The biggest breakthroughs in how we make things lie not in the technology to manipulate materials but in the materials themselves. Such is the thinking behind “4-D printing,” an experimental approach to manufacturing that expands on much-hyped 3-D printing processes. Instead of building static three-dimensional items from layers of plastics or metals, 4-D printing employs dynamic materials that continue to evolve in response to their environment.
This new wrinkle in the maker movement comes courtesy of the Massachusetts Institute of Technology’s Self-Assembly Lab, where director Skylar Tibbits and his team are experimenting with so-called “programmable materials.” The researchers print these substances using a 3-D printer and then watch as the fourth dimension—time—takes over and the materials change shape or automatically reassemble in new patterns.
Improvements in software, computers and assembly processes have enabled more complex designs and greater automation when translating designs into actual things. Tibbits, however, envisions a scenario in which the materials themselves contain the information needed for self-assembly, saving manufacturers time and money.
Scientific American spoke with Tibbits about his efforts to take 3-D printing into new dimensions.
[An edited transcript of the interview follows.]
Your background is in architecture, design computation and computer science. How did you become interested in self-assembling materials?
I was making these experimental structures and installations and showing them at galleries around the world, and that led me to think more about the materials involved. I wasn’t necessarily looking to invent new materials, but rather to combine existing materials in smart ways.
Part of my work had been writing code to digitally design things. If we can write code to operate a machine, why can’t we also use code to get things to assemble themselves?
What are some examples of “smart” materials that already exist?
My favorite example is an old thermostat, one that’s not digital. If you pull off the cover of that thermostat there’s a coil with a bimetallic strip. You have two metals sandwiched together with different expansion rates. When subtle temperature changes happen, it turns the coil to the left or right. That turns a dial to either increase or reduce heat. There’s no motor or traditional sensor. It’s just a material that’s expanding and contracting and turning a dial.
Another smart material that’s been around for a while is nitinol [nickel–titanium], a shape memory alloy used in stents and other biomedical technologies. You have a wire that has a memory and takes a certain shape when heated. When you send a current through [nitinol] it will change shape, which allows you to get medical devices made from that material into tight spaces in the body. In addition to metals, there are also shape-memory polymers—also called “smart plastics”—used in a variety of small-scale applications.
Are you experimenting with these materials or are you creating your own programmable materials?
We use some of these materials, but we try not to rely on them because they’re expensive and already come with defined properties. We would rather use everyday materials like plastics, metals and woods, and combine them in smart ways. Our vision for 4-D printing was to combine and print these materials in different thicknesses and orientations. Thicker materials change properties slower after being printed but proved to be stronger, whereas thinner materials change quickly but are weaker. When combined, these different material properties react differently to their environments—whether they’re placed in light, water or some other medium—in a way that could mimic the movement of machine-assembled devices driven by actuators, motors and sensors.
When you talk about programmable materials containing information about the assembly process, what type of information are you talking about?
This information is in the form of a material’s properties, its shape (or geometry) and the amount of energy used to initiate self-assembly. One of our materials, for example, has properties that cause it to expand and change shape when you dip it in water. To maintain control over how the material changes, we designed it with a particular geometry that determines the direction it will curl, the number of times it will curl and the angles at which it will curl. Now we need to make the material more intuitive to use and easier to control.
How do you program these materials to behave in predictable ways?
You design them around the energy they need to self-assemble. You learn what thresholds of energy they will respond to and how they’ll react by testing a lot of them and then quantifying the results. We printed a 50-foot strand of our material and placed it in a pool for two reasons: to study how it would change when it was submerged and to determine whether we could work with really large structures. Part of the strand was made from a black, rigid plastic that determined its geometry—the angles and orientation as it changed. The strand was also made from a second strip of white plastic that expands 150 percent when placed in water. This reaction is what causes the strand to fold.
What are the biggest challenges facing 3-D printing, and how does 4-D printing address them?
Two of the problems with 3-D printing are the small bed size available in most printers and the difficulty of building things that require embedded electronics. We addressed the first by printing our 50-foot-long strand in the space of a five-inch cube. We tackled the second problem by using multifunctional materials designed to behave as though they have sensors and actuators so that you don’t have to add these electronics to the printed device.
Do you need special equipment to do 4-D printing or can you use a standard 3-D printer?
It’s much easier to do 4-D printing with a multimaterial printer, such as the Stratasys Connex we used. That system spits out droplets of [photopolymer] materials like an inkjet printer and cures them with UV light. The Stratasys printer deposits two different materials at the same time. There could be a way to print 4-D objects using a machine that works with one material, but then you would rely a lot more on the geometry designed into that material.
In addition to the 50-foot self-folding strand, what other 4-D objects have you created—and what’s next?
There’s also a flat sheet that self-folds into a cube when dipped in water. [It takes about 20 minutes to fold, depending on water temperature.] I’m interested in implementing these properties in some real-world products because we want to show this isn’t just some magic trick. [See video below.]
I can’t say whom we’re working with, but one key area for this technology is the sportswear, sports equipment, garment space. There’s an interest there because all of the major sportswear companies are heavily invested in additive manufacturing, and they want to work with higher-performing materials.
We’re not working on this but there are also many, many aspects of the automotive, aerospace and marine industries that rely on shape as a key parameter for performance. It’s all about minimizing resistance and increasing efficiency. As the environment changes, shapes need to change. In the automotive space it could be tires. There are different tires for different kinds of driving and road conditions, whether it’s race-car driving or off-roading. The difference between them is related to the shape, grip and depth of the tire tread.
What are the biggest challenges to developing smart sportswear and tires and anything else that might grow out of 4-D printing?
There’s a lot to work on. One area is the design tools, and we’re working with Autodesk [a maker of 3-D design software] on that. We also need better material properties. The materials we’ve experimented with work right now, but how long would they last if used in a product? And would they perform as needed over time? The programmable materials we have are probably not ready for the market right now. Another challenge is streamlining the 4-D process. We need to have some quantitative idea of how a material will perform ahead of time.
Underlying these problems is the fact that we’re a research lab. Our job is to push knowledge and discover new things. We don’t develop new products; we rely on industry for that. The development of new 4-D printing applications depends on strong collaboration with businesses interested in pursuing this technology.