The unexpected surface of the heart, and the complex vascular structure were produced using a process called Selective Laser Sintering (SLS). The hearts were built with successive layers of powder selectively bound by a laser beam. SLS is also a technique by which parts are built layer by layer. The basic material consists of powder. Successive powder layers are spread on top of each other and then a computer controlled laser beam scans the surface and selectively binds together the powder particles of the corresponding cross section of the heart model. During laser exposure, the powder temperature rises enough to make adjacent particles flow together.