Rational design of network properties in guest-host assembled and shear-thinning hyaluronic acid hydrogels

Dynamic injectable tissue adhesives with strong adhesion and rapid self-healing for regeneration of large muscle injury

Wounds often necessitate the use of instructive biomaterials to facilitate effective healing. Yet, consistently filling the wound and retaining the material in place presents notable challenges. Here, we develop a new class of injectable tissue adhesives by leveraging the dynamic crosslinking chemistry of Schiff base reactions. These adhesives demonstrate outstanding mechanical properties, especially in regard to stretchability and self-healing capacity, and biodegradability. Furthermore, they also form robust adhesion to biological tissues. Their therapeutic potential was evaluated in a rodent model of volumetric muscle loss (VML). Ultrasound imaging confirmed that the adhesives remained within the wound site, effectively filled the void, and degraded at a rate comparable to the healing process. Histological analysis indicated that the adhesives facilitated muscle fiber and blood vessel formation, and induced anti-inflammatory macrophages. Notably, the injured muscles of mice treated with the adhesives displayed increased weight and higher force generation than the control groups. This approach to adhesive design paves the way for the next generation of medical adhesives in tissue repair.

Modular and Photoreversible Polymer-Nanoparticle Hydrogels via Host-Guest Interactions

Polymer-nanoparticle (PNP) hydrogels are a class of nanocomposite materials showing potential as injectable platforms for biomedical applications. Their design is limited by incomplete knowledge of how the binding motif impacts the viscoelastic properties of the material and is generally constrained to non-responsive supramolecular interactions. Expanding the scope of available interactions and advancing the understanding of how defined interactions influence network formation would accelerate PNP hydrogel design. To address this gap in the design of PNP hydrogels, the study designs and investigates a tunable platform based on beta-cyclodextrin (βCD) host-guest cross-links between functionalized polymers and nanoparticles. A host-functionalized polymer (βCD hyaluronic acid) and guest harboring block co-polymer (poly(ethylene glycol)-b-poly(lactic acid)) NPs are synthesized. The presence and accessibility for binding of the host and guest moieties are characterized via isothermal titration calorimetry. PNP hydrogels with varying concentrations of functionalized polymer and NPs reveal a limited window of concentrations for gelation. It is hypothesized that network formation is governed by the capacity of polymer chains to effectively bridge NPs, which is related to the host-guest ratios present in the system. Further, photo-responsive guests are incorporated to engineer photoreversible gelation of PNP hydrogels via exposure to specific wavelengths of light.

Injectable shear-thinning hydrogels promote oligodendrocyte progenitor cell survival and remyelination in the central nervous system

Cell therapy for the treatment of demyelinating diseases such as multiple sclerosis is hampered by poor survival of donor oligodendrocyte cell preparations, resulting in limited therapeutic outcomes. Excessive cell death leads to the release of intracellular alloantigens, which likely exacerbate local inflammation and may predispose the graft to eventual rejection. Here, we engineered innovative cell-instructive shear-thinning hydrogels (STHs) with tunable viscoelasticity and bioactivity for minimally invasive delivery of primary human oligodendrocyte progenitor cells (hOPCs) to the brain of a shiverer/rag2 mouse, a model of congenital hypomyelinating disease. The STHs enabled immobilization of prosurvival signals, including a recombinantly designed bidomain peptide and platelet-derived growth factor. Notably, STHs reduced the death rate of hOPCs significantly, promoted the production of myelinating oligodendrocytes, and enhanced myelination of the mouse brain 12 weeks post-implantation. Our results demonstrate the potential of STHs loaded with biological cues to improve cell therapies for the treatment of devastating myelopathies.

Strain-Stiffening Mechanoresponse in Dynamic-Covalent Cellulose Hydrogels

Mechanical stimuli such as strain, force, and pressure are pervasive within and beyond the human body. Mechanoresponsive hydrogels have been engineered to undergo changes in their physicochemical or mechanical properties in response to such stimuli. Relevant responses can include strain-stiffening, self-healing, strain-dependent stress relaxation, and shear rate-dependent viscosity. These features are a direct result of dynamic bonds or noncovalent/physical interactions within such hydrogels. The contributions of various types of bonds and intermolecular interactions to these behaviors are important to more fully understand the resulting materials and engineer their mechanoresponsive features. Here, strain-stiffening in carboxymethylcellulose hydrogels cross-linked with pendant dynamic-covalent boronate esters using tannic acid is studied and modulated as a function of polymer concentration, temperature, and effective cross-link density. Furthermore, these materials are found to exhibit self-healing and strain-memory, as well as strain-dependent stress relaxation and shear rate-dependent changes in gel viscosity. These features are attributed to the dynamic nature of the boronate ester cross-links, interchain hydrogen bonding and bundling, or a combination of these two intermolecular interactions. This work provides insight into the interplay of such interactions in the context of mechanoresponsive behaviors, particularly informing the design of hydrogels with tunable strain-stiffening. The multiresponsive and tunable nature of this hydrogel system therefore presents a promising platform for a variety of applications.

Biomimetic Non-ergodic Aging by Dynamic-to-covalent Transitions in Physical Hydrogels

Hydrogels are soft materials engineered to suit a multitude of applications that exploit their tunable mechanochemical properties. Dynamic hydrogels employing noncovalent, physically cross-linked networks dominated by either enthalpic or entropic interactions enable unique rheological and stimuli-responsive characteristics. In contrast to enthalpy-driven interactions that soften with increasing temperature, entropic interactions result in largely temperature-independent mechanical properties. By engineering interfacial polymer-particle interactions, we can induce a dynamic-to-covalent transition in entropic hydrogels that leads to biomimetic non-ergodic aging in the microstructure without altering the network mesh size. This transition is tuned by varying temperature and formulation conditions such as pH, which allows for multivalent tunability in properties. These hydrogels can thus be designed to exhibit either temperature-independent metastable dynamic cross-linking or time-dependent stiffening based on formulation and storage conditions, all while maintaining structural features critical for controlling mass transport, akin to many biological tissues. Such robust materials with versatile and adaptable properties can be utilized in applications such as wildfire suppression, surgical adhesives, and depot-forming injectable drug delivery systems.

Modular Multiwell Viscoelastic Hydrogel Platform for Two- and Three-Dimensional Cell Culture Applications

Hydrogels have gained significant popularity as model platforms to study reciprocal interactions between cells and their microenvironment. While hydrogel tools to probe many characteristics of the extracellular space have been developed, fabrication approaches remain challenging and time-consuming, limiting multiplexing or widespread adoption. Thus, we have developed a modular fabrication approach to generate distinct hydrogel microenvironments within the same 96-well plate for increased throughput of fabrication as well as integration with existing high-throughput assay technologies. This approach enables in situ hydrogel mechanical characterization and is used to generate both elastic and viscoelastic hydrogels across a range of stiffnesses. Additionally, this fabrication method enabled a 3-fold reduction in polymer and up to an 8-fold reduction in fabrication time required per hydrogel replicate. The feasibility of this platform for two-dimensional (2D) cell culture applications was demonstrated by measuring both population-level and single-cell-level metrics via microplate reader and high-content imaging. Finally, a 96-well hydrogel array was utilized for three-dimensional (3D) cell culture, demonstrating the ability to support high cell viability. Together, this work demonstrates a versatile and easily adaptable fabrication approach that can support the ever-expanding tool kit of hydrogel technologies for cell culture applications.

Biomaterial engineering strategies for B cell immunity modulations

B cell immunity has a penetrating effect on human health and diseases. Therapeutics aiming to modulate B cell immunity have achieved remarkable success in combating infections, autoimmunity, and malignancies. However, current treatments still face significant limitations in generating effective long-lasting therapeutic B cell responses for many conditions. As the understanding of B cell biology has deepened in recent years, clearer regulation networks for B cell differentiation and antibody production have emerged, presenting opportunities to overcome current difficulties and realize the full therapeutic potential of B cell immunity. Biomaterial platforms have been developed to leverage these emerging concepts to augment therapeutic humoral immunity by facilitating immunogenic reagent trafficking, regulating T cell responses, and modulating the immune microenvironment. Moreover, biomaterial engineering tools have also advanced our understanding of B cell biology, further expediting the development of novel therapeutics. In this review, we will introduce the general concept of B cell immunobiology and highlight key biomaterial engineering strategies in the areas including B cell targeted antigen delivery, sustained B cell antigen delivery, antigen engineering, T cell help optimization, and B cell suppression. We will also discuss our perspective on future biomaterial engineering opportunities to leverage humoral immunity for therapeutics.

