Scientists have cultured small pieces of heart tissue which beat in the same way as the whole organ.
Heart cells grew into cardiac tissue
The US Massachusetts Institute of Technology team hope the work will lead to new ways of repairing heart damage.
They grew the tissue from a few rat heart cells which were placed on an artificial scaffold, and then stimulated with an electric current.
Researchers told Proceedings of the National Academy of Sciences: "Think of it as a patch for a broken heart."
Myocardium, or heart muscle cells, cannot regenerate after injury, limiting the effectiveness of standard therapies.
And heart cells are difficult to culture by conventional methods because they tend to lose their shape and stop functioning.
Researcher Dr Gordana Vunjak-Novakovic said: "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."
The MIT approach involves seeding cardiac cells from a rat onto a 3D polymer scaffold, which slowly biodegrades as the cells develop into a full tissue.
The cell/scaffold constructs, which are a little smaller than a penny piece and about the same thickness, were bathed in a medium that supplies nutrients and gases.
The researchers then applied electrical signals designed to mimic those in a native heart.
They had no idea whether the approach would work, but it turned out that electrical stimulation was crucial for the development of fully functioning cardiac tissue.
After only eight days of cultivation, single cells grew into a tissue with a remarkable level of structural and functional organisation.
Dr Robert Langer, who also worked on the project, said: "The real advance here is we mimicked what the body does itself and got it to work."
The researchers believe that the electrical stimulation helps condition the cells so that they communicate effectively with each other and contract in a synchronized form.
The next stage will be to try to create samples of tissue that are the right thickness for potential use in transplants.
The key here will be to ensure that all the cells throughout the tissue receive enough oxygen.
Dr Charmaine Griffiths, of the British Heart Foundation, said: "Every two minutes someone, somewhere in the UK has a heart attack - often leaving the heart muscle damaged and unable to pump efficiently.
"This interesting early research shows that scientists may have developed a functional patch repair mechanism that might eventually be used to treat damaged heart tissue.
"Although it is very early days for this approach, developments of this kind using living heart cells may provide help for those with damaged heart tissue in the future."