Sticking Together: Injectable Granular Hydrogels with Increased Functionality via Dynamic Covalent Inter-Particle Crosslinking

Dynamic hydrogels for biofabrication: A review

Reversibly crosslinked dynamic hydrogels have emerged as a significant material platform for biomedical applications owing to their distinctive time-dependent characteristics, including shear-thinning, self-healing, stress relaxation, and creep. These physical properties permit the use of dynamic hydrogels as injectable carriers or three-dimensional printable bioinks. It is noteworthy that matrix dynamics can serve as physical cues that stimulate cellular processes. Therefore, dynamic hydrogels are preferred for tissue engineering and biofabrication, which seek to create functional tissue constructs that require regulation of cellular processes. This review summarizes the critical biophysical properties of dynamic hydrogels, various cellular processes and related mechanisms triggered by hydrogel dynamics, particularly in three-dimensional culture scenarios. Subsequently, we present an overview of advanced biofabrication techniques, particularly 3D bioprinting, of dynamic hydrogels for the large-scale production of tissue and organ engineering models. This review presents an overview of the strategies that can be used to expand the range of applications of dynamic hydrogels in biofabrication, while also addressing the challenges and opportunities that arise in the field. This review highlights the importance of matrix dynamics in regulating cellular processes and elucidates strategies for leveraging them in the context of biofabrication.

Supramolecular assembly of multi-purpose tissue engineering platforms from human extracellular matrix

Recapitulating the biophysical and biochemical complexity of the extracellular matrix (ECM) remains a major challenge in tissue engineering. Hydrogels derived from decellularized ECM provide a unique opportunity to replicate the architecture and bioactivity of native ECM, however, they exhibit limited long-term stability and mechanical integrity. In turn, materials assembled through supramolecular interactions have achieved considerable success in replicating the dynamic biophysical properties of the ECM. Here, we merge both methodologies by promoting the supramolecular assembly of decellularized human amniotic membrane (hAM), mediated by host-guest interactions between hAM proteins and acryloyl-β-cyclodextrin (AcβCD). Photopolymerization of the cyclodextrins results in the formation of soft hydrogels that exhibit tunable stress relaxation and strain-stiffening. Disaggregation of bulk hydrogels yields an injectable granular material that self-reconstitutes into shape-adaptable bulk hydrogels, supporting cell delivery and promoting neovascularization. Additionally, cells encapsulated within bulk hydrogels sense and respond to the biophysical properties of the surrounding matrix, as early cell spreading is favored in hydrogels that exhibit greater susceptibility to applied stress, evidencing proper cell-matrix interplay. Thus, this system is shown to be a promising substitute for native ECM in tissue repair and modelling.

Self-Healing Hydrogels: Mechanisms and Biomedical Applications

Hydrogels have emerged as dependable candidates for tissue repair because of their exceptional biocompatibility and tunable mechanical properties. However, conventional hydrogels are vulnerable to damage owing to mechanical stress and environmental factors that compromise their structural integrity and reduce their lifespan. In contrast, self-healing hydrogels with their inherent ability to restore structure and function autonomously offer prolonged efficacy and enhanced appeal. These hydrogels can be engineered into innovative forms including stimulus-responsive, self-degradable, injectable, and drug-loaded variants, thereby enhancing their applicability in wound healing, drug delivery, and tissue engineering. This review summarizes the categories and mechanisms of self-healing hydrogels, along with their biomedical applications, including tissue repair, drug delivery, and biosensing. Tissue repair includes wound healing, bone-related repair, nerve repair, and cardiac repair. Additionally, we explored the challenges that self-healing hydrogels continue to face in tissue repair and presented a forward-looking perspective on their development. Consequently, it is anticipated that self-healing hydrogels will be progressively designed and developed for applications that extend beyond tissue repair to a broader range of biomedical applications.

Injectable microgel and micro-granular hydrogels for bone tissue engineering

Injectable microgels, made from both natural and synthetic materials, are promising platforms for the encapsulation of cells or bioactive agents, such as drugs and growth factors, for delivery to injury sites. They can also serve as effective micro-scaffolds in bone tissue engineering (BTE), offering a supportive environment for cell proliferation or differentiation into osteoblasts. Microgels can be injected in the injury sites individually or in the form of aggregated/jammed ones named micro-granular hydrogels. This review focuses on common materials and fabrication techniques for preparing injectable microgels, as well as their characteristics and applications in BTE. These applications include their use as cell carriers, delivery systems for bioactive molecules, micro-granular hydrogels, bio-inks for bioprinting, three-dimensional microarrays, and the formation of microtissues. Furthermore, we discuss the current and potential future applications of microgels in bone tissue regeneration.

Real-time quantification of microfluidic hydrogel crosslinking via gas-phase electrophoresis

This study presents a novel approach for the controlled synthesis and real-time characterization of crosslinked hyaluronic acid (HA) hydrogels utilizing a microfluidic platform coupled with hyphenated electrospray-differential mobility analysis (ES-DMA). By precisely controlling key synthesis parameters within the microfluidic environment, including pH, temperature, reaction time, and the molar ratio of HA to crosslinker (1,4-butanediol diglycidyl ether, BDDE), we successfully synthesized HA hydrogels with tailored size and properties. The integrated ES-DMA system provides rapid, in-line analysis of hydrogel particle size and distribution, enabling real-time monitoring and optimization of the synthesis process. Furthermore, small-angle x-ray scattering (SAXS) was employed to complement ES-DMA analysis, providing valuable insights into the internal structure and extent of crosslinking within the synthesized hydrogels. The evolution of the number-based particle size distribution revealed a strong correlation with the synthesis conditions, demonstrating the high degree of controllability achieved by this integrated approach. This novel methodology offers a promising platform for the high-throughput synthesis of uniform and well-defined hydrogel nanoparticles with enhanced traceability, paving the way for advancements in various applications including drug delivery, tissue engineering, and biomaterials.

Granular hydrogels as brittle yield stress fluids

While granular hydrogels are increasingly used in biomedical applications, methods to capture their rheological behavior generally consider shear-thinning and self-healing properties or produce ensemble metrics such as the dynamic moduli. Analytical approaches paired with common oscillatory shear tests can describe not only solid-like and fluid-like behavior of granular hydrogels but also transient characteristics inherent in yielding and unyielding processes. Combining oscillatory shear testing with consideration of Brittility (Bt) via the Kamani-Donley-Rogers (KDR) model, we show granular hydrogels behave as brittle yield stress fluids with complex transient rheology. We quantify steady and transient rheology as a function of microgel (composition; diameter) and granular (packing; droplet heterogeneity) assembly properties for mixtures of polyethylene glycol and gelatin microgels. The KDR model with Bt captures granular hydrogel behavior for a wide range of design parameters, reducing the complex transient rheology to a determination of model parameters. We describe the impact of composition on rheological behavior and model parameters in monolithic and mixed granular hydrogels. The model robustly captures self-healing behavior and reveals granular relaxation time depends on strain amplitude. This quantitative framework is an important step toward rational design of granular hydrogels for applications ranging from injection and in situ stabilization to 3D bioprinting.

Microporous annealed particle hydrogels in cell culture, tissue regeneration, and emerging application in cancer immunotherapy

Microporous annealed particle (MAP) hydrogels consist of densely crosslinked and annealed hydrogel particles. Compared to common hydrogels, the inherent porosity within and among these hydrogel particles offers interconnected channels for substance exchange in addition to sufficient growth space for cells, thereby forming a three-dimensional culture system that highly mimics the in vivo microenvironment. Such characteristics enable MAP hydrogels to adapt to various requirements of biomedical applications, along with their excellent injectability and mechanical properties. This review initially provides a comprehensive summary of the fabrication methods and material types of MAP hydrogels, alongside an assessment of their mechanical properties and porosity. In vitro studies are evaluated based on the impact of MAP hydrogels on cellular behaviors, focusing on cell proliferation, differentiation, migration, activity, and phenotype. In vivo research highlights the promising applications of MAP hydrogels in tissue regeneration, as well as their innovative use in cancer immunotherapy. Current challenges and future research directions are outlined, underscoring the potential of MAP hydrogels to significantly improve clinical outcomes in cancer treatment and regenerative medicine.

