Fancy a smoke? No, it’s my last one and I need to get an urgent message to HQ…

Sadly, this line is yet to appear in a spy film, but thanks to George Whitesides and his group at Harvard University, US, it might one day. The group has had another stab at ‘infochemistry’ – using chemical means to convey a message or information without the need for an electrical power supply.

Avid readers of this blog will remember that in June of last year the group first mooted the idea of using ‘infofuses’ soaked in alkali metal solutions to transmit coloured light messages as they burned, and then the follow-up using a microfluidic device with a series of droplets passing by windows in the device to let light through – using intensity, colour and polarisation to encode more information than standard on-off digital signals.

This time, the team have developed their ‘infofuse’ idea. One of the major drawbacks of the original system was the fact that the fuses tended to go out if they were in contact with a surface, and also burned really fast – to keep a message like an SOS call or suchlike repeating for 24 hours would need 2.5km of fuse.

The answers sound simple and almost obvious – use a slower burning fuse and keep most of it lifted off the surface. But it’s never quite as easy as all that. Keeping the fuses off the surface was quite simple – crimping them into a tent-like shape held enough of the nitrocellulose far enough away from whatever surface the fuse was resting on to stop it sinking all the heat and putting out the flame.

But the timing problem required a more considered approach – simply using a slow burning fuse was no good – it would take hours to transmit the message, and most slow burning materials don’t burn hot enough to stimulate thermal emission of the alkali metal ions. What was needed was a combination – a slow burning ‘master’ fuse, with a series of fast ‘slave’ fuses sticking out of it. As the master fuse smoulders up to each slave fuse, it ignites and rapidly transmits its message.

This gives a compact system that can repeat a single, fast message over a long period, or transmit several different messages one after the other. The slow fuse is made from cotton soaked in sodium nitrate – similar to the ‘slow match’ used to ignite gunpowder charges in early matchlock firearms. However, the team showed that one could equally use a cigarette as the slow match – much less conspicuous if you’re an undercover agent…

Phillip Broadwith

Reference: C Kim, S W Thomas III and G M Whitesides, Angew. Chem. Int. Ed., 2010, DOI:
10.1002/anie.201001582