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

Physically Entangled Hydrogels Constructed Through Pre-Stretched Backbone Provide Excellent Comprehensive Mechanical Properties

It is difficult for hydrogels to have both excellent stiffness and toughness in conventional polymer networks. Physically entangled hydrogels provide ideas to solve this problem, but the loss of physical entanglement in the swelling process does not effectively utilize the potential of this approach. In this work, the hydrogel is synthesized including two layers of interpenetrating physically entangled networks, the backbone network and the filler network. According to rubberlike elasticity theory, the backbone network inhibits the swelling of the filler network retaining a large amount of physical entanglements, inheriting and enhancing the advantages of physically entanglement hydrogels. The stiffness of the hydrogel reaches 0.33 MPa, and the fracture toughness reaches 1.93 × 104 J m-2, which shows significant advantages over other hydrogels. The removable physical entanglements tend to favor an ideal uniform network after being subjected to cyclic loading, and the toughness and fracture strain improve more than 2 times to 4.2 MJ m-3 and 7.2, respectively. The fatigue threshold reaches 5.41 × 103 J m-2, which is five times higher than the fatigue-resistant hydrogels. The present work perfectly inherits the physically entangled network structure while realizing the anti-swelling property, which provides a way to break the toughness-stiffness conflict intractable in hydrogels.

Porous hierarchically ordered hydrogels demonstrating structurally dependent mechanical properties

While hierarchical ordering is a distinctive feature of natural tissues and is directly responsible for their diverse and unique properties, efforts to synthesize biomaterials have primarily focused on using molecular-based approaches with little emphasis on multiscale structure. Here, we report a bottom-up self-assembly process to produce highly porous hydrogel fibers that resemble extracellular matrices both structurally and mechanically. Physically crosslinked nanostructured micelles form the walls of micrometer-sized water-rich pores with preferred orientation along the fiber direction. Low elastic moduli (<1 kPa), high elasticity (extending by more than 12 times the initial length), non-linear elasticity (e.g., hyperelasticity), and completely reversible extension are derived from unevenly distributed strain between the micrometer-sized pores and the polymer chains, which is reminiscent of cellular solids. Control of the material microstructure and orientation over many orders of magnitude (e.g., nm-μm), while holding the nanostructure constant, reveals how the multiscale structure directly impacts mechanical properties.

Star Polymer Network Elastomer with Reconfigurable Network Structure via Covalent Bond Exchange through Olefin Metathesis

A covalent adaptable network (CAN) elastomer with a well-defined network structure was fabricated by end-linking monodisperse star polymers via associative dynamic covalent bonds (DCBs). Monodisperse 4-arm star-shaped polyesters with vinyl end groups were synthesized and end-linked by an olefin metathesis reaction, yielding an elastomer with a uniform chain length between cross-links. The well-defined network structure endowed the elastomer with good mechanical properties. The remaining C═C bonds in the network could exchange via olefin metathesis, rendering the network structure reconfigurable. As a result, the elastomer showed stress relaxation and was thermally reprocessable. Moreover, the elastomer was chemically degradable into un-cross-linked polymers under mild conditions through C═C bond exchange. This study demonstrates mechanical robustness and dynamicity in rubbery materials through the combination of a well-defined network structure and associative DCBs.

Ring-Toughened Polymer Networks: The Mighty Impact of Specially Designed Rings on Mechanical Properties

Polymer network materials are gaining significance in daily life and industrial applications. Improving polymer network materials' mechanical properties has long been a focus for chemists and materials scientists. Generally, rings in networks are viewed as adverse elements leading to reduced mechanical performance. In this conceptual article, recent advancements and related strategies in utilizing specially designed rings to enhance the mechanical properties of polymer networks are summarized and discussed. The article concludes by discussing current challenges and future prospects in this field. We aim for this article to offer readers an overview of ring-toughened polymer networks and to catalyze swift progress in this burgeoning area.

Coarse-grained molecular dynamics simulations of slide-ring gels under finite deformation: influence of sliding ring rearrangement on softness and extensibility

Slide-ring (SR) gels are a class of polymer gels known for their unique softness, toughness, and high extensibility. The defining structural feature of SR gels is their figure-of-eight-shaped slidable cross-links, whose sliding dynamics are believed to underpin their mechanical properties. However, the relationship between the sliding mechanics and observed mechanical behavior of SR gels remains unclear because their structure differs considerably from those of conventional fixed cross-link gels and vulcanized rubbers. In this work, we employed coarse-grained molecular dynamics simulations to investigate the mechanical behavior of SR gels up to large deformation. By visualizing the correlated distribution of network strand orientation and stress loading, we found that SR gels under strain exhibit uniform chain orientation and efficient stress dispersion throughout the network, in contrast to gels with fixed cross-links, which display regions of highly oriented and heavily stressed chains. Furthermore, we observed that the distribution of network-strand length changes under deformation, indicating that chains are reconfigured into shorter and longer sections during stretching. Notably, we demonstrated that the finite network-strand length (Nmax) determines the finite extensibility of SR gels, corresponding to the maximum elongation ratio (λmax). These findings provide new insights into the molecular mechanisms driving the high extensibility and toughness of SR gels and offer valuable guidance for designing SR gels with tailored mechanical properties.

High-concentration hydrogels of κ-carrageenan prepared using subcritical water

κ-Carrageenan hydrogels have been prepared at very high concentrations beyond the previous limit of conventional κ-carrageenan hydrogels. By dissolving κ-carrageenan using subcritical water at 150 °C, homogeneous translucent hydrogels have been obtained from 15 wt% to 40 wt%. The high-concentration hydrogels have exceptionally high Young's modulus (E0) ranging 106-107 Pa and exhibit an unknown concentration (c)-dependence of E0 ∝ c1.0-1.1. The distinctive mechanical properties of the high-concentration hydrogels are also demonstrated by modifying the stress (σ)-strain (ε) curves. Curves of σ / E0 versus ε for the high-concentration hydrogels seem to fall on a single curve that is remarkably different from that for the conventional 3 wt% hydrogel. The gel-to-sol transition temperature (Tm) proves high thermal resistance of the high-concentration hydrogels; Tm increases with c and exceeds 100 °C at 30 wt% and 40 wt%. It has been confirmed that separated pieces of high-concentration hydrogels are unified into a single disk above Tm. The endothermic enthalpy of the gel-to-sol transition per unit weight of κ-carrageenan is almost constant regardless of c among the high-concentration hydrogels, which is consistent with the claim that the high-concentration hydrogels have similar distinctive network structure.

Characterizing dynamic heterogeneities during nanogel degradation

Understanding photodegradation of nanogels is critical for dynamic control of their properties and functionalities. We focus on nanogels formed by end-linking of four-arm polyethylene glycol precursors with photolabile groups and characterize dynamic heterogeneities in these systems during degradation. We use our recently developed dissipative particle dynamics framework that captures the controlled scission of bonds between the precursors and diffusion of degraded fragments at the mesoscale. To quantify spatiotemporal fluctuations in the local dynamic behavior, we calculate the self-part of the van-Hove correlation function for the reactive beads for nanogels degrading in various environments. We demonstrate strong deviations from the Gaussian behavior during the degradation and quantify variations in the non-Gaussian parameter as a function of the relative extent of degradation. We show that for the nanogels degrading in a good solvent, the peak values in the non-Gaussian parameter are observed significantly earlier than the reverse gel point, and earlier than the peak values in the dispersity of the broken off fragments. Further, our study shows that a systematic decrease in solvent quality significantly affects the behavior of the non-Gaussian parameter as a function of the relative extent of degradation. The findings of this study allow one to quantify the dynamic heterogeneities during degradation in various environments and can potentially provide guidelines for designing controllably degrading nanocarriers.

