An implant device 12 for use within the body comprises: a data source ; and a non - self -resonant antenna for backscatter communications with an external communication system . The non - self -resonant antenna comprises at least two elec trodes 16 including two conductive patches 16 that are arranged to be spaced apart when the implant device 12 is in use The implant device 12 is arranged to control the back scattering properties of the non - self -resonant antenna in order to thereby transmit data from the data source to the external communications system . The implant device 12 is arranged such that the backscattering properties of the non - self-resonant antenna are controlled by one or more electrical switch(es) 14 including an electrical switch that is arranged to change the impedance of the non -self -resonant antenna by switching between coupling the at least two electrodes 16 via body tissue 18 and coupling the at least two electrodes 16 via a conductive pathway.