Humidity Sensing of Stretchable and Transparent Hydrogel Films for Wireless Respiration Monitoring

Abstract

Respiratory monitoring plays a pivotal role in health assessment and provides an important application prospect for flexible humidity sensors. However, traditional humidity sensors suffer from a trade-off between deformability, sensitivity, and transparency, and thus the development of high-performance, stretchable, and low-cost humidity sensors is urgently needed as wearable electronics. Here, ultrasensitive, highly deformable, and transparent humidity sensors are fabricated based on cost-effective polyacrylamide-based double network hydrogels. Concomitantly, a general method for preparing hydrogel films with controllable thickness is proposed to boost the sensitivity of hydrogel-based sensors due to the extensively increased specific surface area, which can be applied to different polymer networks and facilitate the development of flexible integrated electronics. In addition, sustainable tapioca rich in hydrophilic polar groups is introduced for the first time as a second cross-linked network, exhibiting excellent water adsorption capacity. Through the synergistic optimization of structure and composition, the obtained hydrogel film exhibits an ultrahigh sensitivity of 13,462.1%/%RH, which is unprecedented. Moreover, the hydrogel film-based sensor exhibits excellent repeatability and the ability to work normally under stretching with even enhanced sensitivity. As a proof of concept, we integrate the stretchable sensor with a specially designed wireless circuit and mask to fabricate a wireless respiratory interruption detection system with Bluetooth transmission, enabling real-time monitoring of human health status. This work provides a general strategy to construct high-performance, stretchable, and miniaturized hydrogel-based sensors as next-generation wearable devices for real-time monitoring of various physiological signals.

Dynamic Cross-Linking Network Construction of Carboxymethyl Starch Enabling Temperature and Strain Bimodal Film Sensors

“…A flexible strain sensor is one of the most promising applications of ionic conductive hydrogels. , As the inherent ionic strength or concentration is highly dependent on the environmental alterations, ionic hydrogels can also be used for thermal and/or humidity sensing . For these applications, bulk hydrogel materials, however, are not always well suited because the low surface area-to-volume ratio limits the diffusion of external stimuli inside the hydrogel bulk, lowering response or recovery levels , as a result. Reducing thickness of the bulk is an effective way of improving the responsive sensitivity , and is even able to broaden applications of ionic conductive hydrogels, for instance, in optical fields because of increased transparency .…”

“…However, the hydrogel-based multilayer film as a flexible sensor, to date, has not yet been extensively studied. Wu’s group carried out some pioneering and very interesting studies. ,,− They used polyacrylamide (PAM) and polysaccharide chains to build double-network (DN) structures in hydrogels and a fabricated film sensor by the way of spin-coating using an elastomer or metal as the substrate. As-prepared film sensors were able to detect alterations of temperature or humidity, with good sensitivity, low limit of detection, and fast response, as well as good stability, and therefore revealed promising applications as wearable and multi-functional electronics.…”

“…Reducing thickness of the bulk is an effective way of improving the responsive sensitivity , and is even able to broaden applications of ionic conductive hydrogels, for instance, in optical fields because of increased transparency . However, reducing the thickness is commonly against the stability of the bulk structure, especially as hydrogels are used as strain sensors undergoing repeated stretching, and also unconducive to the water retention because of the increase of surface area-to-volume ratio . In this case, thin-film technology (coating hydrogels on the solid surfaces) has attracted increasing attention …”

High‐Performance Strain Sensors Based on Organohydrogel Microsphere Film for Wearable Human–Computer Interfacing

“…Nonetheless, the cost of these devices increases accordingly, and the hydrogel has poor anti-freeze and anti-drying properties and will swell in water, leading to its failure in various harsh environments such as polar, desert, or underwater. [39][40][41][42][43] Thus, further breakthroughs in the sensitivity of pure hydrogel-based strain sensors are urgently needed for more cost-effective and accurate motion monitoring.…”

Humidity Sensors Based on Metal–Organic Frameworks

“…Normally, humidity−sensing materials are hydrophilic, porous, and stable [ 15 ]. Until now, humidity sensors based on different types of sensing materials such as metal oxides [ 12 ], polymers [ 16 , 17 , 18 , 19 , 20 ], and carbon materials [ 21 , 22 ] have been reported. Metal−organic frameworks (MOFs), types of compounds famous for their characteristics of porosity, adjustable structure, and good chemical stability [ 23 , 24 , 25 ], have been widely applied in the fields of gas separation and storage [ 26 , 27 ], catalysis [ 28 , 29 ], ion batteries [ 30 , 31 ], etc.…”