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A study by Sandia National Laboratories and the University of New Mexico uses tiny microneedles that sip fluid in the middle layer of skin between the layer of dead skin cells and above where veins and nerves reside. Therefore, they’re far less painful than traditional hypodermic needles. (Sandia National Laboratories)

Sandia National Laboratories and University of New Mexico researchers have developed unique microneedle-based sensor technology that they hope can someday be used to help soldiers on vital missions. Ronen Polsky, a Sandia materials scientist who leads the design of the microneedle sensor, said the technology is the first way to extract large volumes of pure interstitial fluid for further study.

Microneedles are a few hairsbreadths wide and can sip the clear fluid between cells in the middle layer of skin. This is below the topmost layer of dead skin cells and above the layer of skin where veins and nerves reside. The clear, colorless fluid is called interstitial fluid and is similar to blood plasma.

The microneedles can contain minuscule sensors or extract the interstitial fluid for further testing. Because microneedles are tiny and don’t go very deep, they’re practically painless. Since the needles are painless and minimally invasive, they could be left in for hours or even a whole day without irritation, allowing constant monitoring, said Polsky.

Continual sampling of important biomarkers in this interstitial fluid could help monitor and diagnose many diseases and disorders. These markers include electrolytes, salts such as potassium and sodium that get out of balance during dehydration; glucose, a sugar that diabetics need to monitor constantly; and lactate, a potential marker of physical exhaustion or life-threatening sepsis.

A small, wearable sensor that can monitor these markers could have many uses. It could help endurance athletes meet their training goals without plunging into dehydration or severe exhaustion. Tracking their physiological conditions would aid soldiers on strenuous missions, alerting them before they get so exhausted it could compromise their objectives. Polsky said the microneedle sensor also could be part of a sense-and-respond device that detects high glucose levels and automatically delivers insulin.

Or the sensors could be used in emergency rooms and critical care facilities to determine which salts are out of balance in cases of severe dehydration or track the response of a septic patient to a course of antibiotics, said Dr. Justin Baca, assistant professor of emergency medicine at UNM who leads the human testing of the sensor. The goal of the first research study in humans was to determine the best length of needle to extract the most interstitial fluid from healthy volunteers and then compare the contents of the interstitial fluid to blood. Typically, the best microneedles are 1.5-millimeter long, or as long as a U.S. penny is thick.

For many applications the researchers envision, the biomarker sensors would be on the very tip of the microneedle to allow continual detection of the conditions inside the body. Future studies will use larger needle arrays to increase sample volume. For other applications, such as early cancer detection, collecting the interstitial fluid may take longer than a standard blood draw, but could provide different clues. Future work includes testing the tip-based sensors in people — they’ve already shown it works well in solutions — and then monitoring the lactate levels in people undergoing strenuous exercise.

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