Integrating microfluidics, hydrogels, and 3D bioprinting for personalized vessel-on-a-chip platforms

Thrombosis, a major cause of morbidity and mortality worldwide, presents a complex challenge in cardiovascular medicine due to the intricacy of clotting mechanisms in living organisms. Traditional research approaches, including clinical studies and animal models, often yield conflicting results due to the inability to control variables in these complex systems, highlighting the need for more precise investigative tools. This review explores the evolution of in vitro thrombosis models, from conventional polydimethylsiloxane (PDMS)-based microfluidic devices to advanced hydrogel-based systems and cutting-edge 3D bioprinted vascular constructs. We discuss how these emerging technologies, particularly vessel-on-a-chip platforms, are enabling researchers to control previously unmanageable factors, thereby offering unprecedented opportunities to pinpoint specific clotting mechanisms. While PDMS-based devices offer optical transparency and fabrication ease, their inherent limitations, including non-physiological rigidity and surface properties, have driven the development of hydrogel-based systems that better mimic the extracellular matrix of blood vessels. The integration of microfluidics with biomimetic materials and tissue engineering approaches has led to the development of sophisticated models capable of simulating patient-specific vascular geometries, flow dynamics, and cellular interactions under highly controlled conditions. The advent of 3D bioprinting further enables the creation of complex, multi-layered vascular structures with precise spatial control over geometry and cellular composition. Despite significant progress, challenges remain in achieving long-term stability, incorporating immune components, and translating these models to clinical applications. By providing a comprehensive overview of current advancements and future prospects, this review aims to stimulate further innovation in thrombosis research and accelerate the development of more effective, personalized approaches to thrombosis prevention and treatment.

Engineering of Polyisoprene Networks Enabled by Host-Guest Thiol-Ene Reaction

Regulating cross-linking of polymers is critical for optimizing the physical properties of polymer networks. Herein, we present a strategic approach for designing polymer networks using dithiol-functionalized metal-organic frameworks (MOFs) with both one- and three-dimensional pore architectures. Upon thermal treatment, thiyl radicals were generated from the MOFs through the dissociation of S-H bonds, as confirmed by electron spin resonance measurements. Unlike in solution and bulk phases, the confinement of these radicals within the MOFs effectively suppressed homocoupling reactions, thus enabling their function as densely packed cross-linkers. The thiol-ene reaction between the MOFs and cis-1,4-polyisoprene (PI) chains, followed by the selective removal of MOF hosts, resulted in PI networks that retained the original structural features. The ordered alignment of the PI chains enhanced their thermal stability compared with the randomly cross-linked PI network.

Physics of polymer gels: Toyoichi Tanaka and after

This review revisits the works of Toyoichi Tanaka on the physics of polymer gels and discusses their scientific significance with the keywords of volume phase transition, structure, dynamics, kinetics and inhomogeneities, followed by some recent topics including defect-free homogeneous gels. Then, the modern physics of polymer gels will be considered from the viewpoints of cross-linking, networking, and percolation, along with the scope of future directions of polymer gels and polymer networks.

Optimizing gelation time for cell shape control through active learning

Hydrogels are popular platforms for cell encapsulation in biomedicine and tissue engineering due to their soft, porous structures, high water content, and excellent tunability. Recent studies highlight that the timing of network formation can be just as important as mechanical properties in influencing cell morphologies. Conventionally, time-dependent properties can be achieved through multi-step processes. In contrast, one-pot synthesis can improve both the efficiency and uniformity of cell encapsulation. Reaction kinetics are sensitive to temperatures and pH conditions, thus, monitoring gelation time across different conditions is essential for formulation. In this work, we choose tetra-poly(ethylene glycol) (TPEG) macromers as a model system to examine the relationship between the rate of polymer network formation and cell morphology. Previous studies of this system focused on reactions at neutral pH and room temperature, leaving much of the formulation space underexplored. We use Gaussian process regression (GPR) to minimize response surface errors by strategically selecting additional investigation points based on prior knowledge. Then we extend the knowledge from pre-trained data at neutral pH to a new surface at physiological pH. We find that the gelation time surface can effectively predict the aspect ratio of the encapsulated cells. Additionally, through focal adhesion kinase inhibition, we show that cell shape is influenced by the properties of the forming network in the initial hours as cells develop connections with the matrix. We demonstrate the utility of a high-throughput microrheology approach in enhancing fabrications of synthetic extracellular matrix and cell assemblies.

Functionalized GelMA/CMCS Composite Hydrogel Incorporating Magnesium Phosphate Cement for Bone Regeneration

Background: Bone regeneration remains a challenging issue in tissue engineering. The use of hydrogels as scaffolds for bone tissue repair has gained attention due to their biocompatibility and ability to mimic the extracellular matrix. This study aims to develop a functionalized GelMA/CMCS composite hydrogel incorporating magnesium phosphate cement (MPC) for enhanced bone regeneration. Methods: These composites were developed by incorporating potassium magnesium phosphate hexahydrate (KMgPO4·6H2O, MPC) powders into methacrylated gelatin/carboxymethyl chitosan (GelMA-C) hydrogels. The material's mechanical properties, antibacterial performance, and cytocompatibility were evaluated. In vitro experiments involved cell viability and osteogenic differentiation assays using rBMSCs as well as angiogenic potential assays using HUVECs. The hydrogel was also assessed for its potential in promoting bone repair in a rat (Sprague-Dawley) model of bone defect. Results: The developed GelMA-CM composite demonstrated improved mechanical properties, biocompatibility, and osteogenic potential compared to individual GelMA or CMCS hydrogels. Incorporation of MPC facilitated the sustained release of ions which promoted osteogenic differentiation of pre-osteoblasts. In vivo results indicated accelerated bone healing in the rat bone defect model. Conclusions: The functionalized GelMA-CM composite could be a viable candidate for clinical applications in bone regeneration therapies.

Transducing chemical energy through catalysis by an artificial molecular motor

Cells display a range of mechanical activities generated by motor proteins powered through catalysis1. This raises the fundamental question of how the acceleration of a chemical reaction can enable the energy released from that reaction to be transduced (and, consequently, work to be done) by a molecular catalyst2-7. Here we demonstrate the molecular-level transduction of chemical energy to mechanical force8 in the form of the powered contraction and powered re-expansion of a cross-linked polymer gel driven by the directional rotation of artificial catalysis-driven9 molecular motors. Continuous 360° rotation of the rotor about the stator of the catalysis-driven motor-molecules incorporated in the polymeric framework of the gel twists the polymer chains of the cross-linked network around one another. This progressively increases writhe and tightens entanglements, causing a macroscopic contraction of the gel to approximately 70% of its original volume. The subsequent addition of the opposite enantiomer fuelling system powers the rotation of the motor-molecules in the reverse direction, unwinding the entanglements and causing the gel to re-expand. Continued powered twisting of the strands in the new direction causes the gel to re-contract. In addition to actuation, motor-molecule rotation in the gel produces other chemical and physical outcomes, including changes in the Young modulus and storage modulus-the latter is proportional to the increase in strand crossings resulting from motor rotation. The experimental demonstration of work against a load by a synthetic organocatalyst, and its mechanism of energy transduction6, informs both the debate3,5,7 surrounding the mechanism of force generation by biological motors and the design principles6,10-14 for artificial molecular nanotechnology.

