, and Figure 2F-H demonstrates the requirements on
the material stiffness of bioelectronics for continuous monitoring of electrophysiological signals to reduce
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noise . These noise signals have a wide frequency range, spanning from 0.01 to nearly 15 Hz, including
respiration (0.11 Hz), heartbeat (0.34 Hz), and gait movements (1-15 Hz). Due to the overlap of these low-
frequency noises with the low-frequency bands of brain, muscle, and electrocardiographic signals, the
conventional approach to filter the signals by bandpass filters will lead to the loss of target signals. Inspired
by the selective removal of dynamic mechanical noise artifacts of the spiders, Park et al. proposed a novel
[79]
bandpass filter based on a viscoelastic gelatin@chitosan hydrogel damper . The hydrogel exhibits
frequency-dependent phase transitions and acts as an adaptive bandpass filter, which enables the acquisition
of high-quality signals without the need of complex signal processing. In brief, the reversible hydrogen
bonds between chitosan and gelatin contribute to the viscoelastic properties of the hydrogel, and the
material transitions from a rubbery state to a glassy state at an applied frequency of 30 Hz. This phase
transition allows the material to selectively transmit target signals (> 30 Hz) while filtering out low-
frequency noise (< 30 Hz). Also, the frequency band for noise absorption can be regulated by the relaxation
time of viscoelastic materials, which is dependent on the elastic modulus and viscoelasticity of the hydrogel.
In addition, the viscoelasticity can be precisely controlled by adjusting the molecular weight of gelatin, water
content, and temperature to obtain high SNRs of the target signals. Therefore, the employment of hydrogel
dampers in bioelectronics is a promising technique for the continuous detection of biological signals
without the need of complex signal processing units.

The mechanical properties of hydrogels are essential for their application in brain-machine interfaces, since
they significantly influence the biocompatibility and overall performance. The challenges of adhesion due to
surface moisture highlight the need for advanced design strategies to enhance stability. The viscoelastic