Blood, Sweat, and… Saliva: How Our Bodily Fluids Can Save Us

Picture a fighter pilot commanding a plane as they engage in aerial combat.  When you think of the greatest threats to the pilot’s safety, you probably think of attacks from other aircraft or the risk of crashing the plane as they swiftly maneuver it between various obstacles.  But what about the potential for harm coming from inside the pilot’s own body?  For example, the stress the pilot is feeling might give him or her a heart attack.  Similarly, their severe dehydration could lead to heat stroke.  Both of these conditions would spell disaster for the pilot.  While often overshadowed by the inherent hazards of weapons and machinery, these ailments pose a very serious threat to the safety of military personnel.  But how can we know if someone is dehydrated or enduring dangerous levels of stress while they’re thousands of feet in the air?

biomarkers of a heart attack
Molecules in our bodily fluids can serve as biomarkers of disease, telling us when something bad is about to happen.

Fortunately, our bodies are way ahead of us.  Our blood, sweat and saliva contain biomarkers. These are substances that, when present at abnormal levels, signify that someone is in an unhealthy state.  Too much cortisol in your saliva, for example, indicates that your body is stressed.  Elevated salt concentration in your sweat signals that you’re dehydrated.  Measuring these biomarkers in real-time could enable a pilot to treat his or herself with the right remedy before it’s too late.

The task of detecting a single substance amidst the multitude flowing in our bodily fluids is anything but trivial.  Suppose you’ve been handed a picture of a random person and asked to find them among the flood of people spilling into the arrivals terminal of an airport.  You could try to search each face and compare it to your picture, but your chances of missing them are high.  A more effective strategy would be to station the person’s mom in the reception area while you sit comfortably on the sidelines.  When she hones in on her child in the crowd, she is going to react (hopefully in a positive way) and alert you to their presence.  Scientists are developing sensors for biomarkers that work the same way—they recognize one particular compound within the complex mixture of substances in our bodies.  Upon detection of that compound, these sensors produce a signal that a scientist can measure.

Detecting Biomarkers of Illness
biomarkers of a heart attack
An aptamer with a unique shape that fits a specific molecular biomarker

If the biomarker that someone’s looking for is a molecule such as cortisol, the sensor typically includes another type of molecule called an aptamer.  An aptamer is a segment of genetic material, like DNA or RNA, that fits together with a biomarker molecule like a complementary puzzle piece.  Different aptamers adopt different shapes depending on their genetic sequence, creating pockets that can bind specific molecules.  The Air Force Research Laboratory (AFRL) developed an aptamer-based sensing tool for cortisol, which they hope to use to monitor pilots’ stress levels in midair.  The device uses an aptamer that comes pre-bound to a molecule with a similar structure to cortisol.  When someone tests a sample of saliva on the device, the pre-bound molecule dislodges from the aptamer as cortisol takes its place.  The free-floating molecule then meets up with a carbon-coated surface on the device, and together, they produce an electrical signal.  In this way, the amount of electrical activity the tool picks up tells how much cortisol a person has in their body.

biomarkers of a heart attack (c) Air Force Research Laboratory
A member of the armed forces wears a sweat patch to monitor his hydration status in the field

If the biomarker someone wants to detect is an ion (a charged particle), the sensor usually incorporates an ion-selective electrode.  These electrodes are long, hollow structures made from different materials designed to recognize different types of ions.  Scientists at the AFRL developed ion-selective electrodes to monitor people’s hydration.  The electrode contains a solution of sodium or potassium ions, similar to the ones in your sweat, that produces a known electrical charge.  When someone’s sweat touches the electrode, the ions in the sweat interact with the outside of the structure, creating their own buildup of charge.  A detector then measures the difference in charge between the two sides of the structure.  This tells how much sodium or potassium is in the sweat sample.

It’s one thing to measure these biomarkers in a laboratory.  It’s another thing to actually use the technology in the air.  While engineers have already figured out how to turn the sweat hydration detector into a skin patch, there’s a lot more work to do to convert other sensors into similarly portable, fast and non-invasive devices.

And as the list of human health and safety biomarkers continues to grow, so will the need for new ways of detecting them.  In a recent research endeavor, scientists searched for compounds that could be used to identify hypoxia, which is when someone doesn’t have enough oxygen in their blood.  This condition frequently affects Air Force pilots because the higher up you go, the less oxygen there is in the air.  To find a compound that could help, scientists tested air that Air Force pilots exhaled at different altitudes. The researchers discovered six molecules that correlated with the amount of oxygen in their breath, which means they could be used as new biomarkers for hypoxia.

Detecting Biomarkers of Health  

The first wave of research in biomarker sensing in Air Force pilots focused on monitoring life-threatening physical conditions.  However, scientists have recently become interested in measuring biomarkers that indicate health and safety in a broader sense.  For example, researchers are trying to develop tools for detecting glucose (sugar), lactate (a molecule released when you’re exercising) and other compounds in sweat that give us information about energy level and fatigue.  This information is useful for customizing training programs for members of the armed forces to optimize their performance.  Additionally, if scientists could detect glucose in someone’s sweat the way they can sodium and potassium, diabetics in the Air Force and beyond wouldn’t have to prick their fingers to test their bodies’ sugar levels.

Scientists are also excited about identifying and detecting biomarkers for mental health conditions such as anxiety, depression, and post-traumatic stress disorder (PTSD).  Researchers at the Air Force Research Laboratory recently developed a sensor for neuropeptide Y, which is a small protein biomarker for PTSD.  The sensor uses microscopic spheres called nanoparticles to detect neuropeptide Y.  First, another protein attaches onto a nanoparticle, turning the particle red. After neuropeptide Y binds to this protein, the particle turns purple.  By measuring the amount of red versus purple spheres, scientists can calculate the amount of neuropeptide Y in a sample, which could potentially help to diagnose PTSD earlier.

Picture again the fighter pilot.  An enemy aircraft shoots at their plane, narrowly missing the engine before speeding past. The pilot clutches the control wheel in both hands, momentarily paralyzed as they replay the close encounter in their mind.  Just then, an alarm goes off, alerting them that their cortisol levels have spiked.  They shakily remove the cap from their anti-cortisol supplements and toss two back.  They fly on, having prevented their heart attack midair.