Controlling Microparticle Aspect Ratio via Photolithography for Injectable Granular Hydrogel Formation and Cell Delivery

Granular hydrogels are injectable and inherently porous biomaterials assembled through the packing of microparticles. These particles typically have a symmetric and spherical shape. However, recent studies have shown that asymmetric particles with high aspect ratios, such as fibers and rods, can significantly improve the mechanics, structure, and cell-guidance ability of granular hydrogels. Despite this, it remains unknown how controlled changes in the particle aspect ratio influence the injectability, porosity, and cell-instructive capabilities of granular hydrogels. Part of the challenge lies in obtaining microparticles with precisely tailored dimensions using fabrication methods such as flow-focusing microfluidics or extrusion fragmentation. In this work, we leveraged facile photolithography and photocurable hyaluronic acid to fabricate rod-shaped microparticles with widths and heights of 130 μm and lengths that varied from 260 to 1300 μm to obtain aspect ratios (ARs) of 2, 4, 6, 8, and 10. All AR microparticles formed porous and injectable granular hydrogels after centrifugation jamming. Interestingly, the longest microparticles neither clogged the needle nor fractured after extrusion from a syringe. This was attributed to a relatively low elastic modulus that permitted microparticle pliability and reversible deformation under shear. Cells (NIH/3T3 fibroblasts) mixed with the jammed microparticles and injected into molds remained viable, adhered to the particles' surface, and showed a significant and rapid rate of proliferation over a period of 7 days compared to bulk hydrogels. The proliferation rate and morphology of the cells were significantly influenced by the particle AR, with higher cell numbers observed with intermediate ARs, likely attributable to the surface area available for cell adhesion. These findings showcase the utility of injectable granular hydrogels made with high-aspect-ratio microparticles for biomedical applications.

Interparticle Crosslinked Ion-responsive Microgels for 3D and 4D (Bio)printing Applications

Microgels offer unique advantages over bulk hydrogels due to their improved diffusion limits for oxygen and nutrients. Particularly, stimuli-responsive microgels with inherently bioactive and self-supporting properties emerge as highly promising biomaterials. This study unveils the development of interparticle-crosslinked, self-supporting, ion-responsive microgels tailored for 3D and 4D (bio)printing applications. A novel strategy was proposed to develop microgels that enabled interparticle crosslinking, eliminating the need for filler hydrogels and preserving essential microscale void spaces to support cell migration and vascularization. Additionally, these microgels possessed unique, ion-responsive shrinking behavior primarily by the Hofmeister effect, reversible upon the removal of the stimulus. Two types of microgels, spherical (µS) and random-shaped (µR), were fabricated, with µR exhibiting superior mechanical properties and higher packing density. Fabricated microgel-based constructs supported angiogenesis with tunable vessel size based on interstitial void spaces while demonstrating excellent shear-thinning and self-healing properties and high print fidelity. Various bioprinting techniques were employed and validated using these microgels, including extrusion-based, embedded, intraembedded, and aspiration-assisted bioprinting, facilitating the biofabrication of scalable constructs. Multi-material 4D printing was achieved by combining ion-responsive microgels with non-responsive microgels, enabling programmable shape transformations upon exposure to ionic solutions. Utilizing 4D printing, complex, dynamic structures were generated such as coiling filaments, grippers, and folding sheets, providing a foundation for the development of advanced tissue models and devices for regenerative medicine and soft robotics, respectively.

Programmed shape transformations in cell-laden granular composites

Tissues form during development through mechanical compaction of their extracellular matrix (ECM) and shape morphing, processes that result in complex-shaped structures that contribute to tissue function. While observed in vivo, control over these processes in vitro to understand both tissue development and guide tissue formation has remained challenging. Here, we use combinations of mesenchymal stromal cell spheroids and hydrogel microparticles (microgels) with varied hydrolytic stability to fabricate programmable and dynamic granular composites that control compaction and tissue formation over time. Mixed microgel populations of varying stability provide a further handle to alter compaction, and the level of compaction guides the uniformity and level of ECM deposition within tissues. Last, spatially patterned granular composites of varying compaction enable shape transformations (i.e., bending/curvature) that are stable with culture and are predicted by finite element models.

Activation of muscle amine functional groups using eutectic mixture to enhance tissue adhesiveness of injectable, conductive and therapeutic granular hydrogel for diabetic ulcer regeneration

Herein, Polydopamine-modified microgels and microgels incorporated with superficial epoxy groups were synthesized and applied as precursors for the fabrication of four granular hydrogels. To enhance the tissue adhesiveness, a ternary deep eutectic solvent was synthesized to activate the muscle amine functional groups facilitating the formation of robust NC bonds at ambient conditions. At a certain shear rate of 10 s-1, hydrogel DMG displayed a viscosity of 9×103 Pa/s, representing the highest complex viscosity among the tested hydrogels primarily driven by quinone groups in PDA which enhanced reversible interactions, thereby increasing particle cohesion. Moreover, the intersection point escalating from about 4×103 to approximately 9×104 as the concentration of DMG increased from 0 % (for MG) to 70% (7D3MG) by weight. There was a decrease in adhesion strength from 0.45 ± 0.08 N in MG to 0.39 ± 0.16 N, 0.35± 0.18 N, and 0.33 ± 0.15 N for 3D7MG, 7D3MG, and DMG respectively, suggesting that MG was capable of forming numerous covalent bonds, thereby enhancing its adhesion to the substrate. The type of eutectic mixture affected the electrical conductivity and a very important point was the changes in resistance value with time. For MG catalyzed by [DES]AZG, the resistance increased only by 1.3 % (from 3.37 to 3.81 kΩ) at day 3 and 37.09 % (from 3.37 to 4.62 kΩ) at day 5. The 3D7MG hydrogel exhibited superior therapeutic efficacy toward diabetic wound regeneration. The proliferation index value for 3D7MG-[DES]AZG and 3D7MG-[DES]AG were calculated 42.3 % and 58.6 %, respectively, while the control group exhibited a lower value of 37.8 %.

A Universal Strategy to Construct High-Performance Homo- and Heterogeneous Microgel Assembly Bioinks

Three dimensional (3D) extrusion bioprinting aims to replicate the complex architectures and functions of natural tissues and organs. However, the conventional hydrogel and new-emerging microgel bioinks are both difficult in achieving simultaneously high shape-fidelity and good maintenance of cell viability/function, leading to limited amount of qualified hydrogel/microgel bioinks. Herein, a universal strategy is reported to construct high-performance microgel assembly (MA) bioinks by using epigallocatechin gallate-modified hyaluronic acid (HA-EGCG) as coating agent and phenylboronic acid grafted hyaluronic acid (HA-PBA) as assembling agent. HA-EGCG can spontaneously form uniform coating on the microgel surface via mussel-inspired chemistry, while HA-PBA quickly forms dynamic phenylborate bonds with HA-EGCG, conferring the as-prepared MA bioinks with excellent rheological properties, self-healing, and tissue-adhesion. More importantly, this strategy is applicable to various microgel materials, enabling the preparation of homo- and heterogeneous MA (homo-MA and hetero-MA) bioinks and the hierarchical printing of complicated structures with high fidelity by integration of different microgels containing multiple materials/cells in spatial and compositional levels. It further demonstrates the printing of breast cancer organoid in vitro using homo-MA and hetero-MA bioinks and its preliminary application for drug testing. This universal strategy offers a new solution to construct high-performance bioinks for extrusion bioprinting.

