Think of a flower growing. Biological tissues have always been able to change shape because cells swell and stretch. Now, polymer scientists and engineers have developed a polymer gel and lithography technique that can follow those kinds of transformations.
Rebecca Boyle reports for Popular Science:
For the first time, engineers have figured out how to induce this action in sheets of synthetic gel, creating self-curling and folding structures that can contort on command.
The new method, called halftone gel lithography, could someday be used in anything from soft robots to tissue engineering, researchers say. It’s like a new method of 3-D printing — call it 3-D curling.
Basically, the polymer gel swells when it is exposed to water. But it won’t swell where the “resist dots” are added to the polymer gel with the lithography process. Making same-size dots in a uniform pattern will make the sheet swell but stay flat when exposed to water. Creating different-size dots in different areas yields a 3-D shape, as shown in the video above and reported in the journal Science. The researchers crafted various shapes, including spheres, cones, and saddles.
Boyle describes how the researchers did it:
Ryan Hayward, Christian Santangelo and colleagues at the University of Massachusetts Amherst worked with ultrathin sheets of an elastic polymer that shrinks when it’s heated. They spread a 10-micrometer-thick layer of polymer onto a substrate, and exposed patches of it to ultraviolet light. The light-exposed portions become crosslinked polymer chains, while areas that were masked will swell and expand when they’re exposed to water. This selective swelling causes the whole sheet to warp and buckle, mimicking the concept of cellular swelling that drives the growth of soft tissues. To start again, just dry out the sheet.
David Bradley provides more details about the polymer chemistry in Chemistry World:
The team focused on poly(N-isopropylacrylamide) copolymers containing pendant benzophenone units. The benzophenone groups allow cross-links to be formed between polymer chains that are readily tuned by the dose of incident ultraviolet irradiation. By using two photo-masks to create patterns of dots, the team was then able to control the pattern of irradiation on a thin layer of this polymer. [...] The process is akin to printing a halftone image in a newspaper where the patterns of ink dots produce an image that looks smooth and realistic to the naked eye.
Paul Topham of Aston University in the U.K. told Chemistry World that “the work is truly ground-breaking.”
Source: “Video: 3-D Curling Method Creates Custom Structures From Flat Plastic Sheets,” Popsci.com, 3/12/12
Source: “Light-sensitive shape-shifters are swell gels,” Chemistry World, 3/8/12
Source: “Self-Shaping Gel Sheet,” YouTube
Rachel Petkewich is a freelance science writer and editor. She has worked as a research scientist in the chemical industry and spent eight years as a staff writer and editor at various science journals and magazines, including Chemical & Engineering News.