Designing Hydrogels for 3D Cell Culture Using Dynamic Covalent Crosslinking

Three-Dimensional Culture for In Vitro Folliculogenesis in the Aspect of Methods and Materials

In vitro ovarian follicle culture is a reproduction technique used to obtain fertilizable oocytes, for overcoming fertility issues due to premature ovarian failure. This requires the establishment of an in vitro culture model that is capable of better simulating the in vivo ovarian growth environment. Two-dimensional (2D) culture systems have been successfully set up in rodent models. However, they are not suitable for larger animal models as the follicles of larger animals cultured in 2D culture systems often lose their shape due to dysfunction in the gap junctions. Three-dimensional (3D) culture systems are more suitable for maintaining follicle architecture, and therefore are proposed for the successful in vitro culturing of follicles in various animal models. The role of different methods, scaffolds, and suspension cultures in supporting follicle development has been studied to provide direction for improving in vitro follicle culture technologies. The three major strategies for in vitro 3D follicle cultures are discussed in this article. First, the in vitro culture systems, such as microfluidics, hanging drop, hydrogels, and 3D-printing, are reviewed. We have focused on the 3D hydrogel system as it uses different materials for supporting follicular growth and oocyte maturation in several animal models and in humans. We have also discussed the criteria used for biomaterial evaluations such as solid concentration, elasticity, and rigidity. In addition, future research directions for advancing in vitro 3D follicle culture system are discussed. Impact statement A new frontier in assisted reproductive technology is in vitro tissue or follicle culture, particularly for fertility preservation. The in vitro three-dimensional (3D) culture technique enhances follicular development and provides mature oocytes, overcoming the limitations of traditional in vitro two-dimensional cultures. Polymer biomaterials have good compatibility and retain the physiological structure of follicles in the 3D culture system. Utilizing hybrid in vitro culture materials by merging matrix, hydrogel, and unique patterned materials may facilitate follicular growth in the future.

Viscoelastic Notch Signaling Hydrogel Induces Liver Bile Duct Organoid Growth and Morphogenesis

Cholangiocyte organoids can be used to model liver biliary disease; however, both a defined matrix to emulate cholangiocyte self-assembly and the mechano-transduction pathways involved therein remain elusive. A series of defined viscoelastic hyaluronan hydrogels to culture primary cholangiocytes are designed and it is found that by mimicking the stress relaxation rate of liver tissue, cholangiocyte organoid growth can be induced and expression of Yes-associated protein (YAP) target genes could be significantly increased. Strikingly, inhibition of matrix metalloproteinases (MMPs) does not significantly affect organoid growth in 3D culture, suggesting that mechanical remodeling of the viscoelastic microenvironment-and not MMP-mediated degradation-is the key to cholangiocyte organoid growth. By immobilizing Jagged1 to the hyaluronan, stress relaxing hydrogel, self-assembled bile duct structures form in organoid culture, indicating the synergistic effects of Notch signaling and viscoelasticity. By uncovering critical roles of hydrogel viscoelasticity, YAP signaling, and Notch activation, cholangiocyte organogenesis is controlled, thereby paving the way for their use in disease modeling and/or transplantation.

Application of "Click" Chemistry in Biomedical Hydrogels

Since "click" chemistry was first reported in 2001, it has remained a popular research topic in the field of chemistry due to its high yield without byproducts, fast reaction rate, simple reaction, and biocompatibility. It has achieved good applications in various fields, especially for the preparation of hydrogels. The development of biomedicine presents new challenges and opportunities for hydrogels, and "click" chemistry provides a library of chemical tools for the preparation of various innovative hydrogels, including cell culture, 3D bioprinting, and drug release. This article summarizes several common "click" reactions, including copper-catalyzed azide-alkyne cycloaddition reactions, strain-promoted azide-alkyne cycloaddition (SPAAC) reaction, thiol-ene reaction, the Diels-Alder reaction, and the inverse electron demand Diels-Alder (IEDDA) reaction. We introduce the "click" reaction in the nucleic acid field to expand the concept of "click" chemistry. This article focuses on the application of "click" chemistry for preparing various types of biomedical hydrogels and highlights the advantages of "click" reactions for cross-linking to obtain hydrogels. This review also discusses applications of "click" chemistry outside the field of hydrogels, such as drug synthesis, targeted delivery, and surface modification, hydrogels have great application potential in these fields in the future and hopefully inspire other applications of hydrogels.

