Transparent Humidity Sensor Research Articles

Flexible and transparent humidity sensors based on hyperbranched poly(ionic liquid)s for wearable sensing

The rapid development of wearable electronics is driving the exploration of flexible and transparent humidity sensors. However, the limited humidity response and mechanical flexibility of sensitive materials hinder the normal operation of the device. In this research, flexible humidity sensors with a fast response time (1 s) were prepared using transparent imidazolium-based hyperbranched poly(ionic liquid)s (HPIL). The sensor performance was improved by regulating the counter anions with different hydrophobicity, such as trifluoromethanesulfonimide ion (TFSI-). The HPIL-TFSI sensor featured an improved response under low humidity condition (6%∼33% RH) with good linearity, excellent bending humidity response, mechanical stability, and long-term stability over a wide humidity range from 6% to 98% RH. Moreover, accurate sensing was achieved for respiratory monitoring, non-contact sensing, and detecting moisture on patched skin. The HPIL-TFSI sensor paves the way for the production of high-performance wearable sensor platforms.

Ultrasensitive, stretchable, and transparent humidity sensor based on ion-conductive double-network hydrogel thin films.

Ion-conductive hydrogels with intrinsic biocompatibility, stretchability, and stimuli-responsive capability have attracted considerable attention because of their extensive application potential in wearable sensing devices. The miniaturization and integration of hydrogel-based devices are currently expected to achieve breakthroughs in device performance and promote their practical application. However, currently, hydrogel film is rarely reported because it can be easily wrinkled, torn, and dehydrated, which severely hinders its development in microelectronics. Herein, thin, stretchable, and transparent ion-conductive double-network hydrogel films with controllable thickness are integrated with stretchable elastomer substrates, which show good environmental stability and ultrahigh sensitivity to humidity (78,785.5%/% relative humidity (RH)). Benefiting from the ultrahigh surface-area-to-volume ratio, abundant active sites, and short diffusion distance, the hydrogel film humidity sensor exhibits 2 × 105 times increased response to 98% RH, as well as 5.9 and 7.6 times accelerated response and recovery speeds compared with the bulk counterpart, indicating its remarkable thickness-dependent humidity-sensing properties. The humidity-sensing mechanism reveals that the adsorption of water improves the ion migration and dielectric constant, as well as establishes the electrical double layer. Furthermore, the noncontact human-machine interaction and real-time respiratory frequency detection are enabled by the sensors. This work provides an innovative strategy to achieve further breakthroughs in device performance and promote the development of hydrogel-based miniaturized and integrated electronics. Electronic Supplementary MaterialSupplementary material is available in the online version of this article at 10.1007/s40843-021-2022-1.

Transparent humidity sensor with high sensitivity via a facile and scalable way based on liquid-phase exfoliated MoO3-x nanosheets

The flourishing of the Internet of Things (IoT) in recent years creates a huge demand for various humidity sensors, since the humidity is closely related to our daily life. Transparent humidity sensors have great application prospects in the fields such as smart furniture and human-machine interface. However, it is difficult for devices made from traditional moisture sensitive materials to have both high optical transparency and sensitive humidity response. Herein, we developed a novel humidity sensor based on 2D MoO3-x nanosheets, in which the nanosheets were liquid-phase exfoliated from α-MoO3 powder. With the assistance of ethyl cellulose, a high concentration dispersion of oxygen-deficient MoO3-x nanosheets can be easily prepared by probe sonicating. The as-prepared humidity sensor shows a transmittance as high as 86% in visible range. Under a constant bias, the current of the sensor increases monotonically over a broad RH range from 11% to 95%. Moreover, the fast response and recovery speed in the subsecond timescale (0.12 s and 0.53 s) enables the transparent humidity sensor to work as a real-time device in the expiration monitoring system. This work demonstrates a promising application of 2D MoO3-x nanosheets and opens up a new avenue for designing transparent humidity sensors.

Textile-Based Humidity-Driven Wearable Electroluminescent for Visual Sensing

Miniaturization and integration have become a trend of modern wearable intelligent electronics. But how to visualize sensing information in a single-level device remains a challenge. Here, we present a humidity-driven textile-based electroluminescent (EL) interactive display that allows for both sensing and visualization of humidity changes. Based on an interdigitated EL structure, a transparent humidity sensor layer with high humidity sensitivity was creatively introduced on the top-emitting layer as a bridging electrode. The visualization and sensing of humidity can be attributed to the electrical conductivity difference of the sensor layer, thus leading to the varied lighting emitting of EL devices on the application of given electric fields. Benefiting from the highly sensitive sensor layer and well-designed device structure, a variety of humidity-based behavior can be read immediately, including hand-writing and finger approach. Furthermore, our devices fabricated from textiles have great flexibility, breathability, and skin affinity, which is very suitable for human wearing. More importantly, this humidity-driven textile-based EL interactive display shows great application potential in breathing monitoring and health assessment.

A highly transparent humidity sensor with fast response speed based on α-MoO3 thin films.

