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August 29, 2016
4 Min Read
By using a process called microstereolithography, researchers from the Massachusetts Institute of Technology (MIT; Cambridge) and Singapore University of Technology and Design were able to print micron-scale three-dimensional structures that “remember” their original shapes. Size matters in shape-memory polymers, because the smaller the object, the more quickly it can respond to stimuli, such as temperature, and shape shift. Ultimately, the researchers want to use body temperature as a trigger. “If we can design these polymers properly, we may be able to form a drug-delivery device that will only release medicine at the sign of a fever,” Nicholas X. Fang, Associate Professor of mechanical engineering at MIT, told Jennifer Chu, who wrote about the research in MIT News.
The shape-memory polymer was used to 3D-print several structures, including a replica of the Eiffel tower.
Microstereolithography, pioneered by the researchers, uses light from a projector to print patterns on successive layers of resin. First, they create a model of a structure using computer-aided design (CAD) software, which is divided into hundreds of slices, each of which is sent through the projector as a bitmap, an image file format that represents each layer as an arrangement of very fine pixels. The projector then shines light in the pattern of the bitmap onto a liquid resin, or polymer solution, etching the pattern into the resin, which then solidifies.
“We’re printing with light, layer by layer,” Fang says. “It’s almost like how dentists form replicas of teeth and fill cavities, except that we’re doing it with high-resolution lenses that come from the semiconductor industry, which give us intricate parts, with dimensions comparable to the diameter of a human hair.”
They are calling the process 4D printing, because the 3D-printed structures are designed to change over the fourth dimension—time.
“Our method not only enables 4-D printing at the micron-scale, but also suggests recipes to print shape-memory polymers that can be stretched 10 times larger than those printed by commercial 3D printers,” says former MIT-SUTD Research Fellow Qi “Kevin” Ge, now an Assistant Professor at SUTD. “This will advance 4D printing into a wide variety of practical applications, including biomedical devices, deployable aerospace structures and shape-changing photovoltaic solar cells.” Ge co-authored the study with Fang, which was published earlier this month in the online journal Scientific Reports.
To produce the shape-memory material, the researchers blended and cured two polymers, one composed of long-chain polymers and the other resembling a stiff scaffold. The resulting material can be stretched and twisted without breaking. Moreover, the material can bounce back to its original printed form, within a specific temperature range, in this case, between 40° and 180° C (104° to 356° F).
The team printed a variety of structures, including coils, flowers and a miniature Eiffel tower. Fang found that the structures could be stretched to three times their original length without breaking. When they were exposed to heat within the aforementioned range, they snapped back to their original shapes within seconds.
“Because we’re using our own printers that offer a much smaller pixel size, we’re seeing much faster response, on the order of seconds,” Fang says. “If we can push to even smaller dimensions, we may also be able to push their response time to milliseconds.”
To demonstrate a simple application for the shape-memory structures, Fang and his colleagues printed a small, rubbery, claw-like gripper. They attached a thin handle to the base of the gripper, then stretched the gripper’s claws open. When they cranked the temperature of the surrounding air to at least 40° C, the gripper closed around whatever the engineers placed beneath it.
“The grippers are a nice example of how manipulation can be done with soft materials,” Fang says. “We showed that it is possible to pick up a small bolt, and also even fish eggs and soft tofu. That type of soft grip is probably very unique and beneficial.” A video on the website shows the gripper in action.
Going forward, he hopes to find combinations of polymers to make shape-memory materials that react to slightly lower temperatures, approaching the range of human body temperatures, to design soft, active, controllable drug-delivery capsules. He says the material may also be printed as soft, responsive hinges to help solar panels track the sun.
“Very often, excessive heat will build up on the back side of the solar cell, so you could use [shape-memory materials] as an actuation mechanism to tune the inclination angle of the solar cell,” Fang says. “So we think there will probably be more applications that we can demonstrate.”
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