Tiny, robotic fish powered by human heart cells suggest that scientists are getting closer to their goal of building replacement hearts from living tissue.

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Scientists have built a robotic fish powered by human heart cells. NPR's Jon Hamilton reports that this feat is part of an effort to help people with failing hearts.

JON HAMILTON, BYLINE: The tiny fish are built from paper, plastic, gelatin and two strips of living heart muscle cells. Sung-Jin Park of Emory University and Georgia Tech was part of the team that made them. He says they tested some early models. Then they put the rest in an incubator for a couple of weeks.

SUNG-JIN PARK: We opened up the incubator. We can see that all of the fish is swimming by themselves.

HAMILTON: The fish kept swimming for more than three months. And Park says the team felt sad when it came time to sacrifice their robots.

PARK: We feel like - I don't know - it's - we have this - kind of the emotional attachments to the fish.

HAMILTON: The experiment isn't really about fish, though. It's about finding a way to repair or replace a damaged human heart. Kit Parker of Harvard, another member of the team, says that's important because the body can't replace heart cells.

KIT PARKER: Once you're born, about two days after you leave the womb, the number of cardiac muscle cells you have then is all you're going to have for the rest of your life.

HAMILTON: Which is a problem if some of those cells get wiped out by a heart attack or inflammation - so why build a fish to test heart muscle? Parker says it's because a fish is really a kind of pump. But instead of pumping blood through the body, it pumps itself through water.

PARKER: I really believe that there's a common design scheme, there's some fundamental laws of muscular pumps that are conserved from marine life forms to the human heart.

HAMILTON: In 2016, Parker's lab built a stingray using rodent heart cells controlled by light. This time, the team relied on stem cell technology to produce human heart cells that don't need any external control. One strip of muscle is attached to the right side of the fish, the other to the left, and Parker says the strips take turns pulling the robot's tail from side to side.

PARKER: And once that cycle starts, one side contracts. The other side is getting stretched. And then that cycle goes back and forth. And these things just start motoring.

HAMILTON: Parker says signals produced by muscles stretching perpetuate the swimming motion.

PARKER: The really interesting thing about these fish which we weren't expecting is how long they would swim and how fast they would swim in the dish.

HAMILTON: Heart cells constantly rebuild themselves about every 20 days, and Parker says these fish swam for more than 100 days.

PARKER: That means that each cell in there has rebuilt itself about five times.

HAMILTON: Lab-grown heart tissue is still years away from use in patients. For now though, this sort of model system could help researchers test heart drugs, says Ritu Raman, a mechanical engineer at MIT.

RITU RAMAN: You really need to know, how is something built in the native context, and how can we recreate that in the lab as closely as possible?

HAMILTON: Raman's lab has made robots powered by skeletal muscle, including one that could recover from an injury.

RAMAN: This robot would get damaged, and then we would heal it. And after a couple days, it was able to produce the same force and move and walk around just as it had before.

HAMILTON: Raman says robots powered by living cells make some people nervous, but they shouldn't.

RAMAN: All you're really doing is replacing an engine or a rotor or another piece that you would make in a machine with something that's made out of cells. So I would not consider them living beings.

HAMILTON: But Raman says as more sophisticated biohybrids come along, her view could change.

Jon Hamilton, NPR News.

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Tags: weird science  bioengineer  robotics  biomedical research  Stem cells