An implant device (12) for use within the body (22 ) com prises: a data source (14 ) ; and an antenna (32 ) for backscat ter communications with an external communication system (24);wherein the implantdevice (12 ) is arranged to control the backscattering properties of the antenna (32 ) in order to thereby transmit data from the data source to the external communications system . The antenna (32 ) can have a con figuration for backscatter communications that comprises a meandering path extending from a start (42 ) of the meander at a capacitive coupling fed by a coupling patch ( 38 ) to an end (46 ) of the meander connected to a ground plane (44). The backscattering properties of the antenna (32 ) can be controlled by one or more electrical switch (es) (50 , 52) that change the load and/or geometry of the antenna (32 ).