Viscoelastic high-molecular-weight hyaluronic acid hydrogels support rapid glioblastoma cell invasion with leader-follower dynamics

Hyaluronic acid (HA), the primary component of brain extracellular matrix, is increasingly used to model neuropathological processes, including glioblastoma (GBM) tumor invasion. While elastic hydrogels based on crosslinked low-molecular-weight (LMW) HA are widely exploited for this purpose and have proven valuable for discovery and screening, brain tissue is both viscoelastic and rich in high-MW (HMW) HA, and it remains unclear how these differences influence invasion. To address this question, hydrogels comprised of either HMW (1.5 MDa) or LMW (60 kDa) HA are introduced, characterized, and applied in GBM invasion studies. Unlike LMW HA hydrogels, HMW HA hydrogels relax stresses quickly, to a similar extent as brain tissue, and to a greater extent than many conventional HA-based scaffolds. GBM cells implanted within HMW HA hydrogels invade much more rapidly than in their LMW HA counterparts and exhibit distinct leader-follower dynamics. Leader cells adopt dendritic morphologies, similar to invasive GBM cells observed in vivo. Transcriptomic, pharmacologic, and imaging studies suggest that leader cells exploit hyaluronidase, an enzyme strongly enriched in human GBMs, to prime a path for followers. This study offers new insight into how HA viscoelastic properties drive invasion and argues for the use of highly stress-relaxing materials to model GBM.

Microinterfaces in biopolymer-based bicontinuous hydrogels guide rapid 3D cell migration

Cell migration is critical for tissue development and regeneration but requires extracellular environments that are conducive to motion. Cells may actively generate migratory routes in vivo by degrading or remodeling their environments or instead utilize existing extracellular matrix microstructures or microtracks as innate pathways for migration. While hydrogels in general are valuable tools for probing the extracellular regulators of 3-dimensional migration, few recapitulate these natural migration paths. Here, we develop a biopolymer-based bicontinuous hydrogel system that comprises a covalent hydrogel of enzymatically crosslinked gelatin and a physical hydrogel of guest and host moieties bonded to hyaluronic acid. Bicontinuous hydrogels form through controlled solution immiscibility, and their continuous subdomains and high micro-interfacial surface area enable rapid 3D migration, particularly when compared to homogeneous hydrogels. Migratory behavior is mesenchymal in nature and regulated by biochemical and biophysical signals from the hydrogel, which is shown across various cell types and physiologically relevant contexts (e.g., cell spheroids, ex vivo tissues, in vivo tissues). Our findings introduce a design that leverages important local interfaces to guide rapid cell migration.

Host-functionalization of macrin nanoparticles to enable drug loading and control tumor-associated macrophage phenotype

Macrophages are critical regulators of the tumor microenvironment and often present an immuno-suppressive phenotype, supporting tumor growth and immune evasion. Promoting a robust pro-inflammatory macrophage phenotype has emerged as a therapeutic modality that supports tumor clearance, including through synergy with immune checkpoint therapies. Polyglucose nanoparticles (macrins), which possess high macrophage affinity, are useful vehicles for delivering drugs to macrophages, potentially altering their phenotype. Here, we examine the potential of functionalized macrins, synthesized by crosslinking carboxymethyl dextran with L-lysine, as effective carriers of immuno-stimulatory drugs to tumor-associated macrophages (TAMs). Azide groups incorporated during particle synthesis provided a handle for click-coupling of propargyl-modified β-cyclodextrin to macrins under mild conditions. Fluorescence-based competitive binding assays revealed the ability of β-cyclodextrin to non-covalently bind to hydrophobic immuno-stimulatory drug candidates (Keq ~ 103 M-1), enabling drug loading within nanoparticles. Furthermore, transcriptional profiles of macrophages indicated robust pro-inflammatory reprogramming (elevated Nos2 and Il12; suppressed Arg1 and Mrc1 expression levels) for a subset of these immuno-stimulatory agents (UNC2025 and R848). Loading of R848 into the modified macrins improved the drug's effect on primary murine macrophages by three-fold in vitro. Intravital microscopy in IL-12-eYFP reporter mice (24 h post-injection) revealed a two-fold enhancement in mean YFP fluorescence intensity in macrophages targeted with R848-loaded macrins, relative to vehicle controls, validating the desired pro-inflammatory reprogramming of TAMs in vivo by cell-targeted drug delivery. Finally, in an intradermal MC38 tumor model, cyclodextrin-modified macrin NPs loaded with immunostimulatory drugs significantly reduced tumor growth. Therefore, efficient and effective repolarization of tumor-associated macrophages to an M1-like phenotype-via drug-loaded macrins-inhibits tumor growth and may be useful as an adjuvant to existing immune checkpoint therapies.

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.

From Nature-Sourced Polysaccharide Particles to Advanced Functional Materials

Polysaccharides constitute over 90% of the carbohydrate mass in nature, which makes them a promising feedstock for manufacturing sustainable materials. Polysaccharide particles (PSPs) are used as effective scavengers, carriers of chemical and biological cargos, and building blocks for the fabrication of macroscopic materials. The biocompatibility and degradability of PSPs are advantageous for their uses as biomaterials with more environmental friendliness. This review highlights the progresses in PSP applications as advanced functional materials, by describing PSP extraction, preparation, and surface functionalization with a variety of functional groups, polymers, nanoparticles, and biologically active species. This review also outlines the fabrication of PSP-derived macroscopic materials, as well as their applications in soft robotics, sensing, scavenging, water harvesting, drug delivery, and bioengineering. The paper is concluded with an outlook providing perspectives in the development and applications of PSP-derived materials.

Rational design of viscoelastic hydrogels for periodontal ligament remodeling and repair

The periodontal ligament (PDL) is a distinctive yet critical connective tissue vital for maintaining the integrity and functionality of tooth-supporting structures. However, PDL repair poses significant challenges due to the complexity of its mechanical microenvironment encompassing hard-soft-hard tissues, with the viscoelastic properties of the PDL being of particular interest. This review delves into the significant role of viscoelastic hydrogels in PDL regeneration, underscoring their utility in simulating biomimetic three-dimensional microenvironments. We review the intricate relationship between PDL and viscoelastic mechanical properties, emphasizing the role of tissue viscoelasticity in maintaining mechanical functionality. Moreover, we summarize the techniques for characterizing PDL's viscoelastic behavior. From a chemical bonding perspective, we explore various crosslinking methods and characteristics of viscoelastic hydrogels, along with engineering strategies to construct viscoelastic cell microenvironments. We present a detailed analysis of the influence of the viscoelastic microenvironment on cellular mechanobiological behavior and fate. Furthermore, we review the applications of diverse viscoelastic hydrogels in PDL repair and address current challenges in the field of viscoelastic tissue repair. Lastly, we propose future directions for the development of innovative hydrogels that will facilitate not only PDL but also systemic ligament tissue repair. STATEMENT OF SIGNIFICANCE.

Salivary Gland Bioengineering

Salivary gland dysfunction affects millions globally, and tissue engineering may provide a promising therapeutic avenue. This review delves into the current state of salivary gland tissue engineering research, starting with a study of normal salivary gland development and function. It discusses the impact of fibrosis and cellular senescence on salivary gland pathologies. A diverse range of cells suitable for tissue engineering including cell lines, primary salivary gland cells, and stem cells are examined. Moreover, the paper explores various supportive biomaterials and scaffold fabrication methodologies that enhance salivary gland cell survival, differentiation, and engraftment. Innovative engineering strategies for the improvement of vascularization, innervation, and engraftment of engineered salivary gland tissue, including bioprinting, microfluidic hydrogels, mesh electronics, and nanoparticles, are also evaluated. This review underscores the promising potential of this research field for the treatment of salivary gland dysfunction and suggests directions for future exploration.

