Sensors and Transducers - Introduction to Biomechatronics - page 82

any compounds close to the surface will have an effect on this wave. Typically, a section
of fiber is left unclad and in contact with a solution of the molecule of interest or with a
reagent that reacts with the molecule. Multiple internal reflections along this section of
the fiber contribute to the overall absorption or fluorescence, which can be measured at
the end of the fiber.

2.4.17.2 Surface Plasmon Resonance
In this technique, the fiber is clad with a metallized layer, typically gold or silver. When
monochromatic light from the laser reaches this layer, it is absorbed by the plasma gener-
ated by the conduction electrons of the metal. This results in a phenomenon called surface
plasmon resonance (SPR), which leads to absorption of the light. The resonance depends
on the angle, wavelength, and polarization state of the incident light and the refractive
index of the metal film and material adjacent to it. Therefore, SPR is an extremely sensi-
tive method of measuring changes in the refractive index of the medium.

2.4.17.3 Optical Fiber Sensors
Optical fibers are small and low cost. In addition, because electrical potentials are not
involved, there is no electrical risk to the patient, and interference from electric and
magnetic fields does not occur.
Most measurements are based on spectral or absorption changes in the medium deter-
mined by the active molecule, either directly or through an indicator mediated reaction.
Most chemicals of medical interest such as hydrogen, oxygen, carbon dioxide, and glucose
require the use of reagents.
In optical fiber-based sensors, light travels efficiently to the end of the fiber where
it exits into the medium and interacts with the molecule or reagent before returning via
the same or a different optical fiber to a detector for analysis and interpretation. Typical
reagent mediated optical fiber-based sensors are shown in Figure 2-81.

2.4.17.4 Measurement of Blood Oxygen Concentration
This is commonly known as oximetry and generally relies on a color change in the blood
as the relative amounts of deoxyhemoglobin (Hb) and oxyhemoglobin (HbO 2 ) vary.

Measurements are performed at two specific wavelengths, the first at 660 nm, where
there is a large difference between the relative absorption of the two molecules, and the
second at 805 nm, where the absorption is independent of blood oxygenation. The

oxygen saturation, O 2 sat , can then be determined from the ratio of the two absorption

≈

FIGURE 2-81
Configurations of
different reagent
mediated fiber optic
sensors. [Adapted
from (Fraden 1996).]