Battery-Less Smart Diaper Based on NFC Technology

Modern disposable diapers are carefully designed to transport fluid away from the skin to prevent irritation and dermatitis. These devices are used not only for babies. In fact, they are increasingly used for the elderly since both average life expectancy and the number of people who need special care are increasing. Diapers must be changed to prevent skin rash caused by the skin being exposed to wetness for a long time and to high the pH levels of urine. However, it is not easy to check the state of diapers (of babies or the elderly) without removing the baby's or the elderly's clothes. In this paper, we propose a smart diaper equipped with a battery-less near field communication (NFC) tag. Moisture is detected by changes in capacitance between two electrodes located on the back sheet of the diaper, while capacitance is determined from the charge time through a high-value resistor using a microcontroller. The change in capacitance as a function of the wet conditions is simulated and measured. The tag is based on an NFC IC with energy harvesting. The power required to feed the electronics and microcontroller is obtained from the magnetic field generated by a smartphone with NFC used as a reader. A simple model is proposed to estimate the volume of urine in the diaper. We compare this approach with others in the literature and study how the capacitance is affected by the body and the materials.