Hyaluronic Acid Hydrogels with Phototunable Supramolecular Cross-Linking for Spatially Controlled Lymphatic Tube Formation

The dynamics of the extracellular matrix (ECM) influences stem cell differentiation and morphogenesis into complex lymphatic networks. While dynamic hydrogels with stress relaxation properties have been developed, many require detailed chemical processing to tune viscoelasticity, offering a limited opportunity for in situ and spatiotemporal control. Here, a hyaluronic acid (HA) hydrogel is reported with viscoelasticity that is controlled and spatially tunable using UV light to direct the extent of supramolecular and covalent cross-linking interactions. This is achieved using UV-mediated photodimerization of a supramolecular ternary complex of pendant trans-Brooker's Merocyanine (BM) guests and a cucurbit[8]uril (CB[8]) macrocycle. The UV-mediated conversion of this supramolecular complex to its covalent photodimerized form is catalyzed by CB[8], offering a user-directed route to spatially control hydrogel dynamics in combination with orthogonal photopatterning by UV irradiation through photomasks. This material thus achieves spatial heterogeneity of substrate dynamics, recreating features of native ECM without the need for additional chemical reagents. Moreover, these dynamic hydrogels afford spatial control of substrate mechanics to direct human lymphatic endothelial cells (LECs) to form lymphatic cord-like structures (CLS). Specifically, cells cultured on viscoelastic supramolecular hydrogels have enhanced formation of CLS, arising from increased expression of key lymphatic markers, such as LYVE-1, Podoplanin, and Prox1, compared to static elastic hydrogels prepared from fully covalent cross-linking. Viscoelastic hydrogels promote lymphatic CLS formation through the expression of Nrp2, VEGFR2, and VEGFR3 to enhance the VEGF-C stimulation. Overall, viscoelastic supramolecular hydrogels offer a facile route to spatially control lymphatic CLS formation, providing a tool for future studies of basic lymphatic biology and tissue engineering applications.

Saltwater-Induced Rapid Gelation of Photoredox-Responsive Mucomimetic Hydrogels

Shear-thinning hydrogels represent an important class of injectable soft materials that are often used in a wide range of biomedical applications. Creation of new shear-thinning materials often requires that factors such as viscosity, injection rate/force, and needle gauge be evaluated to achieve efficient delivery, while simultaneously protecting potentially sensitive cargo. Here, a new approach to establishing shear-thinning hydrogels is reported where a host-guest cross-linked network initially remains soluble in deionized water but is kinetically trapped as a viscous hydrogel once exposed to saltwater. The shear-thinning properties of the hydrogel is then "switched on" in response to heating or exposure to visible light. These hydrogels consist of polynorbornene-based bottlebrush copolymers with porphyrin- and oligoviologen-containing side chains that are cross-linked through the reversible formation of β-cyclodextrin-adamantane inclusion complexes. The resultant viscous hydrogels display broad adhesive properties across polar and nonpolar substrates, mimicking that of natural mucous and thus making it easier to distribute onto a wide range of surfaces. Additional control over the hydrogel's mechanical properties (storage/loss moduli) and performance (adhesion) is achieved post-injection using a low-energy (blue light) photoinduced electron-transfer process. This work envisions these injectable copolymers and multimodal hydrogels can serve as versatile next-generation biomaterials capable of light-based mechanical manipulation post-injection.

Chemical and Biological Engineering Strategies to Make and Modify Next-Generation Hydrogel Biomaterials

There is a growing interest in the development of technologies to probe and direct in vitro cellular function for fundamental organoid and stem cell biology, functional tissue and metabolic engineering, and biotherapeutic formulation. Recapitulating many critical aspects of the native cellular niche, hydrogel biomaterials have proven to be a defining platform technology in this space, catapulting biological investigation from traditional two-dimensional (2D) culture into the 3D world. Seeking to better emulate the dynamic heterogeneity characteristic of all living tissues, global efforts over the last several years have centered around upgrading hydrogel design from relatively simple and static architectures into stimuli-responsive and spatiotemporally evolvable niches. Towards this end, advances from traditionally disparate fields including bioorthogonal click chemistry, chemoenzymatic synthesis, and DNA nanotechnology have been co-opted and integrated to construct 4D-tunable systems that undergo preprogrammed functional changes in response to user-defined inputs. In this Review, we highlight how advances in synthetic, semisynthetic, and bio-based chemistries have played a critical role in the triggered creation and customization of next-generation hydrogel biomaterials. We also chart how these advances stand to energize the translational pipeline of hydrogels from bench to market and close with an outlook on outstanding opportunities and challenges that lay ahead.

Modification and crosslinking strategies for hyaluronic acid-based hydrogel biomaterials

Hyaluronic acid (HA) is an attractive extracellular matrix-derived polymer. The related HA-based hydrogels are emerging to be the hotspots in the cutting edge of biomaterials. The continuous sights concentrate on exploring modification methods and crosslinking strategies to promote the advancement of HA-based hydrogels with enhanced physical/chemical properties and enriched biological performance. Here, the advances on modification methods and crosslinking strategies for fabricating HA-based hydrogels with diverse capacities are summarized. Firstly, the modification reactions that occur on the active hydroxyl, carboxyl and N-acetyl groups of HA molecule are discussed. Next, the emphasis is put on various crosslinking strategies including physical crosslinking, covalent crosslinking and dynamic covalent crosslinking. Finally, we provide a general summary and give a critical viewpoint on the remaining challenges and the future development of HA-based hydrogels. It is hoped that this review can provide new proposals for the specific design of functional hydrogel biomaterials.

Hydrogels with Reversible Crosslinks for Improved Localised Stem Cell Retention: A Review

Successful stem cell applications could have a significant impact on the medical field, where many lives are at stake. However, the translation of stem cells to the clinic could be improved by overcoming challenges in stem cell transplantation and in vivo retention at the site of tissue damage. This review aims to showcase the most recent insights into developing hydrogels that can deliver, retain, and accommodate stem cells for tissue repair. Hydrogels can be used for tissue engineering, as their flexibility and water content makes them excellent substitutes for the native extracellular matrix. Moreover, the mechanical properties of hydrogels are highly tuneable, and recognition moieties to control cell behaviour and fate can quickly be introduced. This review covers the parameters necessary for the physicochemical design of adaptable hydrogels, the variety of (bio)materials that can be used in such hydrogels, their application in stem cell delivery and some recently developed chemistries for reversible crosslinking. Implementing physical and dynamic covalent chemistry has resulted in adaptable hydrogels that can mimic the dynamic nature of the extracellular matrix.

Modular multiwell viscoelastic hydrogel platform for two- and three-dimensional cell culture applications

Hydrogels have gained significant popularity as model platforms to study the reciprocal interactions between cells and their microenvironment. While hydrogel tools to probe many characteristics of the extracellular space have been developed, fabrication approaches remain challenging and time-consuming, limiting multiplexing or widespread adoption. Thus, we have developed a modular fabrication approach to generate distinct hydrogel microenvironments within 96-well plates for increased throughput of fabrication as well as integration with existing high-throughput assay technologies. This approach enables in situ hydrogel mechanical characterization and was used to generate both elastic and viscoelastic hydrogels across a range of stiffnesses. Additionally, this fabrication method enabled a 3-fold reduction in polymer and up to an 8-fold reduction in fabrication time required per hydrogel replicate. The feasibility of this platform for cell culture applications was demonstrated by measuring both population-level and single cell-level metrics via microplate reader and high-content imaging. Finally, the 96-well hydrogel array was utilized for 3D cell culture, demonstrating the ability to support high cell viability. Together, this work demonstrates a versatile and easily adoptable fabrication approach that can support the ever-expanding tool kit of hydrogel technologies for cell culture applications.

