Hydrazone covalent adaptable networks modulate extracellular matrix deposition for cartilage tissue engineering

New Advances in Covalent Network Polymers via Dynamic Covalent Chemistry

Covalent network polymers, as materials composed of atoms interconnected by covalent bonds in a continuous network, are known for their thermal and chemical stability. Over the past two decades, these materials have undergone significant transformations, gaining properties such as malleability, environmental responsiveness, recyclability, crystallinity, and customizable porosity, enabled by the development and integration of dynamic covalent chemistry (DCvC). In this review, we explore the innovative realm of covalent network polymers by focusing on the recent advances achieved through the application of DCvC. We start by examining the history and fundamental principles of DCvC, detailing its inception and core concepts and noting its key role in reversible covalent bond formation. Then the reprocessability of covalent network polymers enabled by DCvC is thoroughly discussed, starting from the significant milestones that marked the evolution of these polymers and progressing to their current trends and applications. The influence of DCvC on the crystallinity of covalent network polymers is then reviewed, covering their bond diversity, synthesis techniques, and functionalities. In the concluding section, we address the current challenges faced in the field of covalent network polymers and speculates on potential future directions.

Advancing Synthetic Hydrogels through Nature-Inspired Materials Chemistry

Synthetic extracellular matrix (ECM) mimics that can recapitulate the complex biochemical and mechanical nature of native tissues are needed for advanced models of development and disease. Biomedical research has heavily relied on the use of animal-derived biomaterials, which is now impeding their translational potential and convoluting the biological insights gleaned from in vitro tissue models. Natural hydrogels have long served as a convenient and effective cell culture tool, but advances in materials chemistry and fabrication techniques now present promising new avenues for creating xenogenic-free ECM substitutes appropriate for organotypic models and microphysiological systems. However, significant challenges remain in creating synthetic matrices that can approximate the structural sophistication, biochemical complexity, and dynamic functionality of native tissues. This review summarizes key properties of the native ECM, and discusses recent approaches used to systematically decouple and tune these properties in synthetic matrices. The importance of dynamic ECM mechanics, such as viscoelasticity and matrix plasticity, is also discussed, particularly within the context of organoid and engineered tissue matrices. Emerging design strategies to mimic these dynamic mechanical properties are reviewed, such as multi-network hydrogels, supramolecular chemistry, and hydrogels assembled from biological monomers.

Crosslinker Architectures Impact Viscoelasticity in Dynamic Covalent Hydrogels

Dynamic covalent crosslinked (DCC) hydrogels represent a significant advance in biomaterials for regenerative medicine and mechanobiology. These gels typically offer viscoelasticity and self-healing properties that more closely mimic in vivo tissue mechanics than traditional, predominantly elastic, covalent crosslinked hydrogels. Despite their promise, the effects of varying crosslinker architecture - side chain versus telechelic crosslinks - on the viscoelastic properties of DCC hydrogels have not been thoroughly investigated. This study introduces hydrazone-based alginate hydrogels and examines how side-chain and telechelic crosslinker architectures impact hydrogel viscoelasticity and stiffness. In hydrogels with side-chain crosslinking (SCX), higher polymer concentrations enhance stiffness and decelerates stress relaxation, while an off-stoichiometric hydrazine-to-aldehyde ratio leads to reduced stiffness and shorter relaxation time. In hydrogels with telechelic crosslinking, maximal stiffness and slowest stress relaxation occurs at intermediate crosslinker concentrations for both linear and star crosslinkers, with higher crosslinker valency further increasing stiffness and relaxation time. Our result suggested different ranges of stiffness and stress relaxation are accessible with the different crosslinker architectures, with SCX hydrogels leading to slower stress relaxation relative to the other architectures, and hydrogels with star crosslinking (SX) providing increased stiffness and slower stress relaxation relative to hydrogels with linear crosslinking (LX). The mechanical properties of SX hydrogels are more robust to changes induced by competing chemical reactions compared to LX hydrogels. Our research underscores the pivotal role of crosslinker architecture in defining hydrogel stiffness and viscoelasticity, providing crucial insights for the design of DCC hydrogels with tailored mechanical properties for specific biomedical applications.

