Bidirectional data transmission for battery-less medical implants

Active implants for monitoring human body functions became increasingly important in recent years, especially due to an increase of heart diseases in an aging society. With the help of a miniaturised multi sensor implant, physiological parameters in cardiovascular areas can be monitored so that early diagnostics, optimisation of therapy and reduction of hospitalisation time can be achieved. In addition to a highly accurate pressure sensor, the developed implant consists of a further sensor elements, such as acceleration and temperature sensor, which are implemented in order to obtain more information about the patient’s condition and thus improve pressure measurement accuracy. For wireless energy and data transmission inductive near-field coupling at a frequency of 13.56 MHz is used. A bi-directional data transmission between a reader unit and the implant is needed, in order to control the state of the implant. A precise analysis of pressure progression requires a continuous data transmission from the implant to the reader unit with a data rate of at least 26 Kbit/s. Since standard RFID protocols only support non continuous data transmission, such as memory access operations, an extended ISO 15693 protocol was used to fulfil these requirements. In RFID systems usually power and data are transmitted over the same set of resonant antenna coils. The so called Quality-Bandwidth-Dilemma describes the problem, that high energy range and high data transmission bandwidth cannot be achieved simulatneously. Nevertheless, small antenna sizes of the implant, implantation inside human body and continuous measurements requires exactly that. In this work, a solution how to solve this problem with antenna coil design and high efficiency power amplifier in the reader unit is presented. A system composed of the antenna coil, an ASIC with integrated rectifier and a hand held reader unit was manufactured. Wireless measurements were done to evaluate the results