In this series, a 3-D printed multimaterial shape-memory minigripper, consisting of shape-memory hinges and adaptive touching tips, grasps a cap screw. Credit: Photo courtesy of Qi (Kevin) Ge; Creative Commons Attribution Non-Commercial No Derivatives license
Heat-responsive materials may aid in controlled drug delivery, solar panel tracking. Engineers from MIT and Singapore University of Technology and Design (SUTD) are using light to print 3D structures that “remember” their original shapes. Even after being stretched, twisted, and bent at extreme angles, the structures – from small coils and multimaterial flowers, to an inch-tall replica of the Eiffel tower – sprang back to their original forms within seconds of being heated to a certain temperature “sweet spot.” For some structures, they could print micron-scale features as small as the diameter of a human hair -dimensions 1/10 as big as what others have been able to achieve with printable shape-memory materials.
Nicholas X. Fang, associate professor of mechanical engineering at MIT, says shape-memory polymers that can predictably morph in response to temperature can be useful for a number of applications, from soft actuators that turn solar panels toward the sun, to tiny drug capsules that open upon early signs of infection. “We ultimately want to use body temperature as a trigger,” Fang says. “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.”
Ge says the process of 3-D printing shape-memory materials can also be thought of as 4D printing, as the structures are designed to change over the fourth dimension – time. “Our method not only enables 4D 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,” Ge says. “This will advance 4D printing into a wide variety of practical applications, including biomedical devices, deployable aerospace structures, and shape-changing photovoltaic solar cells.”
Shape-memory polymers are particularly intriguing: These materials can switch between two states – a harder, low-temperature, amorphous state, and a soft, high-temperature, rubbery state. The bent and stretched shapes can be “frozen” at room temperature, and when heated the materials will “remember” and snap back to their original sturdy form.
To print shape-memory structures with even finer details, Fang and his colleagues used a 3D printing process they have pioneered, called microstereolithography, in which they use light from a projector to print patterns on successive layers of resin. They first create a model of a structure using computer-aided design (CAD) software, then divide the model into hundreds of slices, each of which they send through the projector as a bitmap. 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.
The researchers then looked through the scientific literature to identify an ideal mix of polymers to create a shape-memory material on which to print their light patterns. They picked two polymers, one composed of long-chain polymers, or spaghetti-like strands, and the other resembling more of a stiff scaffold. When mixed together and cured, the material can be stretched and twisted dramatically without breaking. What’s more, the material can bounce back to its original printed form, within a specific temperature range, 40 – 180C (104 – 356F).
The team printed a variety of structures, including coils, flowers, and the miniature Eiffel tower, whose full-size counterpart is known for its intricate steel and beam patterns. Fang found that the structures could be stretched to 3X their original length without breaking. When they were exposed to heat within the range of 40 C to 180 C, they snapped back to their original shapes within seconds. “Because we’re using our own printers that offer 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.
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.” http://news.mit.edu/2016/3-d-printed-structures-remember-shapes-drug-delivery-solar-panel-0826
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