Viscoelasticity of Hyaluronic Acid Hydrogels Regulates Human Pluripotent Stem Cell-derived Spinal Cord Organoid Patterning and Vascularization

Recently, it has been recognized that natural extracellular matrix (ECM) and tissues are viscoelastic, while only elastic properties have been investigated in the past. How the viscoelastic matrix regulates stem cell patterning is critical for cell-ECM mechano-transduction. Here, this study fabricated different methacrylated hyaluronic acid (HA) hydrogels using covalent cross-linking, consisting of two gels with similar elasticity (stiffness) but different viscoelasticity, and two gels with similar viscoelasticity but different elasticity (stiffness). Meanwhile, a second set of dual network hydrogels are fabricated containing both covalent and coordinated cross-links. Human spinal cord organoid (hSCO) patterning in HA hydrogels and co-culture with isogenic human blood vessel organoids (hBVOs) are investigated. The viscoelastic hydrogels promote regional hSCO patterning compared to the elastic hydrogels. More viscoelastic hydrogels can promote dorsal marker expression, while softer hydrogels result in higher interneuron marker expression. The effects of viscoelastic properties of the hydrogels become more dominant than the stiffness effects in the co-culture of hSCOs and hBVOs. In addition, more viscoelastic hydrogels can lead to more Yes-associated protein nuclear translocation, revealing the mechanism of cell-ECM mechano-transduction. This research provides insights into viscoelastic behaviors of the hydrogels during human organoid patterning with ECM-mimicking in vitro microenvironments for applications in regenerative medicine.

Cardiac Matrix-Derived Granular Hydrogel Enhances Cell Function in 3D Culture

Hydrogels derived from decellularized porcine myocardial matrix have demonstrated significant potential as therapeutic delivery platforms for promoting cardiac repair after injury. Our previous study developed a fibrin-enriched cardiac matrix hydrogel to enhance its angiogenic capacities. However, the bulk hydrogel structure may limit their full potential in cell delivery. Recently, granular hydrogels have emerged as a promising class of biomaterials, offering unique features such as a highly interconnected porous structure that facilitates nutrient diffusion and enhances cell viability. Several techniques have been developed for fabricating various types of granular hydrogels, among which extrusion fragmentation is particularly appealing due to its adaptability to many types of hydrogels, low cost, and high scalability. In this study, we first confirmed the effects of the bulk cardiac matrix hydrogel on the viability of encapsulated human umbilical vein endothelial cells and human mesenchymal stem cells. We then tested the feasibility of producing granular hydrogels from both cardiac matrix and fibrin-enriched cardiac matrix through cellular cross-linking of microgels fabricated by extrusion fragmentation. Afterward, we examined the roles of the produced granular hydrogels in the embedded cells and cell spheroids. Our in vitro data demonstrate that cardiac matrix-derived granular hydrogels support optimal viability of encapsulated cells and promote sprouting of human mesenchymal stem cell spheroids. Additionally, granular hydrogel derived from fibrin-enriched cardiac matrix accelerates angiogenic sprouting of embedded human mesenchymal stem cell spheroids. The results obtained from this study lay an important foundation for the future exploration of using cardiac matrix-derived granular hydrogels for cardiac cell therapy.

Granular Hydrogels Improve Myogenic Invasion and Repair after Volumetric Muscle Loss

Skeletal muscle injuries including volumetric muscle loss (VML) lead to excessive tissue scarring and permanent functional disability. Despite its high prevalence, there is currently no effective treatment for VML. Bioengineering interventions such as biomaterials that fill the VML defect to support cell and tissue growth are a promising therapeutic strategy. However, traditional biomaterials developed for this purpose lack the pore features needed to support cell infiltration. The present study investigates for the first time, the impact of granular hydrogels on muscle repair - hypothesizing that their flowability will permit conformable filling of the defect site and their inherent porosity will support the invasion of native myogenic cells, leading to effective muscle repair. Small and large microparticle fragments are prepared from photocurable hyaluronic acid polymer via extrusion fragmentation and facile size sorting. In assembled granular hydrogels, particle size and degree of packing significantly influence pore features, rheological behavior, and injectability. Using a mouse model of VML, it is demonstrated that, in contrast to bulk hydrogels, granular hydrogels support early-stage (satellite cell invasion) and late-stage (myofiber regeneration) muscle repair processes. Together, these results highlight the promising potential of injectable and porous granular hydrogels in supporting endogenous repair after severe muscle injury.

Granular Hydrogels for Harnessing the Immune Response

This review aims to understand the current progress in immune-instructive granular hydrogels and identify the key features used as immunomodulatory strategies. Published work is systematically reviewed and relevant information about granular hydrogels used throughout these studies is collected. The base polymer, microgel generation technique, polymer crosslinking chemistry, particle size and shape, annealing strategy, granular hydrogel stiffness, pore size and void space, degradability, biomolecule presentation, and drug release are cataloged for each work. Several granular hydrogel parameters used for immune modulation: porosity, architecture, bioactivity, drug release, cell delivery, and modularity, are identified. The authors found in this review that porosity is the most significant factor influencing the innate immune response to granular hydrogels, while incorporated bioactivity is more significant in influencing adaptive immune responses. Here, the authors' findings and summarized results from each section are presented and suggestions are made for future studies to better understand the benefits of using immune-instructive granular hydrogels.

Granular Hydrogels in Biofabrication: Recent Advances and Future Perspectives

Granular hydrogels, which are formed by densely packing microgels, are promising materials for bioprinting due to their extrudability, porosity, and modularity. However, the multidimensional parameter space involved in granular hydrogel design makes material optimization challenging. For example, design inputs such as microgel morphology, packing density, or stiffness can influence multiple rheological properties that govern printability and the behavior of encapsulated cells. This review provides an overview of fabrication methods for granular hydrogels, and then examines how important design inputs can influence material properties associated with printability and cellular responses across multiple scales. Recent applications of granular design principles in bioink engineering are described, including the development of granular support hydrogels for embedded printing. Further, the paper provides an overview of how key physical properties of granular hydrogels can influence cellular responses, highlighting the advantages of granular materials for promoting cell and tissue maturation after the printing process. Finally, potential future directions for advancing the design of granular hydrogels for bioprinting are discussed.

Photo-annealable agarose microgels for jammed microgel printing: Transforming thermogelling hydrogel to a functional bioink

Three-dimensional (3D) printing of hydrogel structures using jammed microgel inks offer distinct advantages of improved printing functionalities, as these inks are strain-yielding and self-recovering types. However, interparticle binding in granular hydrogel inks is a challenge to overcome the limited integrity and reduced macroscale modulus prevalent in the 3D printed microgel scaffolds. In this study, we prepared chemically annealable agarose microgels through a process of xerogel rehydration, applying a low-cost and high throughput method of spray drying. The crosslinked jammed microgel matrix is found to have superior mechanical properties with a Young's modulus of 2.23 MPa and extensibility up to 7.2%, surpassing those of traditional biopolymer-based and microgel-based inks. Furthermore, this study addresses the complexities encountered in the existing system of printing thermoresponsive agarose bioink using this jammed microgel printing approach. The jammed agarose microgel ink exhibited to be self-recovering, yield stress fluid and validated the temperature-independent printing. Furthermore, the 3D printed jammed microgel scaffold demonstrated good cell responsiveness as evaluated through the viability and morphological study in-vitro with mesenchymal stem cells cultured in it. This unique fabrication approach offers exciting possibilities to expand on microgel printing for varied requirements in tissue engineering.

