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  • It's an increasingly common sight in hospitals around the world:

  • a nurse measures our height, weight, blood pressure,

  • and attaches a glowing plastic clip to our finger.

  • Suddenly, a digital screen reads out the oxygen level in our bloodstream.

  • How did that happen?

  • How can a plastic clip learn something about our blood

  • without a blood sample?

  • Here's the trick:

  • our bodies are translucent,

  • meaning they don't completely block and reflect light.

  • Rather, they allow some light to actually pass through our skin,

  • muscles, and blood vessels.

  • Don't believe it?

  • Hold a flashlight to your thumb.

  • Light, it turns out, can help probe the insides of our bodies.

  • Consider that medical fingerclip

  • it's called a pulse oximeter.

  • When you inhale, your lungs transfer oxygen into hemoglobin molecules,

  • and the pulse oximeter measures the ratio of oxygenated to oxygen-free hemoglobin.

  • It does this by using a tiny red LED light on one side of the fingerclip,

  • and a small light detector on the other.

  • When the LED shines into your finger,

  • oxygen-free hemoglobin in your blood vessels absorbs the red light

  • more strongly than its oxygenated counterpart.

  • So the amount of light that makes it out the other side

  • depends on the concentration ratio of the two types of hemoglobin.

  • But any two patients will have different-sized blood vessels in their fingers.

  • For one patient, a saturation reading of ninety-five percent

  • corresponds to a healthy oxygen level,

  • but for another with smaller arteries,

  • the same reading could dangerously misrepresent the actual oxygen level.

  • This can be accounted for with a second infrared wavelength LED.

  • Light comes in a vast spectrum of wavelengths,

  • and infrared light lies just beyond the visible colors.

  • All molecules, including hemoglobin,

  • absorb light at different efficiencies across this spectrum.

  • So contrasting the absorbance of red to infrared light

  • provides a chemical fingerprint to eliminate the blood vessel size effect.

  • Today, an emerging medical sensor industry is exploring all-new degrees

  • of precision chemical fingerprinting,

  • using tiny light-manipulating devices no larger than a tenth of a millimeter.

  • This microscopic technology,

  • called integrated photonics,

  • is made from wires of silicon that guide light

  • like water in a pipe

  • to redirect, reshape, even temporarily trap it.

  • A ring resonator device, which is a circular wire of silicon,

  • is a light trapper that enhances chemical fingerprinting.

  • When placed close to a silicon wire,

  • a ring siphons off and temporarily stores only certain waves of light

  • those whose periodic wavelength fits a whole number of times

  • along the ring's circumference.

  • It's the same effect at work when we pluck guitar strings.

  • Only certain vibrating patterns dominate a string of a particular length,

  • to give a fundamental note and its overtones.

  • Ring resonators were originally designed

  • to efficiently route different wavelengths of light

  • each a channel of digital data

  • in fiber optics communication networks.

  • But some day this kind of data traffic routing

  • may be adapted for miniature chemical fingerprinting labs,

  • on chips the size of a penny.

  • These future labs-on-a-chip may easily, rapidly,

  • and non-invasively detect a host of illnesses,

  • by analyzing human saliva or sweat in a doctor's office

  • or the convenience of our homes.

  • Human saliva in particular

  • mirrors the composition of our bodies' proteins and hormones,

  • and can give early-warning signals for certain cancers

  • and infectious and autoimmune diseases.

  • To accurately identify an illness,

  • labs-on-a-chip may rely on several methods,

  • including chemical fingerprinting,

  • to sift through the large mix of trace substances in a sample of spit.

  • Various biomolecules in saliva absorb light at the same wavelength

  • but each has a distinct chemical fingerprint.

  • In a lab-on-a-chip, after the light passes through a saliva sample,

  • a host of fine-tuned rings

  • may each siphon off a slightly different wavelength of light

  • and send it to a partner light detector.

  • Together, this bank of detectors will resolve

  • the cumulative chemical fingerprint of the sample.

  • From this information, a tiny on-chip computer,

  • containing a library of chemical fingerprints for different molecules,

  • may figure out their relative concentrations,

  • and help diagnose a specific illness.

  • From globe-trotting communications to labs-on-a-chip,

  • humankind has repurposed light to both carry and extract information.

  • Its ability to illuminate continues to astonish us with new discoveries.

It's an increasingly common sight in hospitals around the world:

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B2 US TED-Ed chemical hemoglobin oxygen wavelength chip

You are more transparent than you think - Sajan Saini

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    ktyvr258 posted on 2019/10/26
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