U.S. bioengineers say they have successfully mimicked the way embryonic stem cells develop into heart muscle in a lab study.

Duke University researchers say their achievement using mouse embryonic stem cells is a important first step toward growing a living "heart patch" to repair heart tissue damaged by disease.

The scientists said they used a novel mold of their own design to fashion a three-dimensional "patch" made up of heart muscle cells, known as cardiomyocytes. The new tissue exhibited the two most important attributes of heart muscle cells -- the ability to contract and to conduct electrical impulses.

Researchers said they also found cardiomyocytes flourished only in the presence of a class of "helper" cells known as cardiac fibroblasts, which comprise as much as 60 percent of all cells present in a human heart.

"When we tested the patch, we found that because the cells aligned themselves in the same direction, they were able to contract like native cells," said Brian Liau, a graduate student who presented the study in Pittsburgh last week during a meeting of the Biomedical Engineering Society. "They were also able to carry the electrical signals that make cardiomyocytes function in a coordinated fashion."

Professor Nenad Bursac, who led the research, said he believes the experiments represent a proof-of-principle advance, but said there are still many hurdles that must be overcome before such patches could be implanted into humans with heart disease.