How 4D Printing Works

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].

MIT researchers are not the only ones working on 4-D printing, but the school's Self-Assembly Lab is the one that made the earliest splash, largely thanks to the TED talks of its director, architect Skylar Tibbits.

The lab's researchers first entered the world of self-assembly by creating simple, large-scale, self-building robots. When they found the labor and expense unworkable, they turned to making shapes and materials with logic built into them.

In 2010, they created Logic Matter, a set of interlocking shapes that could solve computational problems using only their geometry.

Reduced to its most basic, a computer operates using electronic gates that combine 1s and 0s and return a true or false answer. These gates use Boolean algebra, which asks questions like "are both inputs 1s?" or "is either input a 1?" The Tibbits lab asked the same questions, but using complex polyhedrons instead of the usual electrical on/off states representing 1s and 0s. Input involved clicking shapes into place. This created a new configuration that would allow the next shape — the output — to attach only in an upward (true) or downward (false) orientation, providing the answer.

Logic Matter did not rise to the level of self-assembly — the pieces required human hands to snap them together — but it did constitute an important first step in that direction by showing that matter could have instructions built into it [source: Tibbits]. Over the years that followed, researchers from the Self-Assembly Lab moved increasingly to items more in keeping with their name: geometric shapes that would combine if rolled or shaken in a container, chains that assumed particular shapes when shaken, and so on.

This marked the next important step: combining a built-in geometric tendency with an input of energy (or some other environmental factor) to kick it into gear.

But what is this geometric tendency? Well, if you've ever tried to make something out of cardboard (or wood, or metal), you know that it folds more readily if you score it first. Scoring, then, is a kind of programming, a way to make the material more likely to behave the way you want it to. Now instead of cardboard, imagine a combination of materials, some of which can absorb water and grow while others remain stiff. Toss it in water, and watch its shape change. Get clever enough with your foldings and scorings and, before you know it, you have something truly special.

But first, you need a lot of precise control over the materials you use and the pattern in which your machinery lays them down. And this approach will work better on smaller scales, where energy inputs and material differences can have a greater effect. Multi-material 3-D printing helped provide the control researchers needed, but they also needed the right materials.

When Tibbits mentioned his idea to the folks at Stratasys, a Minnesota-based 3-D printing company, they showed him a material that could grow 150 percent when submerged in water. Water offers a promising means by which to manipulate 4-D objects, since nature provides numerous working models of objects that change shape in response to moisture. We call them plants.

Plants exhibit tropisms, tendencies to grow in certain ways based on environmental factors, such as sunlight (phototropism), water (hydrotropism), gravity (gravitropism), chemicals (chemotropism) and even physical contact (thigmotropism). For example, plants tend to bend toward sunlight because sunlight kills hormones called auxins that encourage growth. Consequently, the side of a plant facing away from the sun grows faster than the side facing it, causing the plant to bend toward the light. With a little imagination, it's easy to see how we might similarly bend the physics that link materials, environments and energy to do our bidding.

Given the inspiration that plants have provided 4-D printing researchers, it's perhaps not surprising that a Harvard team made news in 2016 by creating a 4-D-printed "orchid" that assumed its namesake's shape when placed in water. The flower was printed using a hydrogel composite, which was piped, layer after layer, like icing from a pastry bag, onto the print bed [source: McAlpine].

Two aspects of the printing process explain the flower's behavior. First is the use of hydrogel, which can absorb large amounts of water. The second is the fact that the composite also contained cellulose fibrils — small, strong fibers essential to plant structure. Because the cellulose always flowed in a known direction, the team could carefully pattern it to control which parts of the flower could swell up and which parts would remain stiff once exposed to water [source: McAlpine].

No doubt, as time goes by we'll see many more experiments using a variety of other materials, such as conductors for flexible and dynamic electric circuits. But we'll also likely see the term 4-D printing, like most buzzwords, take on a life of its own, expanding to comprise a wider array of topics. For example, one company, Nervous System, describes its novel technique for 3-D printing clothing — which creates clothes from cleverly arranged nylon petals connected by joints— as "4-D printing" [source: Rosencranz].

Let's look at a few other potential 4-D futures.

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.