The line between human and machine is as blurry as ever. We have humanoid robots that express emotions, “living” bots that self-replicate, and implants that allow people to control prosthetics with their minds.

Now, scientists have managed to make a model fish out of plastic, gelatin, and a layer of human stem cells. Thanks to the living cells that make up its muscle layers, the fish can propel itself autonomously around a small tank.

Researchers described this futuristic creation last week in the journal Science.

Why it matters — The model fish mimics how fish swim in the wild. Its sheet of stem cells expand and contract, causing a seamless back and forth motion that propels the fish through fluids.

This process mimics another one found in nature. In fact, it’s something that you’re experiencing right now. The fish’s muscles expand and contract in the same way that the human hearts pump blood.

Though we’ve long known the anatomy of a human heart, scientists have struggled to replicate the organ’s autonomous pumping motion in a lab. Even if researchers were able to grow something that looks like a heart, getting it to work and a natural one is its own hurdle.

But incremental experiments with other models — such as a biohybrid fish — could pave the way for researchers to understand better how to replicate the motions of the heart.

Here’s the background — This isn’t the first time this research team created a robot made of both cells and nonliving materials. In 2016, many of the authors of the new Science report also helped create a dime-sized stingray that used a layer of rat muscle cells to move around a tank.

The cells on the stingray were designed to pick up on light cues, a process known as optogenetics. Researchers had to shine a blue light on the robot to get it to move, and it was able to swim for six days spontaneously.

This new fish, explains lead study author Keel Yong Lee, has more muscle layers than the stingray, allowing it to swim independently.

“We wanted to increase the complexity of the biohybrid robot,” says Lee, a postdoctoral researcher at Harvard. “The stingray has [just] a single layer of muscles.”

The goal with the fish, Lee says, was to model two main heart functions: pacemaking and electrical signaling.

How it works — For a human heart to pump at regular intervals, it requires a specialized type of cells called cardiomyocytes. Some generate electrical pulses spontaneously, squeezing and relaxing in a consistent rhythm, which is why they’re often called pacemaker cells.

Cardiomyocytes can be grown from mature stem cells in the lab, which is precisely what the researchers did. They arranged numerous amounts of cells into sheets of muscle. Layers of flexible plastic, gelatin, and paper were sandwiched between a left and right muscle to create a swimming fish.

As the fish bent its body back and forth to propel itself, those muscle layers created a cycle where each contraction happened automatically in response to stretching on the other side. “That means when muscles contract, the opposite muscles stretch,” Lee explains.

When the fish were equipped with two layers of cells on each side instead of just one, they could swim spontaneously for over 108 days using the beat of their own muscles.

What’s next — The goal of this research is to eventually build a 3D, working heart model, Lee says. And using human stem cells to create the fish instead of cells from animals was a big step in the right direction.

“If we want to make an artificial heart, we need to use human stem cells,” Lee says. Once scientists can grow a working heart, the next step will be learning to transplant it. They’ll need to ensure that the person who received the heart won’t have it immediately rejected by their body.

Since anyone’s stem cells could theoretically be used to grow heart muscle, Lee thinks that this could open the door to a personalized approach for growing artificial hearts. It could be a way to not only tackle the organ shortage but also make the process of transplants even more streamlined.