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

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.

The treatment of bacterial biofilms cultivated on knee arthroplasty implants using the bioelectric effect

“…The solution acted as a transmission medium, and currents from 1.3 to 1.9 A (15.6–22.8 W) were obtained. The conduction of NS 0.9% has been described to be of 1.45 S/m ( Falco et al, 2022 ).…”

Paper-Based Humidity Sensors as Promising Flexible Devices, State of the Art, Part 2: Humidity-Sensor Performances

“…These sensors exhibit the best linearity in terms of the RH-impedance modulus, minimum hysteresis, and acceptable responses to variations in humidity. As a sensitive material [97,98], or as a component of a composite [95,100], paper can be also employed in impedance humidity sensors. However, it was found that almost all paperonly sensors become sensitive to H 2 O when the relative humidity is higher than 50-60%.…”

“…Some research studies found that it is necessary to functionalize paper to expand a range of relative humidity levels and improve the sensitivity of humidity sensors. In particular, to improve paper's conductivity, the treatment of paper with a solution of NaCl [97] or NaOH [98] was found to be highly powerful. According to Farrell et al [103], when alkali and cellulose are mixed in solution to form reactive EPTAC (epoxy propyl trimethylammonium chloride), trimethyl amine is emitted.…”

“…In the presence of alkali, cellulose converts to EPTAC, together with the partial dissociation of the cellulose hydroxyl group [104]. For these purposes, spray coating, dip coating [97], and immersion in solution [98] can be applied. The influence of the reaction ratio between the NaOH solution and the paper on the impedance characteristics of the sensors upon exposure to varying levels of humidity is shown in Figure 28.…”

On paper characterisation of droplet and evaporation study using impedance spectroscopy

“…21,22 Paper is used as a microuidic substrate for the identication of various biomolecules and the examination of liquids. [23][24][25][26][27] In order to restrict the amount of uids that are dispersed on the paper substrate, hydrophobically patterned paper is utilised in the process of conducting targeted chemical analysis. [28][29][30] Furthermore, the amount of sample needed for this analysis is smaller in comparison to conventional chemical analysis.…”