The remarkable progress in efforts to prepare conductive self-healing hydrogels mimicking human skin’s functions has been witnessed in recent years. However, it remains a great challenge to develop an integrated conductive gel combining excellent self-healing and mechanical properties, which is derived from their inherent compromise between the dynamic cross-links for healing and steady cross-links for mechanical strength. In this work, we design a tough, self-healing, and self-adhesive ionic gel by constructing synergistic multiple coordination bonds among tannic acid-coated cellulose nanocrystals (TA@CNCs), poly(acrylic acid) chains, and metal ions in a covalent polymer network. The incorporated TA@CNC acts as a dynamic connected bridge in the hierarchically porous network mediated by multiple coordination bonds, endowing the ionic gels the superior mechanical performance. Reversible nature of dynamic coordination interactions leads to excellent recovery property as well as reliable mechanical and elect...

Mussel-Inspired Cellulose Nanocomposite Tough Hydrogels with Synergistic Self-Healing, Adhesive, and Strain-Sensitive Properties

Recent advances in soft materials and system integration technologies have provided a unique opportunity to design various types of wearable flexible hybrid electronics (WFHE) for advanced human healthcare and human-machine interfaces. The hybrid integration of soft and biocompatible materials with miniaturized wireless wearable systems is undoubtedly an attractive prospect in the sense that the successful device performance requires high degrees of mechanical flexibility, sensing capability, and user-friendly simplicity. Here, the most up-to-date materials, sensors, and system-packaging technologies to develop advanced WFHE are provided. Details of mechanical, electrical, physicochemical, and biocompatible properties are discussed with integrated sensor applications in healthcare, energy, and environment. In addition, limitations of the current materials are discussed, as well as key challenges and the future direction of WFHE. Collectively, an all-inclusive review of the newly developed WFHE along with a summary of imperative requirements of material properties, sensor capabilities, electronics performance, and skin integrations is provided.

Advanced Soft Materials, Sensor Integrations, and Applications of Wearable Flexible Hybrid Electronics in Healthcare, Energy, and Environment.

Energy and water are of fundamental importance for our modern society, and advanced technologies on sustainable energy storage and conversion as well as water resource management are in the focus of intensive research worldwide. Beyond their traditional biological applications, hydrogels are emerging as an appealing materials platform for energy- and water-related applications owing to their attractive and tailorable physiochemical properties. In this review, we highlight the highly tunable synthesis of various hydrogels, involving key synthetic elements such as monomer/polymer building blocks, cross-linkers, and functional additives, and discuss how hydrogels can be employed as precursors and templates for architecting three-dimensional frameworks of electrochemically active materials. We then present an in-depth discussion of the structure-property relationships of hydrogel materials based on fundamental gelation chemistry, ultimately targeting properties such as enhanced ionic/electronic conductivities, mechanical strength, flexibility, stimuli-responsiveness, and desirable swelling behavior. The unique interconnected porous structures of hydrogels enable fast charge/mass transport while offering large surface areas, and the polymer-water interactions can be regulated to achieve desirable water retention, absorption, and evaporation within hydrogels. Such structure-derived properties are also intimately coordinated to realize multifunctionality and stability for different target devices. The plethora of stimulating examples is expounded with a focus on batteries, supercapacitors, electrocatalysts, solar water purification, and atmospheric water harvesting, which showcase the unprecedented technological potential enabled by hydrogels and hydrogel-derived materials. Finally, we study the challenges and potential ways of tackling them to reveal the underlying mechanisms and transform the current development of hydrogel materials into sustainable energy and water technologies.

Hydrogels and Hydrogel-Derived Materials for Energy and Water Sustainability.

Biocompatible conductive hydrogels with intrinsic flexibility, high sensitivity, linearity and outstanding reliability are highly demanded for wearable devices or implantable sensors. Here we repor...

Ultrastretchable Strain Sensors and Arrays with High Sensitivity and Linearity Based on Super Tough Conductive Hydrogels

The self-healing of zinc-ion batteries (ZIBs) will not only significantly improve the durability and extend the lifetime of devices, but also decrease electronic waste and economic cost. A poly(vinyl alcohol)/zinc trifluoromethanesulfonate (PVA/Zn(CF3 SO3 )2 ) hydrogel electrolyte was fabricated by a facile freeze/thaw strategy. PVA/Zn(CF3 SO3 )2 hydrogels possess excellent ionic conductivity and stable electrochemical performance. Such hydrogel electrolytes can autonomously self-heal by hydrogen bonding without any external stimulus. All-in-one integrated ZIBs can be assembled by incorporating the cathode, separator, and anode into hydrogel matrix since the fabrication of PVA/Zn(CF3 SO3 )2 hydrogel is a process of converting the liquid to quasi-solid state. The ZIBs show an outstanding self-healing and can recover electrochemical performance completely even after several cutting/healing cycles.

