North Carolina State University researchers have created a technique that uses IR light, plastic sheets and black ink to create robust, strong curved shapes. [Image: Amber Hubbard]
Researchers at North Carolina State University (NCSU) have developed ink-imprinted 2-D thermoplastic sheets that, when exposed to infrared (IR) light, can be predictably curved into tubes, bowls and other 3-D structures (Soft Matter, doi: 10.1039/C7SM00088J). The technique is compatible with commercial printing techniques, and can create curved materials strong enough to grip and hold objects nearly a thousand times their own weight.
Sunflower inspiration
So-called active materials that can change shape with an external stimulus are nothing new, and have been demonstrated, for example, in materials that can fold into 3-D polygonal shapes when exposed to heat. Soft materials like hydrogels have also been used to create 3-D curved structures. But the NCSU team wanted to look at a firmer material—a thermoplastic sheet—that could be deterministically bent into 3-D shapes. Such a material could open up new possibilities in manufacturing; for example, the sheets could be efficiently stacked and cheaply transported in flat form, and then reshaped into their final 3-D form at the point of use.
To get there, the researchers looked to a natural example, sunflowers, for inspiration. They noted that the stems and petals of these heliotropic (sun-following) plants bend through two mechanisms: an “indirect” mechanism, in which stresses due to cell growth on one side of the stem force compression of cells on the other side of the stem, causing the entire stem to curve; and a “direct” mechanism, in which a gradient of cell growth throughout the flower’s petals lead them to curve back continuously. The NCSU team applied both strategies to their plastic sheets.
Black-ink patterns
The approach used by the researchers to create their light-curved polymers is surprisingly simple in principle. It begins with a commercially available shape-memory polymer, Grafix Shrink Film, which shrinks when exposed to a specific temperature. The team then applied black ink in computer-modeled patterns on the polymer using a desktop inkjet printer. Because black ink absorbs a higher percentage of energy from IR radiation than the uninked areas of the polymer, the patterning allowed the team to selectively control the amount of shrinkage in different areas, and create curved features in a highly predictable way.
In the spirit of the times, the team has posted a YouTube video (with a pulsing soundtrack that’s a bit ill-suited to stuffy office environments) to show the technique in action:
The NCSU team was able to use both indirect and direct approaches to form different types of curved shapes from the inked plastics; the shapes agreed well with the computer-modeled predictions, demonstrating that the process could be used deterministically. The group even used the approach to create “bio-inspired grippers” that could grab and hold objects 925 times their own weight.
Jan Genzer, one of the corresponding authors on the study, believes that the combination of a robust computer model and a simple manufacturing process with commercially available materials has significant potential. “Ultimately,” he noted in a press release accompanying the work, “we’d like to be able to input a desired 3-D shape into the model and have it create a pattern that we can print and produce.”