Dynamic Covalent Hydrogels: Strong yet Dynamic

Hydrogels are crosslinked polymer networks with time-dependent mechanical response. The overall mechanical properties are correlated with the dynamics of the crosslinks. Generally, hydrogels crosslinked by permanent chemical crosslinks are strong but static, while hydrogels crosslinked by physical interactions are weak but dynamic. It is highly desirable to create synthetic hydrogels that possess strong mechanical stability yet remain dynamic for various applications, such as drug delivery cargos, tissue engineering scaffolds, and shape-memory materials. Recently, with the introduction of dynamic covalent chemistry, the seemingly conflicting mechanical properties, i.e., stability and dynamics, have been successfully combined in the same hydrogels. Dynamic covalent bonds are mechanically stable yet still capable of exchanging, dissociating, or switching in response to external stimuli, empowering the hydrogels with self-healing properties, injectability and suitability for postprocessing and additive manufacturing. Here in this review, we first summarize the common dynamic covalent bonds used in hydrogel networks based on various chemical reaction mechanisms and the mechanical strength of these bonds at the single molecule level. Next, we discuss how dynamic covalent chemistry makes hydrogel materials more dynamic from the materials perspective. Furthermore, we highlight the challenges and future perspectives of dynamic covalent hydrogels.

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

Granular hydrogels are an exciting class of microporous and injectable biomaterials that are being explored for many biomedical applications, including regenerative medicine, 3D printing, and drug delivery. Granular hydrogels often possess low mechanical moduli and lack structural integrity due to weak physical interactions between microgels. This has been addressed through covalent inter-particle crosslinking; however, covalent crosslinking often occurs through temporal enzymatic methods or photoinitiated reactions, which may limit injectability and material processing. To address this, a hyaluronic acid (HA) granular hydrogel is developed with dynamic covalent (hydrazone) inter-particle crosslinks. Extrusion fragmentation is used to fabricate microgels from photocrosslinkable norbornene-modified HA, additionally modified with either aldehyde or hydrazide groups. Aldehyde and hydrazide-containing microgels are mixed and jammed to form adhesive granular hydrogels. These granular hydrogels possess enhanced mechanical integrity and shape stability over controls due to the covalent inter-particle bonds, while maintaining injectability due to the dynamic hydrazone bonds. The adhesive granular hydrogels are applied to 3D printing, which allows the printing of structures that are stable without any further post-processing. Additionally, the authors demonstrate that adhesive granular hydrogels allow for cell invasion in vitro. Overall, this work demonstrates the use of dynamic covalent inter-particle crosslinking to enhance injectable granular hydrogels.

Thermodynamic, kinetic, and mechanistic studies of the thermal guanidine metathesis reaction

We describe studies of the thermal guanidine metathesis (TGM) reaction, a reversible transformation that results in exchange of N-substituents of the guanidine functional group. By comparing the effects of discrete structural variations, we find that steric congestion is an important factor in determining both the equilibrium guanidine composition and the reaction kinetics. The alkyl versus aryl nature of N-substitution also plays an essential role in the reaction rate, up to the point that minimal TGM reactivity is observed when the guanidine contains wholly alkyl substituents. Furthermore, we demonstrate that TGM occurs under thermodynamic control and present evidence that it proceeds by a dissociative mechanism, supported by direct observation of a carbodiimide intermediate.

