Hydrogel fibers for wearable sensors and soft actuators

Repair of hydrogel wearable devices through topological adhesion and cross-shaped sectional enhancement strategy

Hydrogels, recognized for their biocompatibility, are extensively employed in the realm of wearable devices. Nevertheless, their application is often constrained by their low mechanical robustness, rendering them susceptible to damage during operation. The restoration of their load-bearing and sensory functionalities post-damage represents a captivating yet underexplored domain. Conventional repair techniques, reliant on hydrogen bonding or van der Waals forces, falter in the face of hydrogels' high water content. In this study, a novel composite adhesive gel (SGG), integrating sodium alginate, guar gum, and graphene oxide, was engineered to mend impaired hydrogels. Furthermore, an optimized repair approach, utilizing a cross-shaped sectional (CSS) enhancement strategy, was devised to reinstate the hydrogels' load and sensory capabilities. Investigations revealed that the SGG adhesive infiltrated the hydrogel, establishing an intermediary gel stratum, subsequently solidifying to mend the material through topological adhesion. This process reestablished the continuity of the polymer network and the aqueous phase within the hydrogel. Following the application of the CSS augmentation method, the peak tensile strain of the remediated hydrogel exceeded 200 %, with the uppermost observable adhesive energy touching 2526 J/m2. In addition, the ability to respond to strain was significantly rejuvenated, suggesting an effective methodology for the rehabilitation of wearable technology.

Microscopic and Macroscopic Characterization of Hydrogels Based on Poly(vinyl-alcohol)-Glutaraldehyde Mixtures for Fricke Gel Dosimetry

In recent decades, hydrogels have emerged as innovative soft materials with widespread applications in the medical and biomedical fields, including drug delivery, tissue engineering, and gel dosimetry. In this work, a comprehensive study of the macroscopic and microscopic properties of hydrogel matrices based on Poly(vinyl-alcohol) (PVA) chemically crosslinked with Glutaraldehyde (GTA) was reported. Five different kinds of PVAs differing in molecular weight and degree of hydrolysis were considered. The local microscopic organization of the hydrogels was studied through the use of the 1H nuclear magnetic resonance relaxometry technique. Various macroscopic properties (gel fraction, water loss, contact angle, swelling degree, viscosity, and Young's Modulus) were investigated with the aim of finding a correlation between them and the features of the hydrogel matrix. Additionally, an optical characterization was performed on all the hydrogels loaded with Fricke solution to assess their dosimetric behavior. The results obtained indicate that the degree of PVA hydrolysis is a crucial parameter influencing the structure of the hydrogel matrix. This factor should be considered for ensuring stability over time, a vital property in the context of potential biomedical applications where hydrogels act as radiological tissue-equivalent materials.

Soft Fiber/Textile Actuators: From Design Strategies to Diverse Applications

Fiber/textile-based actuators have garnered considerable attention due to their distinctive attributes, encompassing higher degrees of freedom, intriguing deformations, and enhanced adaptability to complex structures. Recent studies highlight the development of advanced fibers and textiles, expanding the application scope of fiber/textile-based actuators across diverse emerging fields. Unlike sheet-like soft actuators, fibers/textiles with intricate structures exhibit versatile movements, such as contraction, coiling, bending, and folding, achieved through adjustable strain and stroke. In this review article, we provide a timely and comprehensive overview of fiber/textile actuators, including structures, fabrication methods, actuation principles, and applications. After discussing the hierarchical structure and deformation of the fiber/textile actuator, we discuss various spinning strategies, detailing the merits and drawbacks of each. Next, we present the actuation principles of fiber/fabric actuators, along with common external stimuli. In addition, we provide a summary of the emerging applications of fiber/textile actuators. Concluding with an assessment of existing challenges and future opportunities, this review aims to provide a valuable perspective on the enticing realm of fiber/textile-based actuators.

