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Page 8 of 23 Du et al. Soft Sci 2024;4:35 https://dx.doi.org/10.20517/ss.2024.31

Biocompatibility
Hydrogel actuators have high biocompatibility, which not only preserves the functions of encapsulated
[66]
drugs and biomolecules but also reduces the inflammatory response when applied . Furthermore,
hydrogels exhibit antibacterial properties that are crucial in the biomedical field. The prevailing view is that
hydrogels bind to the negatively charged bacterial cell walls, causing changes in membrane permeability.
[67]
This inhibition of cellular DNA replication ultimately leads to cell death . To achieve optimal
functionality, further improvement and optimization of hydrogels are necessary.

Hydrogel actuators in skin therapeutics
Wearable hydrogel actuators
Hydrogels serve as an important bridge connecting electronics and biology due to their flexible properties in
mechanics, electronics, and biology. Especially in wearable bioelectronics, they demonstrate better
biocompatibility and flexibility compared to elastomer materials . Hydrogel can be integrated with
[68]
wearable electronics in various forms such as patches, tattoos, and fibers, and serve roles such as adhesion
layers, stimuli-responsive elements, energy storage, and sensing components [69-79] . Hydrogel wearable
devices enable continuous monitoring of physical and biochemical parameters of the human body.
Furthermore, they are often used to modulate drug delivery and neural mechanisms. Their ease of
integration with wireless communication also paves the way for applications in personal home monitoring
and remote medical practices [80,81] .

Transdermal hydrogel actuators
Using hydrogel as a wound dressing can keep the wound moist while promoting healing and avoiding
[82]
secondary injury . The mechanism involves the absorption and retention of wound exudates, which
[49]
contribute to the proliferation of fibrocytes and migration of keratinocytes, thereby promoting healing .
Additionally, the hydrogel acts as a barrier, reducing infection risk due to its inherent antibacterial
properties. In drug delivery, drugs, antibiotics, and other substances can be incorporated into the hydrogel
for continuous treatment. Due to its flexibility, hydrogel can adapt to various wound surfaces, such as
[83]
traumatic, burn, and diabetic wounds . Hydrogel dressings are particularly advantageous for complex
wounds, such as diabetic wounds, where traditional dressings are less effective. Traditional dressings often
serve only as physical barriers and provide short-term drug supplements, which are insufficient for the
[84]
high-glucose environment and biochemical disorders of diabetic wounds . Hydrogel dressings, with their
physicochemical properties similar to the natural ECM, can be easily loaded with drugs or cytokines for
[46]
effective treatment . Additionally, stimuli-responsive hydrogel actuators retain collagen properties and can
change shape, size, or volume in response to stimuli, enhancing treatment effectiveness and drug utilization,
and promoting wound healing.

Monitorable hydrogel actuator
Biosensors are able to sensitively monitor environmental and physiological indicators in humans and
provide real-time feedback. The unique properties of stimuli-responsive hydrogel actuators, which can
undergo reversible phase volume transitions under environmental stimuli, offer additional options for
biosensors . These hydrogels interact specifically with target biomolecules, enabling detectable physical or
[85]
chemical changes within the hydrogel matrix. This specific detection method allows hydrogel actuators to
accurately quantify interactions, thereby enabling the monitoring of various physiological indicators.
Hydrogel sensors can track a range of physiological indicators and biomarkers in the human body,
[86]
facilitating early health warnings, disease detection, and other functions . The excellent mechanical
properties and biocompatibility of hydrogel actuators make them suitable for safe, long-term use in the
human environment. Furthermore, these actuators exhibit high sensitivity and specificity in response to
environmental stimuli, making them particularly useful in disease monitoring applications, such as diabetes