Design News for Mechanical and Design Engineers

Walled Lake, MI--Fiber optics has fueled the growth of the world's communication networks--and is finding increasing use in medical imaging, diagnosis, and therapy.

A prototype fiber optic MEMS pressure transducer has been developed with a diameter of 0.8 mm (small enough to be inserted into blood vessels--arteries, veins or the heart chamber itself) and designed to measure pressures of 0 to 300 mm Hg.

Researchers hope that the new sensor will be placed on the tip of guidewires used in angioplasty surgeries to measure pressures during the procedure to unclog arteries. The system could find use in monitoring after heart surgery, when pressures within the heart chamber must be taken continuously for 72 hr to make certain the heart is recovering properly.

The design is based on a movable, metallic ribbon which works as a reflector to transform the pressure into a light signal. In laboratory tests conducted by Baylor College of Medicine in Houston, TX, the sensor showed high sensitivity and low noise of about 1 mm Hg over the 0 to 300 mm Hg pressure range.

Underwritten by a grant from the National Institutes of Health Small Business Innovative Research Program, Sentec Corp. has worked on development of the fiber optic pressure catheter for three years, says researcher Takeo Sawatari, manually assembling prototypes which have been tested through Baylor College of Medicine.

"The original application," says Sawatari, "was a monitoring probe instead of a fluid filled catheter--a device which is accurate and disposable without the fluidic lumen which can create calibration problems. Our goal is to produce something which is superior and more cost effective.

"The trick is to make a small and highly accurate pressure sensor which is also stable," says Sawatari. The sensor must also enable pressure readings, which are independent of temperature changes in the range of 30 to 50C.

A fiber-optic pressure sensor holds the benefits of safety with no electrical connection to the body. Since its primary signal is optical, it is not subject to electrical interference. The optical leads, very small and flexible, can be included in catheters for multiple sensing. Materials suitable for long-term implantation, such as plastic, can be used in their fabrication. These sensors are sufficiently simple in their design to be disposable.

The sensor operates using white light from an illuminator which passes through a coupler, connector, and a fiber to reach the other end of the fiber, where the sensor head generates "white light fringes." The fringe information is reflected back through the fiber, the connector, and the coupler to spectroscopic detectors. The detectors read the spectroscopic fringes and a computer determines the contrast of the fringes and converts the contrast into a pressure measurement. The contrast varies as the pressure changes and thus the measured pressure is obtained.

The sensor uses a ribbon reflector, which is underneath a polyurethane window, as the key sensing element that translates its mechanical motion, due to pressure, to an optical intensity variation of the return beam. The end of the fiber is coated with a thin film which works as a Fabry-Perot interferometer. It sends back a fixed "white light fringe pattern," which is not affected by the measurement variable (pressure), in addition to the reflected light beam from the reflector, which is modulated by pressure.

The contrasts of these fringes, which are observed through the spectrometer, vary as a function of the amount of light reflected from the ribbon. Since the fringe contrasts are independent of the first order spectrum change, or source fluctuation, one can measure pressure without being influenced by fiber bending or source fluctuation. To further reduce the fiber bending effect, a fiber connector is devised in such a manner that inside the connector, the fiber is sharply bent, curved, or looped, so that the higher order modes of the fiber leak out. Temperature drift is eliminated by the creation of a vent from the sensor head to the outside atmosphere to keep the inside pressure stable at atmospheric pressure.

According to Craig J. Hartley, Professor of Medicine and Cardiovascular Sciences at Baylor College of Medicine, who has conducted animal tests on the catheter for over a year and a half, "we are encouraged by the process and its potential." Hartley has used the 1.4 French microtip pressure catheter to obtain pressure readings in mice hearts.