Scientists who want to build robots from soft materials have long favored hydrogels to serve as artificial muscles in the machines. But these materials usually can’t muster the strength of real muscles. Now, researchers have designed a new hydrogel-based device powerful enough to break a brick in minutes (Science 2022, DOI: 10.1126/science.abm7862).
The research team at Seoul National University took their inspiration for this hydrogel design from plants. Although some plants’ roots can be tender and soft, even a garden weed can crack concrete as it grows. That’s because plant tissues harness osmotic pressure by holding water within their cell walls like a tire pumped full of air.
Study authors Hyeonuk Na, Yong-Woo Kang, and Chang Seo Park realized that this same turgor pressure could be used to make the actuators that power movement in soft robots and other machines.
Na and his colleagues tried to mimic plants’ cell walls by wrapping a dry hydrogel cube in a stiff, semipermeable membrane, creating an actuator about 3 cm long. On its own, the fully hydrated hydrogel is squishy and brittle. But the membrane casing limits how much the hydrogel can swell as it soaks up water. This membrane tension transforms the crumbly jelly into a rock-hard balloon, the authors write in an email.
The actuator takes about 4 days to fully hydrate when it’s left to soak in water. But Na and his colleagues sped up the process when they used a hydrogel network containing charged polymers and soaked it in an electrolyte solution. When the team passed an electric current through the solution, the movement of electrolytes into the hydrogel dragged enough water to build upwards of 2 times the pressure of that in a standard car tire. In less than 10 minutes, the turgor pressure of this charged actuator generated enough force to snap a brick in two.
The new actuator is a “groundbreaking” development in hydrogel technology, polymer scientists Pingan Song and Zhen Jiang of the University of Southern Queensland say in an email. “The key innovation of this work is that such design enables an actuation force several orders of magnitude greater than existing hydrogels,” they write.
Though the new actuator still has a long way to go before powering soft robots of the future, Song and Jiang agree that the plant-inspired design is another example of the benefits of taking lessons from nature.