Photochemical Control of Network Topology in PEG Hydrogels

Hydrogels are often synthesized through photoinitiated step-, chain-, and mixed-mode polymerizations, generating diverse network topologies and resultant material properties that depend on the underlying network connectivity. While many photocrosslinking reactions are available, few afford controllable connectivity of the hydrogel network. Herein, a versatile photochemical strategy is introduced for tuning the structure of poly(ethylene glycol) (PEG) hydrogels using macromolecular monomers functionalized with maleimide and styrene moieties. Hydrogels are prepared along a gradient of topologies by varying the ratio of step-growth (maleimide dimerization) to chain-growth (maleimide-styrene alternating copolymerization) network-forming reactions. The initial PEG content and final network physical properties (e.g., modulus, swelling, diffusivity) are tailored in an independent manner, highlighting configurable gel mechanics and reactivity. These photochemical reactions allow high-fidelity photopatterning and 3D printing and are compatible with 2D and 3D cell culture. Ultimately, this photopolymer chemistry allows facile control over network connectivity to achieve adjustable material properties for broad applications.

'Click' hydrogels from renewable polysaccharide resources: Bioorthogonal chemistry for the preparation of alginate, cellulose and other plant-based networks with biomedical applications

Click chemistry refers to a class of highly selective reactions that occur in one pot, are not disturbed by water or oxygen, proceed quickly to high yield and generate only inoffensive byproducts. Since its first definition by Barry Sharpless in 2001, click chemistry has increasingly been used for the preparation of hydrogels, which are water-swollen polymer networks with numerous biomedical applications. Polysaccharides, which can be obtained from renewable resources including plants, have drawn growing attention for use in hydrogels due to the recent focus on the development of a sustainable society and the reduction of the environmental impact of the chemical industry. Importantly, plant-based polysaccharides are often bioresorbable and exhibit excellent biocompatibility and biomimicry. This comprehensive review describes the synthesis, characterization and biomedical applications of hydrogels which combine the renewable and biocompatible aspects of polysaccharides with the chemically and biomedically favorable characteristics of click crosslinking. The manuscript focuses on click hydrogels prepared from alginate and cellulose, the most widely used polysaccharides for this type of hydrogel, but also click hydrogels based on other plant-derived polymers (e.g. pectin) are discussed. In addition, the challenges are described that should be overcome to facilitate translation from academia to the clinic.

Gel-Gel Phase Separation in Clustered Poly(ethylene glycol) Hydrogel with Enhanced Hydrophobicity

The development of hydrophobic poly(ethylene glycol) (PEG) hydrogels, which are typically hydrophilic, could significantly enhance their application as artificial extracellular matrices. In this study, we designed PEG hydrogels with enhanced hydrophobicity through gel-gel phase separation (GGPS), a phenomenon that uniquely enhances hydrophobicity under ambient conditions, and we elucidated the pivotal role of elasticity in this process. We hypothesized that increased elasticity would amplify GGPS, thereby improving the hydrophobicity and cell adhesion on PEG hydrogel surfaces, despite their inherent hydrophilicity. To test this hypothesis, we engineered dilute oligo-PEG gels via a two-step process, creating polymer networks from tetra-PEG clusters with multiple reaction points. These oligo-PEG gels exhibited significantly higher elasticity, turbidity, and shrinkage upon water immersion compared to dilute PEG gels. Detailed characterization through confocal laser scanning microscopy, rheological measurements, and cell adhesion assays revealed distinct biphasic structures, increased hydrophobicity, and enhanced cell attachability in the dilute oligo-PEG gels. Our findings confirm that elasticity is crucial for effective GGPS, providing a novel method for tailoring hydrogel properties without chemical modification. This research introduces a new paradigm for designing biomaterials with improved cell-scaffolding capabilities, offering significant potential for tissue engineering and regenerative medicine.

Shear-Induced Cycloreversion Leading to Shear-Thinning and Autonomous Self-Healing in an Injectable, Shape-Holding Collagen Hydrogel

In vivo injectable extracellular matrix (ECM) derived hydrogels that are suitable for cell encapsulation have always been the holy grail in tissue engineering. Nevertheless, these hydrogels still fall short today of meeting three crucial criteria: (a) flexibility on the injectability time window, (b) autonomous self-healing of the injected hydrogel, and (c) shape-retention under aqueous conditions. Here we report the development of a collagen-based injectable hydrogel, cross-linked by cycloaddition reaction between furan and maleimide groups, that (a) is injectable up to 48 h after preparation, (b) can undergo complete autonomous self-healing after injection, (c) can retain its shape and size over several years when stored in the buffer, (d) can be degraded within hours when treated with collagenase, (e) is biocompatible as demonstrated by in vitro cell-culture, and (f) is completely resorbable in vivo when implanted subcutaneously in rats without causing any inflammation.

Investigating Temperature-Dependent Microscopic Deformation in Tough and Self-Healing Hydrogel Using Time-Resolved USAXS

Tough and self-healing hydrogels are typically sensitive to loading rates or temperatures due to the dynamic nature of noncovalent bonds. Understanding the structure evolution under varying loading conditions can provide valuable insights for developing new tough soft materials. In this study, polyampholyte (PA) hydrogel with a hierarchical structure is used as a model system. The evolution of the microscopic structure during loading is investigated under varied loading temperatures. By combining ultra-small angle X-ray scattering (USAXS) and Mooney-Rivlin analysis, it is elucidated that the deformation of bicontinuous hard/soft phase networks is closely correlated with the relaxation dynamics or strength of noncovalent bonds. At high loading temperatures, the gel is soft and ductile, and large affine deformation of the phase-separated networks is observed, correlated with the fast relaxation dynamics of noncovalent bonds. At low loading temperatures, the gel is stiff, and nonaffine deformation occurs from the onset of loading due to the substantial breaking of noncovalent bonds and limited chain mobility as well as weak adaptation of phase deformation to external stretch. This work provides an in-depth understanding of the relationship between structure and performance of tough and self-healing hydrogels.

Critical dynamics of the sol-gel transition studied using particle-tracking microrheology

The formation of tetra-PEG gels, a model network with a well-defined structure, was investigated using particle-tracking microrheology. The dynamic scaling and critical relaxation exponents in the sol-gel transition were determined by applying the time-cure superposition method. Some values of the exponents differed from those of the Rouse model theory. This was attributed to the failure of the theoretical assumption that the cluster structure formation follows the percolation model theory on a three-dimensional lattice.

Optimizing a self-solidifying hydrogel as an endoscopically deliverable hydrogel coating system: a proof-of-concept study on porcine endoscopic submucosal dissection-induced ulcers

Endoscopic submucosal dissection (ESD) benefits patients in the early stages of cancer, but it poses various risks of complication. Strategies involving the application of clinically approved products to cover ulcers caused by ESD can reduce these complications, but the fixed nature of their properties limit the understanding of their effects on ulcer healing. This study was focused on Tetra–PEG gel, an innovative hydrogel with controllable physical properties made from a sulfhydryl–maleimide pair. The use of biocompatible polyethylene glycol (PEG) in Tetra–PEG gel may allow for its application as a biomaterial. The aims of our study were to identify the characteristics of a self-solidifying hydrogel for endoscopic application and to develop a new ulcer coating agent for post-ESD treatment. We developed a specialized double-lumen catheter and determined the optimal application conditions of the hydrogel. We examined the hydrodynamic properties of the gelling solutions and elucidated the pressure drop that occurred during device operation. Finally, by considering previous experimental results, we successfully applied the hydrogel to post-ESD ulcers in porcine stomachs. We believed that by further optimizing hydrogels with effectively controlled properties and by continuing to investigate them through animal experiments, we could expand our understanding of the relationships among material and ulcer healing properties and apply this knowledge to clinical applications.

