Engineers have grown most of the human heart in miniature - and it's beating.

The cells that make up the heart were obtained from the cardiovascular tissues of young rats and then grown on a layer of a scaffold printed from a polymer with grooves to guide tissue growth. This flat mesh made the structure mimic the arrangement of the fibers of the human left ventricular heart muscle, from which blood is ejected into the aorta with one strong contraction. The difference of the new model is also that it is made of several layers of cells - just like a real heart.