Design and Fabrication of a High-Strength Hydrogel with Ideally Homogeneous Network Structure from Tetrahedron-like Macromonomers

Double network hydrogels with highly enhanced toughness based on a modified first network

A novel double network (DN) hydrogel with highly enhanced toughness has been prepared using reversible addition-fragmentation transfer (RAFT)-modified poly(2-acrylamide-2-methylpropane sulfonic acid) (PAMPS) as the first network, and polyacrylamide (PAM) as the second network. The mechanical properties of the first-network-modified PAMPS/PAM DN hydrogels have been studied and the new DN hydrogel shows remarkably high fracture energy (3.3 MJ m^-3) in tensile deformation, which is nearly 9 times larger than that of the unmodified PAMPS/PAM DN hydrogel. Synchrotron radiation small-angle X-ray scattering (SAXS) was used to study the microstructures of the first-network single network (SN) and DN hydrogels. It was demonstrated by the SAXS results that the introduction of the RAFT agent into the first network enlarges the size of the ordered cross-linked domains in the SN hydrogel. The large ordered domains are beneficial for entanglement and interpenetration between the first and the second networks to dissipate concentrated stress more efficiently, resulting in the enhanced toughness of the first-network-modified DN hydrogels.

Ultrastiff Hydrogels Prepared by Schiff's Base Reaction of Bis(p-Formylphenyl) Sebacate and Pillar[5]arene Appended with Multiple Hydrazides

Herein a facile method is reported to prepare polymer gels based on the formation of acylhydrazone bond under mild conditions. A pillar[5]arene derivative appended with ten hydrazide groups provides multiple sites for the reaction with the aldehyde groups of bis(p-formylphenyl) sebacate in the presence of a small amount of HCl as the catalyst in dimethyl sulfoxide (DMSO), producing transparent polymer organogels. The mechanical properties of gels can be easily tuned by the molar ratio of the reactant compounds. After solvent exchange from DMSO to water, translucent polymer hydrogels with dramatically enhanced strength and stiffness are obtained. The tensile breaking stress and Young's modulus of hydrogels are 20-60 and 1.2-2.7 MPa, respectively, 100 and 20 times those of the corresponding organogels. These robust hydrogels with ultrahigh stiffness should find applications such as in load-bearing artificial organs. This work should merit designing functional materials using other macrocycles.

Hyperelastic Tough Gels through Macrocross-Linking

The wet and soft nature of hydrogels makes them useful as a mimic for biological tissues, and in uses such as actuators and drug delivery vehicles. For many applications the mechanical performance of the gel is critical, but gels are notoriously weak and prone to fracture. Free radical polymerization is a very powerful technique allowing for fine spatial and temporal control of polymerization, but also allows for the use of a wide range of monomers and mixtures. In this work, it is demonstrated that extremely tough and extensible hydrogels can be readily produced through simple radical polymerization of acrylamide or acrylic acid with a poly(ethylene oxide) macrocross-linker. These gels, with a water content of 85%, are extremely elastic with an extension much more than 15 000% at 9 MPa true stress. They can be compressed over 98% at a stress of 17 MPa. They are notch-insensitive, and the usual trouser tear test does not work because the tear simply does not propagate. This highly extensible nature seems to be related to very long chain lengths between cross-links and efficient incorporation of chains into the network.

Tough Supramolecular Hydrogel Based on Strong Hydrophobic Interactions in a Multiblock Segmented Copolymer

We report the preparation and structural and mechanical characterization of a tough supramolecular hydrogel, based exclusively on hydrophobic association. The system consists of a multiblock, segmented copolymer of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic dimer fatty acid (DFA) building blocks. A series of copolymers containing 2K, 4K, and 8K PEG were prepared. Upon swelling in water, a network is formed by self-assembly of hydrophobic DFA units in micellar domains, which act as stable physical cross-link points. The resulting hydrogels are noneroding and contain 75-92 wt % of water at swelling equilibrium. Small-angle neutron scattering (SANS) measurements showed that the aggregation number of micelles ranges from 2 × 102 to 6 × 102 DFA units, increasing with PEG molecular weight. Mechanical characterization indicated that the hydrogel containing PEG 2000 is mechanically very stable and tough, possessing a tensile toughness of 4.12 MJ/m3. The high toughness, processability, and ease of preparation make these hydrogels very attractive for applications where mechanical stability and load bearing features of soft materials are required.

