Interstitial fluid is clear, colorless, and similar to blood plasma. Continual sampling of important biomarkers in 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.

Tiny microneedles sip fluid in the middle layer of skin between the layer of dead skin cells and above, where veins and nerves reside. As a result, they are far less painful than traditional hypodermic needles. (Sandia National Laboratories)

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.

Microneedles can extract the 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 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 virtually painless and minimally invasive, they could be left in for hours or even a whole day without irritation, allowing constant monitoring. Typically, the most effective microneedles are 1.5 millimeters long, or the length of a U.S. penny.

In addition to the salts and sugars with well-studied roles in physiological monitoring, interstitial fluid contains many proteins and exosomes — free-floating balls carrying genetic information including cancer markers. Interstitial fluid may someday join blood and urine as a fluid doctors routinely test for their clinical diagnoses.

For many applications, 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 have been shown to work in solutions — and then monitoring the lactate levels in people undergoing strenuous exercise.

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