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An artificial fish built from human heart cells could teach us how the organ functions.

The human heart can pump without signals from the brain, a feature known as automaticity. This is coordinated using electrical signals and mechanical feedback within heart cell tissue, but the process isn’t fully understood.

at Harvard University and his colleagues built a biohybrid fish using plastic, gelatine and a two-sided “fin” of human cardiac cells in order to better understand these automatic physical processes. The work follows previous projects at Parker’s lab that built stingray and jellyfish-like biohybrids from heart cells.

“When people are trying to build human hearts for regenerative medicine, they only try to replicate the anatomy, but there are some biophysical principles that are very important in the heart,” says Parker. By understanding the laws that regulate muscular pumps, we might be able to better treat heart disease, he says.

When the fish was placed in a cell culture, one side of its tail contracted and then the other did, producing movement similar to that of species such as zebrafish that use their tail fin to swim. The biohybrid fish swam for 108 days, or 38 million beats, and had a greater swimming speed than a wild fish of a similar size.

“Primary cells isolated from an actual heart from an animal survive maybe for two, three or four weeks if things go very well,” says at King’s College London. “The fact that you can extend that to levels that are almost the lifetime of small animals is amazing.”

Aside from its mirroring of nature, the fish’s movement has also shed light on functions of the heart. Ů��С��Ƶs have long thought that when the human heart relaxes between beats, blood fills the ventricles passively, but the fish’s fin contractions suggest it might be a more active process, says Parker.

He and his colleagues also designed a type of pacemaker, the G-node, to function like the timekeeping sinoatrial node in the human heart and spark the fin to regularly contract. “[It] brings home how much of cardiac regulation happens autonomously at the level of the actual contractile structures,” says Gautel.

Parker says his team will now look at how the work can be used to build artificial hearts. “We’ve taken away what we needed to learn and we’re applying it to the study of paediatric heart disease and regenerative medicine, and we’re moving on to the next [biohybrid].”