Injectable Radiopaque Hyaluronic Acid Granular Hydrogels for Intervertebral Disc Repair

Injectable hydrogels offer minimally-invasive treatment options for degenerative disc disease, a prevalent condition affecting millions annually. Many hydrogels explored for intervertebral disc (IVD) repair suffer from weak mechanical integrity, migration issues, and expulsion. To overcome these limitations, an injectable and radiopaque hyaluronic acid granular hydrogel is developed. The granular structure provides easy injectability and low extrusion forces, while the radiopacity enables direct visualization during injection into the disc and non-invasive monitoring after injection. The radiopaque granular hydrogel is injected into rabbit disc explants to investigate restoration of healthy disc mechanics following needle puncture injury ex vivo and then delivered in a minimally-invasive manner into the intradiscal space in a clinically-relevant in vivo large animal goat model of IVD degeneration initiated through degradation by chondroitinase. The radiopaque granular hydrogel successfully halted loss of disc height due to degeneration. Further, the hydrogel not only enhanced proteoglycan content and reduced collagen content in the nucleus pulposus (NP) region compared to degenerative discs, but also helped to maintain the structural integrity of the disc and promote healthy segregation of the NP and annulus fibrosus regions. Overall, this study demonstrates the great potential of an injectable radiopaque granular hydrogel for treatment of degenerative disc disease.

Granular polyrotaxane microgels as injectable hydrogels for corneal tissue regeneration

Corneal diseases, a leading cause of global vision impairment, present challenges in treatment due to corneal tissue donor scarcity and transplant rejection. Hydrogel biomaterials in the form of corneal implants for tissue regeneration, while promising, have faced obstacles related to cellular and tissue integration. This study develops and investigates the potential of granular polyrotaxane (GPR) hydrogels as a scaffold for corneal keratocyte growth and transparent tissue generation. Employing host-guest driven supramolecular interactions, we developed injectable, cytocompatible hydrogels. By optimizing cyclodextrin (CD) concentrations in thiol-ene crosslinked PEG microgels, we observed improved mechanical properties and thermoresponsiveness while preserving injectability. These microgels, adaptable for precise defect filling, 3D printing or tissue culture facilitate enhanced cellular integration with corneal keratocytes and exhibit tissue-like structures in culture. Our findings demonstrate the promise of GPR hydrogels as a minimally invasive avenue for corneal tissue regeneration. These results have the potential to address transplantation challenges, enhance clinical outcomes, and restore vision.

Methods to achieve tissue-mimetic physicochemical properties in hydrogels for regenerative medicine and tissue engineering

Hydrogels are water-swollen polymeric matrices with properties that are remarkably similar in function to the extracellular matrix. For example, the polymer matrix provides structural support and adhesion sites for cells in much of the same way as the fibers of the extracellular matrix. In addition, depending on the polymer used, bioactive sites on the polymer may provide signals to initiate certain cell behavior. However, despite their potential as biomaterials for tissue engineering and regenerative medicine applications, fabricating hydrogels that truly mimic the physicochemical properties of the extracellular matrix to physiologically-relevant values is a challenge. Recent efforts in the field have sought to improve the physicochemical properties of hydrogels using advanced materials science and engineering methods. In this review, we highlight some of the most promising methods, including crosslinking strategies and manufacturing approaches such as 3D bioprinting and granular hydrogels. We also provide a brief perspective on the future outlook of this field and how these methods may lead to the clinical translation of hydrogel biomaterials for tissue engineering and regenerative medicine applications.

Structure driven bio-responsive ability of injectable nanocomposite hydrogels for efficient bone regeneration

Injectable hydrogels are promising for treatment of bone defects in clinic owing to their minimally invasive procedure. Currently, there is limited emphasis on how to utilize injectable hydrogels to mobilize body's regenerative potential for enhancing bone regeneration. Herein, an injectable bone-mimicking hydrogel (BMH) scaffold assembled from nanocomposite microgel building blocks was developed, in which a highly interconnected microporous structure and an inorganic/organic (methacrylated hydroxyapatite and methacrylated gelatin) interweaved nano structure were well-designed. Compared with hydrogels lacking micro-nano structures or only showing microporous structure, the BMH scaffold enhanced the ingrowth of vessels and promoted the formation of dense cellular networks (including stem cells and M2 macrophages), across the entire scaffold at early stage after subcutaneous implantation. Moreover, the BMH scaffold could not only directly trigger osteogenic differentiation of the infiltrated stem cells, but also provided an instructive osteo-immune microenvironment by inducing macrophages into M2 phenotype. Mechanistically, our results reveal that the nano-rough structure of the BMH plays an essential role in inducing macrophage M2 polarization through activating mechanotransduction related RhoA/ROCK2 pathway. Overall, this work offers an injectable hydrogel with micro-nano structure driven bio-responsive abilities, highlighting harnessing body's inherent regenerative potential to realize bone regeneration.

Nested Biofabrication: Matryoshka-Inspired Intra-Embedded Bioprinting

Engineering functional tissues and organs remains a fundamental pursuit in bio-fabrication. However, the accurate constitution of complex shapes and internal anatomical features of specific organs, including their intricate blood vessels and nerves, remains a significant challenge. Inspired by the Matryoshka doll, here a new method called "Intra-Embedded Bioprinting (IEB)" is introduced building upon existing embedded bioprinting methods. a xanthan gum-based material is used which served a dual role as both a bioprintable ink and a support bath, due to its unique shear-thinning and self-healing properties. IEB's capabilities in organ modeling, creating a miniaturized replica of a pancreas using a photocrosslinkable silicone composite is demonstrated. Further, a head phantom and a Matryoshka doll are 3D printed, exemplifying IEB's capability to manufacture intricate, nested structures. Toward the use case of IEB and employing an innovative coupling strategy between extrusion-based and aspiration-assisted bioprinting, a breast tumor model that included a central channel mimicking a blood vessel, with tumor spheroids bioprinted in proximity is developed. Validation using a clinically-available chemotherapeutic drug illustrated its efficacy in reducing the tumor volume via perfusion over time. This method opens a new way of bioprinting enabling the creation of complex-shaped organs with internal anatomical features.

A multifunctional injectable, self-healing, and adhesive hydrogel-based wound dressing stimulated diabetic wound healing with combined reactive oxygen species scavenging, hyperglycemia reducing, and bacteria-killing abilities

The proficient handling of diabetic wounds, a rising issue coinciding with the global escalation of diabetes cases, poses significant clinical difficulties. A range of biofunctional dressings have been engineered and produced to expedite the healing process of diabetic wounds. This study proposes a multifunctional hydrogel dressing for diabetic wound healing, which is composed of Polyvinyl Alcohol (PVA) and N1-(4-boronobenzyl)-N3-(4-boronophenyl)-N1, N1, N3, N3-teramethylpropane-1, 3-diaminium (TSPBA), and a dual-drug loaded Gelatin methacryloyl (GM) microgel. The GM microgel is loaded with sodium fusidate (SF) and nanoliposomes (LP) that contain metformin hydrochloride (MH). Notably, adhesive and self-healing properties the hydrogel enhance their therapeutic potential and ease of application. In vitro assessments indicate that SF-infused hydrogel can eliminate more than 98% of bacteria within 24 h and maintain a sustained release over 15 days. Additionally, MH incorporated within the hydrogel has demonstrated effective glucose level regulation for a duration exceeding 15 days. The hydrogel demonstrates a sustained ability to neutralize ROS throughout the entire healing process, predominantly by electron donation and sequestration. This multifunctional hydrogel dressing, which integrated biological functions of efficient bactericidal activity against both MSSA and MRSA strains, blood glucose modulation, and control of active oxygen levels, has successfully promoted the healing of diabetic wounds in rats in 14 days. The hydrogel dressing exhibited significant effectiveness in facilitating the healing process of diabetic wounds, highlighting its considerable promise for clinical translation.

