Engineers have fabricated transparent, gel-based robots that move when water is pumped in and out of them. They are made entirely of hydrogel — a tough, rubbery, nearly transparent material that’s composed mostly of water. Each robot consists of hollow, precisely designed hydrogel structures connected to rubbery tubes. When water is pumped into them, the structures inflate in orientations that enable the robots to curl up or stretch out.
Because the robots are both powered by and made almost entirely of water, they have similar visual and acoustic properties to water. These robots, if designed for underwater applications, may be virtually invisible.
Other previous designs for soft robots were made from rubbers like silicones, which are not as biocompatible as hydrogels. As hydrogels are mostly composed of water, they are naturally safer to use in a biomedical setting. And while others have attempted to fashion robots out of hydrogels, their solutions have resulted in brittle, relatively inflexible materials that crack or burst with repeated use.
To apply the hydrogel materials to soft robotics, the researchers first looked to the animal world, focusing on leptocephali, or glass eels — tiny, transparent, hydrogel-like eel larvae that hatch in the ocean and eventually migrate to their natural river habitats.
The team used 3D printing and laser cutting techniques to print the hydrogel recipes into robotic structures and other hollow units that were bonded to small, rubbery tubes connected to external pumps.
To actuate, or move, the structures, syringe pumps were used to inject water through the hollow structures, enabling them to quickly curl or stretch, depending on the overall configuration of the robots. By pumping water in, the team could produce fast, forceful reactions, enabling a hydrogel robot to generate a few newtons of force in one second. For perspective, other researchers have activated similar hydrogel robots by simple osmosis, letting water naturally seep into structures — a slow process that creates millinewton forces over several minutes or hours.
In experiments using several hydrogel robot designs, the team found the structures were able to withstand repeated use of up to 1,000 cycles without rupturing or tearing. They also found that each design, placed underwater against colored backgrounds, appeared almost entirely camouflaged (see figure). The group measured the acoustic and optical properties of the hydrogel robots and found them to be nearly equal to that of water, unlike rubber and other commonly used materials in soft robotics. The team fabricated a hand-like robotic gripper and pumped water in and out of its “fingers” to make the hand open and close. The researchers submerged the gripper in a tank with a goldfish and showed that as the fish swam past, the gripper was strong and fast enough to close around the fish.
Next, the researchers plan to identify specific applications for hydrogel robotics, as well as tailor their recipes to particular uses. For example, medical applications might not require completely transparent structures, while other applications may need certain parts of a robot to be stiffer than others.
Soft manipulators such as hydrogel hands could potentially apply more gentle manipulations to tissues and organs in surgical operations.