They're actually well on their way to making a whole human heart -- back in 2012, they found that if they treat stem cells "with chemicals that sequentially activate and inhibit Wnt signaling pathway," they can create myocardium muscle cells. The myocardium is the muscular part in the center of the heart that stimulates contractions necessary to pump blood throughout the body. Stanford University's Wnt homepage explains that Wnt proteins regulate the proliferation of cells. Wnt signals "predominantly mediate signaling locally, between neighboring cells" and "give shape to tissues as cells are proliferating."

To transform stem cells into epicardium cells, they had to do the opposite. The study's lead author Lance Lian said they found out that "if [they] activate the cells' Wnt signaling pathway again, [they] can re-drive these cardiac progenitor cells to become epicardium cells, instead of myocardium cells."

The team is now developing a method to create cells that make up the inner part of the heart. Even now, though, their research can save lives. Lian says doctors could implant the epicardium cells they create into heart attack patients:

"Heart attacks occur due to blockage of blood vessels. This blockage stops nutrients and oxygen from reaching the heart muscle, and muscle cells die. These muscle cells cannot regenerate themselves, so there is permanent damage, which can cause additional problems. These epicardium cells could be transplanted to the patient and potentially repair the damaged region."

Last year, researchers from the Massachusetts General Hospital developed a technique to repair hearts deemed unsuitable for transplant with stem cells. A team of Japanese researchers also concocted a method to repair patients' hearts with skin stem cells injected into the organ. Clearly, it's not out of the realm of possibility for someone to create a lab-grown heart that actually works.