Injectable Dual Fenton/Enzymatically Cross-Linked Double-Network Hydrogels Based on Acrylic/Phenolic Polymers with Highly Reinforced and Tunable Mechanical Properties

Injectable hydrogels have been extensively used as promising therapeutic scaffolds for a wide range of biomedical applications, such as tissue regeneration and drug delivery. However, their low fracture toughness and brittleness often limit their scope of application. Double-network (DN) hydrogel, which is composed of independently cross-linked rigid and ductile polymer networks, has been proposed as an alternative technique to compensate for the weak mechanical properties of hydrogels. Nevertheless, some challenges still remain, such as the complicated and time-consuming process for DN formation, and the difficulty in controlling the mechanical properties of DN hydrogels. In this study, we introduce a simple, rapid, and controllable method to prepare in situ cross-linkable injectable DN hydrogels composed of acrylamide (AAm) and 4-arm-PPO-PEO-tyramine (TTA) via dual Fenton- and enzyme-mediated reactions. By varying the concentration of Fenton's reagent, the DN hydrogels were rapidly formed with controllable gelation rate. Importantly, the DN hydrogels showed a 13-fold increase in compressive strength and a 14-fold increase in tensile strength, compared to the single network hydrogels. The mechanical properties, elasticity, and plasticity of DN hydrogels could also be modulated by simply varying the preparation conditions, including the cross-linking density and reagent concentrations. At low cross-linker concentration (<0.05 wt %), the plastic DN hydrogel stretched to over 6,500%, whereas high cross-linker concentration (≥0.05 wt %) induced fully elastic hydrogels, without hysteresis. Besides, DN hydrogels were endowed with rapid self-recovery and highly enhanced adhesion, which can be further applied to wearable devices. Moreover, human dermal fibroblasts treated with DN hydrogels retained viability, demonstrating the biocompatibility of the cross-linking system. Therefore, we expect that the dual Fenton-/enzyme-mediated cross-linkable DN hydrogels offer great potential as advanced biomaterials applied for hard tissue regeneration and replacement.

Understanding the Response of Poly(ethylene glycol) diacrylate (PEGDA) Hydrogel Networks: A Statistical Mechanics-Based Framework

Thanks to many promising properties, including biocompatibility and the ability to experience large deformations, poly(ethylene glycol) diacrylate (PEGDA) hydrogels are excellent candidate materials for a wide range of applications. Interestingly, the polymerization of PEGDA leads to a network microstructure that is fundamentally different from that of the "classic" polymeric gels. Specifically, PEGDA hydrogels comprise PEG chains that are interconnected by multifunctional densely grafted rod-like polyacrylates (PAs), which serve as cross-linkers. In this work, we derive a microstructurally motivated model that captures the essential features which enable deformation in PEGDA hydrogels: (1) entropic elasticity of PEG chains, (2) deformation of PA rods, and (3) PA-PA interactions. Expressions for the energy-density functions and the stress associated with each of the three contributions are derived. The model demonstrates the microstructural evolution of the network during loading and reveals the role of key microscopic quantities. To validate the model, we fabricate and compress PEGDA hydrogel discs. The model is in excellent agreement with our experimental findings for a broad range of PEGDA compositions. Interestingly, we show that the response of PEGDA hydrogels with short PEG chains and long PA rods is governed by PA-PA interactions, whereas networks with longer PEG chains are dominated by entropy. To enable design, we employ the model to investigate the influence of key microstructural quantities, such as the length of the PEG and the PA chains, on the macroscopic properties and response. The findings from this work pave the way to the efficient design of PEGDA hydrogels with tunable properties and behaviors, which will enable the optimization of their performance in various applications.

Light from Afield: Fast, High-Resolution, and Layer-Free Deep Vat 3D Printing

Harnessing light for cross-linking of photoresponsive materials has revolutionized the field of 3D printing. A wide variety of techniques leveraging broad-spectrum light shaping have been introduced as a way to achieve fast and high-resolution printing, with applications ranging from simple prototypes to biomimetic engineered tissues for regenerative medicine. Conventional light-based printing techniques use cross-linking of material in a layer-by-layer fashion to produce complex parts. Only recently, new techniques have emerged which deploy multidirection, tomographic, light-sheet or filamented light-based image projections deep into the volume of resin-filled vat for photoinitiation and cross-linking. These Deep Vat printing (DVP) approaches alleviate the need for layer-wise printing and enable unprecedented fabrication speeds (within a few seconds) with high resolution (>10 μm). Here, we elucidate the physics and chemistry of these processes, their commonalities and differences, as well as their emerging applications in biomedical and non-biomedical fields. Importantly, we highlight their limitations, and future scope of research that will improve the scalability and applicability of these DVP techniques in a wide variety of engineering and regenerative medicine applications.

Enhancing cell adhesion in synthetic hydrogels via physical confinement of peptide-functionalized polymer clusters

Artificially synthesized poly(ethylene glycol) (PEG)-based hydrogels are extensively utilized as biomaterials for tissue scaffolds and cell culture matrices due to their non-protein adsorbing properties. Although these hydrogels are inherently non-cell-adhesive, advancements in modifying polymer networks with functional peptides have led to PEG hydrogels with diverse functionalities, such as cell adhesion and angiogenesis. However, traditional methods of incorporating additives into hydrogel networks often result in the capping of crosslinking points with heterogeneous substances, potentially impairing mechanical properties and obscuring the causal relationships of biological functions. This study introduces polymer additives designed to resist prolonged elution from hydrogels, providing a novel approach to facilitate cell culture on non-adhesive surfaces. By clustering tetra-branched PEG to form ultra-high molecular weight hyper-branched structures and functionalizing their termini with cell-adhesive peptides, we successfully entrapped these clusters within the hydrogel matrix without compromising mechanical strength. This method has enabled successful cell culture on inherently non-adhesive PEG hydrogel surfaces at high peptide densities, a feat challenging to achieve with conventional means. The approach proposed in this study not only paves the way for new possibilities with polymer additives but also serves as a new design paradigm for cell culturing on non-cell-adhesive hydrogels.

Reversible two-way tuning of thermal conductivity in an end-linked star-shaped thermoset

Polymeric thermal switches that can reversibly tune and significantly enhance their thermal conductivities are desirable for diverse applications in electronics, aerospace, automotives, and medicine; however, they are rarely achieved. Here, we report a polymer-based thermal switch consisting of an end-linked star-shaped thermoset with two independent thermal conductivity tuning mechanisms-strain and temperature modulation-that rapidly, reversibly, and cyclically modulate thermal conductivity. The end-linked star-shaped thermoset exhibits a strain-modulated thermal conductivity enhancement up to 11.5 at a fixed temperature of 60 °C (increasing from 0.15 to 2.1 W m-1 K-1). Additionally, it demonstrates a temperature-modulated thermal conductivity tuning ratio up to 2.3 at a fixed stretch of 2.5 (increasing from 0.17 to 0.39 W m-1 K-1). When combined, these two effects collectively enable the end-linked star-shaped thermoset to achieve a thermal conductivity tuning ratio up to 14.2. Moreover, the end-linked star-shaped thermoset demonstrates reversible tuning for over 1000 cycles. The reversible two-way tuning of thermal conductivity is attributed to the synergy of aligned amorphous chains, oriented crystalline domains, and increased crystallinity by elastically deforming the end-linked star-shaped thermoset.

Scalable Approach to Molecular Motor-Polymer Conjugates for Light-Driven Artificial Muscles

The integration of molecular machines and motors into materials represents a promising avenue for creating dynamic and functional molecular systems, with potential applications in soft robotics or reconfigurable biomaterials. However, the development of truly scalable and controllable approaches for incorporating molecular motors into polymeric matrices has remained a challenge. Here, it is shown that light-driven molecular motors with sensitive photo-isomerizable double bonds can be converted into initiators for Cu-mediated controlled/living radical polymerization enabling the synthesis of star-shaped motor-polymer conjugates. This approach enables scalability, precise control over the molecular structure, block copolymer structures, and high-end group fidelity. Moreover, it is demonstrated that these materials can be crosslinked to form gels with quasi-ideal network topology, exhibiting light-triggered contraction. The influence of arm length and polymer structure is investigated, and the first molecular dynamics simulation framework to gain deeper insights into the contraction processes is developed. Leveraging this scalable methodology, the creation of bilayer soft robotic devices and cargo-lifting artificial muscles is showcased, highlighting the versatility and potential applications of this advanced polymer chemistry approach. It is anticipated that the integrated experimental and simulation framework will accelerate scalable approaches for active polymer materials based on molecular machines, opening up new horizons in materials science and bioscience.

