It could be the ultimate stress ball for spies. An invisible ink creates secret messages on bendy plastic that are only revealed when you give it a squeeze. Jianping Ge of the East China Normal University in Shanghai and his colleagues embedded an array of silica crystals in a plastic gel. The crystals reflect light at a certain wavelength depending on their spacing, so the relaxed gel appears green, but squeezing or stretching it turns it blue or red. The team then coated the surface with another gel, and put a cut-out of a secret image on top. They shone ultraviolet light on the set-up, which linked the two gels around the cut-out. Once the cutout was removed, its silhouette only appeared when the gels were squeezed (Advanced Functional Materials, doi.org/f2tmtp).
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