**1. Introduction**

Compact resonators and antennas are presented in [1–6]. The efficiency of compact resonators is low. Several types of compact wearable antennas, such as printed dipoles and microstrip antennas, are presented in [2–6]. Printed metamaterial sensors are employed in wireless communication systems and were presented in several publications, see [2–7]. Materials with periodic artificial structures are called metamaterials. The metamaterial structure and components define the electrical properties of the material. In [8–14] metallic posts structures and periodic split-ring resonators (SRRs) are used to produce structures with required permeability and dielectric constant. Metamaterials may be employed to develop efficient sensors for

RF, medical and IoT wearable devices [12–16]. The bandwidth and gain of the antenna presented in [8] are like those of patch antennas. Materials with negative dielectric permittivity are described in [9]. A model and setup to compute and measure the polarity of SRRs are described in [10]. A dual-band transmission-line metamaterial antenna with two transmission-line arms is described in [14]. The radiation efficiency of the antenna at 3.3 GHz is around 60% with 2.6 dBi directivity, and 0.8 dBi gain. Antennas such as patches, loops, and FIPA antennas have low efficiency [2–5, 16–29]. In communication and healthcare systems, the system polarization may be vertical, horizontal, or elliptical. In these systems, the radiating elements must be circular or dual-polarized. In [15–17] compact metamaterials sensors for healthcare devices are presented. Measurements of wearable sensors are presented in [19]. Active antennas for communication, medical, and IoT devices are presented in [20]. As presented in [30] Wearable healthcare devices are used to increase disease cure and prevention. A wireless device with thermal-aware protocol is presented in [30]. Wearable sensors and antennas for healthcare and RF systems are presented in [31–41]. Dual-polarized wearable antennas for healthcare applications are presented in [41]. Dual polarized metamaterial antennas have significant advantages over regular printed antennas, such as high efficiency and gain. The sensors electrical parameters on and near the human body were evaluated, see [2–6], by employing RF CAD software [42, 43].

In this chapter, metamaterials technology is used to develop high-efficiency sensors and antennas with harvesting energy units for medical, communication, and IoT devices. The energy harvesting units connected to the system provide compact selfpowered efficient sensors.

The antennas bandwidth is around 20 to 45%, for VSWR, better than 3:1. The gain of the antennas with CSRRs is around 7.5 dB. The sensors efficiency is higher than 90%.