UV-Crosslinked Poly(N-isopropylacrylamide) Interpenetrated into Chitosan Structure with Enhancement of Mechanical Properties Implemented as Anti-Fouling Materials

High-performance properties of interpenetration polymer network (IPN) hydrogels, based on physically crosslinked chitosan (CS) and chemically crosslinked poly(N-isopropylacrylamide) (PNiPAM), were successfully developed. The IPN of CS/PNiPAM is proposed to overcome the limited mechanical properties of the single CS network. In this study, the viscoelastic behaviors of prepared materials in both solution and gel states were extensively examined, considering the UV exposure time and crosslinker concentration as key factors. The effect of these factors on gel formation, hydrogel structures, thermal stabilities of networks, and HeLa cell adhesion were studied sequentially. The sol-gel transition was effectively demonstrated through the scaling law, which agrees well with Winter and Chambon's theory. By subjecting the CS hydrogel to the process operation in an ethanol solution, its properties can be significantly enhanced with increased crosslinker concentration, including the shear modulus, crosslinking degree, gel strength, and thermal stability in its swollen state. The IPN samples exhibit a smooth and dense surface with irregular pores, allowing for much water absorption. The HeLa cells were adhered to and killed using the CS surface cationic charges and then released through hydrolysis by utilizing the hydrophilic/hydrophobic switchable property or thermo-reversible gelation of the PNiPAM network. The results demonstrated that IPN is a highly attractive candidate for anti-fouling materials.

Continuous Melt Spinning of Adaptable Covalently Cross-Linked Self-Healing Ionogel Fibers for Multi-Functional Ionotronics

Stretchable conductive fibers play key roles in electronic textiles, which have substantial improvements in terms of flexibility, breathability, and comfort. Compared to most existing electron-conductive fibers, ion-conductive fibers are usually soft, stretchable, and transparent, leading to increasing attention. However, the integration of desirable functions including high transparency, stretchability, conductivity, solvent resistance, self-healing ability, processability, and recyclability remains a challenge to be addressed. Herein, a new molecular strategy based on dynamic covalent cross-linking networks is developed to enable continuous melt spinning of the ionogel fiber with the aforementioned properties. As a proof of concept, adaptable covalently cross-linked ionogel fibers based on dimethylglyoximeurethane (DOU) groups (DOU-IG fiber) are prepared. The resultant DOU-IG fiber exhibited high transparency (>93%), tensile strength (0.76 MPa), stretchability (784%), and solvent resistance. Owing to the dynamic of DOU groups, the DOU-IG fiber shows high healing performance using near-infrared light. Taking advantage of DOU-IG fibers, multifunctional ionotronics with the integration of several desirable functionalities including sensor, triboelectric nanogenerator, and electroluminescent display are fabricated and used for motion monitoring, energy harvesting, and human-machine interaction. It is believed that these DOU-IG fibers are promising for fabricating the next generation of electronic textiles and other wearable electronics.

Pressure-Responsive Polymer Chemosensors for Hydrostatic-Pressure-Signal Detection: Poly-l-Lysine-Pyrene Conjugates

Stimulus-responsive polymer materials are an attractive alternative to conventional supramolecular and polymer assemblies for applications in sensing, imaging, and drug-delivery systems. Herein, we synthesized a series of pyrene-labeled α- and ε-poly-l-lysine conjugates with varying degrees of substitution (DSs). Hydrostatic-pressure-UV/vis, fluorescence, and excitation spectroscopies and fluorescence lifetime measurements revealed ground-state conformers and excited-state ensembles emitting fluorescence with variable intensities. The polylysine-based chemosensors demonstrated diverse ratiometric responses to hydrostatic pressure through adjustments in polar solvents, DSs, and polymer backbones. Additionally, the fluorescence chemosensor exhibited a promising glum value of 3.2 × 10-3, indicating potential applications in chiral fluorescent materials. This study offers valuable insights into the development of smart hydrostatic-pressure-responsive polymer materials.