Low-intensity mixing process of high molecular weight polymer chains leads to elastomers of long network strands and high fatigue threshold.

Many polymer networks are prepared by crosslinking polymer chains. The polymer chains and crosslinkers are commonly mixed in internal mixers or roll mills. These intense processes break the polymer chains, lower viscosity, and ease mixing. The resulting polymer networks have short chains and a fatigue threshold of ∼100 J m-2. Here, we show that a low-intensity process, a combination of kneading and annealing, preserves long chains, leading to a network of polybutadiene to achieve a fatigue threshold of 440 J m-2. In a network, each chain has multiple crosslinks, which divides the chain into multiple strands. At the ends of the chain are two dangling strands that do not bear the load. The larger the number of crosslinks per chain, the lower the fraction of dangling strands. High fatigue threshold requires long strands, as well as a low fraction of dangling strands. Once intense mixing cuts chains short, each short chain can only have a few crosslinks; the strands are short and the fraction of dangling strands is high-both lower the fatigue threshold. By contrast, a low-intensity mixing process preserves long chains, which can have many crosslinks; the strands are long and the fraction of dangling strands is low-both increase the fatigue threshold. It is hoped that this work will aid the development of fatigue-resistant elastomers.

Making Highly Elastic and Tough Hydrogels from Doughs.

A hydrogel is often fabricated from preexisting polymer chains by covalently crosslinking them into a polymer network. The crosslinks make the hydrogel swell-resistant but brittle. This conflict is resolved here by making a hydrogel from a dough. The dough is formed by mixing long polymer chains with a small amount of water and photoinitiator. The dough is then homogenized by kneading and annealing at elevated temperatures, during which the crowded polymer chains densely entangle. The polymer chains are then sparsely crosslinked into a polymer network under an ultraviolet lamp, and submerged in water to swell to equilibrium. The resulting hydrogel is both swell-resistant and tough. The hydrogel also has near-perfect elasticity, high strength, high fatigue resistance, and low friction. The method is demonstrated with two widely used polymers, poly(ethylene glycol) and cellulose. These hydrogels have never been made swell-resistant, elastic, and tough before. The method is generally applicable to synthetic and natural polymers, and is compatible with industrial processing technologies, opening doors to the development of sustainable, high-performance hydrogels.

3D Printing Method for Tough Multifunctional Particle-Based Double-Network Hydrogels.

3D printing of hydrogels finds widespread applications in biomedicine and engineering. Artificial cartilages and heart valves, tissue regeneration and soft robots, require high mechanical performance of complex structures. Although many tough hydrogels have been developed, complicated synthesis processes hinder their fabrication in 3D printing. Here, a strategy is proposed to formulate hydrogel inks, which can be printed into various strong and tough particle-based double-network (P-DN) hydrogels of arbitrary shapes without any rheological modifiers. These hydrogel inks consist of microgels and a hydrogel precursor. The microgels are individual highly cross-linked networks. They are prepared by swelling dried microparticles in the hydrogel precursor that consists of monomers, initiators, and cross-linkers. Microgels regulate the rheological properties of the hydrogel ink and enable the direct printing. After printing and curing, the precursor forms a sparsely cross-linked network that integrates the microgels, leading to a P-DN hydrogel. The proposed hydrogel inks allow 3D printing of multifunctional hydrogel structures with high mechanical performance and strong adhesion to diverse materials. This strategy will open new avenues to fabricate multifunctional devices in tissue engineering and soft robotics.

Ionotronic Luminescent Fibers, Fabrics, and Other Configurations.

A family of recently developed devices, hydrogel ionotronics, uses hydrogels as ionic conductors, and uses hydrophobic elastomers as dielectrics. This development has posed a challenge: integrate hydrogels and hydrophobic elastomers-in various manufacturing processes-with strong, stretchable, and transparent adhesion. Here, a multistep dip-coating process is described to enable hydrogel ionotronics of diverse configurations. In doing so, a hydrophobic surface is primed to let a hydrophilic precursor wet it, and then polymers of different layers are interlinked with covalent bonds. As a representative example, an ionotronic luminescent fiber that can be lengthened to ≈2.5 times its original length and keeps functioning after 10 000 cycles of stretching is fabricated. A luminescent fabric that displays movable pixels and other configurations is also demonstrated. The proposed method of fabrication expands the design space for hydrogel ionotronics.

Dual pH-Responsive Hydrogel Actuator for Lipophilic Drug Delivery.

