Biology has given robot makers lots of good ideas about how to solve certain problems, like how to squeeze into tight spaces, how to conserve energy for flying, and how to move around without a skeleton. But now it's giving us the raw materials for building a robot—or perhaps more properly a cyborg. Researchers have not only based the design of their robot on animals like skates and rays, but have used muscle cells to power its movements and light-sensitive proteins to replace its circuitry.

Skates and rays (technically batoid fish) are distinct from other cartilaginous fish in that they have a largely flat body plan and generate propulsion by undulating their bodies rather than by flapping fins. This undulation is a very simple way to generate locomotion, but it's highly energy-efficient and lends itself to soft-bodied robotics. It's also relatively easy to steer, simply by alternating the frequency of undulations on the right and left sides of the body. So the builders (a huge US-Korean team) of the new robot used this as inspiration for their new design.

To mimic this type of fish, the authors built the body out of a flexible polymer called PDMS. Internally, they added a flexible metallic skeleton made of gold so that once an undulation was done, the body would flex back into its original shape.

Normally, when gold is used for applications like this, it's used to conduct the electricity used to power the robot. But the researchers decided to use a very different source of power: glucose, a simple sugar that's the body's primary source of chemical energy. Glucose also works for this robot because its "muscles" are actually composed of muscle cells, specifically rat cardiomyocytes, the cells that normally comprise the heart.

Cardiomyocytes were chosen because they have distinct properties compared to other muscle cells. The muscles in your arms or legs require a push from a nerve cell to trigger a flow of ions, which causes the cell to contract. Cardiomyocytes, however, form connections with each other, so a flow of ions in one cell can trigger the same response in its neighbors, allowing a single signal to propagate long distances, although this takes a bit of time.

So, by carefully patterning the cardiomyocytes into "muscles," the authors were able to create a system where a single trigger produced the entire rhythmic contraction necessary to produce an undulation. The authors managed to do this by laying down a template of a protein that cardiomyocytes stick to. Given a chance to grow, the cardiomyocytes will naturally form a network of muscles. (In fact, they'll do this in a culture dish if you give them a chance.

With everything in place, once the cardiomyocytes are triggered, they drive a quick undulation of the cyborg's body. When the "muscles" of the cyborg relax, its gold skeleton brings it back to its original shape.

But without nerves, how do you trigger the muscles? The authors relied on a tool developed for a technique called optogenetics: a light-sensitive ion channel. When exposed to light of the right wavelength, this channel opens, allowing a flow of ions that mimics the response to a nerve cell trigger. Thus, the muscles can be made to contract simply with a flash of light.

And it works, at least provided you place the cyborg in a salt and sugar solution that keeps the cells happy. With a flash of light, the cyborg flexes and inches forward at about 1.5 millimeters a second. The authors are even able to steer it around an obstacle course by alternating the frequency of light flashes directed to the muscles on the left and right side of its body.

Obviously, since the muscle cells grow on the exterior of the cyborg, this system is limited to tanks filled with a solution that keeps the muscle cells happy. It might be possible to have them grow internally, but that would add to the complexity of a system to keep the solution circulating across the cells. So, it would take a lot of work to get this to be more generally useful.

Still, it's pretty cool in its current form.

Science, 2016. DOI: 10.1126/science.aaf4292 (About DOIs).