Chemistry in Pictures: Secret code

What’s squishy, pink, and capable of encrypting information? This QR-coded hydrogel, printed by University of Calgary biomedical engineering PhD student Zhangkang Li.Li encodes information into hydrogels by exposing a dish of functionalized polyvinyl alcohol (PVA) to visible light, which triggers crosslinking between the polymer strands. By projecting a pattern of light from a stereolithography 3D printer onto the gel, Li can write that pattern into the PVA’s crosslinked structure. But the pattern isn’t immediately visible upon printing. The information contained in the gel is revealed only after the gel is put into a saline solution for several hours. When the gel absorbs the saline, it swells. The amount of swelling in a given area differs based on how dense the crosslinks are, causing the printed pattern to emerge as the gel takes up the liquid. Li says it’s possible to print gels with a wide variety of hidden features. This particular QR code launches a weblink to his lab’s website.

Submitted by Zhangkang Li

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