Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Technology Roadmap for Flexible Sensors.

A review of technological solutions for RFID sensing and their current or envisioned applications is presented. The fundamentals of the wireless sensing technology are summarized in the first part of the work, and the benefits of adopting RFID sensors for replacing standard sensor-equipped Wi-Fi nodes are discussed. Emphasis is put on the absence of batteries and the lower cost of RFID sensors with respect to other sensor solutions available on the market. RFID sensors are critically compared by separating them into chipped and chipless configurations. Both categories are further analyzed with reference to their working mechanism (electronic, electromagnetic, and acoustic). RFID sensing through chip-equipped tags is now a mature technological solution, which is continuously increasing its presence on the market and in several applicative scenarios. On the other hand, chipless RFID sensing represents a relatively new concept, which could become a disruptive solution in the market, but further research in this field is necessary for customizing its employment in specific scenarios. The benefits and limitations of several tag configurations are shown and discussed. A summary of the most suitable applicative scenarios for RFID sensors are finally illustrated. Finally, a look at some sensing solutions available on the market are described and compared.

A Review of RFID Sensors, the New Frontier of Internet of Things.

Pattern-reconfigurable antennas with multiple switchable beams, especially with both boresight and endfire directions, are highly desired for wireless communications. In this letter, a novel pattern-reconfigurable antenna is proposed that provides an efficient solution. By reconfiguring parasitic striplines placed around a radiating dipole and reflecting metal pieces under the dipole using p-i-n diodes, the antenna main beam can be switched to five directions in the elevation plane, approximately from $-90^{\circ }$ (left endfire), $-45^{\circ }$ , 0 $^{\circ }$ (boresight), $+45^{\circ }$ , to $+90^{\circ }$ (right endfire). The proposed antenna operates at 2.45 GHz with dimensions of about 0.57 $\lambda$ $\times\, 0.45\lambda$ $\times\, 0.28\lambda$ . An antenna prototype is fabricated and measured. For all five directional beams, the measured $|S_{11}|$ values are below $-$ 13 dB, and the measured realized gains range from 5.2 to 6.5 dBi. They agree reasonably well with the simulated ones.

/pdf/pattern-reconfigurable-antenna-with-five-switchable-beams-in-2p96gwh2jp.pdf

Pattern-Reconfigurable Antenna With Five Switchable Beams in Elevation Plane

Radio Frequency Identification (RFID) sensors, integrating the features of Wireless Information and Power Transfer (WIPT), object identification and energy efficient sensing capabilities, have been considered a new paradigm of sensing and communication for the futuristic information systems. RFID sensor tags featuring contactless sensing, wireless information transfer, wireless powered, light weight, non-line-of-sight transmission, flexible and pasteable are a critical enabling technology for future Internet-of-Things (IoT) applications, such as manufacturing, logistics, healthcare, agriculture and food. They have attracted numerous research efforts due to their innovative potential in the various application fields. However, there has been a gap between the in-lab investigations and the practical IoT application scenarios, which has motivated this survey of this research to identify the promising enabling techniques and the underlying challenges. This study aims to provide an exhaustive review on the state-of-art RFID sensor technologies from the system implementation perspective by focusing on the fundamental RF energy harvesting theories, the recent technical progresses and commercial solutions, innovative applications and some RFID sensor based IoT solutions, identify the underlying technological challenges at the time being, and give the future research trends and promising application fields in the rich sensing applications of the forthcoming IoT era.

https://www.mdpi.com/1424-8220/19/18/4012/pdf

Radio Frequency Identification and Sensing Techniques and Their Applications-A Review of the State-of-the-Art.

An overview of transmission/reflection-based methods for the electromagnetic characterisation of materials is presented. The paper initially describes the most popular approaches for the characterisation of bulk materials in terms of dielectric permittivity and magnetic permeability. Subsequently, the limitations and the methods aimed at removing the ambiguities deriving from the application of the classical Nicolson–Ross–Weir direct inversion are discussed. The second part of the paper is focused on the characterisation of partially conductive thin sheets in terms of surface impedance via waveguide setups. All the presented measurement techniques are applicable to conventional transmission reflection devices such as coaxial cables or waveguides.

Electromagnetic Characterisation of Materials by Using Transmission/Reflection (T/R) Devices

A preliminary investigation of the modal excitation purity effect on a three dimensional platform is presented. A novel Balanced Inductive Exciter (BIE)is proposed for improving the modal excitation in the considered scenario. The Modal Weighing Coefficient (MWC)and radiated power (P r ) have been accurately investigated in order to assess the performance of the new exciters when compared to previously adopted solutions.

Smart Antennas Mounted on Complex Platforms by Using Phase-Shifted Characteristic Modes

A pattern control for portable devices at 2.45 GHz is proposed by exploiting phase-shifted characteristic modes (CMs). Among all available orthogonal CMs, the pattern control of a rectangular conductive plane has been obtained by a weighted linear superposition of only two current modes (J4 and J 8 ). A proper position of two capacitive coupling exciters (CCEs), hosted above the rectangular conductive plane, realize the selective excitation of the current modes, whereas the phase difference (Δθ) between the CCEs allows obtaining the pattern control. The obtained results prove that the phase-shifted CM technique is reliable and can be applicable to other kind of structures.

Pattern control for portable devices by exploiting phase-shifted characteristic modes

A novel compact and low profile Circularly Polarized (CP) antenna operating in the whole X-band for satellite communications is proposed. A suitable radiative performance is achieved by resorting to the Characteristic Modes Theory (CMT). More in detail, the wideband operation and CP radiated field are achieved by a proper excitation of two orthogonal current modes (Jn) supported by the investigated metasurface (MTS) antenna. Measurements on a realized prototype provide a satisfactory agreement with simulations, confirming the reliability of the proposed approach.

Characteristic Modes Theory Exploitation for Wideband Circularly Polarized Metasurface Antenna Design

A novel compact S-band antenna concept hosted on a 1U form factor CubeSat platform is presented. Non-resonant radiators are designed to exploit the resonant current modes excitable on the platform. The effect of the satellite platform on the radiated performances (efficiency, band, gain) is intrinsically taken into account by the proposed design approach. A great saving of space and weight is achieved and the robustness of the solution to different platform setup is verified. Measurements assess the achievement of the expected radiation performance.

Compact Reconfigurable Antenna for Nanosatellites

A novel strategy for the design of a circularly polarized (CP) transmitarray (TA) is proposed by resorting to the characteristic modes theory (CMT). The described TA working principle relies on the use of two transmitting unit cells that support a pair of characteristic modes characterized by either right-handed CP (RHCP) or left-handed CP (LHCP) scattered field. The measured results emphasized a notable 3 dB gain bandwidth of 15.22 % (7.77 GHz - 9.05 GHz) and a maximum TA gain of around 22 dBic at 8 GHz.

Circularly Polarized Transmitarray Design Using Characteristic Modes Theory

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