A switchable light-responsive azopolymer conjugating protein micropatterns with topography for mechanobiological studies

Stem cell shape and mechanical properties in vitro can be directed by geometrically defined micropatterned adhesion substrates. However, conventional methods are limited by the fixed micropattern design, which cannot recapitulate the dynamic changes of the natural cell microenvironment. Current methods to fabricate dynamic platforms usually rely on complex chemical strategies or require specialized apparatuses. Also, with these methods, the integration of dynamic signals acting on different length scales is not straightforward, whereas, in some applications, it might be beneficial to act on both a microscale level, that is, cell shape, and a nanoscale level, that is, cell adhesions. Here, we exploited a confocal laser-based technique on a light-responsive azopolymer displaying micropatterns of adhesive islands. The laser light promotes a directed mass migration and the formation of submicrometric topographic relieves. Also, by changing the surface chemistry, the surfacing topography affects cell spreading and shape. This method enabled us to monitor in a non-invasive manner the dynamic changes in focal adhesions, cytoskeleton structures, and nucleus conformation that followed the changes in the adhesive characteristic of the substrate. Focal adhesions reconfigured after the surfacing of the topography, and the actin filaments reoriented to coalign with the newly formed adhesive island. Changes in cell morphology also affected nucleus shape, chromatin conformation, and cell mechanics with different timescales. The reported strategy can be used to investigate mechanotransduction-related events dynamically by controlling cell adhesion at cell shape and focal adhesion levels. The integrated technique enables achieving a submicrometric resolution in a facile and cost-effective manner.

Behavior of colloidal gels made of thermoresponsive anisotropic nanoparticles

Amongst colloidal gels, those designed by the assembly of anisotropic colloidal particles tend to form fibrillar gels and are attracting interest as artificial cell growth environments since they have a structure reminiscent of biological extracellular matrices. Their properties can be tuned by controlling the size, shape, and rigidity of the nanoparticles used during their formation. Herein, the relationship between the physical and mechanical properties of the nanocolloidal building blocks and the properties of the resulting gels is investigated. Thermoresponsive particles with different aspect ratios and controlled rigidity were prepared, and the gelation and the properties of the resulting gels were studied. The results show how the aspect ratio and rigidity of polymer colloids tune the properties of the gels. An increase in the aspect ratio of the nanocolloid used led to a sol–gel transition observed at lower particle concentration, but an increase in the rigidity of the nanocolloids delayed the sol–gel transition to higher concentration. However, at a constant concentration, increases in the anisotropy produced gels with higher modulus and lower yield strain. Similarly, an increase in rigidity of the colloids increased the modulus and reduced the yield strain of the resulting gels.

Exploiting Meltable Protein Hydrogels to Encapsulate and Culture Cells in 3D

There is a growing realization that 3D cell culture better mimics complex in vivo environments than 2D, lessening aberrant cellular behaviours and ultimately improving the outcomes of experiments. Chemically cross-linked hydrogels which imitate natural extracellular matrix (ECM) are proven cell culture platforms, but the encapsulation of cells within these hydrogel networks requires bioorthogonal cross-linking chemistries which can be cytotoxic, synthetically demanding and costly. Capsular antigen fragment 1 (Caf1) is a bacterial, polymeric, fimbrial protein which can be genetically engineered to imitate ECM. Furthermore, it can, reversibly, thermally interconvert between its polymeric and monomeric forms even when chemically cross-linked within a hydrogel network. We demonstrate that this meltable feature of Caf1 hydrogels can be utilized to encapsulate neonatal human dermal fibroblasts at a range of cell densities (2 × 105 - 2 × 106 cells/mL of hydrogel) avoiding issues with chemical cytotoxicity. These hydrogels supported cell 3D culture for up to 21 days, successfully inducing cellular functions such as proliferation and migration. This work is significant because it further highlights the potential of simple, robust, Caf1-based hydrogels as a cell culture platform. This article is protected by copyright. All rights reserved.

