The world of nanomachines has a head start on the road of self-assembly, in part because it can draw from nature for examples of efficient, complex designs that self-assemble, rarely make mistakes and self-repair as needed. Moving these principles into the human scale has proven challenging but, if it works, the possibilities are impressive — a fact that is not lost on the U.S. Army, which has already split $855,000 among Harvard University, University of Pittsburgh and University of Illinois to fund research into military applications such as self-building bridges and shelters [source: Campbell-Dollaghan].
We've already mentioned how fashion and furnishings can provide a fun, profitable way to introduce a novel technology, and given the fact that one size very clearly does not fit all, it's a sector ripe for such applications. We could soon see patterns — or hemlines — that change on command.
The point is, much of 3-D and 4-D printing's appeal lies in its flexibility. Via 3-D computer modeling, a company could customize a dress or shoe to fit any body, right out of the gate, without any cutting or sewing — and print it as a one-off [source: Rosencranz]. Using 4-D materials and geometry, the garment could self-adjust in response to forces of stretch and strain. A running shoe could stiffen to provide lateral support and stability while sensing the stresses of a tennis match, for example.
BMW has already shown a concept car that would incorporate 4-D designs in what they call "Alive Geometry." Picture interior or exterior components that could change shape to handle shifting driving conditions. Outside the car, 4-D panels could adjust to temperature, airflow, steering or sensor input to maximize aerodynamic efficiency. Tires and brakes could also change in response to road conditions [source: Vijayenthiran].
In the future, as biomimetics and 4-D printing come together, we could see medical devices tailored to our bodies and even body augmentations that respond to their environments [source: Grunewald]. Now that's what we call personalized medicine.
Of course, 4-D printing will have to overcome numerous limitations before it can reach its full potential. First, the process remains, for now anyway, very, very slow. And its dependence on geometry limits it somewhat in terms of what it can do, but that's likely a temporary impediment. Potentially more serious are the stresses that act on any material that is forced to bend, or the failure points possibly introduced by such geometry. Moreover, in some cases, 4-D materials have trouble un-changing — they stay in their new form rather than reverting back to the old, or fail to switch among states as designed [source: Wassmer].
As to whether 4-D printing constitutes a fad, a curiosity or the next big thing, only time — appropriately enough — will tell.