Overview of Dynamic Bond Based Hydrogels for Reversible Adhesion Processes

Polymeric hydrogels are soft materials with a three-dimensional (3D) hydrophilic network capable of retaining and absorbing large amounts of water or biological fluids. Due to their customizable properties, these materials are extensively studied for developing matrices for 3D cell culture scaffolds, drug delivery systems, and tissue engineering. However, conventional hydrogels still exhibit many drawbacks; thus, significant efforts have been directed towards developing dynamic hydrogels that draw inspiration from organisms' natural self-repair abilities after injury. The self-healing properties of these hydrogels are closely associated with their ability to form, break, and heal dynamic bonds in response to various stimuli. The primary objective of this review is to provide a comprehensive overview of dynamic hydrogels by examining the types of chemical bonds associated with them and the biopolymers utilized, and to elucidate the chemical nature of dynamic bonds that enable the modulation of hydrogels' properties. While dynamic bonds ensure the self-healing behavior of hydrogels, they do not inherently confer adhesive properties. Therefore, we also highlight emerging approaches that enable dynamic hydrogels to acquire adhesive properties.

In vitro development and optimization of cell-laden injectable bioprinted gelatin methacryloyl (GelMA) microgels mineralized on the nanoscale

Bone defects may occur in different sizes and shapes due to trauma, infections, and cancer resection. Autografts are still considered the primary treatment choice for bone regeneration. However, they are hard to source and often create donor-site morbidity. Injectable microgels have attracted much attention in tissue engineering and regenerative medicine due to their ability to replace inert implants with a minimally invasive delivery. Here, we developed novel cell-laden bioprinted gelatin methacrylate (GelMA) injectable microgels, with controllable shapes and sizes that can be controllably mineralized on the nanoscale, while stimulating the response of cells embedded within the matrix. The injectable microgels were mineralized using a calcium and phosphate-rich medium that resulted in nanoscale crystalline hydroxyapatite deposition and increased stiffness within the crosslinked matrix of bioprinted GelMA microparticles. Next, we studied the effect of mineralization in osteocytes, a key bone homeostasis regulator. Viability stains showed that osteocytes were maintained at 98 % viability after mineralization with elevated expression of sclerostin in mineralized compared to non-mineralized microgels, showing that mineralization can effectively enhances osteocyte maturation. Based on our findings, bioprinted mineralized GelMA microgels appear to be an efficient material to approximate the bone microarchitecture and composition with desirable control of sample injectability and polymerization. These bone-like bioprinted mineralized biomaterials are exciting platforms for potential minimally invasive translational methods in bone regenerative therapies.

Embedded Printing of Hydrogels and Watery Suspensions of Cells in Patterned Granular Baths

Bioprinting within support media has emerged as the superior alternative to conventional extrusion printing. Not only because it allows for more freedom over the shapes that can be printed but also because it allows for the printing of inks that would not retain shape fidelity in freeform deposition such as watery liquids. Apart from functioning as mechanical support during embedded printing, hydrogel microparticle support media can provide the unique advantage of offering distinct chemotactic cues to cells printed in the baths by varying the composition of the hydrogel microparticles. There is great potential in compartmentalized granular baths consisting of different hydrogel particle materials in the field of tissue engineering, as these allow for the local inclusion of properties or cues to guide tissue development. In this work, we present a method to create compartmentalized embedding baths by printing multiple granular hydrogel materials that are widely used in tissue engineering. After adapting the volume fraction (φp) of the particles in the bath, we print within them using both inks composed of hydrogel or of cells and other particles suspended in watery liquid. Our process consists of the following three steps: First, the hydrogel microparticles are packed at a φp that allows them to be extruded while being reversibly jammed, facilitating the localized deposition of the granular media to form a compartmentalized bath. Second, each granular media is deposited in succession to create a packed suspension compartment, and by adding liquid post deposition, φp is reduced to allow for embedded printing. Finally, we demonstrate the printing of multiple inks within the compartmentalized embedding bath and highlight the distinct differences between using inks composed of hydrogels or inks composed of particles suspended in watery liquid. This approach combines the advantages of embedded printing through the use of granular media with the added ability to pattern multiple bioactive granular materials to locally affect the behavior of cells printed within the bath. We expect that this workflow will allow researchers to create spatially compartmentalized, customized bioactive embedding baths that allow for the embedded printing of inks composed of hydrogels, cells, and other particles adapted to their need.

Injectable MSC Spheroid and Microgel Granular Composites for Engineering Tissue

Many cell types require direct cell-cell interactions for differentiation and function; yet, this can be challenging to incorporate into 3-dimensional (3D) structures for the engineering of tissues. Here, a new approach is introduced that combines aggregates of cells (spheroids) with similarly-sized hydrogel particles (microgels) to form granular composites that are injectable, undergo interparticle crosslinking via light for initial stabilization, permit cell-cell contacts for cell signaling, and allow spheroid fusion and growth. One area where this is important is in cartilage tissue engineering, as cell-cell contacts are crucial to chondrogenesis and are missing in many tissue engineering approaches. To address this, granular composites are developed from adult porcine mesenchymal stromal cell (MSC) spheroids and hyaluronic acid microgels and simulations and experimental analyses are used to establish the importance of initial MSC spheroid to microgel volume ratios to balance mechanical support with tissue growth. Long-term chondrogenic cultures of granular composites produce engineered cartilage tissue with extensive matrix deposition and mechanical properties within the range of cartilage, as well as integration with native tissue. Altogether, a new strategy of injectable granular composites is developed that leverages the benefits of cell-cell interactions through spheroids with the mechanical stabilization afforded with engineered hydrogels.

Injectable Granular Hydrogels Enable Avidity-Controlled Biotherapeutic Delivery

Protein therapeutics represent a rapidly growing class of pharmaceutical agents that hold great promise for the treatment of various diseases such as cancer and autoimmune dysfunction. Conventional systemic delivery approaches, however, result in off-target drug exposure and a short therapeutic half-life, highlighting the need for more localized and controlled delivery. We have developed an affinity-based protein delivery system that uses guest-host complexation between β-cyclodextrin (CD, host) and adamantane (Ad, guest) to enable sustained localized biomolecule presentation. Hydrogels were formed by the copolymerization of methacrylated CD and methacrylated dextran. Extrusion fragmentation of bulk hydrogels yielded shear-thinning and self-healing granular hydrogels (particle diameter = 32.4 ± 16.4 μm) suitable for minimally invasive delivery and with a high host capacity for the retention of guest-modified proteins. Bovine serum albumin (BSA) was controllably conjugated to Ad via EDC chemistry without affecting the affinity of the Ad moiety for CD (KD = 12.0 ± 1.81 μM; isothermal titration calorimetry). The avidity of Ad-BSA conjugates was directly tunable through the number of guest groups attached, resulting in a fourfold increase in the complex half-life (t1/2 = 5.07 ± 1.23 h, surface plasmon resonance) that enabled a fivefold reduction in protein release at 28 days. Furthermore, we demonstrated that the conjugation of Ad to immunomodulatory cytokines (IL-4, IL-10, and IFNγ) did not detrimentally affect cytokine bioactivity and enabled their sustained release. Our strategy of avidity-controlled delivery of protein-based therapeutics is a promising approach for the sustained local presentation of protein therapeutics and can be applied to numerous biomedical applications.

Emerging granular hydrogel bioinks to improve biological function in bioprinted constructs

Advancements in 3D bioprinting have been hindered by the trade-off between printability and biological functionality. Existing bioinks struggle to meet both requirements simultaneously. However, new types of bioinks composed of densely packed microgels promise to address this challenge. These bioinks possess intrinsic porosity, allowing for cell growth, oxygen and nutrient transport, and better immunomodulatory properties, leading to superior biological functions. In this review, we highlight key trends in the development of these granular bioinks. Using examples, we demonstrate how granular bioinks overcome the trade-off between printability and cell function. Granular bioinks show promise in 3D bioprinting, yet understanding their unique structure-property-function relationships is crucial to fully leverage the transformative capabilities of these new types of bioinks in bioprinting.

