The road to self-assembling houses and shape-shifting robots could begin with something as simple as plumbing. Today when we want to build infrastructure for moving water around a city, we take rigid pipes with fixed capacities and then bury them. And the system works well enough—until we need to increase the flow of water to an area or until a pipe breaks. Then we have to dig the whole thing up and replace it.
A nice alternative would be flexible pipes that could change shape on command or under the right level of pressure or pipes that could heal themselves in the event of a rupture. Advances in computer-aided design (CAD) and materials science are now making such pipes feasible. Those same advances and the new form of design that they have made possible could yield a world of programmable matter—material objects capable of self-assembling, morphing into new shapes or changing properties on command.
Scientists are already building self-assembling machines, but they are tiny—nanoscale devices that work as biochemical sensors, electronics or drug-delivery carriers. We are interested in what happens when programmable matter achieves human scale. There are two primary ways to achieve this goal. One approach involves creating unconnected building blocks that can come together or break apart autonomously to form larger programmable structures. Another tack is to build shape-changing objects as a single, complete structure—objects with hinges, stress points or electronics embedded in just the right spots to allow them to change shape under desired circumstances. We call this second approach 4-D printing. As with 3-D printing, 4-D printing involves constructing preconnected objects by laying down layer after layer of material. In this case, however, those objects can change shape or properties over time after they are printed.
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Programmable matter could yield objects that save materials, energy and labor. Think of a chair that can turn itself into a table. Think about those flexible, self-healing water pipes. It could make it possible to build complex machines without human construction. Such systems would be particularly valuable in hostile environments, such as outer space. One could launch a small, compressed box into space that, on reaching orbit, would reconfigure itself into a functional satellite. Other space-bound devices could be configured to serve multiple purposes—for example, a solar array could be made to transform into a parabolic antenna or a storage capsule.
But programmable matter could also generate new uncertainties. Imagine a material world that could be hacked. Morphable airplane wings could be sabotaged. Buildings could be commanded to disassemble with people inside. Intellectual-property rights could also become more complex as products began to shape-shift from one form to another, creating patent issues that the U.S. Patent and Trademark Office has never even remotely considered. The existence of such risks is all the more reason to begin the discussion of this potentially transformative technology now, so that solutions, control measures and policies can be built in from the beginning.
No Assembly Required
A handful of imaginative scientists have been talking about programmable matter since the early 1990s, but the field received its big boost in 2007, when the U.S. Defense Advanced Research Projects Agency funded a Programmable Matter project. darpa laid out a multiyear plan for designing and constructing microscale robotics systems that could morph into larger military systems such as physical displays and specialized antennas. Researchers shrank robotics to the millimeter scale, around the width of a pencil. Within a few years they had succeeded in demonstrating tiny, shape-shifting robots.
One of us (Tibbits) has been working on ways to use 4-D printing to build such machines without the robotic mechanisms (motors, wires and electronics). At the Massachusetts Institute of Technology's Self-Assembly Lab, he and his colleagues have fashioned, among other things, a snakelike object, made of a special polymer, that folds to form the letters “MIT” when inserted into water; a single strand of polymer that self-transforms from those letters into the letters “SAL” (for Self-Assembly Lab); a flat surface that self-folds into a truncated octahedron; and a flat disk that, when exposed to water, folds into a curved-crease origami structure.
Christopher B. Williams of Virginia Tech has embedded alloy wires and printed circuits into special compliant structures as they are being printed. Once printing is complete, an external signal can be applied to trigger actuation of the compliant structure, changing the shape of the object. This approach has potential implications for robotics, furniture assembly and building construction.
Williams and one of us (Campbell) have gone further and merged 4-D printing with nanomaterials. The insertion of nanomaterials into printed objects can create multifunctional nanocomposites that can change properties in response to electromagnetic waves (visible light and ultraviolet light). For example, this group printed a Virginia Tech logo with embedded nanomaterials that change color under different lighting. With further development, materials such as these could lead to a new class of sensors, which could be embedded in medical devices to test for extremes in blood pressure, insulin levels and other medical metrics.
A Computational Challenge
These days it is easy to print a static “MIT” or “Virginia Tech” logo: simply feed the instructions for the object you want into a 3-D printer. But printing objects that can later change shape involves designing programmable characteristics such as stress and flex points or embedded nanomaterials into the object. This kind of engineering presents thorny computational challenges beyond the abilities of today's CAD software.
Say you want to print something that transforms from a table into a chair. Topologically there are many ways for a table to fold into a chair. Most of those ways will not work in the real world, however, because in the process of folding, the object will hit itself or become tangled up in itself. Finding the best solution is a complex simulation challenge. Researchers have developed a library of physical mechanisms that form the basis for any object we want to design—mechanisms for folding, stretching, twisting, shrinking, and so on. The object's transformation depends on the collective action of those building blocks. We can design objects in a linear fashion—fold, fold, stretch—or we can program them according to logic—if this happens, do this; if that happens, do that.
