Transistors' hour of departure

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 'current' of fluid that can flow along it. The flow through the conduit is proportional to the electric current applied to the heater.

So far fluistors have found commercial applications mainly as pressure regulators for gases in medicine and chromatography.

Recently, a us $7 million cooperative research and development agreement has just been signed with Lawrence Livermore National Laboratory in the us to apply the technology to liquids. In the process, the circuitry controlling the heating element will be embedded in the device's silicon substratum, making the whole contraption smaller, stronger and cheaper.

The device can regulate the flow in a pipe upto 10,000 time more accurately than a mechanical valve can. It is also possible to use fluistors in reverse. If the fluid is allowed to flow through the fluistor to modulate the silicon membrane and a strain gauge is used to generate an electrical signal, the device becomes a highly accurate pressure sensor.

Fluidic asics, combining sensors, regulators and miniature pumps all integrated into a single microchip appear to be only a year or two away.

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