What if there were a material that responds on its own without external stimuli, like muscles contracting and relaxing by themselves? Recently, a research team led by Kim Yeon-su, a professor in the Department of Materials Science and Engineering at Pohang University of Science and Technology POSTECH, comprehensively organized and published the principles, design, applications, and future research directions of self-actuating hydrogel technology.
Hydrogel is a jelly-like substance that holds water and expands or changes transparency depending on temperature, light, or chemical reactions. By incorporating the concept of self-regulation, in which living organisms adjust their own state, self-regulating hydrogels are next-generation smart materials that can expand and contract on their own and change transparency without external stimuli.
In particular, self-regulating hydrogels can implement physical intelligence that generates repetitive changes like living tissue. Conventional hydrogels respond only in a simple on-off manner.
In this paper, the researchers classified self-regulating hydrogels into two types. The continuous control type creates sustained motion under a constant stimulus, such as a mechanical loop (repetitive bending and straightening of the gel), an optical loop (repeated blocking and transmission of light), and a chemical loop (rhythm generation through pH changes or specific chemical reactions). In contrast, the short-cycle control type shows a one-off response that returns to the original state after a single change.
The researchers anticipated that the ability of these self-regulating hydrogels to achieve autonomous and repetitive motion would enable their expansion into various smart materials.
For example, an autonomous walking gel that walks on its own can move without an external power source and be used for environmental monitoring robots or drug delivery platforms, while a phototactic robot can move on its own according to light without electricity and lay the foundation for energy-efficient soft robotics.
In addition, by leveraging the feature in which color can autonomously oscillate and modulate according to periodic nanostructural changes inside the hydrogel, it can be applied to battery-free color sensors, camouflage materials, and interactive displays. It is drawing attention as a next-generation soft robotics and smart materials platform that mimics the autonomous responses of living organisms.
It also showed promising application potential in the medical and information fields, such as therapeutic gels that temporarily close wounds or automatically release drugs, and information storage devices that disappear without a trace after a set period of time.
Professor Kim Yeon-su said, "Hydrogels that mimic the self-regulation principles of nature will expand beyond simple imitation into intelligent materials needed in everyday life."
The paper was published online in Chemical Reviews, the top-tier journal of the American Chemical Society (ACS), on Sep. 6 (local time).
References
Chemical Reviews (2025), DOI: https://doi.org/10.1021/acs.chemrev.5c00358