Fabrication and Materials Integration of Flexible Humidity Sensors for Emerging Applications

Abstract

In the past decade, humidity measurements have ubiquitously gained consideration in the wide range of application paradigms such as industrial predictive maintenance, instrumentation, automation, agriculture, climate monitoring, healthcare, and semiconductor industries. Accurate humidity measurements and cost-effective fabrication processes for large-volume and high-performance sensors with flexible form factors are essential to meet the stringent performance requirements of the emerging application areas. To address this need, recent efforts focus on development of innovative sensing modalities, process technologies, and exploration and integration of new materials to enable low-cost, robust, and flexible humidity sensors with ultrahigh sensitivity and linearity, large dynamic range, low hysteresis, and fast response time. In this review paper, we present an overview of flexible humidity sensors based on distinct sensing mechanisms, employed processing techniques, and various functional sensing layers and substrate materials for specific applications. Furthermore, we present the critical device design parameters considered to be indicative of sensor performance such as relative humidity range, along with a discussion on some of the specific applications and use cases.

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

“…Humidity sensors have become ubiquitous tools for several emerging applications. 2 Here, we demonstrate the diverse applicability of the proposed all-cellulose-derived humidity sensor. The CO 2 -laser-irradiation process is a facile method for forming patterned electrodes on TEMPO-oxidized cellulose paper, and can also be used to cut the paper into various shapes, allowing for versatile configurations of humidity sensors (Fig.…”

“…Humidity sensors are a major attraction and have diverse applications such as in industrial instrumentation and automation, agriculture, healthcare, skincare, and climate monitoring. 1,2 The market size of humidity sensors is estimated to grow from USD 4.0 billion in 2019 to USD 11.85 billion by 2027. 3 The production of humidity sensors is expected to increase concomitantly; therefore, there is a need for the development of new sensors using abundant, ubiquitous, and even renewable materials.…”

Flexible and Transparent Polymer-Based Optical Humidity Sensor

“…A variety of materials can be used for deposition on flexible substrates [3][4][5][6] but the ones that stands out due to easy deposition in the form of thin films, relatively low cost, tailored functionality and fast response are polymers [7]. A number of different polymers, suitable for use as flexible substrates for sensor applications, are reported in the literature, including polyethylene naphthalate (PEN) [8,9], polyethylene terephthalate (PET) [10][11][12][13][14], polydimethylsiloxane PDMS [15,16], polyimide PI [17][18][19][20], polyvinyl alcohol (PVA) [21][22][23][24], polylactide (PLA) [25,26], polyurethane (PU) [27,28], polysulfone (PSU) [29], polyetheretherketone (PEEK) [30,31] and polycarbonate (PC) [32], along with other materials such as common paper [33,34], flexible glass [35] and last, but not least, composite [36] and multilayer substrates [37]. Among the most commonly used flexible substrates, PI provides a great solution for devices with higher annealing temperatures; however, its amber color makes it unsuitable for devices that require transparency.…”

Fabrication and Characterization of Flexible Capacitive Humidity Sensors Based on Graphene Oxide on Porous PTFE Substrates

“…Flexible humidity sensors have recently gained attention in various applications including respiratory devices, incubators, sterilization processes, medicine production, and storage [ 1 , 2 ]. The sensors with both cost-effective fabrication processes and high performance such as high sensitivity and linearity, low hysteresis, and fast response time are essential to meet the requirements of the emerging application areas.…”

“…The sensors with both cost-effective fabrication processes and high performance such as high sensitivity and linearity, low hysteresis, and fast response time are essential to meet the requirements of the emerging application areas. To address this need, the humidity sensors based on flexible substrates such as polyimide (PI), polydimethylsiloxane (PDMS), polyester (PES), polyethylene naphtholate (PEN), polyethylene terephthalate (PET), cellulose, and paper have been developed [ 2 ].…”