An injectable and coagulation-independent Tetra-PEG hydrogel bioadhesive for post-extraction hemostasis and alveolar bone regeneration

Effective control of post-extraction hemorrhage and alveolar bone resorption is critical for successful extraction socket treatment, which remains an unmet clinical challenge. Herein, an injectable Tetra-PEG hydrogel that possesses rapid gelation, firm tissue adhesion, high mechanical strength, suitable degradability, and excellent biocompatibility is developed as a sutureless and coagulation-independent bioadhesive for the management of extraction sockets. Our results demonstrate that the rapid and robust adhesive sealing of the extraction socket by the Tetra-PEG hydrogel can provide reliable protection for the underlying wound and stabilize blood clots to facilitate tissue healing. In vivo experiments using an anticoagulated rat tooth extraction model show that the hydrogel significantly outperformed clinically used cotton and gelatin sponge in hemostatic efficacy, wound closure, alveolar ridge preservation, and in situ alveolar bone regeneration. Histomorphological evaluations reveal the mechanisms for accelerated bone repair through suppressed long-term inflammation, elevated collagen deposition, higher osteoblast activity, and enhanced angiogenesis. Together, our study highlights the clinical potential of the developed injectable Tetra-PEG hydrogel for treating anticoagulant-related post-extraction hemorrhage and improving socket healing.

Cyclodextrin 'Chaperones' Enable Quasi-Ideal Supramolecular Network Formation and Enhanced Photodimerization of Hydrophobic, Red-shifted Photoswitches in Water

Precision-engineered light-triggered hydrogels are important for a diversity of applications. However, fields such as biomaterials require wavelength outside the harsh UV regime to prevent photodamage, typically requiring chromophores with extended π-conjugation that suffer from poor water solubility. Herein, we demonstrate how cyclodextrins can be used as auxiliary agents to not only solubilize such chromophores, but even to preorganize them in a 2 : 2 host-guest inclusion complex to facilitate photodimerization. We apply our concept to styrylpyrene-end-functionalized star-shaped polyethylene glycols (sPEGs). We initially unravel details of the host-guest inclusion complex using spectroscopy and mass spectrometry to give clear evidence of a 2 : 2 complex formation. Subsequently, we show that the resultant supramolecularly linked hydrogels conform to theories of supramolecular quasi-ideal model networks, and derive details on their association dynamics using in-depth rheological measurements and kinetic models. By comparing sPEGs of different arm length, we further elucidate the model network topology and the accessible mechanical property space. The photo-mediated dimerization proceeds smoothly, allowing to transform the supramolecular model networks into covalent ones. We submit that our strategy opens avenues for executing hydrophobic photochemistry in aqueous environments with enhanced control over reactivity, hydrogel topology or programmable mechanical properties.

From Fluorescence-Transfer-Lightening-Printing-Assisted Conductive Adhesive Nanocomposite Hydrogels toward Wearable Interactive Optical Information-Electronic Strain Sensors

The interactive flexible device, which monitors the human motion in optical and electrical synergistic modes, has attracted growing attention recently. The incorporation of information attribute within the optical signal is deemed advantageous for improving the interactive efficiency. Therefore, the development of wearable optical information-electronic strain sensors holds substantial promise, but integrating and synergizing various functions and realizing strain-mediated information transformation keep challenging. Herein, an amylopectin (AP) modified nanoclay/polyacrylamide-based nanocomposite (NC) hydrogel and an aggregation-induced-emission-active ink are fabricated. Through the fluorescence-transfer printing of the ink onto the hydrogel film in different strains with nested multiple symbolic information, a wearable interactive fluorescent information-electronic strain sensor is developed. In the sensor, the nanoclay plays a synergistic "one-stone-three-birds" role, contributing to "lightening" fluorescence (≈80 times emission intensity enhancement), ionic conductivity, and excellent stretchability (>1000%). The sensor has high biocompatibility, resilience (elastic recovery ratio: 97.8%), and strain sensitivity (gauge factor (GF): 10.9). Additionally, the AP endows the sensor with skin adhesiveness. The sensor can achieve electrical monitoring of human joint movements while displaying interactive fluorescent information transformation. This research poses an efficient strategy to develop multifunctional materials and provides a general platform for achieving next-generation interactive devices with prospective applications in wearable devices, human-machine interfaces, and artificial intelligence.

Structuring and Shaping of Mechanically Robust and Functional Hydrogels toward Wearable and Implantable Applications

Hydrogels possess unique features such as softness, wetness, responsiveness, and biocompatibility, making them highly suitable for biointegrated applications that have close interactions with living organisms. However, conventional man-made hydrogels are usually soft and brittle, making them inferior to the mechanically robust biological hydrogels. To ensure reliable and durable operation of biointegrated wearable and implantable devices, mechanical matching and shape adaptivity of hydrogels to tissues and organs are essential. Recent advances in polymer science and processing technologies have enabled mechanical engineering and shaping of hydrogels for various biointegrated applications. In this review, polymer network structuring strategies at micro/nanoscales for toughening hydrogels are summarized, and representative mechanical functionalities that exist in biological materials but are not easily achieved in synthetic hydrogels are further discussed. Three categories of processing technologies, namely, 3D printing, spinning, and coating for fabrication of tough hydrogel constructs with complex shapes are reviewed, and the corresponding hydrogel toughening strategies are also highlighted. These developments enable adaptive fabrication of mechanically robust and functional hydrogel devices, and promote application of hydrogels in the fields of biomedical engineering, bioelectronics, and soft robotics.

Effect of Cross-Link Homogeneity on the High-Strain Behavior of Elastic Polymer Networks

Cross-link heterogeneity and topological defects have been shown to affect the moduli of polymer networks in the low-strain regime. Probing their role in the high-strain regime, however, has been difficult because of premature network fracture. Here, we address this problem by using a double-network approach to investigate the high-strain behavior of both randomly and regularly cross-linked networks with the same backbone chemistry. Randomly cross-linked poly(n-butyl acrylate) networks with target molecular weights between cross-links of 5-30 kg/mol were synthesized via free-radical polymerization, while regularly cross-linked poly(n-butyl acrylate) networks with molecular weights between cross-links of 7-38 kg/mol were synthesized via cross-linking of tetrafunctional star polymers. Both types of networks were then swollen in a monomer/cross-linker mixture, polymerized to form double networks, and characterized via uniaxial tensile testing. The onset of strain stiffening was found to occur later in regular networks than in random networks with the same modulus but was well-predicted by the target molecular weight between cross-links of each sample. These results indicate that the low- and high-strain behavior of polymer networks result from different molecular-scale features of the material and suggest that controlling network architecture offers new opportunities to both further fundamental understanding of architecture-property relationships and design materials with independently controlled moduli and strain stiffening responses.

