ROSSLYN, Va., March 20, 1997 -- Biomedical engineers have built a prototype drug pump the size of a contact lens, a miniature, closed-loop implant that could monitor its own flow rate to ensure a steady stream of medicine.
The pump is still considered large as a microelectromechanical system, in which sensors, actuators and electronics are merged onto a single silicon wafer. The next step will be to shrink the device so it can be mass produced like a computer chip.
The research, in the laboratory of Whitaker Investigator Michael Huff at Case Western Reserve University, is driven partly by managed health care, which wants to keep people out of hospitals, and partly by the needs of diabetics and other patients on regular, intravenous medication.
Huff, who himself is diabetic, hopes his device may be adapted as a closed-loop system for monitoring blood glucose levels and pumping just the right amount of insulin into the bloodstream. "Ideally, an insulin delivery pump would sense the patient¹s blood glucose level and change the dose of insulin accordingly," said Huff, who has begun with a pump and a flow sensor to ensure a constant pumping rate. An insulin sensor would have to be added later.
Huff¹s prototype consists of a rectangular silicon chamber with one of the outer walls made of two thin layers of a titanium-nickel alloy sandwiched around a layer of silicon. The alloy forcefully changes shape when heated to around 60 degrees Celsius (140 F.). "When these materials recover their shape, they can produce very large displacements and forces," Huff said.
To operate the pump, rhythmic pulses of mild electrical current are passed directly through the alloy, setting up a cycle of heating and cooling that causes the metal to flex. This forces the chamber to expand and contract. The expansion pulls fluid into the chamber through an intake valve, and the contraction expels the fluid through an exhaust valve.
The flow sensor is made up of a heater that raises the temperature of the fluid at one point in the flow stream. Two heat sensors downstream detect this hot spot as it passes by. From this measurement the flow rate can be calculated.
The device has been bench tested successfully and is being scaled down for mass production, Huff reported at SPIE¹s Smart Structures and Materials Symposium in San Diego earlier this month.
The field of microelectromechanical systems and micro fabrication research has led to the development of micro devices which have feature sizes down to a micrometer or less. The field, which developed from integrated circuit fabrication techniques, has recently expanded into the medical device market.
"No company wants to build hospital infusion pumps any longer, since the whole health care system is striving to keep people out of hospitals," Huff said. "Drug delivery systems should be based on ambulatory delivery."
Medical care has already begun to feel the influence of microelectromechanical systems. Ten-dollar disposable blood pressure instruments with microscopic pressure sensors are replacing conventional $600 devices that, while reusable, cost another $50 to sterilize and tune up before each use.
About 20 million miniature blood pressure sensors are now being used each year, in addition to other downsized pumps and sensors. But there are still no closed-loop systems on the market that can regulate themselves without a doctor¹s intervention.