Anisotropic Muscle-like Conductive Composite Hydrogel Reinforced by Lignin and Cellulose Nanofibrils

Abstract

Hydrogels with excellent mechanical properties and high conductivity are key materials for the development of flexible electronic devices, smart soft robots, and so forth. However, the preparation of high-performance conductive hydrogels remains a challenge. Enlightened by the strengthening mechanism of skeletal muscles, a green muscle-like conductive hydrogel was prepared through a repeated mechanical training process. Using cellulose nanofibrils (CNFs) as the fiber reinforcing source, partial depolymerized enzyme hydrolyzed lignin as the interfacial binding agent, and Ag+ as the conducting medium in a polyvinyl alcohol (PVA) matrix, the prepared composite hydrogel exhibited anisotropic high strength, high toughness, and excellent conductivity. Through the introduction of double physical enhancement networks and the adoption of a mechanical training method that mimics the muscle strengthening principle, the enhancement effect of CNFs was maximally demonstrated in the PVA composite hydrogel. Meanwhile, this study also provides a new and effective reference for the preparation of high-performance green hydrogels.

Facile Solvent Regulation for Highly Strong and Tough Physical Eutectogels with Remarkable Strain Sensitivity

“…In addition, the conductivity of the SRT 5:5 -PVA 15 eutectogel perpendicular to the training direction was similar to the original value (1.15 mS cm –1 ), while its conductivity along the training direction increased to 2.63 mS cm –1 (Figure e). This result was attributed to the oriented alignment of the polymer network after training, facilitating the transport of ions along the training direction . In conclusion, our SR-PVA eutectogel can be imparted with a directional muscle-like stretching performance and high conductivity through convenient mechanical training operations, which provided the possibility for its application in soft robotics and artificial muscles.…”

“…This result was attributed to the oriented alignment of the polymer network after training, facilitating the transport of ions along the training direction. 45 In conclusion, our SR-PVA eutectogel can be imparted with a directional muscle-like stretching performance and high conductivity through convenient mechanical training operations, which provided the possibility for its application in soft robotics and artificial muscles.…”

“…The oriented structure of the SRT 5:5 -PVA 15 eutectogel was the result of repeated directional stretching, which significantly affected its performance. 45 As shown in Figure 6d, the stress−strain curve of the SRT 5:5 -PVA 15 eutectogel along the training direction exhibited a musclelike "J" shape. 46,47 Its initial Young's modulus decreased from 2.10 to 0.96 MPa before training, and the tensile strength increased from 1.67 to 3.00 MPa before training (Figure S18).…”

Combined Catalysis: A Powerful Strategy for Engineering Multifunctional Sustainable Lignin-Based Materials

“…The strategy of using lignin (Kraft lignin, lignosulfonate, or hydrolyzed lignin) AgNPs to trigger a quinone–catechol redox reaction followed by cross-linking to provide multifunctional hydrogels has been further expanded by several research groups. − Lu and co-workers combined the Ag-catalyzed strategy of an oxidative decarboxylation of citric acid and poly­(acrylamide- co -acrylic acid), a free radical polymerization of acrylic acid, and a quinone–catechol redox reaction to engineer a versatile hydrogel . Besides the multifunctionality of the hydrogel, it was also injectable through a needle and fabricated by electrospinning to create micro/nanofibers.…”

Novel Lignin Hydrogel Sensors with Antiswelling, Antifreezing, and Anticreep Properties

“…Flexible wearable electronic devices have attracted a significant amount of interest in the fields of human motion detection, health monitoring, electronic skins, flexible sensors, and soft robotics because of their large mechanical flexibility and their ability to adapt to different working environments to a certain extent and meet different deformation requirements. − Among a wide variety of sensing materials, hydrogels show great application potential due to their functionality, formability, and chemical editability. − Although the application value of hydrogel sensors in detecting body movement and vital signs has been proven, their practical application is still significantly limited, which is mainly caused by the inherent properties of hydrogels. − The first inherent property that hinders the practical application of hydrogels as flexible wearable electronic devices is swelling behavior. Hydrogel is a kind of soft material composed of a three-dimensional polymer framework with high water content. − For obtaining good biocompatibility, this three-dimensional polymer framework is generally designed as hydrophilic polymer segments, which will lead to the swelling behavior of the hydrogel in the aqueous environment. − Swelling behavior is usually accompanied by the introduction of a large number of solvent substances and the dilution or loss of functional molecules. − Meanwhile, the decrease of hydrogel density caused by swelling behavior also has a significant negative impact on the mechanical properties of hydrogels. − The second inherent property is the low-temperature phase transition behavior of hydrogels. A large number of water molecules contained in hydrogels will phase-change at low temperatures to form ice without flexibility, which is negative for applications in low-temperature environments.…”