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Applications of Long-Wavelength Sources and

Detectors for Medical Monitoring

Jonathon T. Olesberg

Optical Science and Technology Center

and the Department of Chemistry

University of Iowa, Iowa City, IA

There are several exciting possible applications

of optical sensing for medical monitoring. Optical

monitoring of body chemistry offers several advantages

over conventional chemical techniques. Long-wavelength

semiconductor source and detector technology will likely

play a key role in making these applications possible and

practical.

There are several biochemicals for which

medical monitoring would be useful, including urea,

lactate, cholesterol, and creatinine. The best-knownpotential application of non-invasive optical monitoring,

however, is the measurement of blood glucose for

individuals with diabetes. Although the importance of

frequent blood glucose monitoring is clear, and standard

reagent-based technology for measuring blood glucose is

well developed, most individuals with diabetes do not

monitor their blood glucose values nearly as often as they

should. The primary reasons given for inadequate testing

are the pain of drawing a sample of blood and the cost of

the reagent-carrying test strip. Optical measurements can

do away with both of these factors, allowing the

measurement to be performed non-invasively and

reagentlessly.

Although development of optical blood glucose

sensors has been pursued aggressively for several years,

there are still no commercially available instruments. This

is due to a combination of factors, including an extremely

high signal-to-noise requirement and the difficulty of

properly interpreting spectral absorption information. The

factors required for successful noninvasive biochemical

monitoring, especially as they relate to the potential

impact of the development of semiconductor source and

detector technology in the 2.0-10 m wavelength range,

will be discussed.

Biomolecule absorption in the infrared is due to

interaction of the optical field with vibrational modes of

the molecule. In this regard, infrared optical sensing is

much more difficult than the measurement of hemoglobin

oxygenation in pulse oxymetry, which utilizes electronic

transitions in the hemoglobin molecule. The most intense

vibrational interactions are due to bonds involving

hydrogen atoms, such as, O-H, C-H, and N-H bonds.

Fundamental vibrational modes exist in the 4-10 m

wavelength range. Weaker nonlinear combinations of

fundamental modes exist in the 2.0-2.5 m wavelength

range, and yet weaker overtones exist in the 1.5-1.8 m

and 0.8-1.2 m ranges.

Any measurement in a biological material must

deal with the presence of water. Water has significant

absorption throughout the long-wavelength infrared,

requiring the selection of wavelength bands lying in water

absorption windows. The dominant water transmission

windows in the long-wavelength infrared occur between

2.0-2.5 m, 3.3-5.9 m, and 6.2-11 m.

Much of the work in noninvasive sensing has

utilized the shorter wavelength regions (0.8-1.8 m

wavelength) because of the greater penetration depth of

water and the abundance of well-developed source and

detector technology. However, the optical interaction

strength with glucose is very weak in these ranges. Thefield of biochemical sensing will be greatly aided by

advances in device technology in the 2.0-2.5 m and

3-10 m wavelength ranges.

The requirements for biomolecule sensing in

aqueous environments are very different from gas

sensing, where absorption lines are narrow. In an aqueous

environment, absorption features are broad and highly

overlapped. Fortunately, though, the exact structure of the

absorption bands depends on the chemical environment

surrounding the active bonds. Because of this, the

absorption spectrum of each chemical species is unique.

But absorption measurements must be made at several

wavelengths in order to provide enough degrees of

freedom to identify glucose in the presence of other

interfering compounds.

Much of the present non-invasive chemical

sensing research is performed using broad-band optical

sources (e.g., tungsten lamps or glow-bars) and FTIR

spectrometers. Spectral quality is typically limited by

detector noise due to the small optical powers provided by

broad-band sources. In addition, tissue throughput is

small due to water absorption and tissue scattering: peak

transmission in the 2.0-2.5 m wavelength range is

significantly less than 1% per millimeter of tissue.

Tunable laser sources or arrays of laser diodes caneliminate the low signal limitation, even with modest

powers (e.g., 10 mW). Unlike gas sensing applications,

narrow linewidth is not important, but wavelength

stability (for lasers in an array) or wavelength

reproducibility (for a tunable system) is critical.

Successful noninvasive measurements will likely require

very large (>105) signal-to-noise-ratios, so wavelength

deviations must be very small.

Abs. 283, 205th Meeting, 2004 The Electrochemical Society, Inc.