# Planar Microwave Resonant Sensors: A Review and Recent Developments
## 1 Introduction  
Within the paradigm of the internet of things (IoT), or more generally the internet of everything (IoE), and the advent of the fourth industrial revolution (also known as Industry 4.0), the use of sensors has experienced an exponential growth. Moreover, this trend will continue with the progressive implantation of the 5 th generation of mobile networks, designated as 5G, which is expected to satisfy the large-scale connectivity requirements that today's modern society demands (an interesting example is the autonomous and connected vehicle, necessarily equipped with hundreds of sensors). There are many available technologies for the implementation of sensors. Aspects such as diverse as sensor cost, size, and complexity, as well as the type of measurand, among others, dictate the preferred option. Although the most extended technology in modern sensors is probably optics/photonics (examples include optical fiber sensors, laser-based sensors, wearable chemical and biological sensors, nanophotonic biosensors, image sensors, etc.), significant efforts have been dedicated in recent years to the research and development of microwave sensors, especially for applications related to material characterization and composition. Microwaves are very sensitive to the properties of the materials to which they interact. Therefore, microwave technology is very useful for material sensing. Moreover, microwaves exhibit further interesting properties for sensing, such as low-cost generation and detection systems, interaction to the materials at different scales (i.e., through the near-field or the far-field), compatibility with planar technology, wireless connectivity, and robustness against harsh environments, among others. Thus, highly sensitive, robust, low cost, small size, and low-profile microwave sensors and wireless sensors based on the above-cited properties can be implemented.  
This review paper focuses on microwave sensors, and particularly on planar sensors based on resonant elements for material characterization, including solids or liquids. Planar sensors are of special interest as their low-profile is compatible with many applications, where bulk sensors (e.g., cavity sensors or waveguide-based sensors [1, 2], among others) may find a severe limitation, e.g., conformal sensors [3], wearable sensors [4], submersible sensors [5], integrated sensors [6], lab-on-a-chip sensors [7], microfluidic sensors [8, 9, 10, 11], etc. On the other hand, despite the fact that non-resonant planar sensors operating at microwave frequencies have been reported [12, 13, 14], the combination of sensor size and performance (sensitivity) of resonant-type sensor is difficult to achieve with non-resonant methods. Finally, to end this introductory section, let us mention that although probably most planar microwave resonant sensors have been devoted to material characterization (the main interest in this review paper, to be discussed in detail later), there are many reported realizations focused on motion control applications (linear and angular displacement and velocity measurements) [15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30].