Injectable PEG Hydrogels with Tissue-Like Viscoelasticity Formed through Reversible Alendronate-Calcium Phosphate Crosslinking for Cell-Material Interactions

Synthetic hydrogels provide controllable 3D environments, which can be used to study fundamental biological phenomena. The growing body of evidence that cell behavior depends upon hydrogel stress relaxation creates a high demand for hydrogels with tissue-like viscoelastic properties. Here, a unique platform of synthetic polyethylene glycol (PEG) hydrogels in which star-shaped PEG molecules are conjugated with alendronate and/or RGD peptides, attaining modifiable degradability as well as flexible cell adhesion, is created. Novel reversible ionic interactions between alendronate and calcium phosphate nanoparticles, leading to versatile viscoelastic properties with varying initial elastic modulus and stress relaxation time, are identified. This new crosslinking mechanism provides shear-thinning properties resulting in differential cellular responses between cancer cells and stem cells. The novel hydrogel system is an improved design to the other ionic crosslink platforms and opens new avenues for the development of pathologically relevant cancer models, as well as minimally invasive approaches for cell delivery for potential regenerative therapies.

Engineering a Hydrazone and Triazole Crosslinked Hydrogel for Extrusion-Based Printing and Cell Delivery

Covalent adaptable crosslinks, such as the alkyl-hydrazone, endow hydrogels with unique viscoelastic properties applicable to cell delivery and bioink systems. However, the alkyl-hydrazone crosslink lacks stability in biologically relevant environments. Furthermore, when formed with biopolymers such as hyaluronic acid (HA), low molecular weight polymers (<60 kDa), or low polymer content (<2 wt%) hydrogels are typically employed as entanglements reduce injectability. Here, a high molecular weight (>60 kDa) HA alkyl-hydrazone crosslinked hydrogel is modified with benzaldehyde-poly(ethylene glycol)3-azide to incorporate azide functional groups. By reacting azide-modified HA with a multi-arm poly(ethylene glycol) (PEG) functionalized with bicyclononyne, stabilizing triazole bonds are formed through strain-promoted azide-alkyne cycloaddition (SPAAC). Increasing the fraction of triazole bonds within the hydrogel network from 0% to 12% SPAAC substantially increases stability. The slow gelation kinetics of the SPAAC reaction in the 12% SPAAC hydrogel enables transient self-healing properties and a similar extrusion force as the 0% SPAAC hydrogel. Methyl-PEG4-hydrazide is then introduced to further slowdown network evolution, which temporarily lowers the extrusion force, improves printability, and increases post-extrusion mesenchymal stem cell viability and function in the 12% SPAAC hydrogel. This work demonstrates improved stability and temporal injectability of high molecular weight HA-PEG hydrogels for extrusion-based printing and cell delivery.

Nascent matrix deposition supports alveolar organoid formation from aggregates in synthetic hydrogels

Human induced pluripotent stem cell (iPSC) derived alveolar organoids have emerged as a system to model the alveolar epithelium in homeostasis and disease. However, alveolar organoids are typically grown in Matrigel, a mouse-sarcoma derived basement membrane matrix that offers poor control over matrix properties, prompting the development of synthetic hydrogels as a Matrigel alternative. Here, we develop a two-step culture method that involves pre-aggregation of organoids in hydrogel-based microwells followed by embedding in a synthetic hydrogel that supports alveolar organoid growth, while also offering considerable control over organoid and hydrogel properties. We find that the aggregated organoids secrete their own nascent extracellular matrix (ECM) both in the microwells and upon embedding in the synthetic hydrogels. Thus, the synthetic gels described here allow us to de-couple exogenous and nascent ECM in order to interrogate the role of ECM in organoid formation.

Mechanical forces amplify TCR mechanotransduction in T cell activation and function

Adoptive T cell immunotherapies, including engineered T cell receptor (eTCR) and chimeric antigen receptor (CAR) T cell immunotherapies, have shown efficacy in treating a subset of hematologic malignancies, exhibit promise in solid tumors, and have many other potential applications, such as in fibrosis, autoimmunity, and regenerative medicine. While immunoengineering has focused on designing biomaterials to present biochemical cues to manipulate T cells ex vivo and in vivo, mechanical cues that regulate their biology have been largely underappreciated. This review highlights the contributions of mechanical force to several receptor-ligand interactions critical to T cell function, with central focus on the TCR-peptide-loaded major histocompatibility complex (pMHC). We then emphasize the role of mechanical forces in (i) allosteric strengthening of the TCR-pMHC interaction in amplifying ligand discrimination during T cell antigen recognition prior to activation and (ii) T cell interactions with the extracellular matrix. We then describe approaches to design eTCRs, CARs, and biomaterials to exploit TCR mechanosensitivity in order to potentiate T cell manufacturing and function in adoptive T cell immunotherapy.