Collagen fibril-like injectable hydrogels from self-assembled nanoparticles for promoting wound healing

Soft hydrogels are excellent candidate materials for repairing various tissue defects, yet the mechanical strength, anti-swelling properties, and biocompatibility of many soft hydrogels need to be improved. Herein, inspired by the nanostructure of collagen fibrils, we developed a strategy toward achieving a soft but tough, anti-swelling nanofibrillar hydrogel by combining the self-assembly and chemical crosslinking of nanoparticles. Specifically, the collagen fibril-like injectable hydrogel was subtly designed and fabricated by self-assembling methylacrylyl hydroxypropyl chitosan (HM) with laponite (LAP) to form nanoparticles, followed by the inter-nanoparticle bonding through photo-crosslinking. The assembly mechanism of nanoparticles was elucidated by both experimental and simulation techniques. Due to the unique structure of the crosslinked nanoparticles, the nanocomposite hydrogels exhibited low stiffness (G'< 2 kPa), high compressive strength (709 kPa), and anti-swelling (swelling ratio of 1.07 in PBS) properties. Additionally, by harnessing the photo-crosslinking ability of the nanoparticles, the nanocomposite hydrogels were processed as microgels, which can be three-dimensionally (3D) printed into complex shapes. Furthermore, we demonstrated that these nanocomposite hydrogels are highly biocompatible, biodegradability, and can effectively promote fibroblast migration and accelerate blood vessel formation during wound healing. This work presents a promising approach to develop biomimetic, nanofibrillar soft hydrogels for regenerative medicine applications.

Material assembly from collective action of shape-changing polymers

Some animals form transient, responsive and solid-like ensembles through dynamic structural interactions. These ensembles demonstrate emergent responses such as spontaneous self-assembly, which are difficult to achieve in synthetic soft matter. Here we use shape-morphing units comprising responsive polymers to create solids that self-assemble, modulate their volume and disassemble on demand. The ensemble is composed of a responsive hydrogel, liquid crystal elastomer or semicrystalline polymer ribbons that reversibly bend or twist. The dispersions of these ribbons mechanically interlock, inducing reversible aggregation. The aggregated liquid crystal elastomer ribbons have a 12-fold increase in the yield stress compared with cooled dispersion and contract by 34% on heating. Ribbon type, concentration and shape dictate the aggregation and govern the global mechanical properties of the solid that forms. Coating liquid crystal elastomer ribbons with a liquid metal begets photoresponsive and electrically conductive aggregates, whereas seeding cells on hydrogel ribbons enables self-assembling three-dimensional scaffolds, providing a versatile platform for the design of dynamic materials.

3D printing microporous scaffolds from modular bioinks containing sacrificial, cell-encapsulating microgels

Microgel-based biomaterials have inherent porosity and are often extrudable, making them well-suited for 3D bioprinting applications. Cells are commonly introduced into these granular inks post-printing using cell infiltration. However, due to slow cell migration speeds, this strategy struggles to achieve depth-independent cell distributions within thick 3D printed geometries. To address this, we leverage granular ink modularity by combining two microgels with distinct functions: (1) structural, UV-crosslinkable microgels made from gelatin methacryloyl (GelMA) and (2) sacrificial, cell-laden microgels made from oxidized alginate (AlgOx). We hypothesize that encapsulating cells within sacrificial AlgOx microgels would enable the simultaneous introduction of void space and release of cells at depths unachievable through cell infiltration alone. Blending the microgels in different ratios produces a family of highly printable GelMA : AlgOx microgel inks with void fractions ranging from 0.03 to 0.35. As expected, void fraction influences the morphology of human umbilical vein endothelial cells (HUVEC) within GelMA : AlgOx inks. Crucially, void fraction does not alter the ideal HUVEC distribution seen throughout the depth of 3D printed samples. This work presents a strategy for fabricating constructs with tunable porosity and depth-independent cell distribution, highlighting the promise of microgel-based inks for 3D bioprinting.

A Balance between Inter- and Intra-Microgel Mechanics Governs Stem Cell Viability in Injectable Dynamic Granular Hydrogels

Injectable hydrogels are increasingly explored for the delivery of cells to tissue. These materials exhibit both liquid-like properties, protecting cells from mechanical stress during injection, and solid-like properties, providing a stable 3D engraftment niche. Many strategies for modulating injectable hydrogels tune liquid- and solid-like material properties simultaneously, such that formulation changes designed to improve injectability can reduce stability at the delivery site. The ability to independently tune liquid- and solid-like properties would greatly facilitate formulation development. Here, such a strategy is presented in which cells are ensconced in the pores between microscopic granular hyaluronic acid (HA) hydrogels (microgels), where elasticity is tuned with static covalent intra-microgel crosslinks and flowability with mechanosensitive adamantane-cyclodextrin (AC) inter-microgel crosslinks. Using the same AC-free microgels as a 3D printing support bath, the location of each cell is preserved as it exits the needle, allowing identification of the mechanism driving mechanical trauma-induced cell death. The microgel AC concentration is varied to find the threshold from microgel yielding- to AC interaction-dominated injectability, and this threshold is exploited to fabricate a microgel with better injection-protecting performance. This delivery strategy, and the balance between intra- and inter-microgel properties it reveals, may facilitate the development of new cell injection formulations.

In vitro development and optimization of cell-laden injectable bioprinted gelatin methacryloyl (GelMA) microgels mineralized on the nanoscale

Bone defects may occur in different sizes and shapes due to trauma, infections, and cancer resection. Autografts are still considered the primary treatment choice for bone regeneration. However, they are hard to source and often create donor-site morbidity. Injectable microgels have attracted much attention in tissue engineering and regenerative medicine due to their ability to replace inert implants with a minimally invasive delivery. Here, we developed novel cell-laden bioprinted gelatin methacrylate (GelMA) injectable microgels, with controllable shapes and sizes that can be controllably mineralized on the nanoscale, while stimulating the response of cells embedded within the matrix. The injectable microgels were mineralized using a calcium and phosphate-rich medium that resulted in nanoscale crystalline hydroxyapatite deposition and increased stiffness within the crosslinked matrix of bioprinted GelMA microparticles. Next, we studied the effect of mineralization in osteocytes, a key bone homeostasis regulator. Viability stains showed that osteocytes were maintained at 98% viability after mineralization with elevated expression of sclerostin in mineralized compared to non-mineralized microgels, indicating that mineralization effectively enhances osteocyte maturation. Based on our findings, bioprinted mineralized GelMA microgels appear to be an efficient material to approximate the bone microarchitecture and composition with desirable control of sample injectability and polymerization. These bone-like bioprinted mineralized biomaterials are exciting platforms for potential minimally invasive translational methods in bone regenerative therapies.

Design Parameters for Injectable Biopolymeric Hydrogels with Dynamic Covalent Chemistry Crosslinks

Dynamic covalent chemistry (DCC) crosslinks can form hydrogels with tunable mechanical properties permissive to injectability and self-healing. However, not all hydrogels with transient crosslinks are easily extrudable. For this reason, two additional design parameters must be considered when formulating DCC-crosslinked hydrogels: 1) degree of functionalization (DoF) and 2) polymer molecular weight (MW). To investigate these parameters, hydrogels comprised of two recombinant biopolymers: 1) a hyaluronic acid (HA) modified with benzaldehyde and 2) an elastin-like protein (ELP) modified with hydrazine (ELP-HYD), are formulated. Several hydrogel families are synthesized with distinct HA MW and DoF while keeping the ELP-HYD component constant. The resulting hydrogels have a range of stiffnesses, G' ≈ 10-1000 Pa, and extrudability, which is attributed to the combined effects of DCC crosslinks and polymer entanglements. In general, lower MW formulations require lower forces for injectability, regardless of stiffness. Higher DoF formulations exhibit more rapid self-healing. Gel extrusion through a cannula (2 m length, 0.25 mm diameter) demonstrates the potential for minimally invasive delivery for future biomedical applications. In summary, this work highlights additional parameters that influence the injectability and network formation of DCC-crosslinked hydrogels and aims to guide future design of injectable hydrogels.

