Shining a light on shape memory polymers Wednesday, 31 August 2016

Engineers from America and Singapore have succeeded in using light to print three-dimensional structures that 'remember' their original shapes.

The researchers from Massachusetts Institute of Technology (MIT), Singapore University of Technology and Design (SUTD) and Georgia Institute of Technology have been exploring the use of soft, active materials as reliable, pliable tools. These materials, which include shape-memory polymers, can stretch and deform dramatically in response to environmental stimuli such as heat, light, and electricity — properties that researchers have been investigating for use in biomedical devices, soft robotics, wearable sensors, and artificial muscles.

MIT associate professor of mechanical engineering Nicholas Fang 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,” he 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.”

The 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 fabricate these structures, the researchers are using 3D printing, which 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 millimeters. Fang says this size restriction also limits how fast the material can recover its 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 said.

“For example, a flower can release pollen in milliseconds. It can only do that because its actuation mechanisms are at the micron scale.”

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 CAD software, then divide the model into hundreds of slices, each of which they send through the projector as a bitmap file. 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.

His collaborator Qi 'Kevin' Ge, an assistant professor at SUTD says 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.”

[When the claw reaches the bolt, the temperature change triggers its closure around the bolt head. Photo: Qi (Kevin) Ge/MIT]