# A Survey of NFC Sensors Based on Energy Harvesting for IoT Applications
## 3 NFC Sensors  
The progressive introduction of NFC ICs into the market enables the development of low-cost batteryless portable sensors. Figure 12 shows the number of NFC-enabled mobile devices worldwide between 2012 and 2018 [53; 54]. Between 2013 and the end of 2018, worldwide shipments of NFC-enabled cellphones rose by 325%. Market estimations expect that, by 2020, 85% of smartphones will be equipped with NFC. In this section, we review several NFC-based sensors in the literature. Some of these sensors are listed in Table 5. The second column in this table describes the target application or sensor type. The third column shows the NFC IC used in commercial devices or custom IC designs. Although we focused on passive devices, we also included some interesting semi-passive (battery-assisted passive tags) or data logger implementations. Several comments are also included. The references were sorted based on application. Figure 11: (**a**) Equivalent circuit of the reader, including the electromagnetic interference (EMI) filter and matching network. (**b**) Single-ended equivalent circuit. One pioneering work is the NFC-WISP platform [29]. In this case, rectification is done externally using a full-wave rectifier with discrete diodes, and the ISO-14443 protocol is completely implemented in a low-power microcontroller (TI MSP430). Optionally, an E-ink screen can be used to show the measurements. In this case, a thin-film battery or supercapacitor must be used in order to provide the peak current for the E-ink screen. Temperature measurement for cold-chain data logging is shown as an RFID data logger, which provides temperature history for personnel without post-processing via the E-ink screen. Reference [55] reports a system inspired by the NFC-WISP design for monitoring the temperature of newborns in an incubator. The tag was positioned on the mattress inside the incubator and the reader (TI TRF7970A) was placed below the mattress tray. Temperature was recorded periodically. Another implementation for cold-chain temperature monitoring and quality is found in Reference [56], which developed a critical temperature indicator (CTI) based on a solvent melting point. The smart sensor combines irreversible visual color changes and RFID. A Melexis MLX90129 was used to measure the change in resistance of multi-walled carbon nanotubes (MWCNTs) connected to two copper wires. The proposed CTI smart sensor integrates the microfluidic CTI to an RFID tag in order to remotely detect the melting of the solvent once the critical temperature is reached. The CTI smart sensor has a fast response to the critical temperature of 18-19 \({}^{\circ}\)C.  
The medical market is especially poised to take advantage of NFC thanks to smart sensors that can measure the physical conditions of patients and wirelessly transmit the data to a nearby smartphone [57]. The measuring of vital signs for personalized healthcare is generating substantial interest from ambient assisted living solutions [58; 59]. NFCs provide an intuitive user interface that is easy for patients to use. The latency between touching the device and displaying the result is typically less than one second. The main properties of these sensors are that they are wearable, low-cost, and green. Moreover, the tags can be disposed of in order to avoid contamination between patients. Smartphones enabled with NFC technology facilitate integration with cloud services because the same app that is used for sensor data reading can upload the data to the cloud using a mobile or WiFi internet connection. The continuous monitoring of medical parameters can help improve the diagnosis and follow-up of several diseases, while also reducing personal attention. In the long term, incorporating these sensors can help reduce the cost of healthcare in societies with aging populations. Another potential application is the development of devices for fast screening before deciding whether more expensive analysis is required.