3D printed structures “remember” their shapes

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 are said to spring back to their original forms within seconds of being heated to a certain temperature range.

For some structures, the researchers were able to print micron-scale features that are at least one-tenth as big as what others have been able to achieve with printable shape-memory materials.

The MIT team says shape-memory polymers that can predictably morph in response to temperature could be useful for a number of applications, including biomedical devices, deployable aerospace structures, and shape-changing photovoltaic solar cells.

“We ultimately want to use body temperature as a trigger,” said Nicholas Fang, associate professor of mechanical engineering at MIT. “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.”

Fang and others have been exploring the use of soft, active materials as reliable, pliable tools. These new and emerging materials, which include shape-memory polymers, can stretch and deform dramatically in response to environmental stimuli such as heat, light, and electricity.

Shape-memory polymers 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 micron-scale 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,” explained prof Fang. “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 identified 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, 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 temperature range of between 40 and 180°C.

To demonstrate a simple application for the shape-memory structures, prof Fang’s team 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 the temperature was raised to 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 said. “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.”

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.