MIT Builds Transparent, Gel-Based Robots Capable of Evading Underwater Detection

Transparent, gel-based robots that are capable of moving when water is pumped in and out of them have been developed by engineers at MIT. The bots can carry out several fast, forceful tasks, including catching and releasing a live fish and kicking a ball underwater.

The robots are made completely of hydrogel, a tough, rubbery, virtually transparent material that mainly consists of water. Each robot is composed of hollow, precisely engineered hydrogel structures, connected to rubbery tubes. The structures quickly inflate in orientations when the researchers pump water into the hydrogel robots, enabling the bots to stretch out or curl up.

The team designed a number of hydrogel robots, including an articulated appendage that mimics kicking motions, a finlike structure that flaps back and forth, and a soft, hand-shaped robot that can squeeze and relax.

As the robots are both powered by and made almost totally of water, they have similar acoustic and visual properties to water. The researchers suggest that these robots, if engineered for underwater applications, could be nearly invisible.

The team, headed by Xuanhe Zhao, associate professor of mechanical engineering and civil and environmental engineering at MIT, and graduate student Hyunwoo Yuk, is currently working towards adapting hydrogel robots for medical applications.

Engineers at MIT have fabricated transparent gel robots that can perform a number of fast, forceful tasks, including kicking a ball underwater, and grabbing and releasing a live fish (Video: Melanie Gonick/MIT)

Hydrogels are soft, wet, biocompatible, and can form more friendly interfaces with human organs. We are actively collaborating with medical groups to translate this system into soft manipulators such as hydrogel ‘hands,’ which could potentially apply more gentle manipulations to tissues and organs in surgical operations.

Xuanhe Zhao, Associate Professor, MIT

Zhao and Yuk have published their results this week in the journal Nature Communications. Their co-authors include MIT graduate students Shaoting Lin and Chu Ma, postdoc Mahdi Takaffoli, and associate professor of mechanical engineering Nicholas X. Fang.

Robot Recipe

In the last five years, Zhao’s team has been developing “recipes” for hydrogels, mixing solutions of water and polymers, using methods they invented to fabricate strong yet highly stretchable materials. They have also developed methods to glue these hydrogels to a variety of surfaces such as metal, glass, rubber, and ceramic forming highly strong bonds that resist peeling.

The team realized that such flexible, durable, strongly bondable hydrogels are likely to be perfect materials for application in soft robotics. Various research teams have designed soft robots from rubbers like silicones, but Zhao highlights that such materials are not as biocompatible as hydrogels.

As hydrogels mostly consist of water, he says, they are naturally safer to use in a biomedical application. Although others have tried to design robots using hydrogels, their solutions have resulted in fragile, comparatively inflexible materials that burst or crack when used repeatedly.

On the other hand, Zhao’s team found out that their formulations were more applicable to soft robotics.

We didn’t think of this kind of [soft robotics] project initially, but realized maybe our expertise can be crucial to translating these jellies as robust actuators and robotic structures.

Hyunwoo Yuk, Graduate Student, MIT

Fast and Forceful

The team first drew inspiration from the animal world in order to apply their hydrogel materials to soft robotics. They focused closely on glass eels (leptocephali) - miniature, transparent, hydrogel-like eel larvae that hatch in the ocean and in due course migrate to their natural river environments.

“It is extremely long travel, and there is no means of protection,” Yuk says. “It seems they tried to evolve into a transparent form as an efficient camouflage tactic. And we wanted to achieve a similar level of transparency, force, and speed.”

To achieve that, Yuk and Zhao utilized 3D printing and laser cutting methods to print their hydrogel recipes into robotic structures and other hollow units, which they bonded to tiny, rubbery tubes that are connected to outer pumps.

To activate or move the structures, the team used syringe pumps to inject water via the hollow structures, enabling them to swiftly stretch or curl, based on the total configuration of the robots.

Yuk and Zhao discovered that by pumping water in, they could create fast, forceful reactions, enabling a hydrogel robot to produce a few Newtons of force in just one second. Other researchers were able to activate similar hydrogel robots using simple osmosis, allowing water to naturally seep into structures — a slow process that generates millinewton forces in a span of several minutes or hours.

Catch and Release

In experiments utilizing many hydrogel robot designs, the team discovered that the structures could endure repeated use of up to 1,000 cycles without tearing or rupturing. They also discovered that each design, placed underwater against colored backgrounds, appeared almost fully camouflaged.

The team measured the optical and acoustic properties of the hydrogel robots, and found them to be almost equal to that of water, in contrast to rubber and other widely used materials in soft robotics.

In a remarkable demonstration of the technology, the team built a hand-like robotic gripper and pumped water in and out of its “fingers”, which caused the hand to close and open. The researchers submerged the gripper into a tank containing a goldfish and demonstrated that as the fish swam past, the gripper was strong and fast enough to catch the fish.

[The robot] is almost transparent, very hard to see. When you release the fish, it’s quite happy because [the robot] is soft and doesn’t damage the fish. Imagine a hard robotic hand would probably squash the fish.

Xuanhe Zhao, Associate Professor, MIT

Going forward, the researchers hope to identify specific applications for hydrogel robotics, as well as customize their recipes to specific uses. For instance, medical applications might not need totally transparent structures, while other applications may require some parts of a robot to be stiffer than others.

“We want to pinpoint a realistic application and optimize the material to achieve something impactful,” Yuk says. “To our best knowledge, this is the first demonstration of hydrogel pressure-based acutuation. We are now tossing this concept out as an open question, to say, ‘Let’s play with this.’”

This research received supported, in part, from the Office of Naval Research, the MIT Institute for Soldier Nanotechnologies, and the National Science Foundation.