Biomedical Engineering Reference
In-Depth Information
FIGURE 10.40
Fiber optic in vivo pressure sensor.
Courtesy of Fiso Technologies, Quebec, Canada.
hand, fiber optic pressure sensors based on light intensity modulation have a lower sensi-
tivity but involve simpler construction.
The basic operating principle of a fiber optic pressure sensor is based on light intensity
modulation. Typically, white light or light produced by a light emitting diode (LED) is car-
ried by an optical fiber to a flexible mirrored surface located inside a pressure-sensing ele-
ment. The mirror is part of a movable membrane partition that separates the fiber end from
the fluid chamber. Changes in the hydrostatic fluid pressure cause a proportional displace-
ment of the membrane relative to the distal end of the optical fiber. This in turn modulates
the amount of light coupled back into the optical fiber. The reflected light is measured by a
sensitive photodetector and converted to a pressure reading.
A fiber optic pressure transducer for in vivo application based on optical interferometry
using white light was developed by Fiso Technologies (Figure 10.40). The sensing element
is based on a Fabry-P
´
rot principle. A miniaturized Fabry-P
´
rot cavity is defined on one
end by a micromachined silicon diaphragm membrane that acts as the pressure sensing ele-
ment and is bonded on a cup-shaped glass base attached to the opposite side of the optical
fiber. When external pressure is applied to the transducer, the deflection of the diaphragm
causes variation of the cavity length that in turn is converted to a pressure reading. Due to
its extremely small size (dia: 550
m
m), the sensor can be inserted through a hypodermic
needle.
10.6.5 Intravascular Fiber Optic Temperature Sensors
Miniature fiber optic temperature sensors, also commercialized by Fiso Technologies,
are based on a similar Fabry-P´rot principle utilized in the construction of a miniaturized
fiber optic pressure transducer. The Fabry-P´rot cavity in these designs is formed by
two optical fibers assembled into a glass capillary tube or a transparent semiconductor
material. The cavity length changes with temperature variations due to differences in the
thermal expansion coefficient between the glass capillary and optical fibers. Due to their
miniature construction (dia: 210-800
m), the thermal inertia is close to zero, allowing
ultrafast temperature response. The miniature size of the sensor allows the integration
into minimally invasive medical devices for direct in situ measurement in space-restricted
cavities.
m
10.6.6 Indicator-Mediated Fiber Optic Sensors
Since only a limited number of biochemical substances have an intrinsic optical absorp-
tion or fluorescence property that can be measured directly with sufficient selectivity by
standard spectroscopic methods, indicator-mediated sensors have been developed to use
specific reagents that are immobilized either on the surface or near the tip of an optical
