Heat-responsive 3D-printed items ‘remember’ their shapes

Article By : Jennifer Chu

The structures spring back to their original forms within seconds of being heated to a certain temperature “sweet spot.”

Industrial users tend to avoid plastics like a plague due to their low durability, but engineers from the Singapore University of Technology and Design (SUTD) and MIT may have found a way that could change all that.

Using light, the team were able 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, the researchers were able to print micron-scale features as small as the diameter of a human hair—dimensions that are at least one-tenth as big as what others have been able to achieve with printable shape-memory materials. The team’s results were published earlier this month in the online journal Scientific Reports.

Nicholas X. Fang, associate professor of mechanical engineering at MIT, said 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 said. “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.”

Former MIT-SUTD research fellow Qi Ge, now an assistant professor at SUTD, said the process of 3D 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 said. “This will advance 4-D 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 fabricate shape-memory structures, some researchers have looked to 3D printing, as the technology allows them to custom-design structures with relatively fine detail. However, using conventional 3D printers, researchers have only been able to design structures with details no smaller than a few millimetres. Fang said this size restriction also limits how fast the material can recover its original shape.

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.

“We’re printing with light, layer by layer,” Fang said. “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 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—in this case, between 40°C and 180°C.

“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 said. “If we can push to even smaller dimensions, we may also be able to push their response time, to milliseconds.”