These Tiny Liquid Robots Merge and Split Like ‘Terminator’

Our cells are like the ultimate soft robots. Made mostly of a liquid interior wrapped inside a fatty shell, they split, stretch, roam, and squeeze into every nook and cranny of the body.

Actual robots, not so much. Even soft robots made of flexible materials struggle to deform outside of the physical limits of their building blocks.

This month, a team from Korea introduced liquid robots inspired by biological cells. About the size of a grain of rice, each robot is made of water coated with Teflon particles. The gummy-candy-like blobs are controlled using sound waves and can slip through grated fences, chomp up debris, and skim across solid and liquid surfaces.

They can also function as tiny chemical reactors. In a test, the team directed two robots, each loaded with a different chemical, to jump off a ledge and merge together without breaking, allowing the chemicals to react inside their Teflon shells.

Because the robots are biocompatible, they could one day shuttle drugs to hard-to-reach areas of the body—potentially loading up on chemotherapies to kill tumors, for example. Formations with other molecular tools embedded within the bots could also help diagnose diseases.

“It is challenging to emulate biological forms and functions with artificial machines,” wrote the team. “[But] a promising avenue to tackle this problem is harnessing the supreme deformability of liquids while providing stable yet flexible shells around them.”

From T-1000 to Liquid Marbles

Those who have seen Terminator 2: Judgment Day will remember the film’s formidable robot antagonist. Made of liquid metal, the T-1000 deforms, liquifies, and reconstructs itself on demand, instantly healing damage to its body.

Scientists have long sought to capture this versatility in machines (without the killer robot angle, of course). Previous studies have used a variety of liquid metals that change their shape when subjected to electromagnetic fields. These unconventional robots—smaller than a fingertip—can split, merge, and transport cargoes on demand. But their high metal content makes them incompatible with most chemical reactions and biology, limiting their practical use.

Another way to build liquid robots is to encapsulate water or other liquids in an armor-like barrier. It’s a bit like making gummy candy with a squishy but supportive outer casing and a gushy core. In practice, researchers dust a hydrophobic powder onto a liquid drop, the mixture shrinks into a bead-like shape thanks to a physical phenomenon called capillary interaction.

These forces partly stem from the surface tension between a solid and liquid, like when you barely overfill a glass and the water forms a round top. Adding hydrophobic powder to small amounts of liquid stabilizes these forces, pushing water molecules into tiny beads that almost behave like solids.

Appropriately dubbed liquid marbles, these non-stick water drops can roll across surfaces. Researchers can control their movement using gravity and electrical and magnetic fields, allowing them to float and climb across terrain. Some versions can even shuttle ingredients from one place and release their cargo in another.

But classic liquid marbles have a weakness. Small fluctuations in temperature or force, such as squeezing or dropping, causes them to leak or fully collapse. So, the authors developed a stronger shell to make their marbles more durable.

Ice, Ice, Baby

First, the team searched for the best ratio of Teflon dust to water. They found that more dust on the surface led to stronger, more durable shells.

Next, they worked out how to manufacture droplets with higher dust content. Traditional methods use spherical drops, which don’t have a lot of surface area compared to their volume. Cubes are a better starting point because they have more area. So, the team froze water in custom ice trays and coated the cubes with industrial-grade Teflon powder.

This method has another perk. Ice has more volume than water. As the cubes melt, their volume shrinks, squeezing the Teflon particles together on the surface of the droplets, limiting their movement, and forming much stronger armor for each liquid robot.

On the Move

The team pitted these enhanced liquid robots against traditional liquid marbles in a kind of playground with paper-covered foam structures and pools of water.

Both kinds of droplets could deform, such as briefly opening to expose their watery interior. But thanks to their harder shell, the Teflon bots were better able keep their liquid cores intact and survive falls without bursting. The liquid marbles, on the other hand, stuck to surfaces and eventually collapsed.

The team used sound waves to steer the robots around for more difficult tasks. In one task, they piloted the bots across an array of 3D-printed pillars. Upon meeting a pair of pillars, the robots split open, oozed through, and then merged back into their original forms on the other side. In another test, the researchers zapped adjacent bots with sound waves, deforming them into a bridge-like shape. Once touching, the two bots merged into a single, larger blob.

Thanks to their water-repelling nature, the robots could skim over both water and land—sometimes both. Older liquid marbles easily burst when shifting between the two terrains.

Liquid Bot Mission

To fully test the robots, the team designed a mission where two robots worked together. One bot picked up a chemical “toxin” locked behind bars. It then had to find its partner with the “antidote” in a pool of water, merge with the other bot to neutralize the toxin, and dump the final chemical into a safe container.

The team steered the first bot through its prison bars to engulf the toxin and carry it back out. Meanwhile, its partner skimmed across the pool to devour the antidote. The bots dropped from a height multiple times their size to their rendezvous, where they merged toxin and antidote, opened the outer shell, and dumped out the neutralized chemical.

Don’t worry, we’re still a ways from building T-1000s. The liquid robots are tiny and controlled manually. But the team is working to add smart materials for autonomous operation. And though they used water and Teflon here, the same process could be used in the future to mix other ingredients into a variety of liquid robots with different capabilities.