Dynamically Cross-Linked Granular Hydrogels for 3D Printing and Therapeutic Delivery

Granular hydrogels have recently emerged as promising biomaterials for tissue engineering and 3D-printing applications, addressing the limitations of bulk hydrogels while exhibiting desirable properties such as injectability and high porosity. However, their structural stability can be improved with post-injection interparticle cross-linking. In this study, we developed granular hydrogels with interparticle cross-linking through reversible and dynamic covalent bonds. We fragmented photo-cross-linked bulk hydrogels to produce aldehyde or hydrazide-functionalized microgels using chondroitin sulfate. Mixing these microgels facilitated interparticle cross-linking through reversible hydrazone bonds, providing shear-thinning and self-healing properties for injectability and 3D printing. The resulting granular hydrogels displayed high mechanical stability without the need for secondary cross-linking. Furthermore, the porosity and sustained release of growth factors from these hydrogels synergistically enhanced cell recruitment. Our study highlights the potential of reversible interparticle cross-linking for designing injectable and 3D printable therapeutic delivery scaffolds using granular hydrogels. Overall, our study highlights the potential of reversible interparticle cross-linking to improve the structural stability of granular hydrogels, making them an effective biomaterial for use in tissue engineering and 3D-printing applications.

Single versus dual microgel species for forming guest-host microporous annealed particle PEG-MAL hydrogel

Inter-particle secondary crosslinks allow microporous annealed particle (MAP) hydrogels to be formed. Methods to introduce secondary crosslinking networks in MAP hydrogels include particle jamming, annealing with covalent bonds, and reversible noncovalent interactions. Here, we investigate the effect of two different approaches to secondary crosslinking of polyethylene glycol (PEG) microgels via reversible guest-host interactions. We generated a dual-particle MAP-PEG hydrogel using two species of PEG microgels, one functionalized with the guest molecule, adamantane, and the other with the host molecule, β-cyclodextrin (Inter-MAP-PEG). In a different approach, a mono-particle MAP-PEG hydrogel was generated using one species of microgel functionalized with both guest and host molecules (Intra-MAP-PEG). The Intra-MAP-PEG formed a homogenous distribution due to the single type of microgels used. We then compared the mechanical properties of these two types of MAP-PEG hydrogels and found that Intra-MAP-PEG resulted in significantly softer gels with lower yield stress. We investigated the effect of intra-particle guest-host interactions through titrated weight percentage and the concentration of functional groups added to the hydrogel. We found that there was an ideal concentration of guest-host molecules that enables intra- and inter-particle guest-host interactions with sufficient covalent crosslinking. Based on these studies, Intra-MAP-PEG provides a homogeneous guest-host hydrogel that is shear-thinning with reversible secondary crosslinking.

Structurally dynamic self-healable hydrogel cooperatively inhibits intestinal inflammation and promotes mucosal repair for enhanced ulcerative colitis treatment

Hydrogels are a class of biocompatible materials with versatile functions that have been increasing explored for the localized treatment of ulcerative colitis (UC), but various mechanical stimuli may cause premature hydrogel breakage and detachment, impeding their further clinical translation. Here we report a multifunctional mechanically-resilient self-healing hydrogel for effective UC treatment, which is synthesized through the host-guest interaction between dopamine/β-cyclodextrin-modified hyaluronic acid (HA-CD-DA) and amantadine-modified carboxymethyl chitosan (CMCS-AD). The excessive β-CD cavities allow the incorporation of dexamethasone (DEX), while the porous hydrogel network potentiates the encapsulation of basic fibroblast growth factor (bFGF) and L-alanyl-l-glutamine (ALG). DA moieties in HA components allow firm adhesion of the hydrogel to the ulcerative lesions after in-situ implantation, while the reversible host-guest interaction between CD and AD could enhance the persistence of hydrogel. The hydrogel demonstrated favorable biocompatibility and could continuously release DEX to induce M1-to-M2 repolarization of mucosal macrophages through inhibiting the toll-like receptor 4 (TLR4)-nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) axis. Furthermore, the co-delivered bFGF and ALG facilitates the regeneration of ulcerative mucosa and restore its barrier functions to ameliorate UC symptoms. The mechanically resilient hydrogel offers an integrative approach for UC therapy in the clinics.

Direct 3D Bioprinting of Tough and Antifatigue Cell-Laden Constructs Enabled by a Self-Healing Hydrogel Bioink

Three-dimensional (3D) extrusion bioprinting has emerged as one of the most promising biofabrication technologies for preparing biomimetic tissue-like constructs. The successful construction of cell-laden constructs majorly relies on the development of proper bioinks with excellent printability and cytocompatibility. Bioinks based on gelatin methacryloyl (GelMA) have been widely explored due to the excellent biocompatibility and biodegradability and the presence of the arginine-glycine-aspartic acid (RGD) sequences for cell adhesion. However, such bioinks usually require low-temperature or ionic cross-linking systems to solidify the extruded hydrogel structures, which results in complex processes and limitations to certain applications. Moreover, many current hydrogel-based bioinks, even after chemical cross-linking, hardly possess the required strength to resist the mechanical loads during the implantation procedure. Herein, we report a self-healing hydrogel bioink based on GelMA and oxidized dextran (OD) for the direct printing of tough and fatigue-resistant cell-laden constructs at room temperature without any template or cross-linking agents. Enabled by dynamic Schiff base chemistry, the mixed GelMA/OD solution showed the characteristics of a dynamic hydrogel with shear-thinning and self-supporting behavior, which allows bridging the 5 mm gap and efficient direct bioprinting of complex constructs with high shape fidelity. After photo-cross-linking, the resulting tissue constructs exhibited excellent low cell damage, high cell viability, and enhanced mechanical strength. Moreover, the GelMA/OD construct could resist up to 95% compressive deformation without any breakage and was able to maintain 80% of the original Young's modulus during long-term loading (50 cycles). It is believed that our GelMA/OD bioink would expand the potential of GelMA-based bioinks in applications such as tissue engineering and pharmaceutical screening.

Controlling extrudate volume fraction through poroelastic extrusion of entangled looped fibers

When a suspension of spherical or near-spherical particles passes through a constriction the particle volume fraction either remains the same or decreases. In contrast to these particulate suspensions, here we observe that an entangled fiber suspension increases its volume fraction up to 14-fold after passing through a constriction. We attribute this response to the entanglements among the fibers that allows the network to move faster than the liquid. By changing the fiber geometry, we find that the entanglements originate from interlocking shapes or high fiber flexibility. A quantitative poroelastic model is used to explain the increase in velocity and extrudate volume fraction. These results provide a new strategy to use fiber volume fraction, flexibility, and shape to tune soft material properties, e.g., suspension concentration and porosity, during delivery, as occurs in healthcare, three-dimensional printing, and material repair.

Granular Disulfide-Crosslinked Hyaluronic Hydrogels: A Systematic Study of Reaction Conditions on Thiol Substitution and Injectability Parameters

Granular polymer hydrogels based on dynamic covalent bonds are attracting a great deal of interest for the design of injectable biomaterials. Such materials generally exhibit shear-thinning behavior and properties of self-healing/recovery after the extrusion that can be modulated through the interactions between gel microparticles. Herein, bulk macro-hydrogels based on thiolated-hyaluronic acid were produced by disulphide bond formation using oxygen as oxidant at physiological conditions and gelation kinetics were monitored. Three different thiol substitution degrees (SD%: 65%, 30% and 10%) were selected for hydrogel formation and fully characterized as to their stability in physiological medium and morphology. Then, extrusion fragmentation technique was applied to obtain hyaluronic acid microgels with dynamic disulphide bonds that were subsequently sterilized by autoclaving. The resulting granular hyaluronic hydrogels were able to form stable filaments when extruded through a syringe. Rheological characterization and cytotoxicity tests allowed to assess the potential of these materials as injectable biomaterials. The application of extrusion fragmentation for the formation of granular hyaluronic hydrogels and the understanding of the relation between the autoclaving processes and the resulting particle size and rheological properties should expand the development of injectable materials for biomedical applications.

