Highly sensitive and wearable self-powered sensors based on a stretchable hydrogel comprising dynamic hydrogen bond and dual coordination bonds

Multifunctional sodium lignosulfonate/xanthan gum/sodium alginate/polyacrylamide ionic hydrogels composite as a high-performance wearable strain sensor

Recently, conductive hydrogels have shown great promise in flexible electronics and are ideal materials for the preparation of wearable strain sensors. However, developing a simple method to produce conductive hydrogels with excellent mechanical properties, self-adhesion, transparency, anti-freezing, and UV resistance remains a significant challenge. A novel sodium lignosulfonate/xanthan gum/sodium alginate/polyacrylamide/Zn2+/DMSO (SLS/XG/SA/PAM/Zn2+/DMSO) ionic conductive hydrogel was developed using a one-pot method. The resulting ionic conductive hydrogels have excellent mechanical properties (stress: 0.13 MPa, strain: 1629 %), high anti-fatigue properties, self-adhesion properties (iron: 7.37 kPa, pigskin: 4.74 kPa), anti-freezing (freezing point: -33.49 °C) and UV resistance by constructing a chemical and physical hybrid cross-linking network. In particular, the conductivity of G hydrogel reached 6.02 S/m at room temperature and 5.52 S/m at -20 °C. Thus, the hydrogel was assembled into a flexible sensor that could distinguish a variety of large and small scales human movements, such as joint bending, swallowing and speaking in real time with high stability and sensitivity. Moreover, the hydrogel could be used as electronic skin just like human skin and touch screen pen to write.

A zinc-ion battery-type self-powered strain sensing system by using a high-performance ionic hydrogel

Flexible strain sensors based on conductive hydrogels have profound implications for wearable electronics and health-monitoring systems. However, such sensors still need to integrate with energy providing devices to drive their functions. Herein, we develop a soaking-free polyacrylamide/carboxymethyl cellulose/tannic acid (PAAM/CMC/TA) hydrogel containing 2 M ZnSO4 + 0.1 M MnSO4 electrolyte for a novel zinc-ion battery-type self-powered strain sensing system. The synthesized hydrogel possesses desirable stretchability (tensile strain/stress of 622%/132 kPa), self-healing and self-adhesive properties, as well as good ionic conductivity (0.76 ± 0.04 S m-1). A mechanically durable Zn-MnO2 battery is developed using the PAAM/CMC/TA hydrogel and it can deliver a high specific capacity (223.0 mA h g-1) and maintain stable energy outputs under severe mechanical deformations. The electrochemical behavior of the battery can recover even after several self-healing cycles. Due to the excellent strain and pressure sensing properties of the PAAM/CMC/TA hydrogel, the battery combined with a fixed resistor served as a self-powered wearable sensing device, which could translate different human movements into distinguishable electrical signals without an external power supply. Our work provides guidance for the development of next-generation self-powered sensors.

Recent Development of Self-Powered Tactile Sensors Based on Ionic Hydrogels

Hydrogels are three-dimensional polymer networks with excellent flexibility. In recent years, ionic hydrogels have attracted extensive attention in the development of tactile sensors owing to their unique properties, such as ionic conductivity and mechanical properties. These features enable ionic hydrogel-based tactile sensors with exceptional performance in detecting human body movement and identifying external stimuli. Currently, there is a pressing demand for the development of self-powered tactile sensors that integrate ionic conductors and portable power sources into a single device for practical applications. In this paper, we introduce the basic properties of ionic hydrogels and highlight their application in self-powered sensors working in triboelectric, piezoionic, ionic diode, battery, and thermoelectric modes. We also summarize the current difficulty and prospect the future development of ionic hydrogel self-powered sensors.

High stretchable and self-healing nanocellulose-poly(acrylic acid) composite hydrogels for sustainable CO2 shutoff

The injection of CO₂ into oil reservoirs to enhance oil recovery (EOR) has become a widely accepted and effective technical method, which, however, remains subject to the gas channeling caused by the reservoir fractures. Herein, this work developed a novel plugging gel combining excellent mechanical properties, fatigue resistance, elastic and self-healing properties for the CO₂ shutoff purpose. This gel consisting of grafted nanocellulose and polymer network was synthesized via a free-radical polymerization, and reinforced by using Fe³⁺ to cross-link the two networks. The as-prepared PAA-TOCNF-Fe³⁺ gel has a stress of 1.03 MPa and a high strain of 1491 %, and self-heals to its original 98 % in stress and 96 % in strain after rupture, respectively. The introduce of TOCNF/Fe³⁺ improves the excellent energy dissipation and self-healing via the synergy effect of dynamical coordination bonds and hydrogen bonds. Further, the PAA-TOCNF-Fe³⁺ gel is both flexible and high-strength in plugging the multi-round CO₂ injection, during which the CO₂ breakthrough pressure is above 9.9 MPa/m, the plugging efficiency exceeds 96 %, and the self-healing rate is larger than 90 %. Given that above, this gel shows a great potential to plug the high-pressure CO₂ flow, which could offer a new method for CO₂-EOR and carbon storage.

Graphene oxide composite hydrogels for wearable devices

For graphene oxide (GO) composite hydrogels, a two-dimensional GO material is introduced into them, whose special structure is used to improve their properties. GO contains abundant oxygen-containing functional groups, which can improve the mechanical properties of hydrogels and support the application needs. Especially, the unique-conjugated structure of GO can endow or enhance the stimulation response of hydrogels. Therefore, GO composite hydrogels have a great potential in the field of wearable devices. We referred to the works published in recent years, and reviewed from these aspects: (a) structure of GO; (b) factors affecting the mechanical properties of the composite hydrogel, including hydrogen bond, ionic bond, coordination bond and physical crosslinking; (c) stimuli and signals; (d) challenges. Finally, we summarized the research progress of GO composite hydrogels in the field of wearable devices, and put forward some prospects.

Stretchable Thermoelectric-Based Self-Powered Dual-Parameter Sensors with Decoupled Temperature and Strain Sensing

Thermoelectric-based sensors with multifunctional sensing properties that can recognize different stimulations in a self-powered environment by converting low-grade heat into electrical energy have attracted increasing attention. However, the current thermoelectric-based multifunctional sensors are faced with issues such as limited preparation methods, complex structural designs, and hard decoupling, which greatly hinder their further development in the field of wearable electronics. Herein, we have fabricated novel free-standing self-powered temperature-strain sensors based on poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS)/carbon nanotube (CNT)/waterborne polyurethane (WPU) composite films through a simple drop-casting method. The composite films can maintain stable thermoelectric performance after washing 1000 times and withstand repeated bending and stretching. More importantly, based on the Seebeck effect arising from PEDOT:PSS/CNT composites, the assembled sensor successfully detects temperature changes and strain deformations under a self-powered condition. The decoupling of strain stimulation and temperature stimulation is mainly attributed to the good conductive network inside the composite film and the conductive bridge formed by PEDOT:PSS particles between CNTs when the composite film is stretched. Thus, the designed self-powered sensor with dual-parameter sensing prepared by a simple strategy has shown great potential in wearable electronics.