sensors Review A Review of RFID Sensors, the New Frontier of Internet of Things Filippo Costa 1,* , Simone Genovesi 1, Michele Borgese 2 , Andrea Michel 1 , Francesco Alessio Dicandia 3 and Giuliano Manara 1 1 Dipartimento di Ingegneria dell’Informazione, Università di Pisa, 56126 Pisa, Italy; [email protected] (S.G.); [email protected] (A.M.); [email protected] (G.M.) 2 Research and Development Department, Siae Microelettronica, 20093 Cologno Monzese, Italy; [email protected] 3 IDS Ingegneria dei Sistemi SpA, 56121 Pisa, Italy; [email protected] * Correspondence: fi[email protected] Abstract: 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 Citation: Costa, F.; Genovesi, S.; specific scenarios. The benefits and limitations of several tag configurations are shown and discussed. Borgese, M.; Michel, A.; Dicandia, A summary of the most suitable applicative scenarios for RFID sensors are finally illustrated. Finally, F.A.; Manara, G. A Review of RFID a look at some sensing solutions available on the market are described and compared. Sensors, the New Frontier of Internet of Things. Sensors 2021, 21, 3138. Keywords: RFID; sensors; RFID sensors; RFID antennas; RFID readers; chipless RFID; wireless sen- https://doi.org/10.3390/s21093138 sors; wireless sensors networks; internet of things; metamaterials; metasurfaces; printed electronics Academic Editor: Federico Alimenti Received: 9 February 2021 1. Introduction Accepted: 30 March 2021 Published: 30 April 2021 Radio frequency identification (RFID) is a low-cost wireless technology that makes possible the connection of billions of things, enabling consumers and businesses to engage, Publisher’s Note: MDPI stays neutral identify, locate, transact, and authenticate products [1]. The general RFID market has seen with regard to jurisdictional claims in a considerable growth over the past few years in terms of the number of RFID tags sold. published maps and institutional affil- The exploration of allied technologies such as RFID sensors has been enabled thanks to new iations. chipsets, both within the High Frequency (HF) band (NFC—near field communication) at 13.56 MHz, and within the Ultra-High Frequency (UHF) (RAIN—radio frequency identifi- cation) frequency band around 866 MHz (ETSI - European Telecommunications Standards Institute) or 910 MHz (FCC - Federal Communications Commission), which are dedicated to supporting several sensor platforms [2]. Even though battery-powered sensors have an Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. obvious advantage for data transmission over large distances, at the same time, a battery in- This article is an open access article creases the system’s complexity and maintenance issues, reduces system life, and limits the distributed under the terms and temperature range of sensor applications. Small form factors of batteries, such as thin-film conditions of the Creative Commons or other types of micro-batteries, are available on the market, but they need replacement Attribution (CC BY) license (https:// every few days [3]. RFID sensors can be fully passive, or battery powered; in the latter case, creativecommons.org/licenses/by/ they do not need a frequent change of the batteries as in the case of traditional wireless 4.0/). sensor nodes. In case of battery assisted RFID sensors or battery-assisted passive mode, Sensors 2021, 21, 3138. https://doi.org/10.3390/s21093138 https://www.mdpi.com/journal/sensors Sensors 2021, 21, x FOR PEER REVIEW 2 of 35 Sensors 2021, 21, 3138 2 of 34 (http://creativecommons.org/li- or battery powered; in the latter case, they do not need a frequent change of the batteries censes/by/4.0/). as in the case of traditionala simple wireless circuit sensor is nodes. built around In case theof battery memory assisted chip, thusRFID enabling sensors the chip to switch to a or battery-assisted passivelocal mode, energy-assisted a simple circuit mode is only built when around it senses the memory a certain chip, stimulus thus [4]. Instead, chip-based enabling the chip to switchpassive to a sensorslocal energy acquire-assisted the power mode required only when for activation it senses a from certain the reader through wireless stimulus [4]. Instead, chippower-based transfer. passive sensors acquire the power required for activation from the reader through wirelessDifferent power RFID transfer. sensors are currently proposed in terms of architecture, complexity, Different RFID sensorsand are system currently requirements. proposed A in chip-based terms of architecture, design, where complexity, the sensor is integrated inside the and system requirements.chip, A chip provides-based a design, reliable where configuration, the sensor since is integrated the sensing inside and the communication functions chip, provides a reliableare configuration separated. Since, since embedding the sensing the and sensor communication increases both functions tag size and cost, an alternative are separated. Since embeddingsolution the is the sensor functional increases integration both tag ofsize the and antenna cost, an and alternative the sensor component. The chal- solution is the functionallenge integration is then of to the transform antenna theand RFID the sensor tag antenna component. into aThe sensor. chal- In antenna-based RFID lenge is then to transformsensors, the RFID the tag response antenna is moreinto a dependent sensor. In onantenna the environment.-based RFID Today’ssen- generation of passive sors, the response is moretags dependent has the abilityon the toenvironment. sense several Today’s environmental generation parameters of passive such as light, humidity, tags has the ability to senseand temperature,several environmental becoming aparameters key technology such foras “object-based”light, humidity, services. The role of RFID and temperature, becomingsystems a key can technology be extended, for “object and even-based” involve services. ubiquitous The role computing of RFID by moving from simple systems can be extendedpassive, and even tags involve to smart ubiquitous tags that cancomputing perform by different moving functions, from simple thanks to integrated sensors passive tags to smart tagsand that a microcontrollercan perform different unit (MCU). functions Research, thanks is to also integrated active onsen- the so-called chipless RF sors and a microcontrollersensors unit (MCU). that do Research not employ is also Integrated active on Circuits the so- (ICs)called and chipless may offer RF the benefits of a longer sensors that do not employlife Integrated and lower Circuits cost. Chipless (ICs) and RFID, may also offer known the benefits as passive of a longer RFID sensors, are compatible life and lower cost. Chiplesswith planarRFID, also technology, known allowingas passive them RFID to sensors, be produced are compatible by roll-to-roll processing. with planar technology, allowingRFID them sensors to be are produced a new paradigm by roll-to for-roll the processing. internet of things (IoT). They have a limited RFID sensors are a newcost paradigm and negligible for the maintenance, internet of things which (IoT). make They them have appealing a limited for numerous applicative scenarios such as manufacturing, logistics, healthcare, agriculture, and food. They have cost and negligible maintenance, which make them appealing for numerous applicative attracted numerous research efforts due to their innovative potential in various application scenarios such as manufacturing, logistics, healthcare, agriculture, and food. They have fields [5]. In order to also address the relevance of the research topic at an industry level, attracted numerous research efforts due to their innovative potential in various applica- we report some figures obtained from the database Scopus. In particular, the number of tion fields [5]. In order to also address the relevance of the research topic at an industry patents published every year according to the keywords ‘RFID’ and ‘RFID sensor’ are level, we report some figures obtained from the database Scopus. In particular, the num- reported in Figure1a. The patents classified by registration offices are reported Figure1b ber of patents published every year according to the keywords ‘RFID’ and ‘RFID sensor’ to highlight the most relevant countries active on this technology. We observe that a very are reported in Figure 1a. The patents classified by registration offices are reported Figure high number of patents, i.e., more than 129,000,