Researchers have created miniature models of the human heart—and, yes, they are functional.

The models, believed to be the first of their kind, includes cells, structures and even vascular tissue exactly like one would find on the human heart. Each one is roughly the size of a sesame seed and is able to beat in less than a week.

“These minihearts constitute incredibly powerful models in which to study all kinds of cardiac disorders with a degree of precision unseen before,” senior author Aitor Aguirre, assistant professor of biomedical engineering at Michigan State University’s Institute of Quantitative Health Science and Engineering, said in a statement.

The models were created using stem cells, and it’s a process that takes roughly a few weeks. Researchers can then monitor the progress in real time.

“This process allows the stem cells to develop, basically as they would in an embryo, into the various cell types and structures present in the heart,” Aguirre said. “We give the cells the instructions and they know what they have to do when all the appropriate conditions are met.”

Next, the team aims to learn new details about how the human heart develops—and, ideally, improve the models so that they are even more accurate.