3D printable and mechanically tunable hydrogels achieved through hydrophobic and ionic interactions.

Thermal stiffening materials are commonly applied in the aerospace and automotive industries, among others, since their dimensional stabilities and stiffness characteristics improve at high temperatures. In this study, temperature-triggered modulus-tunable hydrogels were prepared by combining Pluronic F-127 with charged polymers. Pluronic F-127, a triblock copolymer micelle, provided three-dimensional printing capabilities of fine resolution with high viscosity, while hydrophobic and ionic interactions among polymer networks provided thermal stiffening. The hydrogel ink's printability was demonstrated by successfully creating complex 3D structures. A calcium ion carrying a hydrophobic propionate and carboxylate group in polymer chains was used to form additional physical crosslinking at high temperature, ultimately leading to the thermal stiffening effect without volume change. The thermal stiffening behavior was found to be fully reversible and repeatable. Finally, to demonstrate the versatility of this work, graphene oxide was added to produce a light-controllable modulus based on its photothermal properties.

Seamless Integration of Conducting Hydrogels in Daily Life: From Preparation to Wearable Application.

Conductive hydrogels (CHs) have received significant attention for use in wearable devices because they retain their softness and flexibility while maintaining high conductivity. CHs are well suited for applications in skin-contact electronics and biomedical devices owing to their high biocompatibility and conformality. Although highly conductive hydrogels for smart wearable devices are extensively researched, a detailed summary of the outstanding results of CHs is required for a comprehensive understanding. In this review, the recent progress in the preparation and fabrication of CHs is summarized for smart wearable devices. Improvements in the mechanical, electrical, and functional properties of high-performance wearable devices are also discussed. Furthermore, recent examples of innovative and highly functional devices based on CHs that can be seamlessly integrated into daily lives are reviewed.

High-Resolution 3D Printing of Mechanically Tough Hydrogels Prepared by Thermo-Responsive Poloxamer Ink Platform.

High-resolution 3D-printable hydrogels with high mechanical strength and biocompatibility are in great demand because of their potential applications in numerous fields. In this study, a material system comprising Pluronic F-127 dimethacrylate (FDMA) is developed to function as a direct ink writing (DIW) hydrogel for 3D printing. FDMA is a triblock copolymer that transforms into micelles at elevated temperatures. The transformation increases the viscosity of FDMA and preserves its structure during DIW 3D printing, whereupon the printed structure is solidified through photopolymerization. Because of this viscosity shift, various functionalities can be incorporated through the addition of other materials in the solution state. Acrylic acid is incorporated into the pregel solution to enhance the mechanical strength, because the carboxylate group of poly(acrylic acid) ionically crosslinks with Fe3+ , increasing the toughness of the DIW hydrogel 37 times to 2.46 MJ m-3 . Tough conductive hydrogels are also 3D printed by homogenizing poly(3,4-ethylenedioxythiophene) polystyrene sulfonate into the pregel solution. Furthermore, the FDMA platform developed herein uses DIW, which facilitates multicartridges 3D printing, and because all the materials included are biocompatible, the platform may be used to fabricate complex structures for biological applications.

Recent Advances in Design Strategies for Tough and Stretchable Hydrogels.

The development of multifunctional hydrogels with excellent stretchability and toughness is one of the most fascinating subjects in soft matter research. Numerous research efforts have focused on the design of new hydrogel systems with superior mechanical properties because of their potential applications in diverse fields. In this Minireview, we consider the most up-to-date mechanically strong hydrogels and summarize their design strategies based on the formation of double networks and dual physical crosslinking. Based on the synthetic approaches and different toughening mechanisms, double-network hydrogels can be further classified into three different categories, namely chemically crosslinked, hybrid physically-chemically crosslinked, and fully physically crosslinked. In addition to the above-mentioned methods, we also discuss few uniquely designed hydrogels with the intention of guiding the future development of these fascinating materials for superior mechanical performance.

Integration of Macro-Cross-Linker and Metal Coordination: A Super Stretchable Hydrogel with High Toughness.

The development of multifunctional hydrogels with high strength and stretchability is one of the most important topics in soft-matter research owing to their potential applications in various fields. In this work, a dual physically cross-linked network was designed for the fabrication of ultrastretchable tough hydrogels. The hydrogels were prepared through in situ polymerization of acrylic acid and acrylamide in the presence of positively charged quaternary poly(ethylene imine) (Q-PEI) and micelle-forming Pluronic F127 diacrylate, thus introducing electrostatic interactions between the positively charged Q-PEI and negatively charged poly(acrylic acid-co-acrylamide). For further mechanical reinforcement, Ca2+ and Cu2+ ions were introduced into the hydrogel network to construct coordination bonds, significantly enhancing tensile strength as well as stretchability. The hydrogel prepared with Ca2+ ion coordination bonds was found to be stretchable to 108 times its original length and exhibited a maximum toughness of 177 MJ·m-3, representing one of the most robust systems with both extraordinary toughness and superstretchability prepared to date. The hydrogels also exhibited excellent recovery of dimensions and reproducibility in terms of mechanical properties, providing a promising ultrastretchable soft-matter system with outstanding mechanical strength.

Temperature-Controllable Hydrogels in Double-Walled Microtube Structure Prepared by Using a Triple Channel Microfluidic System.

Hydrogels in the shape of double-walled microtubes possess great potential for development into artificial human blood vessels. In this work, we have prepared temperature-responsive tubular hydrogels with selectively controllable wall diameters, by using alginate templated photopolymerization in a triple channel microfluidic device. These tubular hydrogels mimic human blood vessels because of the separate thermally active inner and passive outer walls. The different behavior of each wall leads to the expansion of the hollow center volume with increasing temperature. This temperature-based control of the hollow center volume cannot be achieved in the case of conventional hydrogel microtubes. Furthermore, through this method, the hydrogels can be modified to achieve a controllable outer diameter while maintaining the hollow center dimensions simply by changing the position of the hydrogel walls. The ability to change the layer properties of the developed system indicates that the preparation of hydrogels with various monomers is possible.

Microfluidic Fabrication of Multistimuli-Responsive Tubular Hydrogels for Cellular Scaffolds.

Stimuli-responsive hydrogel microfibers and microtubes are in great demand for biomedical applications due to their similarity to the native extracellular matrix. In this study, we prepared pH- and temperature-responsive hydrogel microfibers and microtubes using a microfluidic device through alginate-templated photopolymerization. Hydrogel monomer solutions containing N-isopropylacrylamide (NIPAm) and sodium acrylate (SA) or allyl amine (AA) were irradiated with UV light to invoke in situ photopolymerization. A repulsive force between the ionized SA or AA groups caused by protonation/deprotonation of the acrylate or amine groups, respectively, led to changes in the diameters and wall thicknesses of the fibers and/or tubes depending on the pH of the medium. Poly(NIPAm) is a well-known thermally responsive polymer wherein the NIPAm-based copolymer microfibers exhibited a thermal behavior close to the lower critical solution temperature. We have demonstrated that these multistimuli-responsive volume changes are fully reversible and repeatable. Furthermore, the positively charged microfibers were shown to exhibit cell adhesion, and the number of cells attached to the microfibers could be further increased by supplying nutrients, presenting the possibility of their application in tissue engineering and other biomedical fields.