## 1 Introduction  
Battery-powered electronics systems, such as wearable and mobile devices, wireless sensors, and medical devices are widely used during the everyday life. Implantable medical devices (IMDs) represent a category of electronic systems which have an increasing impact in the improvement of quality of life since they enable monitoring or replacing of sensory functions. The idea of using an IMD to assist patients originates since 1950s when the appearance of transistors opened the possibility of implementing a fully implantable pacemakers [1]. From that, various devices have been developed such as heart rate monitors, cochlear implants, retinal implants and brain-computer interfaces. All these devices have a similar architecture which is summarized in Figure 1.  
It comprises circuital blocks for sensing/actuation, signal processing, communication (transmitter device), and energy management as well as for energy storage (usually imple  
Figure 1: General block diagram of an implantable device with battery. mented by a battery). Although the power required by an IMD is contained, the methods to provide energy to these kind of electronic devices represent the bottleneck of their diffusion. Indeed, the technological progress of batteries does not have a similar trend of the semiconductor transistors scaling, therefore devices are much smaller than the battery needed for their operation, limiting the minimum volume of the implanted device. The pacemakers, for instance, clearly demonstrate such limitation considering that their total volume, dominated by the battery, has hardly changed over decades of evolution. Thus, for implantable devices, the battery is not suitable for its invasive dimensions and its limited lifetime. Moreover, batteries require a new surgery operation for replacement or need to be recharged by wireless system [1; 2]. To solve these issues, the idea of recovering energy from the environment is adopted. A system exploiting this technique, usually referred to in the literature as energy harvesting, is shown in Figure 2. The energy recovered from light, vibrations or temperature gradients, allows extending the system lifetime by reducing the battery capacity or even eliminating it [3; 4; 5; 6; 7; 8; 9; 10]. Battery-less systems allows longer and potentially unlimited lifetime, reduced size, improved bio-compatibility and better eco-friendliness. However, an IMD based on energy harvesting still requires an energy storage device to be used to enable the systems working even when no energy can be collected from the energy harvester. In this case, direct connection between the energy harvester and the energy storage device is usually not possible and the energy management unit is in charge of handling its charging/ discharging. For this purpose, simple capacitors are preferred over batteries due to their superiority in terms of life cycle and compactness. The methods of providing power to IMDs can be categorized into two types, namely the _in-body_ type and the _out-of-body_ type. The in-body energy sources come from the human body. Some examples are vibrations gathered from patient movements, breathing activity, and heartbeats through piezoelectric devices, or temperature gradients between the inner body, the skin and the air that are converted into voltage by thermo-electric generators (TEGs) [11; 12; 13]. Unfortunately, these in-body sources provide extremely low and unpredictable power levels which prevent their adoption in most applications [14; 15; 16]. In out-of-body power delivery, an external energy source is coupled to the energy harvester implanted inside the body, which will charge an accumulation device (rechargeable battery or, more frequently, a capacitor) or will directly feed the IMDs [5; 17]. The external power can be provided through radiofrequency, electromagnetic induction or Ultrasound (US) waves. Among these power transfer systems, only US waves have the capability to enable simultaneous power and data transfer in deep-implanted (\(>\)2 cm) mm-sized devices [18; 19; 20; 21].