Microinterfaces in bicontinuous hydrogels guide rapid 3D cell migration

Cell migration is critical for tissue development and regeneration but requires extracellular environments that are conducive to motion. Cells may actively generate migratory routes in vivo by degrading or remodeling their environments or may instead utilize existing ECM microstructures or microtracks as innate pathways for migration. While hydrogels in general are valuable tools for probing the extracellular regulators of 3D migration, few have recapitulated these natural migration paths. Here, we developed a biopolymer-based (i.e., gelatin and hyaluronic acid) bicontinuous hydrogel system formed through controlled solution immiscibility whose continuous subdomains and high micro-interfacial surface area enabled rapid 3D migration, particularly when compared to homogeneous hydrogels. Migratory behavior was mesenchymal in nature and regulated by biochemical and biophysical signals from the hydrogel, which was shown across various cell types and physiologically relevant contexts (e.g., cell spheroids, ex vivo tissues, in vivo tissues). Our findings introduce a new design that leverages important local interfaces to guide rapid cell migration.

Nanocomposite hydrogel based on chitosan/laponite for sealing and repairing tracheoesophageal fistula

Tracheoesophageal fistula (TEF) is an abnormal connection between the trachea and esophagus that severely impairs quality of life. Current treatment options have limitations, including conservative treatment, surgical repair, and esophageal stent implantation. Here, we introduced laponite (LP) nano-clay to improve chitosan-based hydrogels' rheological properties and mechanical properties and developed an endoscopically injectable nanocomposite shear-thinning hydrogel to seal and repair fistulas as an innovative material for the treatment of TEF. Excellent injectability, rheological properties, mechanical strength, self-healing, biodegradability, biocompatibility, and tissue repair characterize the new hydrogel. The introduction of LP nano-clay improves the gel kinetics problem of hydrogels to realize the sol-gel transition immediately after injection, avoiding gel flow to non-target sites. The addition of LA nano-clay can significantly improve the rheological properties and mechanical strength of hydrogels, and hydrogel with LP content of 3 % shows better comprehensive performance. The nanocomposite hydrogel also shows good cytocompatibility and can promote wound repair by promoting the migration of HEEC cells and the secretion of VEGF and FGF. These findings suggest that this nanocomposite hydrogel is a promising biomaterial for TEF treatment.

Effects of Cell-Adhesive Ligand Presentation on Pentapeptide Supramolecular Assembly and Gelation: Simulations and Experiments

The extracellular matrix (ECM) is a complex, hierarchical material containing structural and bioactive components. This complexity makes decoupling the effects of biomechanical properties and cell-matrix interactions difficult, especially when studying cellular processes in a 3D environment. Matrix mechanics and cell adhesion are both known regulators of specific cellular processes such as stem cell proliferation and differentiation. However, more information is required about how such variables impact various neural lineages that could, upon transplantation, therapeutically improve neural function after a central nervous system injury or disease. Rapidly Assembling Pentapeptides for Injectable Delivery (RAPID) hydrogels are one biomaterial approach to meet these goals, consisting of a family of peptide sequences that assemble into physical hydrogels in physiological media. In this study, we studied our previously reported supramolecularly-assembling RAPID hydrogels functionalized with the ECM-derived cell-adhesive peptide ligands RGD, IKVAV, and YIGSR. Using molecular dynamics simulations and experimental rheology, we demonstrated that these integrin-binding ligands at physiological concentrations (3-12 mm) did not impact the assembly of the KYFIL peptide system. In simulations, molecular measures of assembly such as hydrogen bonding and pi-pi interactions appeared unaffected by cell-adhesion sequence or concentration. Visualizations of clustering and analysis of solvent-accessible surface area indicated that the integrin-binding domains remained exposed. KYFIL or AYFIL hydrogels containing 3 mm of integrin-binding domains resulted in mechanical properties consistent with their non-functionalized equivalents. This strategy of doping RAPID gels with cell-adhesion sequences allows for the precise tuning of peptide ligand concentration, independent of the rheological properties. The controllability of the RAPID hydrogel system provides an opportunity to investigate the effect of integrin-binding interactions on encapsulated neural cells to discern how hydrogel microenvironment impacts growth, maturation, or differentiation.

The biocompatibility and the metabolic impact of thermoresponsive, bile acid-based nanogels on auditory and macrophage cell lines

Deoxycholic acid (DCA), lithocholic acid (LCA), and ursodeoxycholic acid (UDCA) are bile acids that may serve as permeation enhancers when incorporated within the nanogel matrix for drug delivery in the inner ear. In this study, thermoresponsive nanogels were formulated with DCA, LCA and UDCA and their rheological properties and biocompatibility were assessed. The impact of nanogel on cellular viability was evaluated via cell viability assay, the impact of nanogels on cellular bioenergetic parameters was estimated by Seahorse mito-stress test and glycolysis-stress test, while the presence of intracellular free radicals was assessed by reactive oxygen species assay. Nanogels showed a high level of biocompatibility after 24-hour exposure to auditory and macrophage cell lines, with minimal cytotoxicity compared to untreated control. Incubation with nanogels did not alter cellular respiration and glycolysis of the auditory cell line but showed possible mitochondrial dysfunction in macrophages, suggesting tissue-dependent effects of bile acids. Bile acid-nanogels had minimal impact on intracellular reactive oxygen species, with LCA demonstrating the most pro-oxidative behaviour. This study suggests that thermoresponsive nanogels with bile acid, particularly DCA and UDCA, may be promising candidates for inner ear drug delivery.

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.

In Situ Covalent Reinforcement of a Benzene-1,3,5-Tricarboxamide Supramolecular Polymer Enables Biomimetic, Tough, and Fibrous Hydrogels and Bioinks

Synthetic hydrogels often lack the load-bearing capacity and mechanical properties of native biopolymers found in tissue, such as cartilage. In natural tissues, toughness is often imparted via the combination of fibrous noncovalent self-assembly with key covalent bond formation. This controlled combination of supramolecular and covalent interactions remains difficult to engineer, yet can provide a clear strategy for advanced biomaterials. Here, a synthetic supramolecular/covalent strategy is investigated for creating a tough hydrogel that embodies the hierarchical fibrous architecture of the extracellular matrix (ECM). A benzene-1,3,5-tricarboxamide (BTA) hydrogelator is developed with synthetically addressable norbornene handles that self-assembles to form a and viscoelastic hydrogel. Inspired by collagen's covalent cross-linking of fibrils, the mechanical properties are reinforced by covalent intra- and interfiber cross-links. At over 90% water, the hydrogels withstand up to 550% tensile strain, 90% compressive strain, and dissipated energy with recoverable hysteresis. The hydrogels are shear-thinning, can be 3D bioprinted with good shape fidelity, and can be toughened via covalent cross-linking. These materials enable the bioprinting of human mesenchymal stromal cell (hMSC) spheroids and subsequent differentiation into chondrogenic tissue. Collectively, these findings highlight the power of covalent reinforcement of supramolecular fibers, offering a strategy for the bottom-up design of dynamic, yet tough, hydrogels and bioinks.

Hybrid Shear-thinning Hydrogel Integrating Hyaluronic Acid with ROS-Responsive Nanoparticles

Nanoparticle (NP) supra-assembly offers unique opportunities to tune macroscopic hydrogels' mechanical strength, material degradation, and drug delivery properties. Here, synthetic, reactive oxygen species (ROS)-responsive NPs are physically crosslinked with hyaluronic acid (HA) through guest-host chemistry to create shear-thinning NP/HA hydrogels. A library of triblock copolymers composed of poly(propylene sulfide)-bl-poly(N,N-dimethylacrylamide)-bl-poly(N,N-dimethylacrylamide-co-N-(1-adamantyl)acrylamide) are synthesized with varied triblock architectures and adamantane grafting densities and then self-assembled into NPs displaying adamantane on their corona. Self-assembled NPs are mixed with β-cyclodextrin grafted HA to yield eighteen NP/HA hydrogel formulations. The NP/HA hydrogel platform demonstrates superior mechanical strength to HA-only hydrogels, susceptibility to oxidative/enzymatic degradation, and inherent cell-protective, antioxidant function. The performance of NP/HA hydrogels is shown to be affected by triblock architecture, guest/host grafting densities, and HA composition. In particular, the length of the hydrophilic second block and adamantane grafting density of self-assembled NPs significantly impacts hydrogel mechanical properties and shear-thinning behavior, while ROS-reactivity of poly(propylene sulfide) protects cells from cytotoxic ROS and reduces oxidative degradation of HA compared to HA-only hydrogels. This work provides insight into polymer structure-function considerations for designing hybrid NP/HA hydrogels and identifies antioxidant, shear-thinning hydrogels as promising injectable delivery platforms for small molecule drugs and therapeutic cells.

