## 1 Introduction  
Over the last few years, the industrial internet of things (IIoT) has become a hot research and development topic. The IIoT leverages wireless sensor network (WSN) technologies to enable untethered and seamless connectivity in industrial environments. Several solutions have been tailored to the goal of continuously monitoring key components with untethered sensors to track in real-time rotation, position, speed, temperature, and pressure [1]. One of the key desired characteristics of an IIoT wireless sensor node is the capability to operate autonomously from energy harvesting (EH) rather than relying on bulky batteries,which have a limited lifetime, especially at high temperatures, and might require multiple replacements over the lifetime of a monitored artifact, thereby increasing maintenance cost. Several implementations of such sensors have been presented in the literature, e.g., relying on solar [2; 3], wind [3], kinetic [4], thermal [5], thermoelectric [6] or piezoelectric [7] energy harvesting. The main limitations of these solutions are the unreliability of harvesting sources and the need for harvesters to be tailored to the specific deployment scenario. The development of wireless power transfer (WPT) solutions and the reduction of the node power consumption enabled untethered sensor powering with a reliable and well-controlled energy source, with the two-fold advantage of getting rid of battery cost and environmental impact as well as abating the personnel and material cost related to battery replacements [8; 9; 10; 11]. Predictive maintenance is becoming one of the most promising application targets of these battery-free nodes to enable continuous full-lifetime monitoring of the most critical parts of mission-critical equipment to prevent failure [12]. Companies have placed great emphasis on these goals and are seeking solutions to power the condition-monitoring sensor nodes through radiofrequency (RF) waves. As an example, near-field inductive solutions have been adopted in that sense [13; 14; 15], as well as prototypes exploiting far-field transmission [16]. RFID technology for quasi-total metallic environments is well developed. For instance, in [17], an antenna tag topology is presented with the aim of being inserted in close contact with metal tools for industrial plants. In addition, many different approaches have been explored towards concepts of wireless information and power transfer (WIPT) [18], and simultaneous wireless information and power transfer (SWIPT) [19], both for consumer and for industrial purposes. Figure 1 represents a sketch of the fully customized WPT and WSN designed system for a specific automotive application example. A major highlight is the possibility to conjugate the electromagnetic far-field WPT capabilities together with the low-power sensor and communication part that is conceived to work in the same frequency band. This allows the simultaneous wireless transfer of power and information, with possibility to store a certain amount of energy enabling the full-cycle activities of a microcontroller unit, a real-time clock, a three-axis accelerometer, but above all bi-directional communication with a Long Range (LoRa) transceiver at 2.4 GHz. The main practical outcome of this work for the exemplary automotive application is to demonstrate, for the first time, the possibility to deploy battery-less miniaturized wireless sensor nodes to be placed in correspondence of key points of the car engine, wirelessly powered in accordance with regulations, and monitor temperature and tridimensional acceleration, in order to minimize the need of maintenance for the equipment that is obviously highly time- and money-consuming. Figure 1: Illustrative example of the envisioned scenario for a WSN system energized by WPT technology in a harsh automotive scenario. The sensor nodes are represented in blue, the power source (yellow), and the communication hub (green) are also shown in one of the possible locations.