A thin, flexible tube intended to be inserted/implanted into tissue (e.g., brain, liver, muscle) as part of a system used to monitor local tissue chemistry for the early detection of metabolic imbalances and/or cell damage, typically after surgery or during a metabolic disorder. The device has a semi-permeable membrane at its distal end for the diffusion of interstitial fluid to obtain a dialysate sample for analysis. This is a single-use device.