Aug 26, 2016 | By Alec

While industrial users tend to avoid plastics with low durability like the plague, that could all change thanks to a new innovation by engineers from MIT and Singapore University of Technology and Design (SUTD). They have just pioneered 3D printable heat-responsive polymer materials that ‘remember’ the original shapes they were 3D printed in – even after being exposed to extreme pressures and twisted and bent into useless shapes. All these 3D printed objects have to do is reach a certain temperature ‘sweet spot’, and they spring back to their original form in a matter of seconds.

It’s a huge innovation that could make 3D printing suitable for a wide range of applications, and the pioneering researchers are already looking into controlled drug delivery and solar panel tracking solutions. It is also a breakthrough that can be technically listed as 4D printing, or the 3D printing of functional materials that can take on different shapes in certain conditions. An example would be a 3D printed plastic seal that opens or closes when exposed to a certain amount of pressure or water. Just earlier this week, another MIT team applied the same principles to self-assembling 3D printed phones.

But this latest material innovation could be even more widely applicable. This MIT/SUTD breakthrough has also just been published in the journal Nature, under the title Multimaterial 4D Printing with Tailorable Shape Memory Polymers. As MIT associate professor of mechanical engineering and co-author Nicholas X. Fang explained, these shape-memory polymers are ‘programmed’ to predictably morph into shapes under certain temperatures, making it perfect for soft actuators that turn solar panels toward the sun, for drug capsules that unleash their stuff when body temperature rises, and a lot more.

The research team also included SUTD assistant professor Qi “Kevin” Ge and Rutgers University assistant professor Howon Lee (both formerly of MIT). As they explained, these 4D printing principles can theoretically be applied on any scale. “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 -D 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.”

Of course this is not the first time that the possibilities of soft, active materials have been explored. As Fang revealed, other environmental stimuli (such as heat, light, and electricity) have also been investigated for biomedical, robotic and wearable applications. But shape-memory is a particularly useful trait, as it allows switching between harder, and softer, rubbery states. In this particular case, even room temperature can ‘freeze’ these materials in different shapes, while a slightly higher temperature will allow them to snap back into solid states.

But conventional 3D printing has not been ideal for fabricating these materials, Fang revealed, as size affect the speed at which these materials recover their original shape. “The reality is that, if you’re able to make it to much smaller dimensions, these materials can actually respond very quickly, within seconds,” Fang says. “For example, a flower can release pollen in milliseconds. It can only do that because its actuation mechanisms are at the micron scale.”

A new 3D printing solution was therefore necessary, and Fang and his team pioneered microstereolithography – essentially a very small scale version of DLP 3D printing that relies on a projector. “We’re 3D 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.”

The next step was creating an ideal polymer mix for shape-memory materials, and they ultimately settled on a mix of spaghetti-like long-chain polymers, and another, stiffer polymer. When mixed together and cured, the material can easily withstand stretching and bending without breaking. Most importantly, it bounces back to its original shape when heated to anywhere within 40 to 180 degrees Celsius.

This effect was already shown with various 3D printed coils, flowers and even a miniature Eiffel tower. These can be stretched to three times their original length without breaking, and return to their original shapes in a matter of seconds. “Because we’re using our own 3D 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.”

But the crucial step is the microstereolithography 3D printing process, which is far more advanced than nozzle or ink-jet based 3D printing. “The method’s main advantages are faster printing and better structural integrity,” said University of California at San Diego’s professor of nano-engineering Shaochen Chen (not involved in the study).

But the most functional example they have produced so far is the soft, rubbery gripper visible above. 3D printed with a firm grip, the claws were stretched open afterwards. But when placed in a warm environment, the gripper’s hands 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.”

In the future, Fang and his team hope to find other combinations of polymers that react at lower temperatures (in the range of human temperatures). “So we think there will probably be more applications that we can demonstrate,” Fang said. While there’s still a lot of work to be done, 3D printed plastics themselves are about to become a lot more functional.



Posted in 3D Printing Materials



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