Driven by design software, 3-D printers churn out made-to-order objects on a desktop. A myriad of materials from polymers to metal alloys enable virtually anyone to manufacture almost anything they can imagine, including glow-in-the-dark pens, Lego-like building blocks, cogs and gears, electrical circuits and jewelry. Advanced applications produce living tissue for replacement organs in the body, intricate engine designs and parts for spacecraft during deep space missions.