How light technology is changing medicine - Sajan Saini
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“I’m a doctor, not an escalator.”
-Dr. McCoy in “Friday’s Child,” Star Trek television series (1969)
by Dorothy C. Fontana
Contrary to the cantankerous quote of a beloved medical doctor from sci-fi pop culture, modern technologies such as integrated photonics take up many technical roles—in addition to integrated photonics in a Lab-on-a-Chip, check out these TED-Ed videos on its role in high-tech wireless cloud computing and 3D imaging in self-driving cars.
The pulse oximeter is a successful example of this sort of crossover: the LED, a photonics device developed for lighting and imaging, was re-appropriated for a medical diagnostic tool that’s become a hospital standard for check-ups and surgical care in upper income nations—and now championed by the World Health Organization in low/middle income countries. To better understand how scientists reimagine roles for optical technologies, check out a TED talk by Mary Lou Jepsen on using light to see into our bodies; then, tackle an optical model for human skin and a medical device review; and learn how hemoglobin transports oxygen, how its oxygenated molecules have a unique chemical fingerprint that absorbs more infrared but less red light, and how arterial pulsation defines oxygen saturation level. Examine how this technology continues to evolve, today.
Next, take a step back to review a handy NASA guide identifying various wavelengths (or frequencies) of light we interact with, beyond the visible spectrum.
Returning to integrated photonics, this excellent review on ring resonators explains how light is guided by wires of silicon. An online simulation by AIM Photonics Academy shows materials other than silicon guide light too, and how a little light always bleeds out beyond the wire. This evanescent light enables a ring to tap into a nearby wire and siphon optical power, as described in a classic paper. The resonance condition is the key ingredient to draw all the light from a wire, and a musical analogy with a guitar string shows how particular sound waves resonantly build up in a cavity—similar to what waves of light do inside a ring.
Light sensing of an analyte such as saliva can be done in an absorption mode (chemical fingerprint) or polarization mode (see a video on measuring minute changes in refractive index). In both cases, only the evanescent light beyond the wire samples the analyte, so the sensing is a weak optical effect. Here, the ring is a versatile device to enhance optical detection in both modes. And by inserting a slot in the ring, optical sampling can be further enhanced. Ring diameters are sensitive to manufacturing variability, which influences their resonance condition, and this article shows a novel way to fine-tune them. Such fine control can allow groups of rings to precision filter light frequencies, and open the door to on-chip spectral analysis.
The Lab-on-a-Chip (LOC) is a radical leap beyond the pulse oximeter, leveraging microfluidics, integrated photonics, and other techniques to rapidly analyze small saliva, sweat, or blood samples in the palm of your hand. Ring resonators will be a key ingredient in LOCs and other complex chip applications such as quantum and AI computing. To learn more on how light is used in LOCs, check out these informative articles on wearable sensors, sensing methods, DNA detection, and detecting algae in water. Then, dig into a novel technique with plasmonics to enhance detection in LOCs, an efficient method to produce low cost LOCs, and how integrated photonics improve microfluidic flow in LOCs. Lastly, discover how TED Fellow Patience Mthunzi-Kufa is innovating with lasers to counter HIV, and changing rural African healthcare with LOC-based solutions.
More broadly, LOCs partially overlap with a greater family of transformative sensors leading to an Internet of Things (IoT), and revolutionizing manufacturing with an industrial IoT
Integrated photonics in LOCs have traditionally relied on the near infrared wavelengths used in communications; but as these optofluidic LOCs advance, research with mid-infrared light—which is more efficient for chemical fingerprinting—is leading back to integrated photonics innovations in circuits, devices, and design. Sometimes a doctor can teach an escalator a few new tricks.
To learn more on integrated photonics and how the technology is transforming low power cloud computing, hyperfast wireless, smart sensing, and augmented imaging, visit the AIM Photonics Institute and its education program at MIT, AIM Photonics Academy. Then, step back to learn about similar advanced Manufacturing institutes like AIM that are transforming robotics, smart fabrics, flexible electronics, 3D printing, bio-fabrication, and other high-tech fields.
Sajan Saini is a former materials scientist and science writer. He directs the educational curriculum for AIM Photonics Academy at MIT. He has written for Coda Quarterly, MIT Ask an Engineer, Harper's Magazine, and TED-Ed. Learn about Sajan here.
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