Light-Activated Muscle Cells Power a Paper-Thin Swimming Robot Through Watery Mazes

Swimming is deceptively demanding work. Water is roughly eight hundred times denser than air, and every stroke a swimmer takes must push against that resistance with enough force to make progress. For engineers hoping to build tiny robots that move through aquatic environments, this physical reality has long been a barrier: the smaller the machine, the harder it becomes to carry a power source strong enough to overcome drag. Now a team of mechanical engineers at the Massachusetts Institute of Technology has demonstrated that even a single layer of living muscle cells, if engineered with care, can generate enough force to propel a robot through water. Their creation, a paper-thin swimming robot described in the journal Advanced Functional Materials, represents the first example of a very thin, two-dimensional, muscle-powered robot capable of locomotion.

The robot, which the researchers call an aquabot, is built around a skeleton made from a film of gel roughly the length and width of a stick of chewing gum. The two halves of the gel serve as the robot’s fins, and each fin is coated with a layer of live skeletal muscle cells far thinner than a single strand of human hair. These cells are genetically engineered to contract when illuminated, which means the robot can be steered entirely with light. When the team shines light on one fin, the muscle cells on its surface twitch in unison, causing the whole fin to flap with enough force to pull the robot through the water. By flashing light on one fin or the other, and by varying the timing of those flashes, the researchers can control both the direction and the speed of the swimming robot.

In demonstrations, the paper-thin bot swam and swiveled its way through a simple watery maze set inside a large petri dish, following a light source that the researchers maneuvered manually over the surface of the water. At its fastest, the aquabot covered a distance of about four times its body length in one minute. That pace is admittedly modest compared with elite human swimmers, who can cover up to sixty-five body lengths per minute, but it is competitive with more leisurely swimmers in the animal kingdom. The cow shark, for instance, explores the ocean at roughly the same rate relative to its body size. For a robot whose entire propulsion system is a film of muscle cells thinner than a hair, the performance is a striking proof of concept.

It takes a lot of force to move through water versus air, notes study author Ritu Raman, associate professor of mechanical engineering at MIT. The robot’s quite strong, given its size, she says. That strength did not come by accident. It was the product of a deliberate engineering campaign to optimize every parameter of the platform on which the living muscle grows, a process the researchers describe as tuning the robot’s skeleton. The lesson of the work is that in biohybrid robotics, the artificial scaffold matters just as much as the biological engine that sits on top of it.

The new aquabot builds directly on a milestone the same group achieved the previous year. In that earlier effort, Raman’s laboratory created an iris-inspired disk of artificial muscle tissue by stamping a disk of gel with a pattern of concentric and radial grooves and then depositing live muscle cells onto the gel’s surface. The cells grew along the grooves, forming a thin layer, and when stimulated with light they moved in patterns that stretched and squeezed the disk much as a human iris dilates and constricts the pupil of the eye. That work was the first to show that muscle cells could be grown in a very thin layer and in complex patterns that, when stimulated, could move in multiple controllable directions. People hadn’t seen this muscle architecture engineered from scratch before, Raman says, and the cells were moving in multiple directions. But they only moved about one hundred microns. From a robotics perspective, their movements were tiny.

The goal of the new study was to close that gap: to maximize muscle movement so that the tissue could produce enough force to do useful mechanical work, such as powering a swimming robot. The key, the team discovered, was the underlying gel. In the iris-inspired design, the muscle cells had been grown on fibrin, an ultrasoft gel that the researchers realized could quickly shrivel in response to the forces generated by the contracting muscles. A scaffold that deforms under load wastes the very force the cells are trying to deliver. To better support the cells and maximize their output, the engineers focused on tuning three properties of the gel: its composition, its stiffness, and the size and shape of the grooves stamped into its surface. For engineering any type of tissue, it’s known that these are knobs you can tune, Raman says. And we wanted to optimize all these parameters to support live muscle cells.

The optimization process was systematic. The team experimented with multiple formulations of gel at different stiffnesses, each stamped with grooves of different geometries. One groove type resembled a skinny square trough, while another was more of a long curved valley. When muscle cells were deposited onto each type of grooved gel, the researchers found that the cells settled into alignment most effectively in grooves that were more square than curved. This matters because alignment is everything in engineered muscle: cells that line up in the same direction tend to fuse into fibers, and those fibers form a stronger, more coordinated muscle tissue capable of generating useful force in a single direction. Square grooves, the team concluded, were the way to go.

Composition and stiffness proved equally consequential. Instead of fibrin, the researchers switched to gelatin methacrylate, or GelMA, a material widely used in tissue engineering. By preparing different recipes of GelMA, they created skeletons of varying stiffness and observed how muscle cells grew on each. The cells grew in better alignment and produced the most force on stiffer gels, a finding that runs counter to the intuition that softer scaffolds are always gentler on living tissue. The team also varied the thickness of the gel and found that a film just half a millimeter thin offered good support for a single layer of muscle cells while remaining light enough that the cells stayed anchored to the gel when they contracted, rather than peeling away. As a final step, the researchers exercised the muscles, subjecting them to a training routine of flashing lights that strengthened the tissue over time, much as workouts build strength in an athlete.

With the pumped-up cells and the optimized gel in hand, the team assembled a thin, two-finned robot: the gel, stamped on both sides with square-bottomed grooves and lined with muscle cells that fused into fibers and eventually formed a strong, aligned muscle tissue. You can think of the robot as having two independent muscles, Raman says. If we shine a light on just one, only that muscle moves. If shining on both, they both flap. This independent control of each fin is what gives the aquabot its maneuverability, allowing it to turn and navigate obstacles rather than simply drifting forward in a straight line.

The researchers acknowledge that the current design is relatively basic in form; their first aim was simply to show that the bot could produce enough force to swim at all. Their next goal is to optimize the body design to enable faster swimming. But even at slow swim speeds, the possibilities are intriguing. One could imagine a muscle-powered swimmer being used for purposes like environmental monitoring in aquatic environments, Raman says. Biohybrid robots powered by living muscle could one day perform delicate jobs like exploring environments too fragile or unpredictable for conventional hardware, because living tissue is soft, responsive to its surroundings, and can heal itself. The approach also promises practical advantages over existing biohybrid designs, which are typically built from bulky, three-dimensional chunks of lab-grown skeletal muscle that require millions of cells to fabricate. Thinner, less bulky designs like the aquabot could be cheaper to build and could move more efficiently, bringing soft, light-driven, self-healing machines a step closer to real-world duty beneath the water’s surface.

Subject of Research: Biohybrid robotics using engineered skeletal muscle thin films to power light-controlled swimming robots

Article Title: Powered by muscle cells, a paper-thin robot swims through watery maze

Article References: Powered by muscle cells, a paper-thin robot swims through watery maze. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: biohybrid robots, muscle cells, soft robotics, aquabot, MIT, light-activated actuators, GelMA, tissue engineering, swimming robot, skeletal muscle, environmental monitoring, Advanced Functional Materials