Structural heterogeneity in tetra-armed gels revealed by computer simulation: Evidence from a graph theory assisted characterization

Designing homogeneous networks is considered one typical strategy for solving the problem of strength and toughness conflict of polymer network materials. Experimentalists have proposed the hypothesis of obtaining a structurally homogeneous hydrogel by crosslinking tetra-armed polymers, whose homogeneity was claimed to be verified by scattering characterization and other methods. Nevertheless, it is highly desirable to further evaluate this issue from other perspectives. In this study, a coarse-grained molecular dynamics simulation coupled with a stochastic reaction model is applied to reveal the topological structure of a polymer network synthesized by tetra-armed monomers as precursors. Two different scenarios, distinguished by whether internal cross-linking is allowed, are considered. We introduce the Dijkstra algorithm from graph theory to precisely characterize the network structure. The microscopic features of the network structure, e.g., loop size, dispersity, and size distribution, are obtained via the Dijkstra algorithm. By comparing the two reaction scenarios, Scenario II exhibits an overall more idealized structure. Our results demonstrate the feasibility of the Dijkstra algorithm for precisely characterizing the polymer network structure. We expect this work will provide a new insight for the evaluation and description of gel networks and further help to reveal the dynamic process of network formation.

Advances in Hydrogel-Based Drug Delivery Systems

Hydrogels, with their distinctive three-dimensional networks of hydrophilic polymers, drive innovations across various biomedical applications. The ability of hydrogels to absorb and retain significant volumes of water, coupled with their structural integrity and responsiveness to environmental stimuli, renders them ideal for drug delivery, tissue engineering, and wound healing. This review delves into the classification of hydrogels based on cross-linking methods, providing insights into their synthesis, properties, and applications. We further discuss the recent advancements in hydrogel-based drug delivery systems, including oral, injectable, topical, and ocular approaches, highlighting their significance in enhancing therapeutic outcomes. Additionally, we address the challenges faced in the clinical translation of hydrogels and propose future directions for leveraging their potential in personalized medicine and regenerative healthcare solutions.

Disulfide-Cross-Linked Tetra-PEG Gels

The preparation of polymer gels via cross-linking of four-arm star-shaped poly(ethylene glycol) (Tetra-PEG) precursors is an attractive strategy to prepare networks with relatively well-defined topologies. Typically, Tetra-PEG gels are obtained by cross-linking heterocomplementary reactive Tetra-PEG precursors. This study, in contrast, explores the cross-linking of self-reactive, thiol-end functional Tetra-PEG macromers to form disulfide-cross-linked gels. The structure of the disulfide-cross-linked Tetra-PEG gels was studied with multiple-quantum NMR (MQ-NMR) spectroscopy and small-angle neutron scattering (SANS) experiments. In line with earlier simulation studies, these experiments showed a strong dependence of the relative fractions of the different network connectivities on the concentration of the thiol-end functional Tetra-PEG macromer that was used for the synthesis of the networks. Disulfide-cross-linked Tetra-PEG gels prepared at macromer concentrations below the overlap concentration (c = 0.66c*) primarily feature defect connectivity motifs, such as primary loops and dangling ends. For networks prepared at macromer concentrations above the overlap concentration, the fraction of single-link connectivities was found to be similar to that in amide-cross-linked Tetra-PEG gels obtained by heterocomplementary cross-linking of N-hydroxysuccinimide ester and amine functional Tetra-PEG macromers. Since disulfide bonds are susceptible to reductive cleavage, these disulfide-cross-linked gels are of interest, e.g., as reduction-sensitive hydrogels for a variety of biomedical applications.

Advancing homogeneous networking principles for the development of fatigue-resistant, low-swelling and sprayable hydrogels for sealing wet, dynamic and concealed wounds in vivo

Effective sealing of wet, dynamic and concealed wounds remains a formidable challenge in clinical practice. Sprayable hydrogel sealants are promising due to their ability to cover a wide area rapidly, but they face limitations in dynamic and moist environments. To address this issue, we have employed the principle of a homogeneous network to design a sprayable hydrogel sealant with enhanced fatigue resistance and reduced swelling. This network is formed by combining the spherical structure of lysozyme (LZM) with the orthotetrahedral structure of 4-arm-polyethylene glycol (4-arm-PEG). We have achieved exceptional sprayability by controlling the pH of the precursor solution. The homogeneous network, constructed through uniform cross-linking of amino groups in protein and 4-arm-PEG-NHS, provides the hydrogel with outstanding fatigue resistance, low swelling and sustained adhesion. In vitro testing demonstrated that it could endure 2000 cycles of underwater shearing, while in vivo experiments showed adhesion maintenance exceeding 24 h. Furthermore, the hydrogel excelled in sealing leaks and promoting ulcer healing in models including porcine cardiac hemorrhage, lung air leakage and rat oral ulcers, surpassing commonly used clinical materials. Therefore, our research presents an advanced biomaterial strategy with the potential to advance the clinical management of wet, dynamic and concealed wounds.

Mechanically Interlocked Polymers with Dense Mechanical Bonds

ConspectusMechanically interlocked polymers (MIPs) such as polyrotaxanes and polycatenanes are polymer architectures that incorporate mechanical bonds, which represent a compelling frontier in polymer science. MIPs with cross-linked structures are known as mechanically interlocked networks (MINs) and are widely utilized in materials science. Leveraging the motion of mechanical bonds, MINs hold the potential for achieving a combination of robustness and dynamicity. Currently, the reported MINs predominantly consist of networks with discrete mechanical bonds as cross-linking points, exemplified by well-known slide-ring materials and rotaxane/catenane cross-linked polymers. The motion of these mechanically interlocked cross-linking points facilitates the redistribution of tension throughout the network, effectively preventing stress concentration and thereby enhancing material toughness. In these instances, the impact of mechanical bonds can be likened to the adage "small things can make a big difference", whereby a limited number of mechanical bonds substantially elevate the mechanical performance of conventional polymers. In addition to MINs cross-linked by mechanical bonds, there is another type of MIN in which their principal parts are polymer chains composed of dense mechanical bonds. Within these MINs, mechanical bonds generally serve as repeating units, and their unique properties stem from integrating and amplifying the function of a large amount of mechanical bonds. Consequently, MINs with dense mechanical bonds tend to reflect the intrinsic properties of mechanical interlocked polymers, making their exploration critical for a comprehensive understanding of MIPs. Nevertheless, investigations into MINs featuring dense mechanical bonds remain relatively scarce.This Account presents a comprehensive overview of our investigation and insights into MINs featuring dense mechanical bonds. First, we delve into the synthetic strategies employed to effectively prepare MINs with dense mechanical bonds, while critically evaluating their advantages and limitations. Through meticulous control of the core interlocking step, three distinct strategies have emerged: mechanical interlocking followed by polymerization, supramolecular polymerization followed by mechanical interlocking, and dynamic interlocking. Furthermore, we underscore the structure-property relationships of MINs with dense mechanical bonds. The macroscopic properties of MINs originate from integrating and amplifying countless microscopic motions of mechanical bonds, a phenomenon we define as an integration and amplification mechanism. Our investigation has revealed detailed motion characteristics of mechanical bonds in bulk mechanically interlocked materials, encompassing the quantification of motion activation energy, discrimination of varying motion distances, and elucidation of the recovery process. Additionally, we have elucidated their influence on the mechanical performance of the respective materials. Moreover, we have explored potential applications of MINs, leveraging their exceptional mechanical properties and dynamicity. These applications include enhancing the toughness of conventional polymers, engineering mechanically adaptive and multifunctional aerogels, and mitigating Li protrusion as interfacial layers in lithium-ion batteries. Finally, we offer our personal perspectives on the promises, opportunities, and key challenges in the future development of MINs with dense mechanical bonds, underscoring the potential for transformative advancements in this burgeoning field.

Synthesis of Topological Gels by Penetrating Polymerization Using a Molecular Net

Topological gels possess structures that are cross-linked only via physical constraints; ideally, no attractive intermolecular interactions act between their components, which yields interesting physical properties. However, most reported previous topological gels were synthesized based on supramolecular interlocked structures such as polyrotaxane, for which attractive intermolecular interactions are essential. Here, we synthesize a water-soluble "molecular net" (MN) with a large molecular weight and three-dimensional network structure using poly(ethylene glycol). When a water-soluble monomer (N-isopropylacrylamide) is polymerized in the presence of the MNs, the extending polymer chains penetrates the MNs to form an ideal topological MN gel with no specific attractive interactions between its components. The MN gels show unique physical properties as well a significantly high degree of swelling and high extensibility due to slipping of the physical cross-linking. We postulate this method to yield a new paradigm in gel science with unprecedented physical properties.

