A sensor for the in vivo detection of glucose comprises: components of an assay for glucose, a readout of which is a detectable optical signal which can be interrogated transcutaneously by external optical means when the sensor is implanted in vivo and a shell of biodegradable material encapsulating the assay components while allowing analyte to contact the assay components, wherein the biodegradable material comprises a co-polymer having hydrophobic and hydrophilic units. A method of preparing such a sensor preferably comprises coacervation, solvent evaporation and/or extraction, spray drying, spray coating, spray chilling, rotary disk atomisation, fluid bed coating, coextrusion and/or pan coating. A method of detecting glucose using such a sensor suitably comprises implantation of the sensor into the skin of a mammal, transdermal detection or measurement of glucose using external optical means and degradation of the biodegradable material.