As one of the most promising drug delivery carriers, hydrogels have received considerable attention in recent years. Many previous efforts have focused on diffusion-controlled release, which allows hydrogels to load and release drugs in vitro and/or in vivo. However, it hardly applies to lipophilic drug delivery due to their poor compatibility with hydrogels. Herein, we propose a novel method for lipophilic drug release based on a dual pH-responsive hydrogel actuator. Specifically, the drug is encapsulated and can be released by a dual pH-controlled capsule switch. Inspired by the deformation mechanism of Drosera leaves, we fabricate the capsule switch with a double-layer structure that is made of two kinds of pH-responsive hydrogels. Two layers are covalently bonded together through silane coupling agents. They can bend collaboratively in a basic or acidic environment to achieve the "turn on" motion of the capsule switch. By incorporating an array of parallel elastomer stripes on one side of the hydrogel bilayer, various motions (e.g., bending, twisting, and rolling) of the hydrogel bilayer actuator were achieved. We conducted an in vitro lipophilic drug release test. The feasibility of this new drug release method is verified. We believe this dual pH-responsive actuator-controlled drug release method may shed light on the possibilities of various drug delivery systems.

Adhesive Tough Magnetic Hydrogels with High Fe3O4 Content.

Magnetic hydrogels have promising applications in flexible electronics, biomedical devices, and soft robotics. However, most existing magnetic hydrogels are fragile and suffer insufficient magnetic response. In this paper, we present a new approach to fabricate a strong, tough, and adhesive magnetic hydrogel with nontoxic polyacrylamide (PAAm) hydrogel as the matrix and the functional additive [3-(trimethoxysilyl)propyl methacrylate coated Fe3O4] as the inclusions. This magnetic hydrogel not only offers a relatively high modulus and toughness compared to the pure hydrogel but also responds to the magnetic field rapidly because of high magnetic particle content (up to 60%, with respect to the total weight of the polymers and water). The hydrogel can be bonded to hydroxyl-rich hard and soft surfaces. Magnetic hydrogel with polydimethylsiloxane (PDMS) coating exhibits excellent underwater performance. The bonding between magnetic hydrogel and PDMS is very stable even under cyclic loading. An artificial muscle and its magnetomechanical coupling performance are demonstrated using this hydrogel. The adhesive tough magnetic hydrogel will open up extensive applications in many fields, such as controlled drug delivery systems, coating of soft devices, and microfluidics. The strategy is applicable to other functional soft materials.

Mechanical characterization and modeling of sponge-reinforced hydrogel composites under compression.

Load-bearing applications of hydrogels call for materials with excellent mechanical properties. Despite the considerable progress in developing tough hydrogels, there is still a requirement to prepare high-performance hydrogels using simple strategies. In this paper, a sponge-reinforced hydrogel composite is synthesized by combining poly(acrylamide) (PAAm) hydrogel and polyurethane (PU) sponge. Uniaxial compressive testing of the hydrogel composites reveals that both the compressive modulus and the strength of the hydrogel composites are much higher than those of the PAAm hydrogel or sponge. In order to predict the compressive modulus of the hydrogel composite, we develop a theoretical model that is validated by experiments and numerical simulations. The present work may guide the design and manufacture of hydrogel-based composite materials, especially for biomaterial scaffolds and soft transducers.

Bonding dissimilar polymer networks in various manufacturing processes.

Recently developed devices mimic neuromuscular and neurosensory systems by integrating hydrogels and hydrophobic elastomers. While different methods are developed to bond hydrogels with hydrophobic elastomers, it remains a challenge to coat and print various hydrogels and elastomers of arbitrary shapes, in arbitrary sequences, with strong adhesion. Here we report an approach to meet this challenge. We mix silane coupling agents into the precursors of the networks, and tune the kinetics such that, when the networks form, the coupling agents incorporate into the polymer chains, but do not condensate. After a manufacturing step, the coupling agents condensate, add crosslinks inside the networks, and form bonds between the networks. This approach enables independent bonding and manufacturing. We formulate oxygen-tolerant hydrogel resins for spinning, printing, and coating in the open air. We find that thin elastomer coatings enable hydrogels to sustain high temperatures without boiling.

Wearable and Washable Conductors for Active Textiles.

The emergence of stretchable electronics and its potential integration with textiles have highlighted a challenge: Textiles are wearable and washable, but electronic devices are not. Many stretchable conductors have been developed to enable wearable active textiles, but little has been done to make them washable. Here we demonstrate a new class of stretchable conductors that can endure wearing and washing conditions commonly associated with textiles. Such a conductor consists of a hydrogel, a dissolved hygroscopic salt, and a butyl rubber coating. The hygroscopic salt enables ionic conduction and matches the relative humidity of the hydrogel to the average ambient relative humidity. The butyl rubber coating prevents the loss and gain of water due to the daily fluctuation of ambient relative humidity. We develop the chemistry of dip-coating the butyl rubber onto the hydrogel, using silanes to achieve both the cross-link of the butyl rubber and the adhesion between the butyl rubber and the hydrogel. We test the endurance of the conductor by soaking it in detergent while stretching it cyclically and by machine-washing it. The loss of water and salt is minimal. It is hoped that these conductors open applications in healthcare, entertainment, and fashion.