Characterizing rheological properties and microstructure of thioester networks during degradation

Covalent adaptable networks are designed for applications including cell and drug delivery and tissue regeneration. These applications require network degradation at physiological conditions and on a physiological timescale with microstructures that can: (1) support, protect and deliver encapsulated cells or molecules and (2) provide structure to surrounding tissue. Due to this, the evolving microstructure and rheological properties during scaffold degradation must be characterized. In this work, we characterize degradation of covalent adaptable poly(ethylene glycol) (PEG)-thioester networks with different amounts of excess thiol. Networks are formed between PEG-thiol and PEG-thioester norbornene using photopolymerization. These networks are adaptable because of a thioester exchange reaction that takes place in the presence of excess thiol. We measure degradation of PEG-thioester networks with L-cysteine using multiple particle tracking microrheology (MPT). MPT measures the Brownian motion of fluorescent probe particles embedded in a material and relates this motion to rheological properties. Using time-cure superposition (TCS), we characterize the microstructure of these networks at the gel-sol phase transition by calculating the critical relaxation exponent, n, for each network with different amounts of excess thiol. Based on the measured n values, networks formed with 0% and 50% excess thiol are tightly cross-linked and elastic in nature. While networks formed with 100% excess are similar to ideal, percolated networks, which have equal viscous and elastic components. MPT measurements during degradation of these networks also measure a non-monotonic increase in probe motility. We hypothesize that this is network rearrangement near the phase transition. We then measure macroscopic material properties including the equilibrium modulus and stress relaxation. We measure a trend in bulk network properties that agrees with the values of n. Elastic modulus and stress relaxation measurements show that networks with 50% excess thiol are more elastic compared to the other two networks. As the amount of excess thiol is increased from 0% to 50%, the networks become more elastic. Further increasing excess thiol to 100% reduces the elastically effective cross-links. We hypothesize that these properties are due to network non-idealities, resulting in networks with 50% excess thiol that are more elastic. This work characterizes dynamic rheological properties during degradation, which mimics processes that could occur during implantation. This work provides information that can be used in the future design of implantable materials enabling both the rheological properties and timescale of degradation to be specified.

Design Parameters for Injectable Biopolymeric Hydrogels with Dynamic Covalent Chemistry Crosslinks

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

Transformative Materials to Create 3D Functional Human Tissue Models In Vitro in a Reproducible Manner

Recreating human tissues and organs in the petri dish to establish models as tools in biomedical sciences has gained momentum. These models can provide insight into mechanisms of human physiology, disease onset, and progression, and improve drug target validation, as well as the development of new medical therapeutics. Transformative materials play an important role in this evolution, as they can be programmed to direct cell behavior and fate by controlling the activity of bioactive molecules and material properties. Using nature as an inspiration, scientists are creating materials that incorporate specific biological processes observed during human organogenesis and tissue regeneration. This article presents the reader with state-of-the-art developments in the field of in vitro tissue engineering and the challenges related to the design, production, and translation of these transformative materials. Advances regarding (stem) cell sources, expansion, and differentiation, and how novel responsive materials, automated and large-scale fabrication processes, culture conditions, in situ monitoring systems, and computer simulations are required to create functional human tissue models that are relevant and efficient for drug discovery, are described. This paper illustrates how these different technologies need to converge to generate in vitro life-like human tissue models that provide a platform to answer health-based scientific questions.

Dynamic Hydrogels with Viscoelasticity and Tunable Stiffness for the Regulation of Cell Behavior and Fate

The extracellular matrix (ECM) of natural cells typically exhibits dynamic mechanical properties (viscoelasticity and dynamic stiffness). The viscoelasticity and dynamic stiffness of the ECM play a crucial role in biological processes, such as tissue growth, development, physiology, and disease. Hydrogels with viscoelasticity and dynamic stiffness have recently been used to investigate the regulation of cell behavior and fate. This article first emphasizes the importance of tissue viscoelasticity and dynamic stiffness and provides an overview of characterization techniques at both macro- and microscale. Then, the viscoelastic hydrogels (crosslinked via ion bonding, hydrogen bonding, hydrophobic interactions, and supramolecular interactions) and dynamic stiffness hydrogels (softening, stiffening, and reversible stiffness) with different crosslinking strategies are summarized, along with the significant impact of viscoelasticity and dynamic stiffness on cell spreading, proliferation, migration, and differentiation in two-dimensional (2D) and three-dimensional (3D) cell cultures. Finally, the emerging trends in the development of dynamic mechanical hydrogels are discussed.

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.

Coupled bond dynamics alters relaxation in polymers with multiple intrinsic dissociation rates

Dynamic networks containing multiple bond types within a continuous network grant engineers another design parameter - relative bond fraction - by which to tune storage and dissipation of mechanical energy. However, the mechanisms governing emergent properties are difficult to deduce experimentally. Therefore, we here employ a network model with prescribed fractions of dynamic and stable bonds to predict relaxation characteristics of hybrid networks. We find that during stress relaxation, predominantly dynamic networks can exhibit long-term moduli through conformationally inhibited relaxation of stable bonds due to exclusion interactions with neighboring chains. Meanwhile, predominantly stable networks exhibit minor relaxation through non-affine reconfiguration of dynamic bonds. Given this, we introduce a single fitting parameter, ξ, to Transient Network Theory via a coupled rule of mixture, that characterizes the extent of stable bond relaxation. Treating ξ as a fitting parameter, the coupled rule of mixture's predicted stress response not only agrees with the network model's, but also unveils likely micromechanical traits of gels hosting multiple bond dissociation timescales.