Semibatch monomer addition as a general method to tune and enhance the mechanics of polymer networks via loop-defect control

Controlling the molecular structure of amorphous cross-linked polymeric materials is a longstanding challenge. Herein, we disclose a general strategy for precise tuning of loop defects in covalent polymer gel networks. This "loop control" is achieved through a simple semibatch monomer addition protocol that can be applied to a broad range of network-forming reactions. By controlling loop defects, we demonstrate that with the same set of material precursors it is possible to tune and in several cases substantially improve network connectivity and mechanical properties (e.g., ∼600% increase in shear storage modulus). We believe that the concept of loop control via continuous reagent addition could find broad application in the synthesis of academically and industrially important cross-linked polymeric materials, such as resins and gels.

High-water-content and resilient PEG-containing hydrogels with low fibrotic response

Hydrogels such as those based on polyethylene glycol (PEG) are broadly used in biomedicine where high water contents, robust mechanical properties such as resilience and favorable interactions with the body are often simultaneously desirable. However, the mechanical properties of conventional hydrogels often degrade rapidly after swelling or with increasing water content, limiting their potential in many applications. Here we describe a new class of PEG-containing hydrogels that remain highly resilient after maximum swelling. We achieved the hydrogels by incorporating reversible "dual" hydrogen bonding into highly coiled, elastic PEG networks. These hydrogels, due to their high water content and high mechanical resilience, can form highly permeable, yet durable and easy-to-handle cell delivery devices without any additional structural support. In addition, optimization of chemical composition resulted in hydrogels with superior bio-inertness, inducing much less fibrosis upon subcutaneous implantation in mice than a polyhydroxyethylmethacrylate (PHEMA) hydrogel control.

STATEMENT OF SIGNIFICANCE

Hydrogels such as polyethylene glycol (PEG)-based ones are broadly used in the biomedical world. Examples include wound dressings, tissue scaffolds, medical implants, biosensors and drug or cell delivery devices. In many of these applications, robust mechanical property, high water content (or facile mass transfer) and favorable interactions with the body are often simultaneously desirable. However, the mechanical property of hydrogels often degrades rapidly after swelling or with increasing water content. Here we report a new class of PEG-based hydrogels that simultaneously possess high water content, high mechanical resilience and low fibrotic response upon subcutaneous implantation in mice. These hydrogels may therefore find broad applications in biomedicine.

Demonstration of thermo-sensitive tetra-gel with implication for facile and versatile platform for a new class of smart gels

A tertiary branched poly(N-isopropylacrylamide) with controlled molecular weight, distribution and the end amino-functionalization (tetra-PNIPAAm-NH₂) was studied for the ability to form a gel via in situ chain-end reaction with a counterpart tertiary branched poly(ethyleneglycol) bearing N-hydroxysuccinimide end groups (tetra-PEG-NHS), a well-documented class of building block to yield the tetra-gel. Some of these polymers, both comparable and distinct (relative to the counterpart) extended chain length pairs, provided a self-standing and macroscopically homogeneous gel, which was capable of undergoing thermo-sensitive and reversible change in hydration in line with the nature of PNIPAAm. Phantom network model based calculation indicated that a half molar fraction of the polymer chains in the network remained unreacted, revealing further room for optimizing the reaction condition. Since such tetra-PNIPAAm based motif can be readily tailored to a variety of other physicochemical stimuli-responsive analogues, our finding may give important insight into a platform for 'smart' tetra-gels with exceptional mechanical properties and potentially highly controllable molecular cut-off capability.

