Brick-snapping soft hydrogel actuator inspired by plant cell turgor

Inspired by the turgor pressure features of plant cells, which allow plants to support their bodies, researchers modified a hydrogel to swell swiftly enough to break a rigid brick in half, and to build complex underwater structures in minutes. The researchers’ hydrogel design contributed to a uniquely strong and fast hydrogel-based actuator that may have applications in fields including soft robotics, tunable optics, and biomedicine. Due to their lengthy list of unique material properties, hydrogels are promising materials for soft actuators – parts that convert various forms of energy into physical movements – particularly for soft robotics design. They are also well-positioned to bridge the gap between synthetic actuators and biological systems in biomedical applications. However, conventional soft hydrogel actuators generally suffer from weak actuation force and slow response speed due to their osmotic-driven actuation mechanisms. In addition, the materials themselves are prone to failure under high stress and pressures. To address these critical limitations, Hyeonuk Na and colleagues present a method to produce a strong and fast hydrogel actuator based on the strong turgor pressure of plant cells. Turgor pressure is the osmosis-driven hydrostatic pressure confined within a plant cell’s walls. It allows soft plant cells to support a plant’s tall body and growing roots to crack rocks. Na et al. created a hydrogel wrapped in a stiff yet flexible semipermeable membrane, which controlled and confined the osmotic swelling of the hydrogel, allowing for an actuation force of 730 newtons (N) or 3 orders of magnitude higher than existing hydrogel actuators. The force was enough to break a brick, the authors show (see related video). Using the actuator in water, they also demonstrated the rapid construction of a Greek temple structure and an igloo-like structure (see related videos).  Na et al. also show that the actuation speed can be greatly increased using electroosmosis, which reduced the actuation time from more than an hour to several minutes. “Na et al. open an exciting avenue for maximizing actuation force in hydrogels,” write Zhen Jiang and Pingan Song in a related Perspective. “The turgor hydrogel that combines ultrahigh actuation force, high compressibility and fast response will likely help expedite the next generation of aquatic soft robots capable of withstanding high underwater pressures.”