A 3-D printer was used to construct these tiny two-ventricle cylinders at the University of Louisville, in Louisville, Ky. Researchers are working on a project to build a human heart using a 3-D printer and human cells. (AP Photo/Dylan Lovan)
University of Louisville researcher Stuart Williams, director of a program to build a human heart with a 3-D printer and human cells, poses for a picture, in Louisville, Ky. (AP Photo/Dylan Lovan)