Strategies in the preparation of conductive polyvinyl alcohol hydrogels for applications in flexible strain sensors, flexible supercapacitors, and triboelectric nanogenerator sensors: an overview

The increasing demand for cost-efficient and user-friendly wearable electronic devices has led to the development of stretchable electronics that are both cost-effective and capable of maintaining sustained adhesion and electrical performance under duress. In this study, we report on a novel physically crosslinked Poly(vinyl alcohol) (PVA)-based hydrogel that serves as a transparent, strain-sensing skin adhesive for motion monitoring. By incorporating Zn2+ into the ice-templated PVA gel, we observed a densified amorphous structure through optical and scanning electron microscopy, and found that the material can stretch up to 800% strain according to tensile tests. Fabrication in a binary glycerol:water solvent resulted in electrical resistance in the kiloohm range, a gauge factor of 0.84, and ionic conductivity on the scale of 10-4 S cm-1 , making it a potentially low-cost candidate for a stretchable electronic material. This study characterizes the relationship between improved electrical performance and polymer-polymer interactions through spectroscopic techniques, which play a role in the transport of ionic species through the material. This article is protected by copyright. All rights reserved.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/adhm.202300076

A Transparent Poly(Vinyl Alcohol) Ion-Conducting Organohydrogel for Skin-Based Strain-Sensing Applications.

In recent years, wearable electronic skin has garnered significant attention due to its broad range of applications in various fields, including personal health monitoring, human motion perception, human-computer interaction, and flexible display. The flexible multimodal sensor, as the core component of electronic skin, can mimic the multistimulus sensing ability of human skin, which is highly significant for the development of the next generation of electronic devices. This paper provides a summary of the latest advancements in multimodal sensors that possess two or more response capabilities (such as force, temperature, humidity, etc.) simultaneously. It explores the relationship between materials and multiple sensing capabilities, focusing on both active materials that are the same and different. The paper also discusses the preparation methods, device structures, and sensing properties of these sensors. Furthermore, it introduces the applications of multimodal sensors in human motion and health monitoring, as well as intelligent robots. Finally, the current limitations and future challenges of multimodal sensors will be presented.

Research Progress and Application of Multimodal Flexible Sensors for Electronic Skin.

Conductive hydrogels have garnered significant interest in the realm of wearable flexible sensors due to their close resemblance to human tissue, wearability, and precise signal acquisition capabilities. However, the concurrent attainment of an epidermal hydrogel sensor incorporating reliable self-healing capabilities, biodegradability, robust adhesiveness, and the ability to precisely capture subtle electrophysiological signals poses a daunting and intricate challenge. Herein, an innovative MXene-based composite hydrogel (PBM hydrogel) with exceptional self-healing, self-adhesive, and versatile functionality is engineered through the integration of conductive MXene nanosheets into a well-structured poly(vinyl alcohol) (PVA) and bacterial cellulose (BC) hydrogel three-dimensional (3D) network, utilizing multiple dynamic cross-linking synergistic repeated freeze-thaw strategy. The hydrogel harnesses the presence of dynamically reversible borax ester bonds and multiple hydrogen bonds between its constituents, endowing it with rapid self-healing efficiency (97.8%) and formidable self-adhesive capability. The assembled PBM hydrogel epidermal sensor possesses a rapid response time (10 ms) and exhibits versatility in detecting diverse external stimuli and human movements such as vocalization, handwriting, joint motion, Morse code signals, and even monitoring infusion status. Additionally, the PBM hydrogel sensor offers the added advantage of swift degradation in phosphate-buffered saline solution (within a span of 56 days) and H2O2 solution (in just 53 min), maintaining an eco-friendly profile devoid of any environmental pollution. This work lays the groundwork for possible uses in electronic skins, interactions between humans and machines, and the monitoring of individualized healthcare. 

Self-Healable, Self-Adhesive and Degradable MXene-Based Multifunctional Hydrogel for Flexible Epidermal Sensors.

Semi-IPN ionogel based on poly (ionic liquids)/xanthan gum for highly sensitive pressure sensor.

Recently, hydrogels have attracted considerable interests due to their intrinsic flexibility and adjustable mechanical properties. However, the integration of high conductivity, enhanced mechanical performance, and plasticity into one single hydrogel is still challenging. In this work, TEMPO-oxidized cellulose nanofibers (TOCNF, diameter 20–50 nm, length $$\ge$$
 1 um) are used as the efficient dispersant of multiwalled carbon nanotubes (MWCNTs, out diameter 5–15 nm, length 10–30 um). Conductive hydrogels are prepared through a simple one pot process and freeze–thaw cyclic method in combination with Fe3+ ions and TOCNF-MWCNT dispersions in polyvinyl alcohol (PVA) hydrogels. The resulting hydrogels demonstrate high water content (~ 88.8%), mechanical properties (ultimate stress of 1.1 MPa, strain of 336%), and conductivity (0.57 S/m), which can be applied to the preparation of stress or strain sensors. The hydrogel-based resistance sensor is applied to human motion detection, showing a sensitivity of 1.876 kPa−1 with pressure lower than 16 kPa. Similarly, the hydrogel-based capacitive sensor can detect imperceptible pressure of 20 Pa. In addition, the hydrogels also reveal excellent plasticity. In conclusion, the conductive hydrogel possesses excellent comprehensive properties, providing inspiration and potential for the preparation and application of wearable strain sensors.

Multifunctional flexible polyvinyl alcohol nanocomposite hydrogel for stress and strain sensor

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Multifunctional flexible polyvinyl alcohol nanocomposite hydrogel for stress and strain sensor