A transdermal device (10) for sensing one or more analytes in a biological fluid the device (10) comprising a liquid or gel layer (14) a sensor (18) located in the liquid or gel layer (14) for sensing analyte in the gel layer and an outer layer (16) that is adapted to bind with or trap the one or more analytes of interest wherein the device (10) is arranged so that in use the analyte is continuously drawn out of the liquid or gel (14) and into the outer layer (16) and the analyte concentration as a function of position across the gel/liquid has a defined profile.
