Pulsing transplanted heart cells with electrical current helps them grow into mature cardiac cells, bringing doctors closer to a grow-your-own heart patch, U.S. researchers said on Tuesday.

So far tests have only been done in rats but the researchers at the Massachusetts Institute of Technology and Harvard University say they hope to find a way to repair tissue damaged when people have heart attacks.

“We have been trying to engineer a patch of tissue that has the same properties as native heart tissue, or myocardium, that could be attached over injured myocardium,” said Gordana Vunjak-Novakovic of both Harvard and MIT, who led the study.

“Think of it as a patch for a broken heart,” she added in a statement.

Vunjak-Novakovic and her colleagues have been working for years to find ways to make tissue patches for hearts, new blood vessels and other organs. Simply transplanting cells does not work, because they do not grow into the right layers and often do not produce the compounds that cells native to organs do.

The researchers have found that by growing cells under conditions that mimic the living human body, they can make them behave more like the normal tissue.

After growing the rat heart cells for a week with regular electrical pulses coursing through the lab dish, the cells started to look and pulse like mature heart cells. They also produced heart proteins, such as the myosin heavy chain and cardiac troponin I, essential for normal heart function.

“The real advance here is we mimicked what the body does itself and got it to work,” said Robert Langer, who also worked on the study.

One key goal is to get the cells to contract in a synchronized way. “We don’t want them beating at different rates,” said researcher Hyoungshin Park.

The eventual goal is to take a few cells from a heart attack patient, grow them in the lab under the right conditions and then transplant them back to the injured area.