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Page 6 of 23 Du et al. Soft Sci 2024;4:35 https://dx.doi.org/10.20517/ss.2024.31

actuators containing photosensitivity factors can change their properties by absorbing photons to gain
[53]
energy . Light-responsive hydrogel actuators can achieve high sensitivity for remote control and operation
[54]
without the need for complex external equipment . They can produce the necessary structural or
physicochemical changes under light irradiation to meet required treatment modes. Additionally, the light
conditions can be externally adjusted, allowing the hydrogel actuator to respond dynamically to light of
sufficient intensity and appropriate wavelength.

Magnet-responsive hydrogel actuators
Magnet-responsive hydrogel actuators can respond to applied magnetic field stimuli. Unlike other
stimulation modes, magnet-responsive hydrogel actuators are modified by adding paramagnetic or
ferromagnetic substances, which impart sensitivity and precision to external magnetic stimulation . When
[55]
exposed to a magnetic field, the magnetic material in the hydrogel network aligns with the field, causing
part of the hydrogel structure to move. This alignment allows the magnet-responsive hydrogel actuator to
respond and move according to the magnetic field guidance, achieving the desired actuation .
[56]
Additionally, magnetic stimulation can increase the temperature, causing the hydrogel to expand or
contract . The magnetic field can act as a remote switch to control the hydrogel actuator, making it
[57]
suitable for remote actuation. Due to their superparamagnetism and biocompatibility, magnet-responsive
hydrogels can be applied in biomedical fields, such as minimally invasive surgery and targeted drug
administration .
[58]

Redox-responsive hydrogel actuators
Redox-responsive hydrogel actuators are achieved by redox reactions occurring in the internal network
structure of the hydrogel. Adding redox-active components to the hydrogel network can control molecular
interactions, thereby causing swelling or shrinking . Redox points can be established through high
[40]
concentrations of glutathione (GSH) in the cytoplasm and intracellular organelles . In pathological
[59]
[53]
environments, redox potential changes significantly , triggering the hydrogel’s drug delivery. Significant
differences in glutathione concentrations make redox-responsive nanohydrogels an intelligent platform for
controlled and precise drug delivery. Oxidation-responsive hydrogel actuators respond to reactive oxygen
species (ROS), facilitating hydrogel degradation and drug release. During drug delivery, redox-responsive
hydrogel actuators respond to redox phenomena in specific regions and thus function effectively.

Ultrasound-responsive hydrogel actuators
Ultrasound is widely used in medical fields such as disease diagnosis and treatment due to its safety and
non-invasive characteristics. It introduces biological effects into cells and tissues through three main
[49]
mechanisms: heat, cavitation, and acoustic flow . Similar to other exogenous stimuli, ultrasound can be
used to remotely trigger on-demand administration and achieve deep tissue penetrating therapy. For both
treatment and diagnosis, ultrasound can act on living tissues and cells either thermally or mechanically.
Hydrogels serve as carriers for small bioactive molecules such as different drugs, growth factors, and
[60]
antibodies . Ultrasound can promote drug release without disrupting the gel network, allowing for
spatially controlled delivery of therapeutic agents. This can be achieved through different methods,
including pulsed, unidirectional burst release, and prolonged release after in vivo administration .
[61]
Multiple stimuli-responsive hydrogel actuators
Given the variability of disease environments and differences between individuals, it is essential to develop
new hydrogels that can dynamically adapt to their surroundings. Multiple stimuli-responsive hydrogel
actuators can respond simultaneously to various environmental stimuli, allowing for independent
regulation of each stimulus. This combination of multiple stimuli enables more precise drug delivery,
thereby achieving better therapeutic outcomes. In practice, while hydrogel actuators rely on two or more