Engineered biomaterials to guide spheroid formation, function, and fabrication into 3D tissue constructs

Cellular spheroids are aggregates of cells that are being explored to address fundamental biological questions and as building blocks for engineered tissues. Spheroids possess distinct advantages over cellular monolayers or cell encapsulation in 3D natural and synthetic hydrogels, including direct cell-cell interactions and high cell densities, which better mimic aspects of many tissues. Despite these advantages, spheroid cultures often exhibit uncontrollable growth and may be too simplistic to mimic complex tissue structures. To address this, biomaterials are being leveraged to further expand the use of cellular spheroids for biomedical applications. In this review, we provide an overview of recent studies that utilize engineered biomaterials to guide spheroid formation and function, as well as their fabrication into tissues for use as tissue models and for therapeutic applications. First, we describe biomaterial strategies that allow the high-throughput fabrication of homogeneously-sized spheroids. Next, we summarize how engineered biomaterials are introduced into spheroid cultures either internally as microparticles or externally as hydrogel microenvironments to influence spheroid behavior (e.g., differentiation, fusion). Lastly, we discuss a variety of biofabrication strategies (e.g., 3D bioprinting, melt electrowriting) that have been used to develop macroscale tissue models and implantable constructs through the guided assembly of spheroids. Overall, the goal of this review is to provide a summary of how biomaterials are currently being engineered and leveraged to support spheroids in biomedical applications, as well as to provide a future outlook of the field. STATEMENT OF SIGNIFICANCE: Cellular spheroids are becoming increasingly used as in vitro tissue models or as 'building blocks' for tissue engineering and repair strategies. Engineered biomaterials and their processing through biofabrication approaches are being leveraged to structurally support and guide spheroid processes. This review summarizes current approaches where such biomaterials are being used to guide spheroid formation, function, and fabrication into tissue constructs. As the field is rapidly expanding, we also provide an outlook on future directions and how new engineered biomaterials can be implemented to further the development of biofabricated spheroid-based tissue constructs.

Control of the post-infarct immune microenvironment through biotherapeutic and biomaterial-based approaches

Ischemic heart failure (IHF) is a leading cause of morbidity and mortality worldwide, for which heart transplantation remains the only definitive treatment. IHF manifests from myocardial infarction (MI) that initiates tissue remodeling processes, mediated by mechanical changes in the tissue (loss of contractility, softening of the myocardium) that are interdependent with cellular mechanisms (cardiomyocyte death, inflammatory response). The early remodeling phase is characterized by robust inflammation that is necessary for tissue debridement and the initiation of repair processes. While later transition toward an immunoregenerative function is desirable, functional reorientation from an inflammatory to reparatory environment is often lacking, trapping the heart in a chronically inflamed state that perpetuates cardiomyocyte death, ventricular dilatation, excess fibrosis, and progressive IHF. Therapies can redirect the immune microenvironment, including biotherapeutic and biomaterial-based approaches. In this review, we outline these existing approaches, with a particular focus on the immunomodulatory effects of therapeutics (small molecule drugs, biomolecules, and cell or cell-derived products). Cardioprotective strategies, often focusing on immunosuppression, have shown promise in pre-clinical and clinical trials. However, immunoregenerative therapies are emerging that often benefit from exacerbating early inflammation. Biomaterials can be used to enhance these therapies as a result of their intrinsic immunomodulatory properties, parallel mechanisms of action (e.g., mechanical restraint), or by enabling cell or tissue-targeted delivery. We further discuss translatability and the continued progress of technologies and procedures that contribute to the bench-to-bedside development of these critically needed treatments.

Alginate/polyacrylamide host-guest supramolecular hydrogels with enhanced adhesion

Although injectable hydrogels with minimally invasive delivery have garnered significant interest, their potential applications have been restricted by a singular property. In this study, a supramolecular hydrogel system with improved adhesion was constructed through host-guest interactions between alginate and polyacrylamide. The maximum tensile adhesion strength between the β-cyclodextrin and dopamine-grafted alginate/adamantane-grafted polyacrylamide (Alg-βCD-DA/PAAm-Ad, namely AβCDPA) hydrogels and pigskin reached 19.2 kPa, which was 76 % stronger than the non-catechol-based control hydrogel (β-cyclodextrin-grafted alginate/adamantane-grafted polyacrylamide, Alg-βCD/PAAm-Ad). Moreover, the hydrogels demonstrated excellent self-healing, shear-thinning, and injectable properties. The required pressure to extrude the AβCDPA2 hydrogel from a 16G needle at a rate of 2.0 mL/min was 67.4 N. As the polymer concentration and adamantane substitution degree increased, the hydrogels exhibited higher modulus, stronger network structure, and lower swelling ratio and degradation rate. Encapsulating and culturing cells within these hydrogels demonstrated good cytocompatibility. Therefore, this hydrogel can serve as a viscosity extender or bioadhesive, and as a carrier material to deliver encapsulated therapeutic substances into the body through minimally invasive injection methods.

Accelerating aging with dynamic biomaterials: Recapitulating aged tissue phenotypes in engineered platforms

Aging is characterized by progressive decline in tissue function and represents the greatest risk factor for many diseases. Nevertheless, many fundamental mechanisms driving human aging remain poorly understood. Aging studies using model organisms are often limited in their applicability to humans. Mechanistic studies of human aging rely on relatively simple cell culture models that fail to replicate mature tissue function, making them poor surrogates for aged tissues. These culture systems generally lack well-controlled cellular microenvironments that capture the changes in tissue mechanics and microstructure that occur during aging. Biomaterial platforms presenting dynamic, physiologically relevant mechanical, structural, and biochemical cues can capture the complex changes in the cellular microenvironment in a well-defined manner, accelerating the process of cellular aging in model laboratory systems. By enabling selective tuning of relevant microenvironmental parameters, these biomaterials systems may enable identification of new therapeutic approaches to slow or reverse the detrimental effects of aging.

Molecular Tuning of a Benzene-1,3,5-Tricarboxamide Supramolecular Fibrous Hydrogel Enables Control over Viscoelasticity and Creates Tunable ECM-Mimetic Hydrogels and Bioinks

Traditional synthetic covalent hydrogels lack the native tissue dynamics and hierarchical fibrous structure found in the extracellular matrix (ECM). These dynamics and fibrous nanostructures are imperative in obtaining the correct cell/material interactions. Consequently, the challenge to engineer functional dynamics in a fibrous hydrogel and recapitulate native ECM properties remains a bottle-neck to biomimetic hydrogel environments. Here, the molecular tuning of a supramolecular benzene-1,3,5-tricarboxamide (BTA) hydrogelator via simple modulation of hydrophobic substituents is reported. This tuning results in fibrous hydrogels with accessible viscoelasticity over 5 orders of magnitude, while maintaining a constant equilibrium storage modulus. BTA hydrogelators are created with systematic variations in the number of hydrophobic carbon atoms, and this is observed to control the viscoelasticity and stress-relaxation timescales in a logarithmic fashion. Some of these BTA hydrogels are shear-thinning, self-healing, extrudable, and injectable, and can be 3D printed into multiple layers. These hydrogels show high cell viability for chondrocytes and human mesenchymal stem cells, establishing their use in tissue engineering applications. This simple molecular tuning by changing hydrophobicity (with just a few carbon atoms) provides precise control over the viscoelasticity and 3D printability in fibrillar hydrogels and can be ported onto other 1D self-assembling structures. The molecular control and design of hydrogel network dynamics can push the field of supramolecular chemistry toward the design of new ECM-mimicking hydrogelators for numerous cell-culture and tissue-engineering applications and give access toward highly biomimetic bioinks for bioprinting.

