**5. Far-field electromagnetic systems**

As mentioned earlier in this chapter, the main issue in the implanted devices is the battery due to their bulkiness and limited life time, which make them not suitable for long term applications. Thus, it is necessary to power up the implanted device wirelessly through one of the wireless power transfer techniques (i.e. inductive coupling or far-field). Inductive coupling method is used in most conventional wireless power transmission systems, where the transfer of power depends on the coupling between pair of adjacent coils. The main issue in this method is the fact that low frequency electromagnetic waves for power transmission require relatively large coils. On the other hand, wireless implanted devices need to be compact as much as possible for making them allowable to be implanted at different parts of the body (system scalability) and to improve resolution of received signals. This issue can be addressed by utilizing RF systems in order to miniaturize the implanted device and improve the wireless communication link. The amount of harvested power in the RF systems is limited to few hundreds of μW due to the regulation of transmitted power keeping it under the safe level. It is hence needed to have an efficient antenna to receive the signal and efficient rectifying and power management process in order to provide enough power for high-performance implanted biomedical devices. **Figure 9** illustrates a block diagram of power harvesting platform for a wirelessly powered implanted device [32]. The system design employs an integrated on-chip loop antenna. The antenna passes the received power to a multistage rectifier to convert it into DC voltage, which is then passed to the power management unit. It is worth mentioning here that double-gate CMOS transistors can be used and operated at deep threshold region in order to minimize the power consumption and reduce the leakage current.

**Figure 9.** *Block diagram of power harvesting platform for a wirelessly powered implanted device [32].*
