4D Printed Objects Remember Their Shapes

A team of researchers from MIT and Singapore University of Technology and Design have leveraged a novel form of 3D-printing to create objects that can be twisted and stretched almost beyond recognition, yet when heated to a certain temperature return to their original shape.

As the researchers detailed last week in Scientific Reports, their shape-memory objects could see a variety of applications outside the lab, including drug capsules that are triggered to release their contents when an infection is detected and solar panels that follow the sun.

"We ultimately want to use body temperature as a trigger," said Nicholas X. Fang, an 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."

To make this happen, the researchers made use of a cutting-edge 3D printing technique called microstereolithography, which uses light to print on ultra-thin layers of resin.

Microstereolithography works by rendering an object as a 3D computer model which is then digitally divided into hundreds of slices. Each of these slices is saved as an image file which is then run through a projector to etch the image onto liquid resin. These layers are then stacked upon one another until they form the completed object.

This method of 3D printing allowed the researchers to create microscale objects with diameters equivalent to a human hair. By using a special blend of polymers for printing, the objects created by the team, which included an Eiffel tower, a flower, and a claw-like tool, could be stretched to several times their original size without breaking.

When the researchers subjected these distorted objects to a specific heat range (104-356 Fahrenheit), they would return to their original shapes within seconds. Due to this unique property, the team refers to their objects as 4D printed since they operate on a fourth dimension—time.

To demonstrate a simple application of their shape-memory technique, the team designed a soft rubber gripper that would slowly ball up into a claw when the surrounding air was heated to 40 Celsius, grabbing whatever was placed beneath it.

Going forward, the team hopes to create shape-memory materials that are responsive to lower temperatures, a crucial requirement to make them practical outside of a laboratory setting.

"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 3-D printers," said Qi Ge, an assistant professor at Singapore University of Technology and Design. "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."