# Low-Power Rectennas in Microwave Wireless Power Transmission
## 4 Emerging Application Scenarios
Figure 24 presents a flexible and wearable hybrid RF and solar energy harvesting system consisting of a flexible transparent antenna, a flexible transparent rectifier, and an amorphous silicon solar cell. The system can cover the n78/n79 5G communication bands. Under a light intensity of 210 Lux, the system achieves an additional 35.6% to 769.5% output power compared to a single solar cell within an input power range of 4 dBm to 10 dBm [135]. Figure 25 demonstrates wireless power transfer to electronic devices placed in the gastrointestinal (GI) tract using 1.2 GHz  
Figure 23: An image of a fabricated planar fully integrated rectenna array for orientation-oblivious energy harvesting: **(a)** Top view, **(b)** Bottom view [134]. Figure 22: (a) IoT temperature sensor, **(b)** IoT light sensor based on ultra-thin Huygens antenna [133]. mid-field antennas. One antenna is positioned externally, while the other is placed in an anesthetized pig's esophagus, stomach, and colon. The received power levels at these locations are 37.5 \(\upmu\)W, 123 \(\upmu\)W, and 173 \(\upmu\)W, respectively. Wireless powering of medical devices is realized while maintaining radiation exposure below permissible safety standards [136]. Figure 26 depicts a fully wireless implanted asynchronous cardiac pacemaker consisting of a 1.2 GHz implanted electrode antenna, a rectifier, and a pacing circuit. The implanted electrode prototype was tested in vivo in sheep; the RF energy was transmitted from a horn antenna placed 25 cm above the sheep's thorax to the implanted electrode. Asynchronous pacing was successfully achieved while ensuring radiation safety [137]. Figure 26: A picture of a fully wireless implanted asynchronous cardiac pacemaker implanted in a sheep’s thorax [137]. Figure 24: Flexible and wearable hybrid RF-solar energy harvesting system: (**a**) Illustration of system, (**b**) Measurement on human arm model [135]. Figure 25: The locations where the 1.2 GHz mid-field antenna is placed [136]. Furthermore, in the context of renewable energy and space exploration, space solar power stations (SSPSs) have been proposed to collect solar energy in orbit and transmit it wirelessly to Earth via RF beams, representing a visionary application for wireless power transmission [138; 139; 140]. According to the development roadmap for the SSPS outlined by the Japan Aerospace Exploration Agency (JAXA), the 100 MW level space demonstration is scheduled for completion by 2030, with commercial operation of gigawatt SSPSs expected before 2040 [141]. In [142], the challenges and solutions for improving the conversion efficiency of rectennas under both low and high input power levels are discussed. The paper also outlines the requirements of rectennas for SSPSs and introduces key technologies for achieving efficient rectification at different input power levels. Figure 27 shows a demonstration system designed to simulate the operational mode of microwave power transmission in a future SSPS. Operating at a frequency of 5.8 GHz, the system achieves a transmission distance of 55 m between the transmitting antenna and the rectenna. The transmitted microwave power is approximately 1600 W, with a DC output power exceeding 300 W. The beam control accuracy is better than 0.4\({}^{\circ}\), which can promote the development of wireless transmission systems for SSPSs [143].