Taking inspiration from the defense mechanism of the bombardier beetle, researchers at ETH Zurich have developed a film that, when damaged, instantly releases a hot foam to discourage malicious actions. The technology could be used as a simple, yet elegant and reliable way of discouraging theft and vandalism on ATM machines.

The bombardier beetle is a small insect only about one centimeter long that is highly adaptable and lives in all continents, except for Antarctica. The beetle owes its name to its peculiar defense mechanism: when threatened, it ejects a potent chemical spray that fends off all the aggressors in its habitat, from ants to frogs.

The beetle stores hydroquinone and hydrogen peroxide in two separate reservoirs inside its abdomen. When danger is near, it directs the reagents into a reaction chamber. Here the reagents mix with catalytic enzymes, quickly reaching a temperature close to the boiling point of water and triggering an explosion that ejects the hot spray in the direction of the aggressor.

The team led by Prof. Wendelin Jan Stark sought inspiration from this mechanism to create a film that, when damaged, releases a hot foam that could fend off potential aggressors. The researchers created several plastic honeycomb structures, filling them with either hydrogen peroxide and manganese dioxide, and then stacking them on top of each other. When the structure is damaged, the two substances mix and create a hot foam reaching temperatures of 80° C (180° F).

One application in which the foam could prove especially useful is in protecting ATMs against vandals and thieves. Current anti-theft measures for cash machines are quite complex and expensive, and need to rely on mechanical systems as well as a source of electricity.

In a test, the researchers added both blue dye and DNA nanoparticles to their plastic honeycomb structure. As soon as the film was damaged, it released both the foam and the dye, making the cash useless. Moreover, the DNA particles marked the banknotes so that they can be traced more easily. The researchers believe one square meter of film would be much more reliable than current solutions and cost much less – approximately US$40.

The research appears in the journal Journal of Material Chemistry A.

Source: ETH Zurich