These combinations quickly become so complex that it is difficult to predict their behavior, which is why developing new types of design software is the first step in making programmable matter a reality. Designers need computers to simulate the transformations of 4-D-printed objects and to translate their designs into instructions that a printer will understand. They need software that can help them avoid problems that are hard to foresee—such as an object getting tangled up in itself when it changes shape. As a first step toward this goal, Tibbits's group worked with the design software firm Autodesk to develop Project Cyborg, which simulates and optimizes the dynamics of 4-D-printed objects. Using Cyborg for design, a multimaterial 3-D printer built by the company Stratasys and a new, Stratasys-developed polymer that expands by 150 percent when submerged in water, Tibbits's group created the self-folding M.I.T. logo and other 4-D-printed objects.
So far most of the objects that programmable matter researchers have designed have been fairly simple, involving more or less one type of joint and two materials. But the materials already exist to build more complex devices, and once we increase that variety further, we are limited only by our computational ability, our imagination and the laws of physics.
Building Blocks
A useful conceptual tool for thinking about programmable matter is the “voxel,” or volumetric pixel. In computing, a voxel is a pixel in three-dimensional space. In programmable matter, a voxel is a fundamental unit from which complex devices could be built. A voxel could be a synthetic particle of varying size made from materials ranging from silicon to ceramics to plastics to titanium. Voxels could be tailored to behave as any one of a wide range of subsystems—an energy-storage device, an actuator, a sensor, a conductor, an insulator, a protective shell, an antenna or even a microcomputer. Voxels could be assembled and, together, programmed to change shape or function and collectively form different objects.
In their recent book Fabricated: The New World of 3D Printing, Hod Lipson and Melba Kurman use voxels to draw an analogy between programmed matter and biological life. The many proteins in living things are, after all, made of 22 building blocks—amino acids. “If fewer than two dozen element types give rise to all biological life, a few basic voxel types can also open a large range of possibilities,” Lipson and Kurman write. There could be hard and soft voxels, conductive voxels for wiring, electrical circuits composed of resistor, capacitor, inductor and transistor voxels. “Add actuator and sensor voxels,” they add, “and you have robots.”
Robots of this sort are of great interest to the U.S. military. The U.S. Army and Navy are already developing ways to 3-D print spare parts on ships or in the field because avoiding the transport and storage of thousands of spare parts could save time, expense and space. Programmable matter could amplify those benefits. Imagine having a bucket of voxels on a submarine. If a part breaks or you need a specific tool, you simply take a collection of voxels and program them to form that tool. When the tool is no longer needed, you command it to disassemble, leaving the voxels available for making other tools or parts.
Beyond parts and tools, programmable matter could provide uniforms that adjust insulation and cooling to the surrounding environment and the biometrics of the individual. This year the army invested nearly $1 million in a project that would use 4-D printing to create dynamic camouflage. Think very long term—and use a fair bit of imagination—and it is conceivable that programmable matter could be used to build morphable robots that can shape-shift around and through obstacles, similar to the T-1000 robot in the movie Terminator 2.
Programmable matter could one day be used in large-scale construction, both in military and in civilian contexts. Consider the possibility of self-assembling buildings. Instead of casting brick or pouring concrete, we pour a building-size volume of programmable matter into a foundation and then tell the elements to “grow” or “stabilize” into a finished structure, complete with electricity and plumbing. This might seem unnecessarily complicated for your average new-home construction, but in hostile environments—say, in a war zone or on the surface of Mars—self-assembly becomes attractive.
The Self-Assembling Future
We have mentioned only a handful of the ways in which programmable-matter researchers might one day deploy their inventions. How about airplane wings that change shape in response to shifting air pressure or temperature? Or tires whose gripping surface changes depending on road and weather conditions? Self-healing materials could protect aircraft or help bridges adapt to sudden increases in traffic or even earthquakes. And what about self-assembling furniture? Anyone who has shopped at Ikea would appreciate a new dresser that is packaged flat but automatically folds into shape on command.
These concepts might sound magical, but they are grounded in real engineering and science research. Yet big hurdles remain. In addition to the computation challenge it poses, programmable matter will push the limits of materials science and manufacturing. To create those self-folding M.I.T. and light-sensitive Virginia Tech logos, we needed entirely new polymers. What types of new materials will it take to build a self-assembling house or a morphing airplane wing? Once the building blocks have been developed, we still face the challenge of assembling them into large, complex objects. How do we make voxels stick together? How should we program them, and what types of energy can they use to self-assemble?
Assuming we are successful in solving these problems, we will still face the challenges mentioned earlier, including exposure to hacking and complicated intellectual-property issues. We should soon have the opportunity to work through these challenges. For the past year and a half Tibbits has been working with several companies to develop shape-shifting materials, products and construction systems, and Campbell and Williams have been in discussions with a company to apply 4-D printing with nanomaterials as an anticounterfeiting system. The self-assembling house might not be as far off as it seems.