At its core, 4-D printing is a combination of 3-D printing and another cutting-edge field, self-assembly.
Self-assembly is exactly what it sounds like — the spontaneous ordering of pieces into a larger, functional whole. The field is popular in nanotechnology circles for two very good reasons. First, self-assembly already happens at the nanoscale and provides the driving force behind processes ranging from protein folding to crystal formation [source: Boncheva and Whitesides]. Second, we don't have hammers, wrenches and screwdrivers than can build a molecule-sized machine. It needs to make do on its own.
But if we could scale up self-assembly to human proportions, it could allow us to make current products cheaper and more simply, or to create otherwise impossible new technologies [source: Boncheva and Whitesides]. It's painstaking and often frustrating work. Even under ideal circumstances, it requires breaking down an assembly sequence, developing programmable parts and coming up with an energy source that will get your contraption going. Building in some error correction is not a bad idea either [source: Tibbits]. Mainly, though, you need the right tools and materials for the job.
Enter 3-D printing. Although new approaches continue to emerge, traditionally, 3-D printing has entailed repeatedly laying down carefully defined layers of polymer on a print bed. As each new layer hardens and fuses with the ones below, a three-dimensional shape emerges. Early models could print with only one material at a time, but newer 3-D printers allow for a wider array of printing media and for printing with more than one material at a time. That's an important breakthrough for 4-D printing, because varying materials allows developers to build in areas that stiffen, flex or swell, or that "want" to fold in certain ways. They can have zones that soak up water like a sponge, or that generate electric current when exposed to light. The sky's the limit, as long as you've built in the right geometry.
This is what the Self-Assembly Lab at MIT calls programmable matter — an approach to science, engineering and materials that focuses on matter that can be encoded to reshape itself or change its function. One application of programmable matter is 4-D printing [source: MIT].