To print a 3-D prototype, the first thing you need is a 3-D model. Using special software, it's possible to make virtual designs which can be fed into a 3-D printer and presto -- you have a desktop visual of your product.
CAD (computer-aided design) software was first pioneered in the early 1960s, and it became popular on the commercial scene in the 1970s. Since then, an enormous field of engineers and designers of all sorts have made use of the technology to design everything from airplane engines to the kitchen sink.
SolidWorks is one company that makes top-of-the-line mechanical CAD software, with a variety of tools and functions to design, manipulate, compile, correlate and evaluate prospective products. SolidWorks software enables users to transform 2-D to 3-D, use a variety of tools to swiftly and efficiently develop their design, simulate the assembly and operation of the prototype, compare and share their finished product, and much more.
While using the SolidWorks software, it's possible to not only manipulate 2-D images and create professional drawings ready for the production line, but also to make detailed 3-D image maps. These can be sent to 3-D printers, and you could have a prototype of your design in a matter of minutes (or hours depending on the size).
One example of an application of the SolidWorks 3-D CAD software was created by the MAKO Surgical Group. They designed and built the MAKO Tactile Guidance System -- a robotic arm that assists surgeons perform less invasive, more successful surgeries. The machine not only helps control the surgeon's movement, it can also let them know where it's safe and appropriate to cut. The SolidWorks software proved useful not only for part design, but also for mapping the virtual space the surgeons navigate.
The guys on "Prototype This!" aren't making surgical equipment with their tools, but read about another tool they're using to make their own unique prototypes.