3D bioprinting of dynamic hydrogel bioinks enabled by small molecule modulators

Three-dimensional bioprinting has emerged as a promising tool for spatially patterning cells to fabricate models of human tissue. Here, we present an engineered bioink material designed to have viscoelastic mechanical behavior, similar to that of living tissue. This viscoelastic bioink is cross-linked through dynamic covalent bonds, a reversible bond type that allows for cellular remodeling over time. Viscoelastic materials are challenging to use as inks, as one must tune the kinetics of the dynamic cross-links to allow for both extrudability and long-term stability. We overcome this challenge through the use of small molecule catalysts and competitors that temporarily modulate the cross-linking kinetics and degree of network formation. These inks were then used to print a model of breast cancer cell invasion, where the inclusion of dynamic cross-links was found to be required for the formation of invasive protrusions. Together, we demonstrate the power of engineered, dynamic bioinks to recapitulate the native cellular microenvironment for disease modeling.

A new boronate ester-based crosslinking strategy allows the design of nonswelling and long-term stable dynamic covalent hydrogels

Dynamic hydrogels are viscoelastic materials that can be designed to be self-healing, malleable, and injectable, making them particularly interesting for a variety of biomedical applications. To design dynamic hydrogels, dynamic covalent crosslinking reactions are attracting increasing attention. However, dynamic covalent hydrogels tend to swell, and often lack stability. Boronate ester-based hydrogels, which result from the dynamic covalent reaction between a phenylboronic acid (PBA) derivative and a diol, are based on stable precursors, and can therefore address these limitations. Yet, boronate ester formation hardly occurs at physiological pH. To produce dynamic covalent hydrogels at physiological pH, we performed a molecular screening of PBA derivatives in association with a variety of diols, using hyaluronic acid as a polymer of interest. The combination of Wulff-type PBA (wPBA) and glucamine stood out as a unique couple to obtain the desired hydrogels. We showed that optimized wPBA/glucamine hydrogels are minimally- to non-swelling, stable long term (over months), tunable in terms of mechanical properties, and cytocompatible. We further characterized their viscoelastic and self-healing properties, highlighting their potential for biomedical applications.

Adhesive hydrogels in osteoarthritis: from design to application

Osteoarthritis (OA) is the most common type of degenerative joint disease which affects 7% of the global population and more than 500 million people worldwide. One research frontier is the development of hydrogels for OA treatment, which operate either as functional scaffolds of tissue engineering or as delivery vehicles of functional additives. Both approaches address the big challenge: establishing stable integration of such delivery systems or implants. Adhesive hydrogels provide possible solutions to this challenge. However, few studies have described the current advances in using adhesive hydrogel for OA treatment. This review summarizes the commonly used hydrogels with their adhesion mechanisms and components. Additionally, recognizing that OA is a complex disease involving different biological mechanisms, the bioactive therapeutic strategies are also presented. By presenting the adhesive hydrogels in an interdisciplinary way, including both the fields of chemistry and biology, this review will attempt to provide a comprehensive insight for designing novel bioadhesive systems for OA therapy.

Dynamically Crosslinked Poly(ethylene-glycol) Hydrogels Reveal a Critical Role of Viscoelasticity in Modulating Glioblastoma Fates and Drug Responses in 3D

Glioblastoma multiforme (GBM) is the most prevalent and aggressive brain tumor in adults. Hydrogels have been employed as 3D in vitro culture models to elucidate how matrix cues such as stiffness and degradation drive GBM progression and drug responses. Recently, viscoelasticity has been identified as an important niche cue in regulating stem cell differentiation and morphogenesis in 3D. Brain is a viscoelastic tissue, yet how viscoelasticity modulates GBM fate and drug response remains largely unknown. Using dynamic hydrazone crosslinking chemistry, a poly(ethylene-glycol)-based hydrogel system with brain-mimicking stiffness and tunable stress relaxation is reported to interrogate the role of viscoelasticity on GBM fates in 3D. The hydrogel design allows tuning stress relaxation without changing stiffness, biochemical ligand density, or diffusion. The results reveal that increasing stress relaxation promotes invasive GBM behavior, such as cell spreading, migration, and GBM stem-like cell marker expression. Furthermore, increasing stress relaxation enhances GBM proliferation and drug sensitivity. Stress-relaxation induced changes on GBM fates and drug response are found to be mediated through the cytoskeleton and transient receptor potential vanilloid-type 4. These results highlight the importance of incorporating viscoelasticity into 3D in vitro GBM models and provide novel insights into how viscoelasticity modulates GBM cell fates.

