SITTING in a culture dish, a layer of chicken heart cells beats in synchrony. But this muscle layer was not sliced from an intact heart, nor even grown laboriously in the lab. Instead, it was "printed", using a technology that could be the future of tissue engineering.

Gabor Forgacs, a biophysicist at the University of Missouri in Columbia, described his "bioprinting" technique last week at the Experimental Biology 2006 meeting in San Francisco. It relies on droplets of "bioink", clumps of cells a few hundred micrometres in diameter, which Forgacs has found behave just like a liquid.

This means that droplets placed next to one another will flow together and fuse, forming layers, rings or other shapes, depending on how they were deposited. To print 3D structures, Forgacs and his colleagues alternate layers of supporting gel, dubbed "biopaper", with the bioink droplets. To build tubes that could serve as blood vessels, for instance, they lay down successive rings containing muscle and endothelial cells, which line our arteries and veins. "We can print any desired structure, in principle," Forgacs told the meeting.