Three-dimensional bioprinting of artificial blood vessel: Process, bioinks, and challenges

The coronary artery bypass grafting is a main treatment for restoring the blood supply to the ischemic site by bypassing the narrow part, thereby improving the heart function of the patients. Autologous blood vessels are preferred in coronary artery bypass grafting, but their availability is often limited by due to the underlying disease. Thus, tissue-engineered vascular grafts that are devoid of thrombosis and have mechanical properties comparable to those of natural vessels are urgently required for clinical applications. Most of the commercially available artificial implants are made from polymers, which are prone to thrombosis and restenosis. The biomimetic artificial blood vessel containing vascular tissue cells is the most ideal implant material. Due to its precision control ability, three-dimensional (3D) bioprinting is a promising method to prepare biomimetic system. In the 3D bioprinting process, the bioink is at the core state for building the topological structure and keeping the cell viable. Therefore, in this review, the basic properties and viable materials of the bioink are discussed, and the research of natural polymers in bioink, including decellularized extracellular matrix, hyaluronic acid, and collagen, is emphasized. Besides, the advantages of alginate and Pluronic F127, which are the mainstream sacrificial material during the preparation of artificial vascular graft, are also reviewed. Finally, an overview of the applications in the field of artificial blood vessel is also presented.

Hydrogel mechanics regulate fibroblast DNA methylation and chromatin condensation

Cellular mechanotransduction plays a central role in fibroblast activation during fibrotic disease progression, leading to increased tissue stiffness and reduced organ function. While the role of epigenetics in disease mechanotransduction has begun to be appreciated, little is known about how substrate mechanics, particularly the timing of mechanical inputs, regulate epigenetic changes such as DNA methylation and chromatin reorganization during fibroblast activation. In this work, we engineered a hyaluronic acid hydrogel platform with independently tunable stiffness and viscoelasticity to model normal (storage modulus, G' ∼ 0.5 kPa, loss modulus, G'' ∼ 0.05 kPa) to increasingly fibrotic (G' ∼ 2.5 and 8 kPa, G'' ∼ 0.05 kPa) lung mechanics. Human lung fibroblasts exhibited increased spreading and nuclear localization of myocardin-related transcription factor-A (MRTF-A) with increasing substrate stiffness within 1 day, with these trends holding steady for longer cultures. However, fibroblasts displayed time-dependent changes in global DNA methylation and chromatin organization. Fibroblasts initially displayed increased DNA methylation and chromatin decondensation on stiffer hydrogels, but both of these measures decreased with longer culture times. To investigate how culture time affected the responsiveness of fibroblast nuclear remodeling to mechanical signals, we engineered hydrogels amenable to in situ secondary crosslinking, enabling a transition from a compliant substrate mimicking normal tissue to a stiffer substrate resembling fibrotic tissue. When stiffening was initiated after only 1 day of culture, fibroblasts rapidly responded and displayed increased DNA methylation and chromatin decondensation, similar to fibroblasts on static stiffer hydrogels. Conversely, when fibroblasts experienced later stiffening at day 7, they showed no changes in DNA methylation and chromatin condensation, suggesting the induction of a persistent fibroblast phenotype. These results highlight the time-dependent nuclear changes associated with fibroblast activation in response to dynamic mechanical perturbations and may provide mechanisms to target for controlling fibroblast activation.

Supramolecular Hydrogels for Precisely Controlled Antimicrobial Peptide Delivery for Diabetic Wound Healing

Diabetic wound patients are often exposed to bacterial infections with delayed healing process due to hyperglycemia in the damaged skin tissue. Antimicrobial peptides (AMPs) have been investigated for the treatment of infection-induced diabetic wounds, but their low stability and toxicity have limited their further applications to diabetic chronic wound healing. Here, we developed a precisely controlled AMP-releasing injectable hydrogel platform, which could respond to infection-related materials of matrix metalloproteinases (MMPs) and reactive oxygen species (ROS). The injectable supramolecular hydrogel was prepared by the simple mixing of hyaluronic acid modified with cyclodextrin (HA-CD) and adamantane (Ad-HA). Ad-HA was conjugated with AMP via the cyclic peptide linker composed of MMP and ROS cleavable sequence (Ad-HA-AMP). Remarkably, only when the AMP-tethered hydrogel was exposed to both MMP and ROS simultaneously, AMP was released from the hydrogel, enabling the controlled release of AMP without causing cytotoxicity. In addition, we confirmed the enhanced serum stability of the Ad-HA-AMP conjugate. The antimicrobial activity of Ad-HA-AMP was maintained much longer than that of the native AMP. Finally, we could demonstrate the greatly improved wound-healing effect of AMP-tethered hydrogels with enhanced safety for the treatment of infection-induced diabetic chronic wounds. Taken together, we successfully demonstrated the feasibility of sHG-AMP for diabetic chronic wound healing.

Influence of Microgel and Interstitial Matrix Compositions on Granular Hydrogel Composite Properties

Granular hydrogels are an emerging class of biomaterials formed by jamming hydrogel microparticles (i.e., microgels). These materials have many advantageous properties that can be tailored through microgel design and extent of packing. To enhance the range of properties, granular composites can be formed with a hydrogel interstitial matrix between the packed microgels, allowing for material flow and then stabilization after crosslinking. This approach allows for distinct compartments (i.e., microgels and interstitial space) with varied properties to engineer complex material behaviors. However, a thorough investigation of how the compositions and ratios of microgels and interstitial matrices influence material properties has not been performed. Herein, granular hydrogel composites are fabricated by combining fragmented hyaluronic acid (HA) microgels with interstitial matrices consisting of photocrosslinkable HA. Microgels of varying compressive moduli (10-70 kPa) are combined with interstitial matrices (0-30 vol.%) with compressive moduli varying from 2-120 kPa. Granular composite structure (confocal imaging), mechanics (local and bulk), flow behavior (rheology), and printability are thoroughly assessed. Lastly, variations in the interstitial matrix chemistry (covalent vs guest-host) and microgel degradability are investigated. Overall, this study describes the influence of granular composite composition on structure and mechanical properties of granular hydrogels towards informed designs for future applications.

Rheology as a Tool for Fine-Tuning the Properties of Printable Bioinspired Gels

Over the last decade, efforts have been oriented toward the development of suitable gels for 3D printing, with controlled morphology and shear-thinning behavior in well-defined conditions. As a multidisciplinary approach to the fabrication of complex biomaterials, 3D bioprinting combines cells and biocompatible materials, which are subsequently printed in specific shapes to generate 3D structures for regenerative medicine or tissue engineering. A major interest is devoted to the printing of biomimetic materials with structural fidelity after their fabrication. Among some requirements imposed for bioinks, such as biocompatibility, nontoxicity, and the possibility to be sterilized, the nondamaging processability represents a critical issue for the stability and functioning of the 3D constructs. The major challenges in the field of printable gels are to mimic at different length scales the structures existing in nature and to reproduce the functions of the biological systems. Thus, a careful investigation of the rheological characteristics allows a fine-tuning of the material properties that are manufactured for targeted applications. The fluid-like or solid-like behavior of materials in conditions similar to those encountered in additive manufacturing can be monitored through the viscoelastic parameters determined in different shear conditions. The network strength, shear-thinning, yield point, and thixotropy govern bioprintability. An assessment of these rheological features provides significant insights for the design and characterization of printable gels. This review focuses on the rheological properties of printable bioinspired gels as a survey of cutting-edge research toward developing printed materials for additive manufacturing.

