The promise of printing human organs began in 1983 when Charles Hull invented stereolithography. This special type of printing relied on a laser to solidify a polymer material extruded from a nozzle. The instructions for the design came from an engineer, who would define the 3-D shape of an object in computer-aided design (CAD) software and then send the file to the printer. Hull and his colleagues developed the file format, known as .stl, that carried information about the object’s surface geometry, represented as a set of triangular faces.

At first, the materials used in stereolithography weren’t sturdy enough to create long-lasting objects. As a result, engineers in the early days used the process strictly as a way to model an end product — a car part, for example — that would eventually be manufactured using traditional techniques. An entire industry, known as rapid prototyping, grew up around the technology, and in 1986, Hull founded 3D Systems to manufacture 3-D printers and the materials to go in them.