A Self-Healing Integrated All-in-One Zinc-Ion Battery.

Self-healing gels, which are able to restore their original shape/condition after damage, have recently attracted considerable interest. The present review provides an update on the current status of self-healing gels for use in soft self-healing devices, with the main focus on wearable devices. Following a brief introduction of conventional self-healing gels, this review summarizes the current strategies for synthesizing self-healing gels, comparing the properties of polymeric and small molecular self-healing gels. Some of the strengths and weaknesses of these two types of self-healing gels are clearly highlighted. Several typical instances to outline the emerging applications of self-healing gels with partially or fully self-healing devices/materials, including wearable sensors, supercapacitors, lithium batteries, and coatings, are provided. Finally, the current challenges and future perspectives regarding the further development of self-healing gels are also discussed.

Self-Healable Gels for Use in Wearable Devices

Herein, carboxymethyl cellulose sodium (CMCNa)/graphene oxide (GO) hydrogel microparticles (CGs) with diameters of 2.2–3.6 μm were prepared facilely via spray drying. The as-prepared CGs were used as adsorbents for the removal of methylene blue (MB), Eosin Y and various heavy metal ions. The results revealed that the adsorption capacities of the CG with 8 wt% GO increased by 340–354% for the dyes and 350–500% for heavy metal ions, in comparison with the pristine CMCNa reference. Moreover, the microparticle structure also endowed the CGs with high adsorption kinetics as the adsorption equilibrium for all pollutants was commonly reached within 30 min. The adsorption mechanisms for dyes were due to both electrostatic and π–π interactions, while those for heavy metals were the synergistic effect of electrostatic interactions, surface complexation and ion exchange. Further investigation indicated that the adsorption isotherms and kinetics can be well fitted by the Langmuir isotherm model and the pseudo-second-order model, respectively. By adjusting the GO content and solution pH, the adsorption capacities of our CGs can be further improved. The unique adsorption properties of our CGs, together with the spray drying method, make them very promising in the mass production of broad-spectrum adsorbents for water purification.

Facile fabrication of carboxymethyl cellulose sodium/graphene oxide hydrogel microparticles for water purification

Herein, a metal-free 1,3-dipolar cycloaddition click reaction was used to prepare hydrogels. First, copolymer of N,N-dimethylacrylamide and 2-hydroxyethyl methacrylate was synthesized via free radical copolymerization with an initiator of 2,2’-azobisisobutyronitrile and a solution of 1,4-dioxane. Second, maleimide-functionalized polymer was obtained by a coupling reaction of the above synthesized copolymer and N-maleoyl alanine with a dehydrating agent of N,N’-dicyclohexylcarbodiimide. Similarly, azido-modified polymer was prepared by the reaction between azidoacetic acid and the as-prepared copolymer. Third, the hydrogels were obtained by the reaction of the azidomodified polymer and maleimide-functionalized copolymer without any initiator or catalyst. Gelation times of different conditions were measured by a cuvette inversion method. The samples of different steps were analysized by means of FTIR, NMR, SEM, etc. The swelling behaviour of the gels was investigated via a general gravimetric method. Finally, release mechanism was explored with a model drug molecule of 5-fluorouracil. It was found temperature, concentration and solvent had an impact on the gelation time. And the swelling behavior and drug-controlled release profiles of the hydrogels were relevant to the concentration of polymers used for the hydrogels.

Click synthesis of hydrogels by metal-free 1,3-dipolar cycloaddition reaction between maleimide and azide functionalized polymers

The invention belongs to the field of water-retention sustained/controlled-release fertilizers and particularly discloses a preparation method for an isobutylidene diurea water-retention sustained/controlled-release fertilizer. According to the preparation method, isobutylidene diurea with extensive application is produced through a reaction between isobutylaldehyde and urea, so that the utilization ratio of a nitrogen fertilizer is increased, and material wasting and environmental pollution are reduced. During the synthesis of isobutylidene diurea, hydrophilic monomers, a cross-linking agent and an initiator are simultaneously added, a hydrophilic polymer network is formed through heating-up polymerization, and the isobutylidene diurea water-retention sustained/controlled-release fertilizer is prepared by a one-pot method. The water-retention sustained/controlled-release fertilizer not only has good sustained-release performance, but also has certain water retention property, and the preparation method is simple and is wide in application prospect.

Gang Wang

Papers

Self-Healable Gels for Use in Wearable Devices

Facile fabrication of carboxymethyl cellulose sodium/graphene oxide hydrogel microparticles for water purification

Click synthesis of hydrogels by metal-free 1,3-dipolar cycloaddition reaction between maleimide and azide functionalized polymers

Preparation method for isobutylidene diurea water-retention sustained-release fertilizer