Injectable phase-separated tetra-armed poly(ethylene glycol) hydrogel scaffold allows sustained release of growth factors to enhance the repair of critical bone defects

With the rising prevalence of bone-related injuries, it is crucial to improve treatments for fractures and defects. Tissue engineering offers a promising solution in the form of injectable hydrogel scaffolds that can sustain the release of growth factors like bone morphogenetic protein-2 (BMP-2) for bone repair. Recently, we discovered that tetra-PEG hydrogels (Tetra gels) undergo gel-gel phase separation (GGPS) at low polymer content, resulting in hydrophobicity and tissue affinity. In this work, we examined the potential of a newer class of gel, the oligo-tetra-PEG gel (Oligo gel), as a growth factor-releasing scaffold. We investigated the extent of GGPS occurring in the two gels and assessed their ability to sustain BMP-2 release and osteogenic potential in a mouse calvarial defect model. The Oligo gel underwent a greater degree of GGPS than the Tetra gel, exhibiting higher turbidity, hydrophobicity, and pore formation. The Oligo gel demonstrated sustained protein or growth factor release over a 21-day period from protein release kinetics and osteogenic cell differentiation studies. Finally, BMP-2-loaded Oligo gels achieved complete regeneration of critical-sized calvarial defects within 28 days, significantly outperforming Tetra gels. The easy formulation, injectability, and capacity for sustained release makes the Oligo gel a promising candidate therapeutic biomaterial.

Tough Hydrogels with Different Toughening Mechanisms and Applications

Load-bearing biological tissues, such as cartilage and muscles, exhibit several crucial properties, including high elasticity, strength, and recoverability. These characteristics enable these tissues to endure significant mechanical stresses and swiftly recover after deformation, contributing to their exceptional durability and functionality. In contrast, while hydrogels are highly biocompatible and hold promise as synthetic biomaterials, their inherent network structure often limits their ability to simultaneously possess a diverse range of superior mechanical properties. As a result, the applications of hydrogels are significantly constrained. This article delves into the design mechanisms and mechanical properties of various tough hydrogels and investigates their applications in tissue engineering, flexible electronics, and other fields. The objective is to provide insights into the fabrication and application of hydrogels with combined high strength, stretchability, toughness, and fast recovery as well as their future development directions and challenges.

Functional hemostatic hydrogels: design based on procoagulant principles

Uncontrolled hemorrhage results in various complications and is currently the leading cause of death in the general population. Traditional hemostatic methods have drawbacks that may lead to ineffective hemostasis and even the risk of secondary injury. Therefore, there is an urgent need for more effective hemostatic techniques. Polymeric hemostatic materials, particularly hydrogels, are ideal due to their biocompatibility, flexibility, absorption, and versatility. Functional hemostatic hydrogels can enhance hemostasis by creating physical circumstances conducive to hemostasis or by directly interfering with the physiological processes of hemostasis. The procoagulant principles include increasing the concentration of localized hemostatic substances or establishing a physical barrier at the physical level and intervention in blood cells or the coagulation cascade at the physiological level. Moreover, synergistic hemostasis can combine these functions. However, some hydrogels are ineffective in promoting hemostasis or have a limited application scope. These defects have impeded the advancement of hemostatic hydrogels. To provide inspiration and resources for new designs, this review provides an overview of the procoagulant principles of hemostatic hydrogels. We also discuss the challenges in developing effective hemostatic hydrogels and provide viewpoints.

In situ forming, mechanically resilient hydrogels prepared from 4a-[PEG-b-PTMC-Ac] and thiolated chondroitin sulfate for nucleus pulposus cell delivery

Intervertebral disk degeneration (IVDD) is a common condition that causes severe back pain and affects patients' mobility and life quality considerably. IVDD originates within the central region of the disk called the nucleus pulposus (NP). Removing the damaged tissue and replacing it with NP cells (NPCs) delivered within an in situ forming hydrogel is a promising treatment approach. Herein we describe a hydrogel formulation based on 4-arm [poly(ethylene glycol)-b-poly(trimethylene carbonate)-acrylate] (4a[PEG-b-PTMC-Ac]) crosslinked with thiolated chondroitin sulfate via Michael-type reaction for this purpose. A library of hydrogels based on 15 kDa 4a-[PEG] with PTMC blocks of varying molecular weight were prepared and characterized. The instantaneous moduli of the hydrogels were adjustable from 24 to 150 kPa depending on the length of the PTMC block and the polymer volume fraction. The influence of each of these parameters was effectively explained using both scaling or mean field theories of polyelectrolyte hydrogels. The hydrogels were resistant to cyclic compressive loading and degraded gradually over 70 days in vitro. A hydrogel formulation with an instantaneous modulus at the high end of the range of values reported for human NP tissue was chosen to assess the ability of these hydrogels for delivering NPCs. The prepolymer solution was injectable and formed a hydrogel within 30 minutes at 37 °C. Bovine NPCs were encapsulated within this hydrogel with high viability and proliferated throughout a 28 day, hypoxic culture period. The encapsulated NPCs formed clusters and deposited collagen type II but no collagen type I within the hydrogels. Despite an initial gradual decrease, a steady-state modulus was reached at the end of the 28 day culture period that was within the range reported for healthy human NP tissue. This in situ forming hydrogel formulation is a promising approach and with further development could be a viable clinical treatment for IVDD.

Elaboration and rheological characterization of nanocomposite hydrogels containing C60 fullerene nanoplatelets

Nanocomposite hydrogels were elaborated that consisted of a physical network formed by an amphiphilic polymer in which C60 fullerene nanoplatelets were embedded. Characterization showed that the nanoplatelets within the polymer network were aggregated. The presence of these nanoplatelets led to an increase of the shear modulus of the hydrogels, that cannot be explained by a filler effect alone. The nanocomposite gels displayed similar rheological behavior, both in linear and non-linear domains, as neat hydrogels at higher polymer concentrations. We suggest that the particles reinforced the gels by forming additional connections between the polymer chains.

The Toughness-Enhanced Atelocollagen Double-Network Gel for Biomaterials

A tough gel composed of atelocollagen, which lacks an immunogenetic site, is a promising material for biomedical application. In this study, we created a composite hydrogel composed of atelocollagen gel cross-linked with glutaraldehyde (GA) and poly-(N,N-dimethylacrylamide) gel exhibiting biocompatibility based on the double-network (DN) gel principle. The tensile toughness of atelocollagen gel remained constant regardless of the amount of cross-linker (GA) used. In contrast, tensile tests of the DN gel indicated that mechanical properties, such as fracture stress and toughness, were significantly higher than those of the atelocollagen gel. Moreover, fibroblast cells adhered and spread on the gels, the Schiff bases of which were treated via reductive amination for detoxification from GA. These findings demonstrate the potential of the proposed gel materials as artificial alternative materials to soft tissues with sub-MPa fracture stress.

