Chemical communications

A new system for non-electronic communication that can transmit alphanumeric information encoded as pulses of light, over intervals of hours, without needing electricity and so remaining operational even without batteries in remote, hazardous or poor locations.

Increasingly, we rely on electronics to transmit information from person to person, whether that is the myriad text messages, twitter posts and Facebook updates, or the fast-becoming quaint idea of speaking to one another on a phone. Some devices use light for their internal communications, music systems commonly use a laser to transmit information from the surface of a plastic disk, a CD to the speakers, and of course few personal computers lack a builtin DVD player these days. Future devices will exploit photons in a much more direct manner, substituting the shuffling of electrons between components for the shuttling of photons.

Ultimately, all of these systems will use electricity, and if they're portable, they will require some kind of portable power supply, batteries, fuel cells or photovoltaics, for instance. But, while powered electronic communication is a relatively modern phenomenon, albeit quite ubiquitous, even in the developing world, there are those who seek alternatives that preclude the need for batteries, solar power, or fuel cells.

George Whitesides and his colleagues, Choongik Kim and Samuel Thomas, III, at Harvard University in Cambridge (Massachusetts, USA) have turned to a much more fundamental method of information transfer - chemical communication. They have now developed a concept that allows transmission of alphanumeric information in the form of light pulses without needing an electricity supply. They call their new technology "infofuse" and suggest, in the journal Angewandte Chemie, that one day it might be possible to use this approach to build highly sophisticated systems that function under conditions in which electronics or batteries simply do not work or are unavailable.

Previously, the team demonstrated the concept of "infochemistry" using a prototype infofuse produced by printing a pattern of different alkali metal salts on to a very thin strip of explosive nitrocellulose using an ink jet-printer. When they ignite the strip, the flame travels forward and reaches the dots of lithium, caesium etc, in sequence. The heat causes each element to emit light at characteristic frequency and so combinations of three different alkali metal salts can be produced. The prototype has capacity to relay 49 different signals.

Unfortunately, the prototype suffered from a serious deficit in conflagrational longevity - the flame tended to go out too soon, in other words. By using a different material, such as fibreglass, for the substrate, the team reasoned that they could reduce heat loss by conduction. They also figured that the strips could be deposited over a crimped trench, so that they are no longer lying flat on the surface, which would also reduce heat transfer to the substrate.

There was an additional problem to be addressed in that the flame front progressed far too quickly in the older system, which led to very short transmission times. The nitrocellulose strips burn at a rate of several dozen millimetres per second, so an infofuse that transmitted for 24 hours would have to be 2.6 kilometres long, which is somewhat impractical.

The team has now improved on the prototype design in three ways. First, they have extended the transmission time, without resorting to extraordinarily long infofuses so that they can transmit information for several hours rather than mere seconds. Secondly, they have been able to extend the length of messages they can transmit. Thirdly, they have avoided the problem of accidental flame extinction. These characteristics improve the functionality and potential for practical use of infofuses, and make them more convenient to use as a test bed for infochemistry, the team says.

The solution to the problems facing the prototype infofuse was a dual-speed arrangement. The team attached branches of the fast-burning infofuse to a slow-burning central fuse. They can vary the distance between branches as necessary and so obtain a flame front that moves at only 1 to 2 m per second. This, they explain, allows them to repeat a packet of information several times.

Thomas and colleagues point out that a colour camera or fibre optic cable could be coupled to a spectrometer to read the signal at a distance of 30 metres, even in full daylight. "We hope that it will be possible to develop a light, portable, non-electric system of information transmission that can be integrated into modern information technology," says Whitesides. "For example, it could be used to gather and transmit environmental data or to send messages by emergency services."