A YOUNG VICTIM OF A BOMB BLAST IN BAGHDAD arrives at a U.S. army hospital requiring facial reconstruction. Surgeons upload data from scans of the boy’s skull into a 3-D laser printer, which produces a ceramic bone substitute of his jaw within hours. Known as additive fabrication, or rapid prototyping, the process—in which cross-sections of liquid, powder or sheet materials are fused into objects of ceramic, plastic and metal—is used to create prosthetic devices and models of organs, joints and bones. Recently, surgeons in Dallas used a model of a conjoined skull, replicating bone structure and vasculature, to guide them in separating twins.