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THREE-DIMENSIONAL PRINTING IS BLOWING UP. From the obvious—hand tools and chess pieces—to the less obvious—body parts and shelf-stable food—just about every item imaginable is being subjected to the two-step process of digitization and fabrication that is 3D printing. One of the factors fueling the excitement is the ease with which 3D printing can be used to build items with hollow internal spaces—that is, to build something around nothing. This task is always hard and often impossible by conventional manufacturing methods, which usually involve creating an item by removing material from a larger mass and so are referred to as subtractive manufacturing methods. A hollow ball, for example, might be crafted in one of two subtractive ways: by making a hole in a solid sphere and reaching inside to scrape out the center, then covering the hole; or by casting two separate hemispheres, sticking the halves together, and obscuring the seam. A 3D printer, on the other hand, manufactures additively by stacking thin layers on top of one another until a 3D object is formed. So it could whip out a hollow ball in no time without hidden holes or seams. Los Alamos chemist Alex Mueller leads a team that is using 3D printing to create next-generation high explosives. Since the First World War, scientists have known that the behavior of trinitrotoluene (TNT) can be altered by the addition of certain materials, such as detergent or sand. Infusing TNT with bubbles or grains rendered it more sensitive 2 1663 March 2016 and easier to detonate—but no one knew why, so TNT was (and still is) difficult to control. The Los Alamos team is working to understand not only what goes on inside an One thing making 3D printing so popular is its ability to build items with hollow internal spaces. explosive during detonation but also how to control and tailor it through manipulation of its internal hollow spaces and microstructures. And a 3D printer is the ideal tool for the job. Hot spots and not spots Explosives are categorized as either low or high, depending on whether they burn (low) or detonate (high). Detonation involves an explosive shock front traveling through the material faster than sound can travel through it, while burning is entirely subsonic. In a conventional high explosive (CHE), such as TNT, the chemical reactions and supersonic shock front are relatively easy to initiate, and therefore CHEs are not immune to accidental detonation. Indeed, fires, accidents, or other munitions can cause a CHE to detonate