Non-invasive extraction of interstitial fluid is notoriously difficult. Until recently it was only possible to extract minute amounts and only at a few timepoints. Not anymore. The M-Duo Technology uses small needles placed close to each other and work in pair. One needle injects a carrier fluid. The other aspirates it. The aspirated fluid is a mixture of the carrier fluid, interstitial fluid, and all soluble signals present in the locality of the needles. Extraction of interstitial fluid can be done continuously without removing the needles.