Soft, Dynamic Hydrogel Confinement Improves Kidney Organoid Lumen Morphology and Reduces Epithelial-Mesenchymal Transition in Culture

Pluripotent stem cell-derived kidney organoids offer a promising solution to renal failure, yet current organoid protocols often lead to off-target cells and phenotypic alterations, preventing maturity. Here, various dynamic hydrogel architectures are created, conferring a controlled and biomimetic environment for organoid encapsulation. How hydrogel stiffness and stress relaxation affect renal phenotype and undesired fibrotic markers are investigated. The authors observe that stiff hydrogel encapsulation leads to an absence of certain renal cell types and signs of an epithelial-mesenchymal transition (EMT), whereas encapsulation in soft, stress-relaxing hydrogels leads to all major renal segments, fewer fibrosis or EMT associated proteins, apical proximal tubule polarization, and primary cilia formation, representing a significant improvement over current approaches to culture kidney organoids. The findings show that engineering hydrogel mechanics and dynamics have a decided benefit for organoid culture. These structure-property-function relationships can enable the rational design of materials, bringing us closer to functional engraftments and disease-modeling applications.

Regulating Bacterial Behavior within Hydrogels of Tunable Viscoelasticity

Engineered living materials (ELMs) are a new class of materials in which living organism incorporated into diffusive matrices uptake a fundamental role in material's composition and function. Understanding how the spatial confinement in 3D can regulate the behavior of the embedded cells is crucial to design and predict ELM's function, minimize their environmental impact and facilitate their translation into applied materials. This study investigates the growth and metabolic activity of bacteria within an associative hydrogel network (Pluronic-based) with mechanical properties that can be tuned by introducing a variable degree of acrylate crosslinks. Individual bacteria distributed in the hydrogel matrix at low density form functional colonies whose size is controlled by the extent of permanent crosslinks. With increasing stiffness and elastic response to deformation of the matrix, a decrease in colony volumes and an increase in their sphericity are observed. Protein production follows a different pattern with higher production yields occurring in networks with intermediate permanent crosslinking degrees. These results demonstrate that matrix design can be used to control and regulate the composition and function of ELMs containing microorganisms. Interestingly, design parameters for matrices to regulate bacteria behavior show similarities to those elucidated for 3D culture of mammalian cells.

Spatial and Temporal Modulation of Cell Instructive Cues in a Filamentous Supramolecular Biomaterial

Supramolecular materials provide unique opportunities to mimic both the structure and mechanics of the biopolymer networks that compose the extracellular matrix. However, strategies to modify their filamentous structures in space and time in 3D cell culture to study cell behavior as encountered in development and disease are lacking. We herein disclose a multicomponent squaramide-based supramolecular material whose mechanics and bioactivity can be controlled by light through co-assembly of a 1,2-dithiolane (DT) monomer that forms disulfide cross-links. Remarkably, increases in storage modulus from ∼200 Pa to >10 kPa after stepwise photo-cross-linking can be realized without an initiator while retaining colorlessness and clarity. Moreover, viscoelasticity and plasticity of the supramolecular networks decrease upon photo-irradiation, reducing cellular protrusion formation and motility when performed at the onset of cell culture. When applied during 3D cell culture, force-mediated manipulation is impeded and cells move primarily along earlier formed channels in the materials. Additionally, we show photopatterning of peptide cues in 3D using either a photomask or direct laser writing. We demonstrate that these squaramide-based filamentous materials can be applied to the development of synthetic and biomimetic 3D in vitro cell and disease models, where their secondary cross-linking enables mechanical heterogeneity and shaping at multiple length scales.

3D-Printed Porous Scaffolds of Hydrogels Modified with TGF-β1 Binding Peptides to Promote In Vivo Cartilage Regeneration and Animal Gait Restoration

The treatment of cartilage injury and osteoarthritis has been a classic problem for many years. The idea of in situ tissue regeneration paves a way for osteochondral repair in vivo. Herein, a hydrogel scaffold linked with bioactive peptides that can selectively adsorb transforming growth factor β1 (TGF-β1) was hypothesized to not only afford cell ingrowth space but also induce the endogenous TGF-β1 recruitment for chondrogenesis promotion. In this study, bilayered porous scaffolds with gelatin methacryloyl (GelMA) hydrogels as a matrix were constructed via three-dimensional (3D) printing, of which the upper layer was covalently bound with bioactive peptides that can adsorb TGF-β1 for cartilage repair and the lower layer was blended with hydroxyapatite for subchondral regeneration. The scaffolds showed promising therapeutic efficacy proved by cartilage and osteogenic induction in vitro and osteochondral repair of rats in vivo. In particular, the animal gait behavior was recovered after the in situ tissue regeneration, and the corresponding gait analysis demonstrated the promotion of tissue regeneration induced by the porous hydrogels with the binding peptides.

