Optoelectronic Sensors in Medical ApplicationsSeptember 1, 2003 By: Ray King, Texas Advanced Optoelectronic Solutions, Inc. (TAOS), Inc. (TAOS)
Electronic components have been incorporated into medical equipment designs for years. Until recently, medical electronics has been focused primarily on the institutional side of the medical market, in expensive diagnostic equipment, such as MRI and CAT scanning machinery for use in hospitals and clinics. An aging and expanding population is accelerating the development of new and different medical equipment. Professionals in health-care settings want real-time, reliable, and accurate diagnostic results provided by devices that can go to the patient, whether he or she is in a hospital or clinic or at home, being monitored remotely. The equipment should be appropriate for both home care and ambulatory treatment; it must be patient friendly, both technically and physically; and it must be small, light, and energy efficient.
Electronics-based medical equipment requires sensors to convert the various types of stimuli (optical, mechanical, etc.) into electrical form. Many different types of sensors have been included in medical equipment designs. Sensors in current and future medical equipment need to be highly reliable, small, packaged with surface-mount technology (for a smaller footprint on the PCB and for ease of manufacture), low cost, highly accurate, and have no lead content. This article will address why optoelectronic sensors work particularly well in the medical equipment environment, in both institutional and home-care equipment designs.
Optoelectronic Sensor Basics
By definition, an optoelectronic sensor is a device that produces an electrical signal proportional to the amount of light incident on its active area. A number of devices meet this definition, but none is more prevalent than the semiconductor photodiode. Over the years, this two-terminal device has become the mainstay for light sensing.
Designers today—especially medical equipment designers—are now asking for more than just a light-to-current transducer. They are looking for more functionality integrated around this semiconductor workhorse, to provide themselves with improved performance and reliability, and lower system cost in the highly cost-sensitive medical equipment market. Added integrated functionality also removes the need for certain subsystem circuit designs and thus helps shorten the design cycle.
Integrated optoelectronic sensors are designed to respond to light so that they can recognize things such as patterns, images, motion, intensity, and color. The sensor's ability to perform this recognition (and the complexity of the recognition possible) depends upon
the type of integrated optoelectronic sensor. Some of the basic types of integrated optoelectronic sensors currently in use are outlined in Figure 2.
All of these types of sensors can be, and are, used in medical equipment applications. They help eliminate human error while providing more accurate readings and faster results. Rather than rely on human judgment to match colors or identify changes in light intensity, the sensors are designed to read or measure light— a real-world signal considered to be very stable and highly accurate—in a reliable, repeatable way. Data from the optoelectronic measurements are fed directly into the computer system, removing another possible source of error. The sensors are noncontact, able to perform their sensing or measurement functions without the need for physical contact with specimens such as blood, urine, or other bodily fluids. This is critical because if the specimens are tainted in any way, the resulting readings and measurements may not be accurate.
Current Medical Applications
Current medical applications that use optoelectronic sensors include pulse oximetry, measuring the amount of oxygen in the blood (see "Optical Sensors in Pulse Oximetry"); heart-rate monitors; blood diagnostics, such as blood glucose monitoring; urine analysis; and dental color matching.
Light-to-voltage and light-to-frequency converters serve as the platform on which the other integrated optoelectronic sensors are built, e.g., the IR optoelectronic sensors currently used in pulse oximetry systems and personal heart-rate monitors. These types of optoelectronic sensors integrate other functions such as current-to-voltage conversion, amplification, and A/D conversion, which results in smaller, less costly, and more reliable diagnostic systems. Integrated optoelectronic sensors have helped establish pulse oximetry as a viable medical procedure. Prior to pulse oximetry, medical professionals relied on blood samples to determine blood oxygen content, but these measurements could not give real-time results. Early pulse oximetry systems were large, bulky, and expensive, costing approximately $10,000. With the advent of integrated optoelectronic sensors and better LEDs, modern pulse oximetry systems became possible, enabling real-time, accurate, and noninvasive measurements of blood oxygen content. The use of integrated optoelectronic sensors in personal heart-rate monitors has resulted in similar benefits.
Light-to-voltage and light-to-frequency converters also serve as the platforms for color sensors, which are currently used in blood glucose monitors designed for home use. Blood glucose monitors (see "Integrated Color Sensors in Blood Glucose Meters") presently use one of two different methods:
- The optoelectronic sensor method, in which a drop of blood on a special test strip is read by an optoelectronic sensor, which measures a color change or the reflectance of a particular wavelength of light to determine blood glucose levels.
- The electrochemical method, in which a drop of blood on a special test strip provides an electroresistive measurement to determine glucose levels.
Of the two approaches, the optoelectronic sensor method offers several benefits: The test strips tend to be cheaper to manufacture; testing requires extremely low specimen volumes, leading to less pain for the patient; the method is less sensitive to the presence of prescription and over-the-counter drugs in the patient's system; and compared to the electrochemical method, its accuracy is less affected by the number of red blood cells present.
The accuracy and speedy results enabled by integrated color sensors also make them the optoelectronic sensors of choice for urine analysis and dental color matching applications. They allow medical professionals to automate analyses that used to rely on human judgments. Prior to optoelectronic sensors, urine analysis involved dipping a test strip into a specimen and trying to match the resulting color to a chart; dental color matching consisted basically of eyeballing color strips.
The combination of small size, sophisticated functionality, solid-state robustness, and low-power operation makes integrated optoelectronic sensors a natural choice in medical equipment design. The ability to create smaller, portable, noninvasive test equipment that provides quick, accurate results has led to the development of pulse oximeters, personal heart-rate monitors, and blood glucose meters. Integrated optoelectronic sensors have provided medical professionals with a quick, accurate procedure for analyzing urine specimens, as well as helping eliminate the need to look at color strips in dental color matching.
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