Trifunctional Microgel-Mediated Preparation and Toughening of Printable High-Performance Chitosan Hydrogels for Underwater Communications

Natural and biocompatible chitosan has demonstrated wide applications. However, rapidly fabricating high-performance chitosan hydrogels in one-step controllable processes is still a challenge for some advanced applications. Here, we report a trifunctional microgel-mediated photochemical (TMMP) strategy to achieve the fabrication of printable tough chitosan-based hydrogels (PTCHs) in seconds. Such microgels help the slow release of persulfate anions and their uniform dispersion in an aqueous solution of cationic chitosan. The released persulfates are available for preparing multiple networks of phenolic coupling of modified chitosan and radical polymerization of Pluronic F127 via orthogonal tris(bipyridine)ruthenium(II)-based photochemistry, respectively. Trifunctional microgels have reversible Ca2+-crosslinked networks that further improve the hydrogels' mechanical properties and toughness. The maximum stress and toughness increase by >20 folds compared to the chitosan and F127 hydrogels with single network structures. Moreover, these microgels enable the precursor to have a good shearing-thinning property and benefit the controllable preparation of PTCHs in a short time, as low as ∼4 s under visible light irradiation. It, therefore, is compatible with standard printing techniques to make complex structures. Strain sensors based on structured PTCHs have stable mechanical and responsive properties in the water, which are applied for real-time underwater communications (<0.4 s). It is anticipated that this one-step TMMP strategy opens new horizons for designing advanced chitosan hydrogels.

Structurally Dynamic Gelatin-Based Hydrogels with Self-Healing, Shape Memory, and Cytocompatible Properties for 4D Printing

Three-dimensional (3D) printable hydrogels with a shape memory effect have emerged as a new class of 4D printing materials recently and found wide applications in various fields. However, synergistically endowing such materials with good mechanical strength and biocompatibility for biomedical uses remains challenging. In this study, a series of multiresponsive hydrogels have been prepared through a dynamic covalent imine/Diels-Alder network from biocompatible starting materials of modified gelatin and poly(ethylene glycol)-based polymers. By further secondary crosslinking with a hyperbranched triethoxysilane reagent (HPASi) that contains multiple supramolecular hydrogen bonding, the hydrogels presented a strengthened self-healing and temperature-responsive shape memory effect. With the additional features of superior stretchability (elongation at break up to 523%), good cytocompatibility, and 3D printable properties, these multifunctional hydrogels showed great potential for broad biomedical applications.

Dynamic covalent crosslinked hyaluronic acid hydrogels and nanomaterials for biomedical applications

Hyaluronic acid (HA), one of the main components of the extracellular matrix (ECM), is extensively used in the design of hydrogels and nanoparticles for different biomedical applications due to its critical role in vivo, degradability by endogenous enzymes, and absence of immunogenicity. HA-based hydrogels and nanoparticles have been developed by utilizing different crosslinking chemistries. The development of such crosslinking chemistries indicates that even subtle differences in the structure of reactive groups or the procedure of crosslinking may have a profound impact on the intended mechanical, physical and biological outcomes. There are widespread examples of modified HA polymers that can form either covalently or physically crosslinked biomaterials. More recently, studies have been focused on dynamic covalent crosslinked HA-based biomaterials since these types of crosslinking allow the preparation of dynamic structures with the ability to form in situ, be injectable, and have self-healing properties. In this review, HA-based hydrogels and nanomaterials that are crosslinked by dynamic-covalent coupling (DCC) chemistry have been critically assessed.

Injectable Microporous Annealed Particle Hydrogel Based on Guest-Host-Interlinked Polyethylene Glycol Maleimide Microgels

Microporous annealed particle (MAP) hydrogels have emerged as a versatile biomaterial platform for regenerative medicine. MAP hydrogels have been used for the delivery of cells and organoids but often require annealing post injection by an external source. We engineered an injectable, self-annealing MAP hydrogel with reversible interparticle linkages based on guest-host functionalized polyethylene glycol maleimide (PEG-MAL) microgels. We evaluated the effect of guest-host linkages on different types of microgels fabricated by either batch emulsion or mechanical fragmentation methods. Batch emulsion generated small spherical microgels with controllable 10-100 μm diameters and mechanical fragmentation generated irregular microgels with larger diameters (100-200 μm). Spherical microgels (15 μm) showed self-healing behavior and completely recovered from high strain while fragmented microgels (133 μm) did not recover. Guest-host interactions significantly contributed to the mechanical properties of spherical microgels but had no effect on fragmented microgels. Spherical microgels were superior to the fragmented microgels for co-injection of immune cells and pancreatic islets due to their lower force of injection, demonstrating more homogeneously distributed cells and greater cell viability after injection. Based on these studies, the spherical guest-host MAP hydrogels provide a controllable, injectable scaffold for engineered microenvironments and cell delivery applications.

Recent Advances in Macroporous Hydrogels for Cell Behavior and Tissue Engineering

Hydrogels have been extensively used as scaffolds in tissue engineering for cell adhesion, proliferation, migration, and differentiation because of their high-water content and biocompatibility similarity to the extracellular matrix. However, submicron or nanosized pore networks within hydrogels severely limit cell survival and tissue regeneration. In recent years, the application of macroporous hydrogels in tissue engineering has received considerable attention. The macroporous structure not only facilitates nutrient transportation and metabolite discharge but also provides more space for cell behavior and tissue formation. Several strategies for creating and functionalizing macroporous hydrogels have been reported. This review began with an overview of the advantages and challenges of macroporous hydrogels in the regulation of cellular behavior. In addition, advanced methods for the preparation of macroporous hydrogels to modulate cellular behavior were discussed. Finally, future research in related fields was discussed.

Self-Healing Injectable Hydrogels for Tissue Regeneration

Biomaterials with the ability to self-heal and recover their structural integrity offer many advantages for applications in biomedicine. The past decade has witnessed the rapid emergence of a new class of self-healing biomaterials commonly termed injectable, or printable in the context of 3D printing. These self-healing injectable biomaterials, mostly hydrogels and other soft condensed matter based on reversible chemistry, are able to temporarily fluidize under shear stress and subsequently recover their original mechanical properties. Self-healing injectable hydrogels offer distinct advantages compared to traditional biomaterials. Most notably, they can be administered in a locally targeted and minimally invasive manner through a narrow syringe without the need for invasive surgery. Their moldability allows for a patient-specific intervention and shows great prospects for personalized medicine. Injected hydrogels can facilitate tissue regeneration in multiple ways owing to their viscoelastic and diffusive nature, ranging from simple mechanical support, spatiotemporally controlled delivery of cells or therapeutics, to local recruitment and modulation of host cells to promote tissue regeneration. Consequently, self-healing injectable hydrogels have been at the forefront of many cutting-edge tissue regeneration strategies. This study provides a critical review of the current state of self-healing injectable hydrogels for tissue regeneration. As key challenges toward further maturation of this exciting research field, we identify (i) the trade-off between the self-healing and injectability of hydrogels vs their physical stability, (ii) the lack of consensus on rheological characterization and quantitative benchmarks for self-healing injectable hydrogels, particularly regarding the capillary flow in syringes, and (iii) practical limitations regarding translation toward therapeutically effective formulations for regeneration of specific tissues. Hence, here we (i) review chemical and physical design strategies for self-healing injectable hydrogels, (ii) provide a practical guide for their rheological analysis, and (iii) showcase their applicability for regeneration of various tissues and 3D printing of complex tissues and organoids.