U.S. researchers say they've developed tiny robots that are half animal, half machine, powered by actual muscle tissue activated by electric current.

When a current is applied to their biologically-based muscles, the tiny 'bio-bots' can move across surfaces or though liquids, scientists are the University of Illinois at Urbana-Champaign say.

"Biological actuation driven by cells is a fundamental need for any kind of biological machine you want to build," says study leader and bioengineering Professor Rashid Bashir. "We're trying to integrate these principles of engineering with biology in a way that can be used to design and develop biological machines and systems for environmental and medical applications."

The bio-bots the researchers have been building are smaller than a quarter-inch in size, created using 3D-printed flexible hydrogels combined with living cells.

The chosen design is based on the bone-tendon-muscle structure found in nature, they say, with the hydrogel acting as a stiff yet flexible backbone to which a length of muscle cells is attached using two posts, the way tendons attach muscles to bone.

The posts serve as the bio-bots' feet, allowing it to move when the muscles contract and then release, flexing and then unflexing the hydrogel backbone.

The speed at which the bio-bots move is determined by the frequency of electrical pulses delivered to them, with higher frequency moving the muscles faster.

One use of such bio-bots, the researchers say, could be as autonomous sensors that could live within a person's body, able to move toward a toxin when they sense it and then release drugs to target the toxin.

Other uses could include smart implants and devices for robotic surgery, Bashir says.

The 3D printing of the bio-bots' basic underlying structure can allow engineers to quickly create and test different designs, Bashir and his research colleagues said.

In fact, they said, the plan to offer a unit in the university's undergraduate laboatory curriculum to allow students to design different types of bio-bots.

"The goal of "building with biology" is not a new one -- tissue engineering researchers have been working for many years to reverse engineer native tissue and organs, and this is very promising for medical applications," graduate student Ritu Raman, one of the study's co-authors, says.

"But why stop there? We can go beyond this by using the dynamic abilities of cells to self-organize and respond to environmental cues to forward engineer non-natural biological machines and systems," Raman says.