Fabrication Strategies Towards Hydrogels for Biomedical Application: Chemical and Mechanical Insights

This review aims at giving selected chemical and mechanical insights on design criteria that should be taken into account in hydrogel production for biomedical applications. Particular emphasis will be given to the chemical aspects involved in hydrogel design: macromer chemical composition, cross-linking strategies and chemistry towards "conventional" and smart/stimuli responsive hydrogels. Mechanical properties of hydrogels in view of regenerative medicine applications will also be considered.

A Photoannealed Granular Hydrogel Facilitating Hyaline Cartilage Regeneration via Improving Chondrogenic Phenotype

Hydrogel-based chondrocyte implantation presents a promising tissue engineering strategy for cartilage repair. However, the widely used elastic hydrogels usually restrict cell volume expansion and induce the dedifferentiation of encapsulated chondrocytes. To address this limitation, a photoannealed granular hydrogel (GH) composed of hyaluronic acid, polyethylene glycol, and gelatin was formulated for cartilage regeneration in this study. The unannealed GH prepared by Diels-Alder cross-linked microgels could be mixed with chondrocytes and delivered to cartilage defects by injection, after which light was introduced to anneal the scaffold, leading to the formation of a stable and microporous chondrocyte deploying scaffold. The in vitro studies showed that GH could promote the volume expansion and morphology recovery of chondrocytes and significantly improve their chondrogenic phenotype compared to the nongranular hydrogel (nGH) with similar compositions. Further in vivo studies of subcutaneous culture and the rat full-thickness cartilage defect model proved that chondrocyte loaded GH could significantly stimulate hyaline cartilage matrix deposition and connection, therefore facilitating hyaline-like cartilage regeneration. Finally, the mechanistic study revealed that GH might improve chondrogenic phenotype via activating the AMP-activated protein kinase/glycolysis axis. This study proves the great feasibility of GHs as in situ chondrocyte deploying scaffolds for cartilage regeneration and brings new insights in designing hydrogel scaffold for cartilage tissue engineering.

Chemical strategies to engineer hydrogels for cell culture

Two-dimensional and three-dimensional cell culture systems are widely used for biological studies, and are the basis of the organoid, tissue engineering and organ-on-chip research fields in applications such as disease modelling and drug screening. The natural extracellular matrix of tissues, a complex scaffold with varying chemical and mechanical properties, has a critical role in regulating important cellular functions such as spreading, migration, proliferation and differentiation, as well as tissue morphogenesis. Hydrogels are biomaterials that are used in cell culture systems to imitate critical features of a natural extracellular matrix. Chemical strategies to synthesize and tailor the properties of these hydrogels in a controlled manner, and manipulate their biological functions in situ, have been developed. In this Review, we provide the rational design criteria for predictably engineering hydrogels to mimic the properties of the natural extracellular matrix. We highlight the advances in using biocompatible strategies to engineer hydrogels for cell culture along with recent developments to dynamically control the cellular environment by exploiting stimuli-responsive chemistries. Finally, future opportunities to engineer hydrogels are discussed, in which the development of novel chemical methods will probably have an important role.

Effect of viscoelastic properties of cellulose nanocrystal/collagen hydrogels on chondrocyte behaviors

Cartilage tissue engineering technology provides a solution for treating osteoarthritis. Based on the viscoelastic nature of articular cartilage, many viscoelastic hydrogel scaffolds have been developed for investigating the effects on chondrocyte behaviors. However, cellulose nanocrystal/collagen (CNC/COL) hydrogels have not been used as a viscoelastic microenvironment to study chondrocyte growth. Here, we prepared CNC/COL hydrogels with tunable viscoelastic properties and investigated their influences on chondrocyte behaviors. The results showed that CNC and COL within the hydrogels are bonded by hydrogen bonds. The hydrogels had a microporous structure, and the viscoelastic properties were enhanced by increasing the concentration of CNC. Moreover, enhancing the hydrogel viscoelastic properties, including stress relaxation, creep, storage modulus, and loss modulus, promoted the cell shape change, proliferation, and matrix deposition and reduced the IL-1β level. Using a principal component analysis (PCA), stress relaxation was assessed to have the strongest correlation with chondrocytes behaviors, with an authority weight value of 62.547%. More importantly, FAK and YAP were involved in the chondrocytes’ response to the rapid relaxing hydrogel by immunofluorescence staining.

