redwood Microsystems (rm), a California-based firm is looking ahead to market fluistor, the fluidic equivalent of a transistor. Fluistor regulates the flow of liquid down a pipe by squeezing it, just as a transistor uses an electrostatic field to restrict the flow of electrons. The fluister was invented in 1986 at Stanford University, California, by Mark Zdelick, who went on to establish rm.

The firm foresaw a $5 billion-a year market, in the us alone, for electro-mechanical valves and regulators and pioneered the product's marketing.

When fluistors are arranged into an equivalent of what semiconductor designers call an applications specific integrated circuit (asic), to administer drugs intravenously, they can bring the price of advanced pharmacological equipment down from us $150,000 to $20,000.

For making a fluistor, the microscopic design is printed repeatedly across the surface of a wafer of silicon and then etched in 90 chips that can fit on a single 7.5 cm wafer of silicon. The company is now moving to 10 cm wafers, allowing it to produce 150 chips at one go. The yield of good chips per wafer is said to be higher than 70 per cent.

The key to the design is a flexible membrane formed by etching silicon away to leave a thin layer in the middle of the device. One side of the hollowed out part is sealed off with a glass lid to make a small cavity; the other forms a conduit for whatever fluid the device uses.

The cavity holds a tiny drop of liquid. A gold element evaporated on to the glass can heat the liquid and make it expand, pushing the membrane out into the channel. As the membrane bulges out or falls back, it changes the width of the channel, modulating the