Most of the muscles in our bodies work only in response to incoming nerve signals, which must stimulate each individual muscle cell to contract or relax. But the heart muscle is different. The impulses that cause the heart muscle to contract are transmitted from one muscle cell to its neighbors, resulting in a wave of regular contractions. This is so integrated into the system that a sheet of cardiomyocytes in the implant dish begins to contract spontaneously.
Now, researchers have taken advantage of some of the unique properties of heart cells to build a robotic fish that swims powered only by sugar. And while they were trying to make the heart’s equivalent of a pacemaker, it turned out that it wasn’t necessary: the correct arrangement of the muscle cells made the fish spontaneously swim.
Building a heart-like muscle
In some ways, the paper describing the new robot fish is an appreciation of our increasing ability to control stem cell growth. The researchers behind the research, based at Harvard University, decided to use heart muscle cells to power their robot. Two years ago, this meant dissecting a heart from an experimental animal before isolating heart cells and growing them in culture.
For the thickness of the robot, the stem cells were better. That’s because stem cells are easier to genetically manipulate, and they are easier to grow into a uniform population. So, the team started with a set of human stem cells and went through the process needed to direct their growth so that they could form heart muscle cells.
A thin layer of these cells was placed inside a thin slice of gelatin, which holds the cells in place on either side of the “fish” (one slice on both sides). The center of the fish was flexible, so a contraction of the muscle of the right flank would pull the tail to the right, and the same was true for the other side. Alternating between right and left contractions, the fish pulls its tail from side to side, pushing it forward. Moreover, the fish has a large dorsal “fin” that contains a buoyancy device to keep the beast upright and prevent it from drowning. Everything was supported by placing it in a solution with sugar, which was absorbed by the cells of the heart muscle.
Perhaps because of this simplicity, the robot was so durable that it was able to swim for more than three months after it was built. Performance was good at first but improved during the first month as the heart cells were better integrated into a cohesive muscle. In the end, the fish was able to travel more than its body length per second. At that pace, the robot was remarkably efficient—per unit of muscle mass, its swimming speed was better than that of actual fish.
In and out of control
One of the things that helped enable the robot fish’s efficiency can be seen in its absence in the image above: any kind of control circuit. Researchers have already tested a number of ways to control the muscles, but in the end they found that the simpler option was the best.
The first attempt at muscle control relied on a little genetic engineering. Muscles are stimulated to contract by an influx of ions, which is usually caused by nerve impulses. But the researchers have identified some proteins that act as light-activated ion channels, which will create a flux of ions in response to specific wavelengths of light. Therefore, the researchers designed the cells on one side to be sensitive to red light and those on the other side to be sensitive to blue. This worked well, as alternating flashes of red and blue light allowed the fish to swim forward.
The second method the researchers tried was inspired by the architecture of the heart, which contains a group of cells that act as a pacemaker by causing a contraction that spreads from there. The researchers formed a ball of heart cells to act as a pacemaker and made a bridge of cells connecting the heart cells to the wing muscles. The flow of ions initiated in the pacemaker cells can spread to the muscles, causing contraction.
This worked to some extent but turned out to be of secondary importance. Researchers discovered that the two muscles speed up each other’s contractions.
Cardiac muscle cells also contain stretch receptors. Pull the cell a lot, and the receptor will be activated and cause contraction. It turns out that this provides internal coordination of the flank muscles. When one side shrank on the right, it caused the cells on the other side to stretch. Once it reaches a critical point, the stretch receptors on the left side will stimulate that muscle to contract, and stretch to the right. This extension then restarted the cycle.
This won’t work indefinitely, and the two muscles will eventually get out of sync. The pacemaker can then help get them back into a regular cycle.
Overall, this is more impressive than useful (unless you’re the type who just admires useful stuff). There aren’t a lot of situations that require a robot to swim through a sugar solution, after all. But the fact that the researchers were able to discover how to use the basic biological properties of these cells to make an efficient machine certainly fits my definition of admiration.
Science, 2022. DOI: 10.1126 / Science. abh0474 (About DOIs).
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