A data communication device (1) for attachment to an object has a flexible substrate, a transmitter (3) adapted to transmit data in the form of electromagnetic radiation and a programmable memory (7). The device has a controller (5) for controlling the transmission of the data and a programming component for programming the device. A power source for the device comprises a power storage device (8) and a power extraction component (9) capable of extracting power from the environment in which the data communication device is located when in use. A data communication system includes the device and a reader for receiving electromagnetic radiation from the device. The reader includes a perceptive filter adapted to distinguish radiation received from the device from radiation received from other sources.