June 24, 1999 Many researchers are racing to develop miniature high-tech devices each smaller than a postage stamp to treat medical conditions, test new drugs and monitor pollutants. All of these require a tiny pump that can repeatedly and reliably push small amounts of liquid through narrow channels for rapid analysis.
To accomplish this, engineers at The Johns Hopkins University have invented a micropump powered by the repeated growth and collapse of a single bubble. Because the bubble is generated by heat, it is easy to control, the inventors say. And because the bubble-powered pump has no moving mechanical parts, it is unlikely to wear out too quickly.
"It's very different from having a pump with a valve that has to open and close any number of times," says Andrea Prosperetti, the university's Charles A. Miller Jr. Distinguished Professor of Mechanical Engineering. "With no moving parts, the bubble-powered pump's prospects of failure are minimal."
Prosperetti, an internationally respected expert on the physics of bubbles, worked with two other Johns Hopkins mechanical engineering researchers in his lab, Hasan Oguz and He Yuan, to design, build and test a prototype bubble-based pump. They presented their results at recent conferences sponsored by the Acoustical Society of America and the Defense Advanced Research Projects Agency
The team's prototype utilizes two main tubes. One is 1.6 millimeters wide, about the width of the wire used to make a large paperclip. The other tube is half that diameter, but the inventors say the same principles would apply to smaller tubes as well. The prototype's main tubes are connected by an even narrower passage or "throat," measuring 0.5 millimeters in diameter. The engineers inserted steel needles into each of the larger tubes, connected the needles to a power source and filled the tubes with a salt solution to complete the circuit. The current was "squeezed" as it passed through the narrow throat, causing the water to boil or vaporize at that location. This led to the formation of a bubble in the throat. As it expanded like a balloon into the wider channel, the bubble pushed fluid ahead of it.
When the electrical current is stopped, the bubble shrinks. Its final collapse, however, takes place not in the throat but in the wider channel. As a result, bubbles created in this system pump fluid through the tubes as they repeatedly expand and collapse. "You pump in the direction of the bubble's expansion," Prosperetti says. The prototype is capable of generating about five bubbles per second, but the inventors say it will be easy to move to much higher rates.
The bubble-powered pump could play a crucial role, for instance, in cutting-edge technology called micro total analysis systems. Engineers in this field are creating tiny devices on silicon chips, capable of detecting and analyzing small samples of fluid, then directing that some action take place. Their tiny size makes them ideal for a range of specialized tasks. For example, a small chip implanted in the body of a person with diabetes might regularly check the person's blood sugar level and order the release of the appropriate amount of insulin.
Also, pharmaceutical companies sometimes make minute quantities of thousands of potential drug compounds, looking for one variation that produces a beneficial chemical reaction. Each compound is costly to produce, but a micro-device would require less than a drop to analyze. Similarly, micro-devices could provide a low-cost method of simultaneously monitoring liquid pollutants produced by a factory at a large number of locations inside and outside of the plant.
"Every time you test something in these systems, you have to move fluids around in very tiny channels," says Prosperetti. "So there's been a search on for a means of actuation pumping without moving mechanical parts. We think bubbles are a good candidate for achieving this." Other researchers are developing different types of micropumps, including some that use miniaturized versions of traditional valves and other mechanical parts. But Prosperetti believes his bubble-powered pump is superior because it has no moving parts to wear out.
Although the test device built by the Johns Hopkins engineers creates bubbles with electrical current and a salt solution, Prosperetti says the system could also work with fluids that do not conduct electricity. For such liquids, tiny heaters embedded in the walls of the tubing could be used to form the bubbles.
Prosperetti and his colleagues have applied for a U.S. patent covering the bubble-based micropump technology. Their research was sponsored by the U.S. Air Force Office of Scientific Research and DARPA.
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