The Intracellular and Extracellular Microenvironment of Tumor Site: The Trigger of Stimuli-Responsive Drug Delivery Systems

The tumor microenvironment (TME), including intracellular and extracellular microenvironment, contains many biochemical indicators (such as acidity/alkalinity, oxygen content, and enzymatic activity) that are different from the normal physiological environment. These abnormal biochemical indicators can accelerate the heterogeneity of tumors, but on the other hand, they also provide opportunities for the design of intelligent drug delivery systems (DDSs). The TME-responsive DDSs have shown great potential in reducing the side effects of chemotherapy and improving the curative effect of tumors. In this review, the abnormal biochemical indicators of TME are introduced in detail from both the extracellular and intracellular aspects. In view of the various physiological barriers encountered during drug delivery, the strategy of constructing TME-responsive DDSs is discussed. By summarizing the typical research progress, the authors prospect the development of TME-responsive DDS in the future.

3D Bioprinting of Multifunctional Dynamic Nanocomposite Bioinks Incorporating Cu-Doped Mesoporous Bioactive Glass Nanoparticles for Bone Tissue Engineering

Bioprinting has seen significant progress in recent years for the fabrication of bionic tissues with high complexity. However, it remains challenging to develop cell-laden bioinks exhibiting superior physiochemical properties and bio-functionality. In this study, a multifunctional nanocomposite bioink is developed based on amine-functionalized copper (Cu)-doped mesoporous bioactive glass nanoparticles (ACuMBGNs) and a hydrogel formulation relying on dynamic covalent chemistry composed of alginate dialdehyde (oxidized alginate) and gelatin, with favorable rheological properties, improved shape fidelity, and structural stability for extrusion-based bioprinting. The reversible dynamic microenvironment in combination with the impact of cell-adhesive ligands introduced by aminated particles enables the rapid spreading (within 3 days) and high survival (>90%) of embedded human osteosarcoma cells and immortalized mouse bone marrow-derived stroma cells. Osteogenic differentiation of primary mouse bone marrow stromal stem cells (BMSCs) and angiogenesis are promoted in the bioprinted alginate dialdehyde-gelatin (ADA-GEL or AG)-ACuMBGN scaffolds without additional growth factors in vitro, which is likely due to ion stimulation from the incorporated nanoparticles and possibly due to cell mechanosensing in the dynamic matrix. In conclusion, it is envisioned that these nanocomposite bioinks can serve as promising platforms for bioprinting complex 3D matrix environments providing superior physiochemical and biological performance for bone tissue engineering.

Three-Dimensional Avian Hematopoietic Stem Cell Cultures as a Model for Studying Disease Pathogenesis

Three-dimensional (3D) cell culture is attracting increasing attention today because it can mimic tissue environments and provide more realistic results than do conventional cell cultures. On the other hand, very little attention has been given to using 3D cell cultures in the field of avian cell biology. Although mimicking the bone marrow niche is a classic challenge of mammalian stem cell research, experiments have never been conducted in poultry on preparing in vitro the bone marrow niche. It is well known, however, that all diseases cause immunosuppression and target immune cells and their development. Hematopoietic stem cells (HSC) reside in the bone marrow and constitute a source for immune cells of lymphoid and myeloid origins. Disease prevention and control in poultry are facing new challenges, such as greater use of alternative breeding systems and expanding production of eggs and chicken meat in developing countries. Moreover, the COVID-19 pandemic will draw greater attention to the importance of disease management in poultry because poultry constitutes a rich source of zoonotic diseases. For these reasons, and because they will lead to a better understanding of disease pathogenesis, in vivo HSC niches for studying disease pathogenesis can be valuable tools for developing more effective disease prevention, diagnosis, and control. The main goal of this review is to summarize knowledge about avian hematopoietic cells, HSC niches, avian immunosuppressive diseases, and isolation of HSC, and the main part of the review is dedicated to using 3D cell cultures and their possible use for studying disease pathogenesis with practical examples. Therefore, this review can serve as a practical guide to support further preparation of 3D avian HSC niches to study the pathogenesis of avian diseases.

