# Shortest circuit

Flowing fluids solve a vexing problem of computation

###### Published: Tuesday 15 May 2001

a salesman needs to visit a dozen odd cities to sell his wares. What is the most efficient route for him to take? The problem may seem a trivial one. But if the cities he has to visit are increased to, say, a hundred? Even for the most powerful computers it becomes a challenging task. And lest it seem just a mathematical problem of little value, it demands the same logical structuring that is needed for planning telecommunication networks or airline flight routes. Now, a network of wells and channels with a fluid running through them might be able to provide a solution.

The travelling salesman problem is called the ' np complete problem'. These are problems where the solution is easily found by exploring all the possibilities when the options to be explored are limited. However, as the options considered increase, the number of possibilities that need to be scanned increase exponentially. To circumvent the use of computers, G Whitesides of Harvard University, us , and his colleagues have tried an ingenious way out -- they have used water flowing down specifically designed channels to find the right option. Though they were not able to beat the computer's speed of processing but the experiment has opened doors for future research. The idea that propelled the experiment was to use parallel processing and explore several solutions at the same time. This is very difficult proposition when it comes to using conventional computer design. Whitesides used microfluidics to mimic a parallel processing computer. They built a tiny system of half millimetre wide channels (about the size of a postage stamp), liquid reservoirs and wells in slabs of plastic, using a cheap and simple technique called soft lithography ( Proceedings of the National Academy of Sciences , Vol 98, p2961).

Several channels were arranged within interconnected layers. Each layer had a reservoir that was linked to a series of wells. To identify the channel with the greatest flow, a suspension of tiny fluorescent beads was added to the fliud. The total flow down the superimposed channels of each layer was brightest where the flow of fluid was the greatest. This 'computer' was then used to solve a variant of the travelling salesman problem. The idea is to let water flow decide the optimal path between two points. After all, this is the method used by nature to determine the flow of rivers and streams. The researchers used a four-layer microfluidic stack to solve the three-city problem and a 16-layer stack to study the six-city case.