Metal oxide based humidity sensors are important indicators in environmental monitoring. However, most of them are non-transparent and have a long response time, which cannot meet the application of real-time humidity sensing in transparent electronics. Here, we report a metal oxide humidity sensor based on chemically synthesized molybdenum oxide (α-MoO3) thin films. By a green reaction in an ice water bath, the stable precursor containing nanocrystalline colloids was obtained. Molybdenum oxide films with controllable morphology were fabricated through one-step spin coating. The α-MoO3 based humidity sensor exhibits extremely high transparency (85%) in the visible region and has short response and recovery times (0.97 and 12.11 s). In addition, it also shows high sensitivity, good logarithmic linearity and selectivity in a wide relative humidity range of 11% to 95%. The mechanism of humidity sensing was further studied by complex impedance spectroscopy. This novel metal oxide humidity sensor combined with high transparency and fast response speed is expected to broaden the application ranges of humidity sensors.

Flexible Smart Noncontact Control Systems with Ultrasensitive Humidity Sensors.

The development of noncontact humidity sensors with high sensitivity, rapid response, and a facile fabrication process is urgently desired for advanced noncontact human-machine interaction (HMI) applications. Here, a flexible and transparent humidity sensor based on MoO3 nanosheets is developed with a low-cost and easily manufactured process. The designed humidity sensor exhibits ultrahigh sensitivity, fast response, great stability, and high selectivity, exceeding the state-of-the-art humidity sensors. Furthermore, a wearable moisture analysis system is assembled for real-time monitoring of ambient humidity and human breathing states. Benefiting from the sensitive and rapid response to fingertip humidity, the sensors are successfully applied to both a smart noncontact multistage switch and a novel flexible transparent noncontact screen for smart mobile devices, demonstrating the potential of the MoO3 nanosheets-based humidity sensors in future HMI systems.

A fully inkjet-printed transparent humidity sensor based on a Ti3C2/Ag hybrid for touchless sensing of finger motion.

Inkjet-printing was used to prepare a flexible and transparent humidity sensor with a Ti3C2/Ag hybrid as the humidity-sensitive film and poly(diallyldimethylammonium chloride) (PDDA) as the adhesive layer. The sensor demonstrates an ultrahigh sensitivity (106 800%), a rapid response (80 ms), and excellent bending resistance. We demonstrate that an array of sensors can track moving fingers in a non-contact way and map the distance of the fingers away from the sensor surface. Therefore, our humidity sensors have great potential for novel human-machine interfacing such as touchless control of electronics and collision control between robots and humans in a cobot setting.

Highly transparent humidity sensor with thin cellulose acetate butyrate and hydrophobic AF1600X vapor permeating layers fabricated by screen printing

A highly transparent humidity sensor has been developed on ITO patterned glass substrate. The fabrication included coating the fluoropolymer insulating layer and cellulose acetate butyrate sensing layer on the ITO interdigitated electrode arrays by screen printing process. The transmittance in the visible band and the capacitive response to the relative humidity ranging from 0% to 90% of the printed sensors were tested experimentally. The fabricated sensors showed a high transparency of 90.74%–92.29% and a linearity of 0.96 of capacitance response to relative humidity in the range of 0%–90%. Fabricated by the low-cost screen printing process which is widely used in display industry, the transparent sensors showed a great application prospect to be integrated on displays in smart devices.

Highly efficient direct friction method for flexible transparent electronic devices based on graphite films featured with economical, environmental and energy saving advantages

In this research, we developed a simple direct friction method to fabricate transparent conductive films. It is a highly efficient technology as the progress consumes only several minutes from raw materials to final products and the adhesion between matrix and functional layer is strong. Flexible transparent conductive films (TCFs) based on graphite and polyethylene terephthalate substrates exhibited satisfied properties with surface resistivity from 5.07 to 83.6 KΩ sq−1, optical transparency from 70 to 85% at 550 nm wavelength, and surface stability strong enough to protect them from aging under destructive surface treatments and bending. The main mechanisms are based on three key factors: (1) The lubrication property of two dimensional layered graphite, (2) Suitable wear properties of target transparent polymer matrix and rubbing medium, (3) A soft interlayer between glass block and rubbing medium. Furthermore, the samples show some potential applications such as flexible transparent heaters, flexible transparent humidity sensors and flexible transparent strain sensors. This technology has great potential to meet the requirements of transparent conductive electrodes for future optoelectronics: lightweight, flexible, economic, and especially compatible with large-scale manufacture. This method and the TCFs have some economical, environmental and energy saving advantages.

Transparent humidity sensor using cross-linked polyelectrolyte membrane

This paper describes the fabrication of a porous cross-linked polyelectrolyte membrane and the characterization of its humidity sensitivity performance. Electrostatic self-assembly, combined with acid treatment, and post-deposition annealing produced the membrane. The fabrication process offers the ability to control the thickness of the membrane, as well as enabling the engineering of the humidity sensitivity properties. A transparent humidity sensor was fabricated by integrating the membrane between two parallel electrodes. In order to improve the moisture absorption and diffusion, both the polyelectrolyte layer and the electrode were made porous. The membrane was cross-linked to enhance the durability in high humid environments. Such a polyelectrolyte membrane showed high sensitivity to relative humidity variation over a range of 25%-99%. The see-through property of the structure adds extra features and benefits to the sensor.