From crosslinking strategies to biomedical applications of hyaluronic acid-based hydrogels: A review

Hyaluronic acid (HA) is not only a natural anionic polysaccharide with excellent biocompatibility, biodegradability, and moisturizing effect, but also an essential factor that can affect angiogenesis, inflammation, cell behavior, which has a wide range of applications in the biomedical field. Among them, HA-based hydrogels formed by various physical or chemical crosslinking strategies are particularly striking. They not only retain the physiological function of HA, but also have the skeleton function of hydrogel, which further expands the application of HA. However, HA-based natural hydrogels generally have problems such as insufficient mechanical strength and susceptibility to degradation by hyaluronidase, which limits their application to a certain extent. To solve such problems, researchers have prepared a variety of HA-based multifunctional hydrogels with remarkable properties in recent years by adopting various structural modification methods or novel crosslinking strategies, as well as introducing functionally reactive molecules or moieties, which have extended the application scope. This manuscript systematically introduced common crosslinking strategies of HA-based hydrogels and highlighted the development of novel HA-based hydrogels in anticancer drug delivery, cartilage repair, three-dimensional cell culture, skin dressing and other fields. We hope to provide some references for the subsequent development of HA-based hydrogels in the biomedical field.

Harnessing cell-material interactions to control stem cell secretion for osteoarthritis treatment

Osteoarthritis (OA) is the most common debilitating joint disease, yet there is no curative treatment for OA to date. Delivering mesenchymal stromal cells (MSCs) as therapeutic cells to mitigate the inflammatory symptoms associated with OA is attracting increasing attention. In principle, MSCs could respond to the pro-inflammatory microenvironment of an OA joint by the secretion of anti-inflammatory, anti-apoptotic, immunomodulatory and pro-regenerative factors, therefore limiting pain, as well as the disease development. However, the microenvironment of MSCs is known to greatly affect their survival and bioactivity, and using tailored biomaterial scaffolds could be key to the success of intra-articular MSC-based therapies. The aim of this review is to identify and discuss essential characteristics of biomaterial scaffolds to best promote MSC secretory functions in the context of OA. First, a brief introduction to the OA physiopathology is provided, followed by an overview of the MSC secretory functions, as well as the current limitations of MSC-based therapy. Then, we review the current knowledge on the effects of cell-material interactions on MSC secretion. These considerations allow us to define rational guidelines for next-generation biomaterial design to improve the MSC-based therapy of OA.

Advanced supramolecular design for direct ink writing of soft materials

The exciting advancements in 3D-printing of soft materials are changing the landscape of materials development and fabrication. Among various 3D-printers that are designed for soft materials fabrication, the direct ink writing (DIW) system is particularly attractive for chemists and materials scientists due to the mild fabrication conditions, compatibility with a wide range of organic and inorganic materials, and the ease of multi-materials 3D-printing. Inks for DIW need to possess suitable viscoelastic properties to allow for smooth extrusion and be self-supportive after printing, but molecularly facilitating 3D printability to functional materials remains nontrivial. While supramolecular binding motifs have been increasingly used for 3D-printing, these inks are largely optimized empirically for DIW. Hence, this review aims to establish a clear connection between the molecular understanding of the supramolecularly bound motifs and their viscoelastic properties at bulk. Herein, extrudable (but not self-supportive) and 3D-printable (self-supportive) polymeric materials that utilize noncovalent interactions, including hydrogen bonding, host-guest inclusion, metal-ligand coordination, micro-crystallization, and van der Waals interaction, have been discussed in detail. In particular, the rheological distinctions between extrudable and 3D-printable inks have been discussed from a supramolecular design perspective. Examples shown in this review also highlight the exciting macroscale functions amplified from the molecular design. Challenges associated with the hierarchical control and characterization of supramolecularly designed DIW inks are also outlined. The perspective of utilizing supramolecular binding motifs in soft materials DIW printing has been discussed. This review serves to connect researchers across disciplines to develop innovative solutions that connect top-down 3D-printing and bottom-up supramolecular design to accelerate the development of 3D-print soft materials for a sustainable future.

Cyclodextrin regulated natural polysaccharide hydrogels for biomedical applications-a review

Cyclodextrin and its derivative (CDs) are natural building blocks for linking with other components to afford functional biomaterials. Hydrogels are polymer network systems that can form hydrophilic three-dimensional network structures through different cross-linking methods and are developing as potential materials in biomedical applications. Natural polysaccharide hydrogels (NPHs) are widely adopted in biomedical field with good biocompatibility, biodegradability, low cytotoxicity, and versatility in emulating natural tissue properties. Compared with conventional NPHs, CD regulated natural polysaccharide hydrogels (CD-NPHs) maintain good biocompatibility, while improving poor mechanical qualities and unpredictable gelation times. Recently, there has been increasing and considerable usage of CD-NPHs while there is still no review comprehensively introducing their construction, classification, and application of these hydrogels from the material point of view regarding biomedical fields. To draw a complete picture of the current and future development of CD-NPHs, we systematically overview the classification of CD-NPHs, and provide a holistic view on the role of CD-NPHs in different biomedical fields, especially in drug delivery, wound dressing, cell encapsulation, and tissue engineering. Moreover, the current challenges and prospects of CD-NPHs are discussed rationally, providing an insight into developing vibrant fields of CD-NPHs-based biomedicine, and facilitating their translation from bench to clinical medicine.

A hybrid cartilage extracellular matrix-based hydrogel/poly (ε-caprolactone) scaffold incorporated with Kartogenin for cartilage tissue engineering

Despite extensive studies, hydrogels are unable to meet the mechanical and biological requirements for successful outcomes in cartilage tissue engineering. In the present study, beta cyclodextrin (β-CD)-modified alginate/cartilage extracellular matrix (ECM)-based interpenetrating polymer network (IPN) hydrogel was developed for sustained release of Kartogenin (KGN). Furthermore, the hydrogel was incorporated within a 3D-printed poly (ε-caprolactone) (PCL)/starch microfiber network in order to reinforce the construct for cartilage tissue engineering. All the synthesized compounds were characterized by H¹-NMR spectroscopy. The hydrogel/microfiber composite with a microfiber strand size and strand spacing of 300 μm and 2 mm, respectively showed a compressive modulus of 17.2 MPa, resembling the properties of the native cartilage tissue. Considering water uptake capacity, degradation rate, mechanical property, cell cytotoxicity and glycosaminoglycan secretions, β-CD-modified hydrogel reinforced with printed PCL/starch microfibers with controlled release of KGN may be considered as a promising candidate for using in articular cartilage defects.

HYDRHA: Hydrogels of hyaluronic acid. New biomedical approaches in cancer, neurodegenerative diseases, and tissue engineering

In the last decade, hyaluronic acid (HA) has attracted an ever-growing interest in the biomedical engineering field as a biocompatible, biodegradable, and chemically versatile molecule. In fact, HA is a major component of the extracellular matrix (ECM) and is essential for the maintenance of cellular homeostasis and crosstalk. Innovative experimental strategies in vitro and in vivo using three-dimensional (3D) HA systems have been increasingly reported in studies of diseases, replacement of tissue and organ damage, repairing wounds, and encapsulating stem cells for tissue regeneration. The present work aims to give an overview and comparison of recent work carried out on HA systems showing advantages, limitations, and their complementarity, for a comprehensive characterization of their use. A special attention is paid to the use of HA in three important areas: cancer, diseases of the central nervous system (CNS), and tissue regeneration, discussing the most innovative experimental strategies. Finally, perspectives within and beyond these research fields are discussed.

