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Photonics and Optoelectronics are Reshaping Medical Measurements and Designers’ Opportunities

You’re almost certainly familiar with the widely used finger clip-on unit for measuring a patient’s blood oxygen level (SpO2), formally known as peripheral capillary oxygen saturation.1 This optical technique, which was widely adopted beginning in the 1980s, totally transformed SpO2 measurement from a time-consuming, risky, and non-real-time process of drawing blood, into a trivial, non-invasive, real-time measurement.

Of course, developing this device was neither quick nor easy. It took decades of research and experimentation, gathering data on blood oxygen and optical characteristics, and algorithm development. This effort was concurrent with the development of LEDs for different wavelengths with consistent performance, as well as complementary phototransistors.

The optical SpO2 test is now so well understood that IC vendors offer easy-to-use ICs and evaluation kits for those who want to develop their own meters, such as the highly integrated MAX30102 oximeter and heartrate monitor from Maxim Integrated. This includes the entire analog front end, management functions, and I/O, and is supported by Maxim Integrated’s MAX30102ACCEVKIT evaluation kit.

Figure 1: The MAX30102 from Maxim Integrated provides a highly integrated, single IC solution for driving the LEDs of an SpO2 unit, as well as digitizing its phototransistor output and providing a digital I/O port. (Image source: Maxim Integrated)

While this is undoubtedly the most widespread example of how the merging of optics and electronics is transforming medical testing for personal use and in the lab, there are many more. Much of this work is still experimental or in the long qualification stage that medical products have by necessity, but these developments are fascinating and worth watching.

The ones which are easiest to grasp like the SpO2 meter use LEDs or laser diodes, usually in conjunction with phototransistors. For example, a research team at Michigan State University is developing a fingertip device which uses an optical sensor on top of a force sensor to also report blood pressure readings on a continuous basis.2

While obtaining blood pressure readings using traditional arm cuff methods is still fairly quick, non-invasive, and risk-free, it is inconvenient for frequent readings or unobtrusive monitoring of this important vital sign.

Figure 2: By combining a blue light source with a wide field microscope and video camera (right), an experimental system from MIT provides automated, fingertip-based counting of white blood cells. (Image source: MIT)

Not all optoelectronic medical devices rely only on LED/laser diode light sources plus one or more optical sensors; video cameras and image analysis are powerful tools, too. A research team at MIT is working on a non-invasive, simple to use, near instant system for assessing white blood cell count in patients undergoing chemotherapy.3 Their system uses a wide field microscope combined with a blue light source; this light penetrates about 50 to 150 microns below the skin and is reflected back to a video camera connected to the microscope (Figure 2). The arrangement is focused at the base of the nail where the capillaries are very close to the skin surface and are so narrow that the white blood cells must pass through in single file, making them easier to count.

The MIT system is an application of basic electronics and optics, but there’s also a lot of activity in photonic-based systems which use electronics for control and data collection, in support of advanced photonics. Many of the designs exploit advanced techniques such as Raman spectroscopy, which relies on inelastic scattering of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range. The laser light interacts with molecular vibrations, phonons, or other excitations in the system, causing the energy (and thus wavelength) of the laser photons to be shifted up or down.

This shift in energy/wavelength, in turn, yields data which provides a structural fingerprint by which molecules can be identified. With today’s technology, implementing a Raman spectroscopy system is not only feasible but is actually straightforward.

Among the many advanced medical systems which use Raman spectroscopy is a lensless endoscope for biosensing and imaging from the Institut Fresnel (Marseille, France). In this approach, a flexible hollow core fiber steers light to the internal organ of interest using a piezoelectric actuator, which also captures its spectral emissions4, 5 (Figure 3). The objective is to provide real-time, barely invasive biopsy of malignant tissue.

Figure 3: Using a hollow optical fiber to convey a laser beam to the site of interest and a piezo probe to steer it, this photonic “biopsy” technique could replace needle-based tests.4 (Image source: Institut Fresnel, via Laser Focus World)

Compare this to the standard biopsy where a piece of the tissue under investigation must be removed with a needle, chemically stabilized, dehydrated, embedded in wax, stained with selective dyes, and then examined under a microscope. This painful process takes days and is traumatic to the patient mentally, physically, and financially.

Many electrical engineers spent at least some of their education studying the basics of optics including lenses, optical wavelengths, reflection, refraction, light sources and sensors, which is necessary and good. Now, by using these concepts in conjunction with the latest in LEDs, laser diodes, photodiodes, and phototransistors, some amazing, widely used and enabling devices have been created, such as the finger SpO2 sensor.

But today’s electro-optics go far beyond just a simple “linear” combination of light sources and lenses, with electronics for control as well as data capture and analysis. Advanced physics and photonic principles are routinely being adapted to medical instrumentation, implementing ideas which were largely impractical until recently or perhaps not even envisioned. Which is why the smart move for any engineer doing instrumentation is to study up on these new techniques and technologies.

Even if you’re not presently in the medical test area, many of these techniques are also being adapted by other test and measurement applications (chemical, materials), and there is a lot of cross-pollination. At the minimum, it’s worth the time to regularly check out photonic web site publications to get a sense of where the action is, where it’s headed, and how you can take advantage of it.

 

References:

1 – Digi-Key, “Understanding and Solving Blood Oxygen Level Monitoring Design Challenges

2 – Michigan State University, “New Blood Pressure App and Hardware Rivals Arm Cuff Accuracy

3 – MIT, “Monitor detects dangerously low white blood cell levels

4 – Institut Fresnel, “Hollow core photonic crystal fiber for flexible nonlinear imaging endoscopes and Raman probes

5 – Laser Focus World, “Multimodal endoscopy targets real-time biopsy

About this author

Image of Bill Schweber

Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.

At Analog Devices, Inc. (a leading vendor of analog and mixed-signal ICs), Bill was in marketing communications (public relations); as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these.

Prior to the MarCom role at Analog, Bill was associate editor of their respected technical journal, and also worked in their product marketing and applications engineering groups. Before those roles, Bill was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls.

He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented on-line courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.

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