3-D printing allows you build something by successively layering a substance. 4-D printing adds another dimension: time. It's when you program a material so that it changes shape in reaction to a stimulus—say, water, air, heat, or an electromagnetic field.
The ability to give materials "memory" could open up all sorts of possibilities in the future. For example, we could set biomedicine devices to do certain things within the body. We could program pipes or valves to change shape underground. Or, Ikea might develop self-assembling furniture, so we don't have to go through the agony ourselves.
It's early days yet, but experiments at Harvard give a sense of what's coming. Scientists there have developed "hydrogel composites" that change shape in the presence of water, becoming orchid-like structures. See the video here:
Like 3-D printing, a nozzle deposits a substance on a sheet, creating a shape. What's different is the make-up of the ink composites. These are composed of a hydrogel—a Jello-like substance of water and a polymer—and cellulose "fibrils" derived from wood. When exposed to water, the fibrils swell in particular directions, like the way plants react to their environment. By varying the direction of the printing, the scientists can achieve differing effects.
"The ability to preferentially orient them by controlling the print-path is what allows [us] to program the desired shape change," says Jennifer Lewis, who leads 4-D printing research at Harvard's Wyss Institute, in an email. "The printed structure swells less along the printing direction than [at right angles] to the printing direction. By printing two layers—with different printing orientations—we can encode the desired behavior: bending, twisting, ruffling."
Lewis says possible applications could include smart textiles, soft electronics, biomedical devices and tissue engineering—though there's a lot of work to be done yet. For now, we can just admire the elegance of slowly changing shapes and the re-engineering of natural processes for human purposes.