A new painless and minimally invasive microneedle technology can extract large volumes of pure interstitial fluid for further study. Developed at Sandia Labs and the University of New Mexico, the microneedles are a few hairs-breadths 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.
Because microneedles are tiny and don't go very deep, they're practically painless, so they could be left in for hours or even a whole day without irritation, allowing constant monitoring. 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. 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.
A small, wearable sensor that can monitor these markers could have many uses. 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. It could also 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. The microneedle sensor also could be part of a sense-and-respond device that detects high glucose levels and automatically delivers insulin.
The researchers now have a sense of the average length that should be used for most people, but some people's skin is a little thicker or a little thinner in that area so the flow rate might be decreased. Typically, the best microneedles are 1.5 millimeters long, or as long as a U.S. penny is thick.
For many possible 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.