By Sarah Faulkner

Engineers from MIT and Singapore University of Technology and Design are using light to create 3D structures that remember their shape when triggered by external stimuli like temperature. These structures, also known as shape-memory polymers, could be useful as delivery vessels for drugs in the human body.

The materials are made from a mix of polymers, long-chain molecules that act as scaffolding. These particular shape-memory polymers are at least one-tenth as big as what other groups have been able to achieve. Nicholas Fang, associate professor of mechanical engineering at MIT, and his colleagues published their results in the online journal Scientific Reports earlier this month.

The structures are printed using microstereolithography, a technique pioneered by Fang and his colleagues. Conventional 3D printers can only print structures with details no smaller than a few millimeters. Fang said in a press release that this alternative printing technique produces smaller structures that respond more quickly to stimuli.

“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.” Using their technique, these researchers have printed shape-memory polymers with features as small as the diameter of a human hair.

They begin by creating a computer model of their structure, which is then sliced into hundreds of layers. Each layer is sent through a projector of light using a bitmap, or an image file representing each slice as an arrangement of fine pixels. The light shines on a liquid resin, made of a mix of polymers, etching the desired pattern which then solidifies and forms the structure.

Shape-memory polymers can be designed to respond to a number of stimuli. These structures bounce back to their original shape within a temperature range of 40 – 180 degrees Celsius (or 104 – 356 degrees Fahrenheit). Moving forward, the team will search for polymers that react to the range of temperatures found in the human body. “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.”