After decades of being seen as rigid, metallic creations, robots are starting to soften up. Spongey materials like hydrogels could make for robots that are more flexible, safer to be around and simpler to build and use than their harder counterparts. Now researchers at Rutgers University have developed a smart gel that can be 3D printed into a variety of shapes, and electrically-activated to make it "walk" underwater, grab and move objects.

Hydrogels keep proving themselves to be excellent materials to make soft robots. Composed of over 70 percent water, they're easy to 3D print into whatever shape is needed, like an octopus or a caterpillar. They're responsive to triggers like light, temperature and electricity, allowing them to expand and contract to make them move, carry objects and perform other functions.

The Rutger researchers created their hydrogel by shining light onto a light-sensitive material while it was being 3D printed. The end result was a humanoid figure that stands about 1 in (2.5 cm) tall, and can be made to walk by placing it in a saltwater solution and zapping it with small electric currents.

The hydrogel contracts when an electric field is applied, so switching the power on and off can make the device shuffle forward, close around small objects and release them somewhere else. Since thinner parts of the hydrogel robot will move faster than thicker ones, changing the thickness of the gel can allow for finer control, as can changing the saltiness of the electrolyte and the strength of the electric field.

The researchers say that the smart gel can be used to build actuating parts for soft robots that can move underwater, performing inspections or other jobs. They could also be made to move through the human body to deliver drugs or diagnose diseases, and it might even be useful to make 3D printable artificial organs and muscles.

"Our 3D-printed smart gel has great potential in biomedical engineering because it resembles tissues in the human body that also contain lots of water and are very soft," says Howon Lee, senior author of the study. "It can be used for many different types of underwater devices that mimic aquatic life like the octopus."

The research was published in the journal ACS Applied Materials & Interfaces, and the team demonstrates the hydrogel in the video below.