LOS ANGELES — These bots were made for walking — out of rat heart cells and hydrogel.

LOS ANGELES — These bots were made for walking — out of rat heart cells and hydrogel.

Scientists have paired these unlikely ingredients to create simple biological machines that look something like a front-loaded inchworm and can step their way through fluid at speeds up to 236 micrometers per second.

Bioengineers working at the boundary between organics and mechanics dream of harnessing the power of biology's nuts and bolts. Some have built tweezers out of DNA; others have made sensors by sticking bacteria on a chip. It's no easy task — plastic and metal are far more predictable materials to work with.

But in work published recently in Scientific Reports, a team of bioengineers described how they came up with a way to manufacture biobots quickly and efficiently with a 3-D printer.

The printer builds preprogrammed shapes layer by ultrathin layer, until it has constructed a soft "body" out of hydrogel, a class of polymers that includes the stuff contact lenses are made from, said senior author Rashid Bashir, a bioengineer at the University of Illinois at Urbana-Champaign.

The researchers designed a roughly 7-millimeter bot that looks something like an asymmetrical see-saw: The boxy base is shifted toward one side, creating one long leg and one short one.

Then the scientists seeded living rat heart cells onto one side of the body, where they beat in sync. When the heart cells contracted, the robot's long leg flexed and pushed off the ground, causing it to "step" forward, in the direction of its shorter side. (Asymmetry, they found, was key: A more symmetrical design got nowhere.)

The shape of the biobot — where the base was placed along the length of the robot's body, for example — dictated how the bot moved and how far it went. The one in the study can only move forward. But the researchers plan to experiment with other designs that may allow the bots to move in different directions.

"They can have multiple legs; they could be able to go in different directions; they might go right, or left; they might reverse," said Roger Kamm, a bioengineer at the Massachusetts Institute of Technology who was not involved in the study.

And they'll be built quickly with the 3-D printing technology, which can cut the construction time from a day or so to a matter of minutes, said Columbia University bioengineer Henry Hess, who also was not involved in the study. (There's no way to make the rat heart cells grow any faster, though, he pointed out.)

Scientists have used 3-D printers before, and others have built synthetic critters, including walking bots and jellyfish-like swimmers. But combining bioengineering with this high-tech printing method is what makes these biobots so exciting, Kamm said.

"It opens up a lot of new potential," Kamm said. "You can go so many different directions with this basic design."

The technology is in the very early stages, and its future uses are unclear. But the researchers see some possible practical applications down the line.

For example, Kamm said, they could take muscle cells and attach them to neural cells that sense toxins or other chemicals. When a specific chemical is detected, the neurons could send the muscle cells a signal to move.

The work helps lay the foundation for making bigger, more complex biological machines using components from all over the plant and animal kingdoms, he added.