Structural Homogeneity of Macromolecular Networks by End-to-End Click Chemistry between Discrete Tetrahedral Star Macromolecules

Cross-linking via the end-to-end click chemistry of multiarm star polymers creates polymer networks with minimal inhomogeneities. Although it has been suggested that the mechanical and swelling properties of such networks depend on the absence of defects, the structural details of homogeneous networks created by this method have not yet been studied at the molecular level. Here, we report the synthesis of discrete tetrahedral star macromolecules (dTSMs) composed of polylactide (PLA) arms with discrete molecular weight and sequence. Polymer networks prepared by 4 × 4 cross-linking by Cu-free strain-promoted cyclooctyne-azide click chemistry (SPAAC) reaction exhibited a high degree of swelling (>40 fold by weight) in solvents without sacrificing mechanical robustness (elastic modulus >4 kPa). The structural details of the networks were investigated by network disassembly spectrometry (NDS) using MALDI-TOF mass spectrometry. By implementing a cleavable repeating unit in the discrete PLA arms of dTSM in a sequence-specific manner, the networks could be disassembled into fragments having discrete molecular weights precisely representing their connectivity in the network. This NDS analysis confirmed that end-to-end click reactions of dTSM networks resulted in the formation of a homogeneous network above the critical concentration (∼10 w/v%) of building blocks in the solution.

One-Pot Approach to Synthesize Tough and Cell Adhesive Double-Network Hydrogels Consisting of Fully Synthetic Materials of Self-Assembling Peptide and Poly(ethylene glycol)

Hydrogels with a double network (DN) structure are compelling biomaterials, holding potential for use as artificial extracellular matrices. Generally, the DN approach imparts hydrogels with high mechanical strength and cell-adhesive properties. However, achieving this often demands a complex multistep process involving potentially hazardous free-radical polymerization, which can result in toxicity. This limits their broad biological applications. In this work, we introduce a straightforward yet biocompatible method to fabricate tough and cell-adhesive DN hydrogels using entirely synthetic materials: the self-assembling peptide (RADA16) and poly(ethylene glycol) (PEG). An in situ mixing of these components leads to the sequential formation of DN hydrogels─first through the self-assembly of the RADA16 peptide and then via chemical cross-linking between PEG molecules. Hydrogels produced this way exhibited up to a 10-fold increase in fracture energy, and cells seeded on their surfaces showcased good attachment. Our design underscores the efficacy of the DN approach and the promising applications of peptides in tissue engineering.

An elastomer with ultrahigh strain-induced crystallization

Strain-induced crystallization (SIC) prevalently strengthens, toughens, and enables an elastocaloric effect in elastomers. However, the crystallinity induced by mechanical stretching in common elastomers (e.g., natural rubber) is typically below 20%, and the stretchability plateaus due to trapped entanglements. We report a class of elastomers formed by end-linking and then deswelling star polymers with low defects and no trapped entanglements, which achieve strain-induced crystallinity of up to 50%. The deswollen end-linked star elastomer (DELSE) reaches an ultrahigh stretchability of 12.4 to 33.3, scaling beyond the saturated limit of common elastomers. The DELSE also exhibits a high fracture energy of 4.2 to 4.5 kJ m-2 while maintaining low hysteresis. The heightened SIC and stretchability synergistically promote a high elastocaloric effect with an adiabatic temperature change of 9.3°C.

Dual Ratiometric Fluorescence Monitoring of Mechanical Polymer Chain Stretching and Subsequent Strain-Induced Crystallization

Tracking the behavior of mechanochromic molecules provides valuable insights into force transmission and associated microstructural changes in soft materials under load. Herein, we report a dual ratiometric fluorescence (FL) analysis for monitoring both mechanical polymer chain stretching and strain-induced crystallization (SIC) of polymers. SIC has recently attracted renewed attention as an effective mechanism for improving the mechanical properties of polymers. A polyurethane (PU) film incorporating a trace of a dual-emissive flapping force probe (N-FLAP, 0.008 wt %) exhibited a blue-to-green FL spectral change in a low-stress region (<20 MPa), resulting from conformational planarization of the probe in mechanically stretched polymer chains. More importantly, at higher probe concentrations (∼0.65 wt %), the PU film showed a second spectral change from green to yellow during the SIC growth (20-65 MPa) due to self-absorption of scattered FL in a short wavelength region. The reversibility of these spectral changes was demonstrated by load-unload cycles. With these results in hand, the degrees of the polymer chain stretching and the SIC were quantitatively mapped and monitored by dual ratiometric imaging based on different FL ratios (I525/I470 and I525/I600). Simultaneous analysis of these two mappings revealed a spatiotemporal gap in the distribution of the polymer chain stretching and the SIC. The combinational use of the dual-emissive force probe and the ratiometric FL imaging is a universal approach for the development of soft matter physics.

Percolation-induced gel-gel phase separation in a dilute polymer network

Cosmic large-scale structures, animal flocks and living tissues can be considered non-equilibrium organized systems created by dissipative processes. Replicating such properties in artificial systems is still difficult. Herein we report a dissipative network formation process in a dilute polymer-water mixture that leads to percolation-induced gel-gel phase separation. The dilute system, which forms a monophase structure at the percolation threshold, spontaneously separates into two co-continuous gel phases with a submillimetre scale (a dilute-percolated gel) during the deswelling process after the completion of the gelation reaction. The dilute-percolated gel, which contains 99% water, exhibits unexpected hydrophobicity and induces the development of adipose-like tissues in subcutaneous tissues. These findings support the development of dissipative structures with advanced functionalities for distinct applications, ranging from physical chemistry to tissue engineering.

Force-Triggered Self-Destructive Hydrogels

Self-destructive polymers (SDPs) are defined as a class of smart polymers that autonomously degrade upon experiencing an external trigger, such as a chemical cue or optical excitation. Because SDPs release the materials trapped inside the network upon degradation, they have potential applications in drug delivery and analytical sensing. However, no known SDPs that respond to external mechanical forces have been reported, as it is fundamentally challenging to create mechano-sensitivity in general and especially so for force levels below those required for classical force-induced bond scission. To address this challenge, the development of force-triggered SDPs composed of DNA crosslinked hydrogels doped with nucleases is described here. Externally applied piconewton forces selectively expose enzymatic cleavage sites within the DNA crosslinks, resulting in rapid polymer self-degradation. The synthesis and the chemical and mechanical characterization of DNA crosslinked hydrogels, as well as the kinetics of force-triggered hydrolysis, are described. As a proof-of-concept, force-triggered and time-dependent rheological changes in the polymer as well as encapsulated nanoparticle release are demonstrated. Finally, that the kinetics of self-destruction are shown to be tuned as a function of nuclease concentration, incubation time, and thermodynamic stability of DNA crosslinkers.

Exceptionally Fast Temperature-Responsive, Mechanically Strong and Extensible Monolithic Non-Porous Hydrogels: Poly(N-isopropylacrylamide) Intercalated with Hydroxypropyl Methylcellulose

Exceptionally fast temperature-responsive, mechanically strong, tough and extensible monolithic non-porous hydrogels were synthesized. They are based on divinyl-crosslinked poly(N-isopropyl-acrylamide) (PNIPAm) intercalated by hydroxypropyl methylcellulose (HPMC). HPMC was largely extracted after polymerization, thus yielding a 'template-modified' PNIPAm network intercalated with a modest residue of HPMC. High contents of divinyl crosslinker and of HPMC caused a varying degree of micro-phase-separation in some products, but without detriment to mechanical or tensile properties. After extraction of non-fixed HPMC, the micro-phase-separated products combine superior mechanical properties with ultra-fast T-response (in 30 s). Their PNIPAm network was highly regular and extensible (intercalation effect), toughened by hydrogen bonds to HPMC, and interpenetrated by a network of nano-channels (left behind by extracted HPMC), which ensured the water transport rates needed for ultra-fast deswelling. Moreover, the T-response rate could be widely tuned by the degree of heterogeneity during synthesis. The fastest-responsive among our hydrogels could be of practical interest as soft actuators with very good mechanical properties (soft robotics), while the slower ones offer applications in drug delivery systems (as tested on the example of Theophylline), or in related biomedical engineering applications.