Dynamic regulable sodium alginate/poly(γ-glutamic acid) hybrid hydrogels promoted chondrogenic differentiation of stem cells

Traditional hydrogels often fail to match the dynamic interactions between mechanical and cellular behaviors exhibited by the natural cartilage extracellular matrix. In this research, we constructed a novel hybrid hydrogels system based on sodium alginate and polyglutamic acid. By controlling the grafting rate and concentration of polymer, the gelation time and mechanical strength can be adjusted between range of 8-28 s and 60-144 kPa. By adding microcrystalline cellulose into the system, so that the degradation time was prolonged (125%) and the swelling rate was reduced (470%). Additionally, the presence of hydrazone bonds gives the system some dynamic response characteristics, and the hydrogel exhibits excellent self healing and injectable ability. It was found that the system had positive cytocompatibility (80%), which accelerated regulatory gene expression in cartilage tissue. In conclusion, this injectable hydrogel with self-healing and customizable mechanical strength will have broad application prospects in future biomedical engineering.

Tuning Hydrogels by Mixing Dynamic Cross-Linkers: Enabling Cell-Instructive Hydrogels and Advanced Bioinks

Rational design of hydrogels that balance processability and extracellular matrix (ECM) biomimicry remains a challenge for tissue engineering and biofabrication. Hydrogels suitable for biofabrication techniques, yet tuneable to match the mechanical (static and dynamic) properties of native tissues remain elusive. Dynamic covalent hydrogels possessing shear-thinning/self-healing (processability) and time-dependent cross-links (mechanical properties) provide a potential solution, yet can be difficult to rationally control. Here, the straightforward modular mixing of dynamic cross-links with different timescales (hydrazone and oxime) is explored using rheology, self-healing tests, extrusion printing, and culture of primary human dermal fibroblasts. Maintaining a constant polymer content and cross-linker concentration, the stiffness and stress relaxation can be tuned across two orders of magnitude. All formulations demonstrate a similar flow profile after network rupture, allowing the separation of initial mechanical properties from flow behavior during printing. Furthermore, the self-healing nature of hydrogels with high hydrazone content enables recyclability of printed structures. Last, a distinct threshold for cell spreading and morphology is observed within this hydrogel series, even in multi-material constructs. Simple cross-linker mixing enables fine control and is of general interest for bioink development, targeting viscoelastic properties of specific cellular niches, and as an accessible and flexible platform for designing dynamic networks.

Human mini brains and spinal cords in a dish: Modeling strategies, current challenges, and prospective advances

Engineered three-dimensional (3D) in vitro and ex vivo neural tissues, also known as "mini brains and spinal cords in a dish," can be derived from different types of human stem cells via several differentiation protocols. In general, human mini brains are micro-scale physiological systems consisting of mixed populations of neural progenitor cells, glial cells, and neurons that may represent key features of human brain anatomy and function. To date, these specialized 3D tissue structures can be characterized into spheroids, organoids, assembloids, organ-on-a-chip and their various combinations based on generation procedures and cellular components. These 3D CNS models incorporate complex cell-cell interactions and play an essential role in bridging the gap between two-dimensional human neuroglial cultures and animal models. Indeed, they provide an innovative platform for disease modeling and therapeutic cell replacement, especially shedding light on the potential to realize personalized medicine for neurological disorders when combined with the revolutionary human induced pluripotent stem cell technology. In this review, we highlight human 3D CNS models developed from a variety of experimental strategies, emphasize their advances and remaining challenges, evaluate their state-of-the-art applications in recapitulating crucial phenotypic aspects of many CNS diseases, and discuss the role of contemporary technologies in the prospective improvement of their composition, consistency, complexity, and maturation.