Rapidly recoverable, anti-fatigue, super-tough double-network hydrogels reinforced by macromolecular microspheres

In this study, a novel strategy was designed to prepare rapidly recoverable, anti-fatigue, super-tough double-network hydrogels by introducing macromolecular microspheres (MMs) as cross-linking centers for hydrophobic associations. MMs were prepared via emulsion polymerization using butyl acrylate (BA) as a main component and dicyclopentyl acrylate (DCPA) as a cross-linker. Then, a double-network (DN) hydrogel was prepared using gelatin as the first network and a copolymer of acrylamide and hexadecyl methacrylate stabilized by MMs as the second network. As a result, the DN hydrogels that were toughened by MMs exhibited an excellent fracture strength of 1.48 MPa and a fracture strain of 2100%. Moreover, the hydrogels exhibited rapid recoverability and fatigue resistance. Therefore, the strategy would open up a novel avenue for the toughening of DN hydrogels for biomedical applications.

Small-angle neutron scattering and molecular dynamics structural study of gelling DNA nanostars

DNA oligomers with properly designed sequences self-assemble into well defined constructs. Here, we exploit this methodology to produce bulk quantities of tetravalent DNA nanostars (each one composed of 196 nucleotides) and to explore the structural signatures of their aggregation process. We report small-angle neutron scattering experiments focused on the evaluation of both the form factor and the temperature evolution of the scattered intensity at a nanostar concentration where the system forms a tetravalent equilibrium gel. We also perform molecular dynamics simulations of one isolated tetramer to evaluate the form factor numerically, without resorting to any approximate shape. The numerical form factor is found to be in very good agreement with the experimental one. Simulations predict an essentially temperature-independent form factor, offering the possibility to extract the effective structure factor and its evolution during the equilibrium gelation.

Dual physically crosslinked double network hydrogels with high toughness and self-healing properties

Toughness and self-healing properties are desirable characteristics in engineered hydrogels used for many practical applications. However, it is still challenging to develop hydrogels exhibiting both of these attractive properties in a single material. In this work, we present the fabrication of fully physically-linked Agar/PAAc-Fe³⁺ DN gels. These hydrogels exhibited dual physical crosslinking through a hydrogen bonded crosslinked agar network firstly, and a physically linked PAAc-Fe³⁺ network via Fe³⁺ coordination interactions secondly. Due to this dual physical crosslinking, the fabricated Agar/PAAc-Fe³⁺ DN gels exhibited very favorable mechanical properties (tensile strength 320.7 kPa, work of extension 1520.2 kJ m^-3, elongation at break 1130%), fast self-recovery properties in Fe³⁺ solution (100% recovery within 30 min), in 50 °C conditions (100% recovery within 15 min), and under ambient conditions (100% recovery of the initial properties within 60 min), as well as impressive self-healing properties under ambient conditions. All of the data indicate that both the hydrogen bonds in the first network and the ionic coordination interactions in the second network act as reversible sacrificial bonds to dissipate energy, thus conferring high mechanical and recovery properties to the prepared Agar/PAAc-Fe³⁺ DN gels.

Construction of Tough, in Situ Forming Double-Network Hydrogels with Good Biocompatibility

Hydrogels are required to have high mechanical properties, biocompatibility, and an easy fabrication process for biomedical applications. Double-network hydrogels, although strong, are limited because of the complicated preparation steps and toxic materials involved. In this study, we report a simple method to prepare tough, in situ forming polyethylene glycol (PEG)-agarose double-network (PEG-agarose DN) hydrogels with good biocompatibility. The hydrogels display excellent mechanical strength. Because of the easily in situ forming method, the resulting hydrogels can be molded into any form as needed. In vitro and in vivo experiments illustrate that the hydrogels exhibit satisfactory biocompatibility, and cells can attach and spread on the hydrogels. Furthermore, the residual amino groups in the network can also be functionalized for various biomedical applications in tissue engineering and cell research.

Biomolecularly stimuli-responsive tetra-poly(ethylene glycol) that undergoes sol–gel transition in response to a target biomolecule

We designed biotin-conjugated four-armed poly(ethylene glycol) (biotinylated Tetra-PEG) as biomolecularly stimuli-responsive polymers that underwent the phase transition from a sol to a gel state in response to avidin as a target biomolecule.

