Battery-less NFC Sensors for the Internet of Things

4Case Study 3: NFC Sensor for pH Monitoring

4.1. Motivation

Color analysis by microfluidic paper-based devices (Martinez et al. 2010) is widespread in applications such as medical devices (Oncescu et al. 2013; Sicard et al. 2015) and pH measurements (Vuong et al. 2010; Loh et al. 2011; Kim et al. 2017). The main advantages of these devices are that they are inexpensive, do not require an energy source, allow easy transport of fluids and are easy to use. However, since diagnosis depends on optical measurements, accuracy is poor especially when classification between different levels is required, as for instance in pH measurements.

pH measurement has a wide range of applications. It is used, for example, to check the water condition of swimming pools, tap water and aquariums. It is also used in medical applications such as sweat or saliva analysis and skin control in wounds, where it can predict an infection before it becomes visible. However, electronic pH probes are expensive and require continuous calibration. In consequence, the cost is prohibitive for home applications. In medical applications such as urine tests (Hashimoto et al. 2017), the probes must be sterilized each time they are used in order to avoid contamination between samples. For this reason, paper-based color strips have been extended to become the simplest, cheapest and quickest pH indicator. Since the integration of high-quality cameras in smartphones, several methods for colorimeter measurement have been ...