Sustained release of drug-loaded nanoparticles from injectable hydrogels enables long-term control of macrophage phenotype

Injectable hydrogels may be pre-formed through dynamic crosslinks, allowing for injection and subsequent retention in the tissue by shear-thinning and self-healing processes, respectively. These properties enable the site-specific delivery of encapsulated therapeutics; yet, the sustained release of small-molecule drugs and their cell-targeted delivery remains challenging due to their rapid diffusive release and non-specific cellular biodistribution. Herein, we develop an injectable hydrogel system composed of a macrophage-targeted nanoparticle (cyclodextrin nanoparticles, CDNPs) crosslinked by adamantane-modified hyaluronic acid (Ad-HA). The polymer-nanoparticle hydrogel uniquely leverages cyclodextrin's interaction with small molecule drugs to create a spatially discrete drug reservoir and with adamantane to yield dynamic, injectable hydrogels. Through an innovative two-step drug screening approach and examination of 45 immunomodulatory drugs with subsequent in-depth transcriptional profiling of both murine and human macrophages, we identify celastrol as a potent inhibitor of pro-inflammatory (M1-like) behavior that furthermore promotes a reparatory (M2-like) phenotype. Celastrol encapsulation within the polymer-nanoparticle hydrogels permitted shear-thinning injection and sustained release of drug-laden nanoparticles that targeted macrophages to modulate cell behavior for greater than two weeks in vitro. The modular hydrogel system is a promising approach to locally modulate cell-specific phenotype in a range of applications for immunoregenerative medicine.

Macrocyclic Supramolecular Assemblies Based on Hyaluronic Acid and Their Biological Applications

Hyaluronic acid (HA), which contains multiple carboxyl, hydroxyl, and acetylamino groups and is an agent that targets tumors, has drawn great attention in supramolecular diagnosis and treatment research. It can not only assemble directly with macrocyclic host-guest complexes through hydrogen bonding and electrostatic interactions but also can be modified with macrocyclic compounds or functional guest molecules by an amidation reaction and used for further assembly. Macrocycles play a main role in the construction of supramolecular drug carriers, targeted imaging agents, and hydrogels, such as cyclodextrins and cucurbit[n]urils, which can encapsulate photosensitizers, drugs, or other functional guest molecules via host-guest interactions. Therefore, the formed supramolecular assemblies can respond to various stimuli, such as enzymes, light, electricity, and magnetism for controlled drug delivery, enhance the luminescence intensity of the assembly, and improve drug loading capacity. In addition, the nanosupramolecular assembly formed with HA can also improve the biocompatibility of drugs, reduce drug toxicity and side effects, and enhance cell permeability; thus, the assembly has extensive application value in biomedical research. This Account mainly focuses on macrocyclic supramolecular assemblies based on HA, especially their biological applications and progress in the field, and these assemblies include (i) guest-modified HA, such as pyridinium-, adamantane-, peptide-, and other functional-group-modified HA, along with their cyclodextrin and cucurbit[n]uril assemblies; (ii) macrocycle-modified HA, such as HA modified with cyclodextrins and cucurbit[n]uril derivatives and their assembly with various guests; (iii) direct assembly between unmodified HA and cyclodextrin- or cucurbit[n]uril-based host-guest complexes. Particularly, we discussed the important role of macrocyclic host-guest complexes in HA-based supramolecular assembly, and the roles included improving the water solubility and efficacy of hydrophobic drugs, enhancing the luminescent intensity of assemblies, inducing room temperature phosphorescence and providing energy transfer systems, constructing multi-stimulus-responsive supramolecular assemblies, and in situ formation of hydrogels. Additionally, we believe that obtaining in-depth knowledge of these HA-based macrocyclic supramolecular assemblies and their biological applications encompasses many challenges regarding drug carriers, targeted imaging agents, wound healing, and biomedical soft materials and would certainly contribute to the rapid development of supramolecular diagnosis and treatment.

Recent advances in engineering hydrogels for niche biomimicking and hematopoietic stem cell culturing

Hematopoietic stem cells (HSCs) provide a life-long supply of haemopoietic cells and are indispensable for clinical transplantation in the treatment of malignant hematological diseases. Clinical applications require vast quantities of HSCs with maintained stemness characteristics. Meeting this demand poses often insurmountable challenges for traditional culture methods. Creating a supportive artificial microenvironment for the culture of HSCs, which allows the expansion of the cells while maintaining their stemness, is becoming a new solution for the provision of these rare multipotent HSCs. Hydrogels with good biocompatibility, excellent hydrophilicity, tunable biochemical and biophysical properties have been applied in mimicking the hematopoietic niche for the efficient expansion of HSCs. This review focuses on recent progress in the use of hydrogels in this specialized application. Advanced biomimetic strategies use for the creation of an artificial haemopoietic niche are discussed, advances in combined use of hydrogel matrices and microfluidics, including the emerging organ-on-a-chip technology, are summarized. We also provide a brief description of novel stimulus-responsive hydrogels that are used to establish an intelligent dynamic cell microenvironment. Finally, current challenges and future perspectives of engineering hydrogels for HSC biomedicine are explored.

Microcarriers in application for cartilage tissue engineering: Recent progress and challenges

Successful regeneration of cartilage tissue at a clinical scale has been a tremendous challenge in the past decades. Microcarriers (MCs), usually used for cell and drug delivery, have been studied broadly across a wide range of medical fields, especially the cartilage tissue engineering (TE). Notably, microcarrier systems provide an attractive method for regulating cell phenotype and microtissue maturations, they also serve as powerful injectable carriers and are combined with new technologies for cartilage regeneration. In this review, we introduced the typical methods to fabricate various types of microcarriers and discussed the appropriate materials for microcarriers. Furthermore, we highlighted recent progress of applications and general design principle for microcarriers. Finally, we summarized the current challenges and promising prospects of microcarrier-based systems for medical applications. Overall, this review provides comprehensive and systematic guidelines for the rational design and applications of microcarriers in cartilage TE.

Recent advancements in polymeric nanofibers for ophthalmic drug delivery and ophthalmic tissue engineering

Nanofibers due to their unique properties such as high surface-to-volume ratio, porous structure, mechanical strength, flexibility and their resemblance to the extracellular matrix, have been researched extensively in the field of ocular drug delivery and tissue engineering. Further, different modifications considering the formulation and process parameters have been carried out to alter the drug release profile and its interaction with the surrounding biological environment. Electrospinning is the most commonly used technique for preparing nanofibers with industrial scalability. Advanced techniques such as co-axial electrospinning and combined system such as embedding nanoparticles in nanofiber provide an alternative approach to enhance the performance of the scaffold. Electrospun nanofibers offers a matrix like structure for cell regeneration. Nanofibers have been used for ocular delivery of various drugs like antibiotics, anti-inflammatory and various proteins. In addition, lens-coated medical devices provide new insights into the clinical use of nanofibers. Through fabricating the nanofibers researchers have overcome the issues of low bioavailability and compatibility with ocular tissue. Therefore, nanofibers have great potential in ocular drug delivery and tissue engineering and have the capacity to revolutionize these therapeutic areas in the field of ophthalmology. This review is mainly focused on the recent advances in the preparation of nanofibers and their applications in ocular drug delivery and tissue engineering. The authors have attempted to emphasize the processing challenges and future perspectives along with an overview of the safety and toxicity aspects of nanofibers.

Leveraging ultra-low interfacial tension and liquid-liquid phase separation in embedded 3D bioprinting

Many recently developed 3D bioprinting strategies operate by extruding aqueous biopolymer solutions directly into a variety of different support materials constituted from swollen, solvated, aqueous, polymer assemblies. In developing these 3D printing methods and materials, great care is often taken to tune the rheological behaviors of both inks and 3D support media. By contrast, much less attention has been given to the physics of the interfaces created when structuring one polymer phase into another in embedded 3D printing applications. For example, it is currently unclear whether a dynamic interfacial tension between miscible phases stabilizes embedded 3D bioprinted structures as they are shaped while in a liquid state. Interest in the physics of interfaces between complex fluids has grown dramatically since the discovery of liquid-liquid phase separation (LLPS) in living cells. We believe that many new insights coming from this burst of investigation into LLPS within biological contexts can be leveraged to develop new materials and methods for improved 3D bioprinting that leverage LLPS in mixtures of biopolymers, biocompatible synthetic polymers, and proteins. Thus, in this review article, we highlight work at the interface between recent LLPS research and embedded 3D bioprinting methods and materials, and we introduce a 3D bioprinting method that leverages LLPS to stabilize printed biopolymer inks embedded in a bioprinting support material.