Paper and Salt: Biodegradable NaCl-Based Humidity Sensors for Sustainable Electronics

Aniello Falco,José F. Salmerón,Almudena Rivadeneyra,Alfonso Salinas-Castillo,Florin C. Loghin,Antonio Marín-Sánchez,Encarnación Castillo

doi:10.3389/felec.2022.838472

Abstract

Flexible and thin-film humidity sensors are currently attracting the attention of the scientific community due to their portability and reduced size, which are highly useful traits for use in the Internet o Things (IoT) industry. Furthermore, in order to perform efficient and profitable mass production, it is necessary to develop a cost-effective and reproducible fabrication process and materials. Green fabrication methods and biodegradable materials would also minimize the environmental impact and create a sustainable IoT development. In this paper, flexible humidity sensors based on a common salt (NaCl) sensing layer are reported. Our sensors and the fabrication techniques employed, such as dip and spray coating, provide a biodegradable, low cost, and highly reproducible device. One of the sensors reported presents a typical resistive behaviour from 40% RH up to 85% RH with a sensitivity of −0.21 (Z/%RH). The performance of the sensors obtained with several fabrication techniques is studied and reported at multiple frequencies from 100 Hz to 10 MHz, showcasing its versatility and robustness.

Highlights

Monitoring environmental variables is gaining relevance in key trend technologies like the Internet of Things (IoT) (Bouzembrak et al, 2019)
The physical characterization of the devices is performed by the analysis of optical microscope and scanning electron microscope (SEM) images
It can be seen that for paper-based sensors the salt crystals are wrapped around the celulose fibers, becoming part of the substrate itself, while for the devices manufactured on polyethylene terephthalate (PET) the salt agglomarates are on the substrate surface

Summary

Introduction

Monitoring environmental variables is gaining relevance in key trend technologies like the Internet of Things (IoT) (Bouzembrak et al, 2019). Controlling humidity is fundamental for developing IoT solutions like smart food packaging and environmental control systems for industrial and manufacturing processes (Najeeb et al, 2018). In this context, fabrication of flexible and thin-film humidity sensors is an important field of research in terms of device portability and compatibility. The fabrication of a flexible humidity sensor with satisfying performance in a wide range of humidities and temperatures has become an important goal in the literature (Chen and Lu, 2005). Characterizing the device performance at different frequencies is essential to differentiate between its capacitive and resistive behavior at several frequencies, to establish its range of operation and to integrate it into a more complex sensor

Methods

Results

Conclusion