An assay implantation apparatus is provided comprising a sensor for use for the in vivo measurement of an analyte and means for implanting said sensor within an upper layer of the skin from which it is naturally ejected over time by growth of the skin and progressive replacement of the outer layer of the skin. The sensor may comprise microparticles containing a chemical assay system into which an analyte may diffuse from intercellular fluid and which assay system can be interrogated remotely by a light signal and a fluorescence sensor.