Interstitial fluid (ISF) is the colorless liquid that fills spaces between cells in the body. This fluid contains some of the same biomarkers found in blood, as well as some unique ones. However, ISF lacks the cells and clotting agents that complicate blood analysis. Recently, researchers developed microneedle patches that can easily and painlessly collect ISF from skin. The fluid collected by the tiny needles goes through a multi-step process of biomarker extraction, centrifugation, sample loading and analysis to detect biomarkers. Srikanth Singamaneni, Mark Prausnitz and colleagues wanted to streamline this procedure so that ISF collected by the skin patch could be analyzed directly with surface-enhanced Raman scattering (SERS) -- a technique that can detect and quantify molecules by looking at how nanomaterials scatter light.

The researchers developed a skin patch with nine microneedles, each about the diameter of a human hair and less than 1 millimeter long. The device also included a strip of plasmonic paper that contained gold nanorods coated with a negatively charged polymer that would attract a model analyte, a positively charged dye. The researchers injected the dye into the rats' bloodstreams and then placed the patches on their skin. The dye entered the ISF and from there, the device. The team then analyzed the plasmonic paper with SERS. The new method could detect the dye as sensitively as the previous multi-step procedure, but with much less time and effort.