A Printed Flexible Humidity Sensor with High Sensitivity and Fast Response Using a Cellulose Nanofiber/Carbon Black Composite

Abstract

In the emerging Internet of Things (IoT) society, there is a significant need for low-cost, high-performance flexible humidity sensors in wearable devices. However, commercially available humidity sensors lack flexibility or require expensive and complex fabrication methods, limiting their application and widespread use. We report a high-performance printed flexible humidity sensor using a cellulose nanofiber/carbon black (CNF/CB) composite. The cellulose nanofiber enables excellent dispersion of carbon black, which facilitates the ink preparation and printing process. At the same time, its hydrophilic and porous nature provides high sensitivity and fast response to humidity. Significant resistance changes of 120% were observed in the sensor at humidity ranging from 30% RH to 90% RH, with a fast response time of 10 s and a recovery time of 6 s. Furthermore, the developed sensor also exhibited high-performance uniformity, response stability, and flexibility. A simple humidity detection device was fabricated and successfully applied to monitor human respiration and noncontact fingertip moisture as a proof-of-concept.

SnO2-Based Ultra-Flexible Humidity/Respiratory Sensor for Analysis of Human Breath

“…Multiwalled carbon nanotube composite film via a poly-L-lysine modification (MWCNTs/PLL)-based humidity sensor on PI substrate at an annealing temperature of 60 • C shows a response of 659.97% at 91.5% RH [21,23]. SnS 2 /rGO, graphdiyne, and Cellulose NF/Carbon black-based humidity sensors printed on flexible substrates were also demonstrated [18,19,21,24]. In addition, ecofriendly cellulose paper is not only used as a flexible substrate but also as a humidity-sensing material [27,28].…”

“…To fulfill the fast-growing demand for wearable electronics in daily life for continuous respiratory monitoring, it is important to develop ultra-flexible, lightweight, and comfortable respiratory monitoring sensors. In recent years, there have been many flexible relative humidity sensors reported on substrates such as polyethylene terephthalate (PET/0.2-0.7 mm) [17][18][19][20][21], polyester (PE/0.1-0.5 mm) [20], polyimide (PI/0.125 mm) [21][22][23], polyethylene napthalate (PEN/50 µm) [21,24], poly carbonate (PC) [25], epoxy [21,26], cellulose paper [21,27,28], and textiles [29,30]. A comparison of these works with the key parameters (sensing material, substrate, deposition method, annealing temperature, dynamic range of sensor, and response and response/recovery time) is shown in Table S1 [17][18][19][22][23][24]27,28,30].…”

“…In recent years, there have been many flexible relative humidity sensors reported on substrates such as polyethylene terephthalate (PET/0.2-0.7 mm) [17][18][19][20][21], polyester (PE/0.1-0.5 mm) [20], polyimide (PI/0.125 mm) [21][22][23], polyethylene napthalate (PEN/50 µm) [21,24], poly carbonate (PC) [25], epoxy [21,26], cellulose paper [21,27,28], and textiles [29,30]. A comparison of these works with the key parameters (sensing material, substrate, deposition method, annealing temperature, dynamic range of sensor, and response and response/recovery time) is shown in Table S1 [17][18][19][22][23][24]27,28,30]. Specifically speaking, graphene/-Ag colloid film on PET substrate annealed by infrared light exhibits a response of 3.5% at 97% RH [17,21].…”

Carboxyl Graphene Oxide Functionalized Cotton Textile with Superwettable Patterns for Humidity Sensing

“…Humidity sensors, which could be realized through the interaction between moisture and sensing materials, have been extensively studied for applications in industries, agriculture, environmental monitoring, and human activity detectors. − To improve the performance of humidity sensors, different sensing materials have been developed including carbon materials, − metal oxide − and composites − with different architectures. Among all these materials, graphene oxide (GO) with multiple oxygen functional groups has been identified as an ideal candidate to fabricate humidity sensors due to its high hydrophilicity and proton conductivity. − Meanwhile, the fundamental mechanism for humidity sensing via GO has been proposed as the proton immigration due to asymmetric moisturizing. − The oxygen-containing groups on the GO surface have a unique affinity for H 2 O, therefore, these functional groups could absorb ambient moisture and generate ionized protons, which further leads to proton movement.…”

A Cellulose Nanofiber Capacitive Humidity Sensor with High Sensitivity and Fast Recovery Characteristics

“…For example, KOH was introduced into regenerated cellulose as a humidity-sensing electrolyte to improve the sensitivity of cellulose sensors [16]. A porous conductive film was prepared by adding carbon black to cellulose nanofibers (CNFs), and the response and recovery times of the corresponding humidity sensors were shortened to 10 s and 6 s, respectively [17].…”