Flexible Capacitive Humidity Sensors Based on Ionic Conductive Wood-Derived Cellulose Nanopapers

Abstract

With the growing requirements for the renewability and sustainability of electronic products, environmentally friendly cellulosebased materials have attracted immense research interests and gained increasing prominence for electronic devices. Humidity sensors play an essential role in industries, agriculture, climatology, medical services, and daily life. Here, for the first time, we fabricate capacitive humidity sensors based on ionic conductive wood-derived cellulose nanopapers (WCNs). The WCN-based humidity sensors exhibited ultrahigh sensitivity, fast response, small hysteresis, and more importantly, a wide working range of relative humidity (RH). The sensors showed >10 4 times increase in the sensing signal over the 7−94% RH range at 20 Hz, while many reported humidity sensors with high sensitivity often have the working range limited to high RH levels. Our sensors can realize the distinction of nuances in humidity and exhibit outstanding noncontact skin humidity sensing properties. Flexible WCN-based humidity sensors were also fabricated, and they displayed excellent sensing properties with long-time stability, endowing them with multifunctional applications. The contrast humidity sensing experiment compared to the existing commercial humidity sensor further demonstrated the higher and faster response of our WCN-based sensors. Thus, this work provides effective guidance for the design of high-performance humidity sensors using nanopapers and opens a new dimension for a variety of future applications.

All-cellulose-derived humidity sensor prepared via direct laser writing of conductive and moisture-stable electrodes on TEMPO-oxidized cellulose paper

“…The response and recovery times were defined as the time required to reach 90% of the change in sensor resistance. 1,38 The sensitivity (S) was calculated using the following equation: 17,39…”

“…[13][14][15][16] In addition, the high density of hydrophilic groups in cellulose molecules makes them sensitive to water vapor, thus providing the ability to use cellulose as a humidity-sensing material. 11,[17][18][19][20] Various electrodes have been fabricated on cellulosic materials using different methods, 21 such as printing, 11 pasting, 20 painting, 22,23 and spraying, 24 for flexible and wearable humidity sensing. For example, a flexible humidity sensor comprising a 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofiber as the sensing material and gold as the electrodes has been reported.…”

“…For example, a flexible humidity sensor comprising a 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofiber as the sensing material and gold as the electrodes has been reported. 17 Degradable humidity sensors have also been developed using cellulose nanofiber paper as the sensing material and silver as the electrodes. 11 However, the use of non-renewable electrodes is still necessary to meet many requirements, such as having a sufficiently higher electrical conductivity than humiditysensing materials and stability against moisture, making it difficult to realize humidity sensors made from fully renewable materials.…”

Water‐Saturated Ion Gel for Humidity‐Independent High Precision Epidermal Ionic Temperature Sensor

“…This sensitive humidity dependence is mainly attributed to the change in the bulk conductivity (𝜎) and permittivity (𝜖) depending on water concentration in the ion gel. [27,28] The EDL capacitance change at the gel-electrode interface can be an additional cause of the impedance profile change. [29,30] Encapsulation with a hydrophobic elastomer like polydimethylsiloxane (PDMS) has been investigated to minimize the moisture effect, [31,32] however this approach cannot completely block the moisture penetration for a long time in the current state where a perfect stretchable passivation layer has not been developed yet.…”

Nanobioengineered Sensing Technologies Based on Cellulose Matrices for Detection of Small Molecules, Macromolecules, and Cells

“…Due to its structural form, cellulose exhibits unique properties such as high young’s modulus, biocompatibility, biodegradability, high mechanical strength, transparency, and thermal stability, which allows high chemical modifications [ 7 , 8 , 9 ]. As well as appealing piezoelectricity, mechanical performance, dialectricity, and convertibility [ 10 ] have also been recorded. Cellulose has its application in a broad range of fields, such as in the pharmaceutical industry for coating of tablets, pellets, beads, granules [ 11 ], and in various other industries [ 12 ].…”