Researchers Create a Robotic Stingray with Light-Activated Living Cells

Researchers at the Wyss Institute for Biologically Inspired Engineering at Harvard have utilized living heart cells of a rat to power a robotic ray.

The research, led by Sung-Jin Park and Professor Kevin Kit Parker at Harvard, has led to a robotic ray which relies on contraction of the rat cells to copy the undulating movement of a stingray’s fins. The robot is one-tenth the size of a real stingray.

Stingrays have a highly stable and efficient method of swimming. With the undulating movement of the fins, stingrays produce a travelling wave from the front to back of their body. This leads to a force which propels them forward in the water. Because of this, stingrays were also examined to design more efficient submersibles a few years ago.

The robotic ray is almost one-tenth the size of a real-life fish. Image courtesy of MIT Technology Review

The Robotic Ray’s Structure

The muscle layer of the robot uses about 200,000 rat cells. Obviously, placing the cells in certain arrangements could lead to a higher speed and efficiency. The scientists spent a lot of time taking apart the muscles of a ray and analyzing them so that they could successfully mimic the synchronized undulations that propel the animal forward. After numerous trials, they chose a muscle arrangement very much like that of a real-life stingray.
However, there is a big difference between the structure of the robot and that of a stingray. The robot incorporates only one layer of rat cells which can produce a downstroke when contracted. To generate the upstroke, researchers built a multipronged gold skeleton which could act just like a spring. This gold skeleton, which is printed on a thin polymer body, returns the fins to their initial state as the muscle relaxes.
To develop a simple method of controlling the cells, the research team turned to optogenetics, which uses light-responsive molecules to trigger cell signaling. By genetically modifying the cells, the Harvard researchers achieved rat cells which contracted when illuminated by blue light. As a result, the light pulses could be used to externally control the robot.
Now, to make the robot steer, the researchers needed to pulse the light faster on one side than the other. This would make the muscles of one side contract and relax faster and effectively the robot would turn.

It Grows, Feeds, and Somehow Gets Old!

Since the robot relies on living cells as its power source, it needs some time–– about seven days–– to grow!
Moreover, the living cells must be fed. The cells get their energy from sugar dissolved in water. Actually, strictly speaking, it is not water: it is a special nutrient bath called “Tyrode’s Solution". The solution contains all the necessary ingredients to fuel the cells and needs to be warm so that the cells can operate properly. Without this solution, if you put the robot in water, it will not move at all no matter how much light pulse you apply to it.
The robot, which is 16.3 mm long and weighs about 10 grams, can swim at a speed of 3.2 mm/s. This is not fast enough to break the world record of swimming, but it is quite good for such a small creature. As the following video shows, the researchers applied light pulses and successfully guided the robot through a 250 mm long course.

After the robot is fully grown, it can stay at 80% of its efficiency for as long as six days–provided that you feed the cells.

The Precursor: A Jellyfish Robot

Kevin Kit Parker, a professor of bioengineering at Harvard University, is one of those responsible for the stingray robot's progress. Previously, however, Parker had built a robotic jellyfish by overlaying heart cells on a silicone cup. To Parker, the rhythmic pumping of a jellyfish reminded him of a heartbeat. He decided to put the heart muscle cells into a sheet of silicon in the shape of a shallow cup.

By applying electricity, the cells contracted and, consequently, the cup squeezed inward. In this way, a propelling force was generated which could push the robot forward in its bath. Similar to the robotic ray project, Parker had to submerge the cells in a salt-sugar solution so that the cells could survive. Compared to the robotic jellyfish, the new stingray robot is of higher complexity.

The Potential Applications of the Robotic Ray

Scientists are becoming increasingly interested in artificial creatures which can perform certain tasks inside the human body. To this end, they need to engineer sensor-rich tissues. Interestingly, the Harvard robot uses the cells as both the sensors and the actuators and we can expect that this adorable innovation can pave the way for many ambitious goals.
According to Parker, the research not only paves the way for building more advanced bio-hybrid robots, but it can also ultimately lead to an artificial human heart. If you can meld cells and artificial materials into a pulsating structure, you may get one step closer to building an artificial human heart.
The professor notes that he is planning to build an artificial heart but it is not possible to go from zero to a whole heart overnight! Parker sees the robotic ray and the robotic jellyfish as training exercises which may replicate some of the heart’s functionalities and reveal some of its secrets.
Moreover, the study can help marine biologists to better understand the swimming patterns of a ray.
Parker calls the robotic ray a work of art. Everyone sees something different in it and this is the beauty of transdisciplinary science. He is moving onto his next project but refuses to give any details about it.

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Considering that even adding a second layer of muscle cells to the Harvard robot is daunting, there is a long way to go before we can see a working artificial heart. However, the achievements of this study are admirable.
We can expect that, one day, biology could be melded with other fields of science to create systems which are far more efficient than what we could achieve without an interdisciplinary approach.

The details of the study are discussed in the journal Science.