As if the line between human and machine wasn't already blurry enough, researchers in Tokyo have developed a new method for using living rat muscle tissue in robotics.

The “biohybrid” design, described today in the journal Science Robotics, simulates the look and movements of a human finger. Video shows how it bends at the joint, picks up a loop, and places it down. It's a seemingly simple movement but one that researchers say lays the groundwork for more advanced—and even more lifelike—robots. (Meet Sophia, the robot that looks almost human.)

“If we can combine more of these muscles into a single device, we should be able to reproduce the complex muscular interplay that allows hands, arms, and other parts of the body to function,” says study author Shoji Takeuchi, a mechanical engineer at the University of Tokyo. “Although this is just a preliminary result, our approach might be a great step toward the construction of a more complex biohybrid system.”

The research group began looking at living muscle tissue because plastic and metal provided a limited range of movement and flexibility. To grow their robot's muscles, they layered hydrogel sheets filled with myoblasts—rat muscle cells—on a robotic skeleton. The grown muscle is then stimulated with an electric current that forces it to contract.

Because they use living tissue, Takeuchi says, these robots can only survive and function in water.

In his previous work using living tissue in robots, Takeuchi had issues with the muscles shrinking after use to the point that they no longer functioned. This time, he layered muscles parallel to each other to simulate what's known as an antagonistic pair.

In the human body, the bicep and tricep in your forearm are considered antagonistic—as one muscle contracts, the other expands. The antagonistic pairing prevents wear, he says, allowing the muscles to be used for a longer amount of time. In the latest tests, for instance, the muscle tissues lasted for just over a week.

In addition to boosting longevity, the antagonistic pairing allowed the robot to have a full 90-degree range of movement.

The research team thinks that future versions of muscle-driven robots could help engineers create more nimble prosthetics. The human-like muscle system could also one day help scientists test drugs and toxins, reducing the need for animal testing, Takeuchi says.

However, a number of obstacles stand in the way of perfecting the biohybrid. The machine's joints produce friction that makes its movement somewhat stilted, so the researchers are looking at possible lubricants.