Sensor casts a new light on pressure readings

St. Foy, Quebec, Canada -The prescription for medical pressure sensors can be tough to fill. They have to be ultra small to fit within the tight confines of a catheter. And given the life-saving importance of some physiological pressure readings, they need to be accurate even when plagued by electrical interference from nearby surgical or diagnostic equipment. These disposable gauges also have to be cheap enough to throw away. Fiso Technologies Inc. has now come up with a fiber-optic pressure gauge that could be just what the doctor ordered.

Called FOP-M, this new transducer targets a variety of invasive pressure-sensing applications-including intrauterine-, intracranial-, and blood-pressure monitoring. With respective tip and optical cable diameters of 0.55 mm and 0.25 mm, the FOP-M sensor fits easily within typical catheter housings. It provides pressure readings accurate to within 1 mmHg, reports Fiso president Claude Belleville. And when supplied in OEM quantities, the sensor costs about $15.00.

Like Fiso's earlier fiber-optic sensors for pressure and strain, the FOP-M operates on a "white light interferometric interrogation" principle. As Belleville explains, Fiso builds its sensors around a Fabry-Perot interferometer, a device that consists of two opposing mirrors separated by a cavity whose length changes in response to pressure. These pressure-induced variations in the cavity length modulate the resonant wavelength of broadband light that bounces between two mirrors. Fiso applies patented technology to correlate the wavelengths to absolute cavity lengths, which in turn translate to specific pressures. The company creates these microelectromechanical sensors through a silicon micromachining process that can produce features as small as 15 microns.

The fiber-optics and the interferometric operating principle could add up to several advantages over liquid-filled, piezoelectric, or intensity-based optical sensors. These include:

Swapping probes. Belleville notes that the probe's cavity length and signal have a linear relationship, eliminating the need for complex calibration routines when switching probes. "Our probes are completely interchangeable," he says. "You just connect the probe and zero it."

Noise and interference. Belleville says that optical transmission of pressure signals protects the sensor from EMI and RFI generated by electric scalpels, MRI machines, and other medical equipment.

Fighting kinks. It isn't sensitive to bent cabling.