Modular mixing of benzene-1,3,5-tricarboxamide supramolecular hydrogelators allows tunable biomimetic hydrogels for control of cell aggregation in 3D

Few synthetic hydrogels can mimic both the viscoelasticity and supramolecular fibrous structure found in the naturally occurring extracellular matrix (ECM). Furthermore, the ability to control the viscoelasticity of fibrous supramolecular hydrogel networks to influence cell culture remains a challenge. Here, we show that modular mixing of supramolecular architectures with slow and fast exchange dynamics can provide a suitable environment for multiple cell types and influence cellular aggregation. We employed modular mixing of two synthetic benzene-1,3,5-tricarboxamide (BTA) architectures: a small molecule water-soluble BTA with slow exchange dynamics and a telechelic polymeric BTA-PEG-BTA with fast exchange dynamics. Copolymerisation of these two supramolecular architectures was observed, and all tested formulations formed stable hydrogels in water and cell culture media. We found that rational tuning of mechanical and viscoelastic properties is possible by mixing BTA with BTA-PEG-BTA. These hydrogels showed high viability for both chondrocyte (ATDC5) and human dermal fibroblast (HDF) encapsulation (>80%) and supported neuronal outgrowth (PC12 and dorsal root ganglion, DRG). Furthermore, ATDC5s and human mesenchymal stem cells (hMSCs) were able to form spheroids within these viscoelastic hydrogels, with control over cell aggregation modulated by the dynamic properties of the material. Overall, this study shows that modular mixing of supramolecular architectures enables tunable fibrous hydrogels, creating a biomimetic environment for cell encapsulation. These materials are suitable for the formation and culture of spheroids in 3D, critical for upscaling tissue engineering approaches towards cell densities relevant for physiological tissues.

Dynamic hydrogels can allow cells to form complex multicellular aggregates. Herein, we show that the dynamics of the hydrogel environment can directly influence the speed and size of cellular aggregates formed by using a modularly tunable supramolecular hydrogel.

Engineering Hydrogels for Modulation of Material-Cell Interactions

Hydrogels are a recurrent platform for Tissue Engineering (TE) strategies. Their versatility and the variety of available methods for tuning their properties highly contribute to hydrogels' success. As a result, the design of advanced hydrogels has been thoroughly studied, in the quest for better solutions not only for drugs- and cell-based therapies but also for more fundamental studies. The wide variety of sources, crosslinking strategies, and functionalization methods, and mostly the resemblance of hydrogels to the natural extracellular matrix, make this 3D hydrated structures an excellent tool for TE approaches. The state-of-the-art information regarding hydrogel design, processing methods, and the influence of different hydrogel formulations on the final cell-biomaterial interactions are overviewed herein. This article is protected by copyright. All rights reserved.

Stress Relaxation and Composition of Hydrazone-Crosslinked Hybrid Biopolymer-Synthetic Hydrogels Determine Spreading and Secretory Properties of MSCs

The extracellular matrix plays a critical role in mechanosensing and thereby influences the secretory properties of bone-marrow-derived mesenchymal stem/stromal cells (MSCs). As a result, interest has grown in the development of biomaterials with tunable properties for the expansion and delivery of MSCs that are used in cell-based therapies. Herein, stress-relaxing hydrogels are synthesized as hybrid networks containing both biopolymer and synthetic macromer components. Hyaluronic acid is functionalized with either aldehyde or hydrazide groups to form covalent adaptable hydrazone networks, which are stabilized by poly(ethylene glycol) functionalized with bicyclononyne and heterobifunctional small molecule crosslinkers containing azide and benzaldehyde moieties. Tuning the composition of these gels allows for controlled variation in the characteristic timescale for stress relaxation and the amount of stress relaxed. Over this compositional space, MSCs are observed to spread in formulations with higher degrees of adaptability, with aspect ratios of 1.60 ± 0.18, and YAP nuclear:cytoplasm ratios of 6.5 ± 1.3. Finally, a maximum MSC pericellular protein thickness of 1.45 ± 0.38 µm occurred in highly stress-relaxing gels, compared to 1.05 ± 0.25 µm in non-adaptable controls. Collectively, this study contributes a new understanding of the role of compositionally defined stress relaxation on MSCs mechanosensing and secretion.

