I. Introduction
In Recent years, a great attention has been devoted to wearable electronic devices for healthcare, fitness, and military applications [1], [2]. A key challenge for their practical usability is represented by the energy supply. To overcome the drawbacks of primary batteries, many papers were published on wearable harvesting systems based on piezoelectric, thermoelectric, photovoltaic, or electromagnetic transducers [3], [4]. For example, thermal harvesting units embedded in T-shirts and watches were proposed in [5], photovoltaic (PV) cells covered jackets in [6], shoes and knee braces equipped with harvesters in [7] and [8], and backpack energy harvesters in [9]–[13]. The latter have proved particularly suitable for outdoor activities and military applications [14], [15].