A new invisible ink that reveals secret messages when squeezed could be useful in preventing fraud.
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.
Previously, Jianping Ge of the East China Normal University in Shanghai, China, and his colleagues created invisible inks that appear when submerged underwater or exposed to a magnetic field. Now they've made an ink you can reveal just by squeezing with your hand. The team first embedded an array of silica crystals in a plastic gel. The crystals reflect light at a certain wavelength depending on their spacing and the angle of viewing, so the relaxed gel appears green, but squeezing or stretching it turns it red or blue.
Next, the team coated the surface with another clear plastic gel, and put a cut-out template of a secret image on top. They shone ultraviolet light on the set-up, which linked the two gels around the cut-out, but left them separate in the parts covered by the template. The linked gels are firmer, so they don't change colours when squeezed. After the cut-out was removed, its silhouette only appeared when the gels were squeezed.
Ge says he is talking to companies about using the technique to protect against counterfeit goods. "These invisible photonic patterns can be potential anti-fake labels," he says. Jon Kellar, a materials engineer at the South Dakota School of Mines and Technology in Rapid City, South Dakota, agrees that the hidden images could help combat fraud, but he thinks that the fabrication process will need to be simplified for commercial use.