In context: Making robots more biologically compatible has been a challenge scientists have been tackling for years. Until now, they have primarily been able to create lab-grown muscle fibers that contract along only one plane. That works well for a robotic arm that bends at a single joint, but it falls short when it comes to more complex movements. In contrast, the human body’s muscles are far more versatile thanks to fibers arranged in intricate, crisscrossing patterns.

Now, MIT engineers have taken a major step toward developing robots that replace rigid gears with something much softer – almost like real, living muscle tissue. In other words, they have found a way to grow artificial muscle that can flex in multiple directions.

To achieve this, they devised a new stamping technique that 3D prints tiny grooves into a soft gel. The actual process is complex, but to simplify: when muscle cells are seeded onto the gel, they grow along these grooved patterns. As a proof of concept, the researchers even fabricated an artificial iris using this method.

The iris is the ring of muscle that controls pupil dilation in the human eye. It consists of an inner set of concentric circular fibers and an outer layer of radiating fibers. The stamped gel replicated some of these patterns, resulting in a muscle-powered pupil that can dilate and contract, just like the real thing.

“We believe we’ve created the first skeletal muscle-powered robot that generates force in more than one direction,” said Ritu Raman, a tissue engineer at MIT. “That was enabled by this new stamping approach.”

Of course, there were challenges to overcome. For one, the gel is extremely delicate. Raman explained that it’s much softer than Jello and difficult to cast because it tears easily. Overcoming this required trial and error, but the results were well worth it.

The stamping technique is highly versatile, allowing researchers to essentially “blueprint” any desired muscle architecture by simply programming the stamp’s groove pattern. The stamps themselves can be produced using a standard tabletop 3D printer.

This flexibility opens up a vast range of possibilities, potentially enabling researchers to replicate everything from the swirling contractions of heart muscles to the coiled movements of the digestive tract.

Looking ahead, the team plans to experiment with different layouts and cell types beyond just skeletal muscle. Once robotic engineers begin integrating these programmable bio-tissues into designs, a new era of machines may emerge.

Raman envisioned one interesting use case.

“Instead of using rigid actuators that are typical in underwater robots, if we can use soft biological robots, we can navigate and be much more energy-efficient, while also being completely biodegradable and sustainable,” she said. “That’s what we hope to build toward.”