Manufacturing of self-standing multi-layered 3D-bioprinted alginate-hyaluronate constructs by controlling the cross-linking mechanisms for tissue engineering applications

3D bioprinting of self-supporting stable tissue and organ structure is critically important in extrusion-based bioprinting system, especially for tissue engineering and regenerative medicine applications. However, the development of self-standing bioinks with desired crosslinking density, biocompatibility, tunable mechanical strength and other properties like self-healing, in situ gelation, drug or protein incorporation is still a challenge. In this study, we report a hydrogel bioink prepared from alginate (Alg) and hyaluronic acid (HA) crosslinked through multiple crosslinking mechanisms, i.e., acyl-hydrazone, hydrazide interactions and calcium ions. These Alg-HA gels were highly dynamic and shear-thinning with exceptional biocompatibility and tunable mechanical properties. The increased dynamic nature of the gels is mainly chemically attributed to the presence of acyl-hydrazone bonds formed between the amine groups of the acyl-hydrazide of alginate and the monoaldehyde of the hyaluronic acid. Among the different combinations of Alg-HA gel compositions prepared, the A5H5 (Alginate-acyl-hydrazide: HA-monoaldehyde, ratio 50:50) one showed a gelation time of ~60 s, viscosity of ~400 Pa.s (at zero shear rate), high stability in various pH solutions and increased degradation time (>50 days) than the other samples. The A5H5 gels showed high printability with increased post-printing stability as observed from the 3D printed structures (e.g., hollow tube (~100 layers), porous cube (~50 layers), star, heart-in, meniscus and lattice). The scanning electron microscopy analysis of the 3D constructs and hydrogels showed the interconnected pores (~181 µm) and crosslinked networks. Further, the gels showed sustained release of 5-amino salicylic acid and bovine serum albumin. Also, the mechanical properties were tuned by secondary crosslinking via different calcium concentrations. In vitro assays confirmed the cytocompatibility of these gels, where the 3D bioprinted lattice and tubular (~70 layers) constructs demonstrated high cell viability under fluorescence analysis. In in vivo studies, Alg-HA gel showed high biocompatibility (>90%) and increased angiogenesis (3 folds) and reduced macrophage infiltration (2-fold decrease), demonstrating the promising potential of these hydrogels in 3D bioprinting applications for tissue engineering and regenerative medicine with tunable properties.

Viscoelastic Biomaterials for Tissue Regeneration

The extracellular matrix (ECM) mechanical properties regulate key cellular processes in tissue development and regeneration. The majority of scientific investigation has focused on ECM elasticity as the primary mechanical regulator of cell and tissue behavior. However, all living tissues are viscoelastic, exhibiting both solid- and liquid-like mechanical behavior. Despite increasing evidence regarding the role of ECM viscoelasticity in directing cellular behavior, this aspect is still largely overlooked in the design of biomaterials for tissue regeneration. Recently, with the emergence of various bottom-up material design strategies, new approaches can deliver unprecedented control over biomaterial properties at multiple length scales, thus enabling the design of viscoelastic biomaterials that mimic various aspect of the native tissue ECM microenvironment. This review describes key considerations for the design of viscoelastic biomaterials for tissue regeneration. We provide an overview of the role of matrix viscoelasticity in directing cell behavior towards regenerative outcomes, highlight recent strategies utilizing viscoelastic hydrogels for regenerative therapies, and outline remaining challenges, potential solutions, and emerging applications for viscoelastic biomaterials in tissue engineering and regenerative medicine.

Viscoelastic Chondroitin Sulfate and Hyaluronic Acid Double-Network Hydrogels with Reversible Cross-Links

Viscoelastic hydrogels are gaining interest as they possess necessary requirements for bioprinting and injectability. By means of reversible, dynamic covalent bonds, it is possible to achieve features that recapitulate the dynamic character of the extracellular matrix. Dually cross-linked and double-network (DN) hydrogels seem to be ideal for the design of novel biomaterials and bioinks, as a wide range of properties required for mimicking advanced and complex tissues can be achieved. In this study, we investigated the fabrication of chondroitin sulfate/hyaluronic acid (CS/HA)-based DN hydrogels, in which two networks are interpenetrated and cross-linked with the dynamic covalent bonds of very different lifetimes. Namely, Diels-Alder adducts (between methylfuran and maleimide) and hydrazone bonds (between aldehyde and hydrazide) were chosen as cross-links, leading to viscoelastic hydrogels. Furthermore, we show that viscoelasticity and the dynamic character of the resulting hydrogels could be tuned by changing the composition, that is, the ratio between the two types of cross-links. Also, due to a very dynamic nature and short lifetime of hydrazone cross-links (∼800 s), the DN hydrogel is easily processable (e.g., injectable) in the first stages of gelation, allowing the material to be used in extrusion-based 3D printing. The more long-lasting and robust Diels-Alder cross-links are responsible for giving the network enhanced mechanical strength and structural stability. Being highly charged and hydrophilic, the cross-linked CS and HA enable a high swelling capacity (maximum swelling ratio ranging from 6 to 12), which upon confinement results in osmotically stiffened constructs, able to mimic the mechanical properties of cartilage tissue, with the equilibrium moduli ranging from 0.3 to 0.5 MPa. Moreover, the mesenchymal stromal cells were viable in the presence of the hydrogels, and the effect of the degradation products on the macrophages suggests their safe use for further translational applications. The DN hydrogels with dynamic covalent cross-links hold great potential for the development of novel smart and tunable viscoelastic materials to be used as biomaterial inks or bioinks in bioprinting and regenerative medicine.

