Wireless Energy Transfer and Wireless Communication for In-body Sensors

Wireless communications in itself is a never-ending growing technology. One growing field of research relates to the implantable biomedical devices which have found applications in a wide range of areas. Some implants use traditional batteries to supply power for the electronic circuits within the sensor. However, any battery has a limited energy storage and life span, and percutaneous links are susceptible to infection and reliability problems. Wireless power transfer (WPT) offer the opportunity to provide power for longer periods without the risk of infection from a percutaneous lead. Inductive power transfer is the most common method of wireless power transfer to the implantable sensors which consist of a primary external power circuit and a secondary implantable power pick-up unit. A common characteristic associated with biomedical applications is loose coupling between the primary and secondary coils. Compensation for loose coupling can be achieved through the use of resonance circuits which enables the voltage or current at the secondary to boost up to useful levels even in the presence of low coupling coefficients. The ability to achieve power transfer is dependent on the match between the resonant frequency of the primary with the resonant frequency of the secondary. Resonance-based wireless power delivery is investigated for improved energy transfer efficiency and reduced dependence on the distance between the primary and secondary coils. However, in practice the resonant frequency of the secondary pick-up circuit is often mismatched with the operating frequency of the primary because of the variations in load, coupling and other circuit parameters. When mismatching occurs, the voltage magnitude control approach can only respond by operating at a high magnitude to attempt to maintain the power flow to the load. One of the main constraints of the system is to achieve the minimum power required by the application by still keeping the implant size small enough for the living subject’s body. This report also focuses on the design issues associated with the wireless exchange of data between the implant and the external world and also with the telemetry of power through the inductive link.