# A Survey of NFC Sensors Based on Energy Harvesting for IoT Applications
## 3 NFC Sensors
Double-sided medical adhesive tape is used below the patch, while, above the patch, a medical textile covering is added to protect it and improve visual aesthetics. A non-invasive, flexible, and wireless pH-sensing system for monitoring wound healing and identifying the possibility of early-stage infection was reported in Reference [63]. Low pH is beneficial since it helps counteract microbial colonization from many human-pathogenic microorganisms that require a more alkaline environment for growth. The sensors consist of a working electrode and a reference electrode. The electrical potential across the two electrodes is a function of the concentration of H+ ions in the solution. The sensor is interfaced to an NFC SL13 chip from AMS with a buffer amplifier (AD8603, Analog Devices Inc., Norwood, MA, USA). The pH sensor exhibits a linear sensitivity of \(-55\) mV/pH and stable performance under mechanical bending in a pH range of 4 to 10. A low-power complementary metal-oxide semiconductor (CMOS) ion-sensitive field-effect transistor (ISFET) array for pH sensing was inductively powered using NFC in Reference [64]. Each pixel in a 3 \(\times\) 3 array contains an ISFET operating in weak inversion that detects changes in pH as a current. The output for all pixels is then averaged, and the resulting signal modulates the frequency of a ring oscillator. This provides simple analog-to-digital conversion suitable for reading and transmitting. The application-specific integrated circuit (ASIC) power consumption was 6 \(\mu\)W (at a 1.2 V supply). The SIC4310 from Silicon Craft NFC IC was used in the study. The literature contains specialized ASIC designs that integrate sensing, signal processing, energy harvesting, and NFC communication. A batteryless wearable electrocardiogram (ECG) monitoring system-in-a-patch assembled by biocompatible and pliable silicon-in-parylene technology was reported in Reference [65]. The system is able to process the acquired ECG signal and detect arrhythmia using a built-in digital signal processor (DSP). An NFC communication system is used to interface the external reader. The system requires an additional power source. The energy harvested from a 5 \(\times\) 5 cm\({}^{2}\) thermoelectric generator (TEG) module (60 W of output power) can be powered and stored in a supercapacitor. An integrated system-on-chip (SoC) for long-term implantable continuous glucose monitoring was reported in Reference [66]. This integrates an amperometric glucose sensor interface, an NFC wireless front-end, and a fully digital switched mode power management unit for supply regulation and on-board battery charging. It uses the 13.56-MHz (ISM) band to harvest energy and backscatter data to an NFC reader. However, it does not use standardized protocol, and custom ASK demodulator circuits extract the modulating frequency that encodes the glucose concentration. Another ASIC for a wireless fully implantable glucose sensor was reported in Reference [67]. In this case, the NFC was based on ISO15693 for passive wireless readout through an NFC interface. The IC is used as the core interface to a fluorescent, glucose transducer to enable a fully implantable sensor-based continuous glucose monitoring system. The whole system (photodiodes, transimpedance amplifier (TIA), ADC, electrically erasable programmable read-only memory (EEPROM), and NFC), except for an external LED, is integrated into the IC. Chemical gas sensors based on NFC technology were recently reported in the literature [68; 69]. Portable gas sensors are used for diagnosing point-of-care diseases, detecting explosives and dangerous chemical agents, indicating food ripening, and monitoring environmental pollution [69]. Reference [68] presents a fully passive flexible multigas-sensing tag for determining oxygen, carbon dioxide, ammonia, and relative humidity, readable by smartphone. The tag is based on NFC technology for energy harvesting and data transmission to a smartphone.