You are more transparent than you think Sajan Saini
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.