A pH, glucose, and dopamine triple-responsive, self-healable adhesive hydrogel formed by phenylborate–catechol complexation

A pH, glucose, and dopamine triple-responsive, self-healable and adhesive polyethylene glycol hydrogel was developed via the formation of phenylborate–catechol complexation.

Defect-free network formation and swelling behavior in ionic liquid-based electrolytes of tetra-arm polymers synthesized using a Michael addition reaction

Gelation reaction of TetraPEGs and the ion gel swollen with an ionic-liquid electrolyte.

Synthesis of an injectable, self-healable and dual responsive hydrogel for drug delivery and 3D cell cultivation

An injectable, self-healable and dual pH and temperature responsive hydrogel was facilely prepared and applied as a potential carrier for drug delivery and cell cultivation.

Restorable, high-strength poly(N-isopropylacrylamide) hydrogels constructed through chitosan-based dual macro-cross-linkers with rapid response to temperature jumps

Thermo-responsive, high mechanical poly(N-isopropylacrylamide) hydrogels with excellent recoverability and rapid response rate are constructed on chitosan-based dual macro-cross-linkers.

The energy dissipation and Mullins effect of tough polymer/graphene oxide hybrid nanocomposite hydrogels

Polyacrylamide/graphene oxide hybrid NC gels exhibited high strength, high toughness and rapid self-recovery properties.

F127DA micelle cross-linked PAACA hydrogels with highly stretchable, puncture resistant and self-healing properties

F127DA micelle cross-linked PAACA hydrogels with highly stretchable, puncture resistant and self-healing properties are prepared.

Inspired by elastomers: fabrication of hydrogels with tunable properties and re-shaping ability via photo-crosslinking at a macromolecular level

Fabrication of hydrogels with tunable properties and re-shaping abilityviaphoto-crosslinking at a macromolecular level.

New design of hydrogels with tuned electro-osmosis: a potential model system to understand electro-kinetic transport in biological tissues

A series of charged polymer gels with precisely controlled magnitude and direction of electro-osmotic flow was prepared and opens up the possibility for understanding the contribution of electro-osmosis to transport phenomenon in native biological tissues.

Polyetheramine (PEA): a versatile platform to tailor the properties of hydrogels via H-bonding interactions

Hydrogels with excellent mechanical properties, fast pH response and reshaping ability were prepared by dual H-bonding interactions.

A novel high-strength polymer hydrogel with identifiability prepared via a one-pot method

A simple one-pot method was developed to fabricate a novel hydrogel with good biocompatibility, excellent mechanical properties, and identifiability via photopolymerization and sol–gel processes.

Mechanically strong interpenetrating network hydrogels for differential cellular adhesion

Hydrogels as “soft-and-wet” materials have been widely used as tissue engineering scaffolds due to their similarity to natural extracellular matrix.

Origin of nanostructural inhomogeneity in polymer-network gels

Polymer-network gels often display nano- to microstructural inhomogeneity; this article reviews multiple types of origin of this structural feature.

Hydrogels with high mechanical strength cross-linked by a rosin-based crosslinking agent

A novel type of DN hydrogel, prepared by micellar copolymerization of acrylamide and rosin-based crosslinking agent in a micellar solution of SDS. The hydrogels could form both chemical crosslinks and hydrophobic association crosslinked centers.

Mechanical and responsive properties of temperature-responsive gels prepared via atom transfer radical polymerization

Temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm) gels were prepared via atom transfer radical polymerization (ATRP), and their mechanical and responsive properties were investigated from the viewpoint of their network homogeneity.

High strength and self-healable gelatin/polyacrylamide double network hydrogels

Gelatin/polycrylamide double-network (DN) hydrogels composed of two different polymer networks with strong asymmetry are excellent structural platforms to integrate different mechanical properties into a single material.

Mussel-mimetic hydrogels with defined cross-linkers achieved via controlled catechol dimerization exhibiting tough adhesion for wet biological tissues

Mussel-mimetic hydrogels possessing ultrahigh adhesion energy on wet biological tissues via enhancing both the interfacial adhesion and bulk cohesion are fabricated.