tem cells are viewed on a computer screen on August 27, 2010 in Farmington, Connecticut. AFP Scientists on Monday said that for the first time they had printed 3D objects using human embryonic stem cells, furthering the quest to fabricate transplantable organs.

Once fine-tuned, the technology should allow scientists to make three-dimensional human tissue in the lab, eliminating the need for organ donation or testing on animals, they reported.

Human embryonic stem cells (hESCs) can replicate indefinitely and become almost any type of cell in the human body.

They are touted as a source of replacement tissue, fixing nearly anything from malfunctioning hearts and lungs, to damaged spines, Parkinson's disease or even baldness.

Scientists have previously tested 3D printing, which uses inkjet technology, with other types of cells, including adult stem cells.

But until now hESCs, which are more versatile than mature ones, have proven too fragile.

"This is a scientific development which we hope and believe will have immense valuable long-term implications for reliable, animal-free drug testing and in the longer term, to provide organs for transplant on demand," said Jason King from British stem cell company Roslin Cellab, which took part in the work.

The team used a specially-designed "valve-based" printer that deposited a "bio ink" of liquid containing laboratory-cultivated hESCs.

The cells are forced out with a tiny blast of air, and the flow is controlled by the opening and closing of a microvalve.

"We are able to print millions of cells within minutes," co-author Will Shu of the Heriot-Watt University in Edinburgh, told AFP.

"The printer is similar to the size of a standard desktop-size laser printer."

The living cells are printed onto a culture dish and left to aggregate together to form "what we call a spheroid, like... a little ball," said Shu.

Each spheroid was smaller than a millimetre.

The study appears in Biofabrication, a journal published by Britain's Institute of Physics (IOP).

The experiment was not designed to create anything but to demonstrate a method that did not damage the delicate cells.

"Most importantly, the printed hESCs maintained their pluripotency -- the ability to be differentiated into any other cell type," the IOP said in a statement.

Theoretically, the team can print any shape, but is not yet able to recreate a human organ, which needs a mesh of blood vessels.

"The challenge for printing a whole organ is to have this vascular structure inside it to feed it, allowing the tissue to survive in the long term," explained Shu. "We have our first step towards that."

Another big hurdle is fine-tuning the science of instructing embryonic stem cells to become specific types of tissue.

In the short term, said Shu, his team is seeking to print 3D liver tissue, which has one of the simplest of biological structures.

This could then be used for laboratory drug testing, "which would hopefully eliminate the use of animals," he said.

"I expect this technology can be realised in one or two years' time."

One idea behind the search for replacement organs is to grow the cells using a patient's own DNA to avert transplant rejection.

But the sector has been dogged by objections over the use of early-stage embryos, where the most adaptable, or pluripotent, cells are found.