Desymmetrization via Activated Esters Enables Rapid Synthesis of Multifunctional Benzene-1,3,5-tricarboxamides and Creation of Supramolecular Hydrogelators

Supramolecular materials based on the self-assembly of benzene-1,3,5-tricarboxamide (BTA) offer an approach to mimic fibrous self-assembled proteins found in numerous natural systems. Yet, synthetic methods to rapidly build complexity, scalability, and multifunctionality into BTA-based materials are needed. The diversity of BTA structures is often hampered by the limited flexibility of existing desymmetrization routes and the purification of multifunctional BTAs. To alleviate this bottleneck, we have developed a desymmetrization method based on activated ester coupling of a symmetric synthon. We created a small library of activated ester synthons and found that a pentafluorophenol benzene triester (BTE) enabled effective desymmetrization and creation of multifunctional BTAs in good yield with high reaction fidelity. This new methodology enabled the rapid synthesis of a small library of BTA monomers with hydrophobic and/or orthogonal reactive handles and could be extended to create polymeric BTA hydrogelators. These BTA hydrogelators self-assembled in water to create fiber and fibrous sheet-like structures as observed by cryo-TEM, and the identity of the BTA conjugated can tune the mechanical properties of the hydrogel. These hydrogelators display high cytocompatibility for chondrocytes, indicating potential for the use of these systems in 3D cell culture and tissue engineering applications. This newly developed synthetic strategy facilitates the simple and rapid creation of chemically diverse BTA supramolecular polymers, and the newly developed and scalable hydrogels can unlock exploration of BTA based materials in a wider variety of tissue engineering applications.

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.

Cellular modulation by the mechanical cues from biomaterials for tissue engineering

Mechanical cues from the extracellular matrix (ECM) microenvironment are known to be significant in modulating the fate of stem cells to guide developmental processes and maintain bodily homeostasis. Tissue engineering has provided a promising approach to the repair or regeneration of damaged tissues. Scaffolds are fundamental in cell-based regenerative therapies. Developing artificial ECM that mimics the mechanical properties of native ECM would greatly help to guide cell functions and thus promote tissue regeneration. In this review, we introduce various mechanical cues provided by the ECM including elasticity, viscoelasticity, topography, and external stimuli, and their effects on cell behaviours. Meanwhile, we discuss the underlying principles and strategies to develop natural or synthetic biomaterials with different mechanical properties for cellular modulation, and explore the mechanism by which the mechanical cues from biomaterials regulate cell function toward tissue regeneration. We also discuss the challenges in multimodal mechanical modulation of cell behaviours and the interplay between mechanical cues and other microenvironmental factors.

Mechanobiological Strategies to Enhance Stem Cell Functionality for Regenerative Medicine and Tissue Engineering

Stem cells have been extensively used in regenerative medicine and tissue engineering; however, they often lose their functionality because of the inflammatory microenvironment. This leads to their poor survival, retention, and engraftment at transplantation sites. Considering the rapid loss of transplanted cells due to poor cell-cell and cell-extracellular matrix (ECM) interactions during transplantation, it has been reasoned that stem cells mainly mediate reparative responses via paracrine mechanisms, including the secretion of extracellular vesicles (EVs). Ameliorating poor cell-cell and cell-ECM interactions may obviate the limitations associated with the poor retention and engraftment of transplanted cells and enable them to mediate tissue repair through the sustained and localized presentation of secreted bioactive cues. Biomaterial-mediated strategies may be leveraged to confer stem cells enhanced immunomodulatory properties, as well as better engraftment and retention at the target site. In these approaches, biomaterials have been exploited to spatiotemporally present bioactive cues to stem cell-laden platforms (e.g., aggregates, microtissues, and tissue-engineered constructs). An array of biomaterials, such as nanoparticles, hydrogels, and scaffolds, has been exploited to facilitate stem cells function at the target site. Additionally, biomaterials can be harnessed to suppress the inflammatory microenvironment to induce enhanced tissue repair. In this review, we summarize biomaterial-based platforms that impact stem cell function for better tissue repair that may have broader implications for the treatment of various diseases as well as tissue regeneration.

Enhancing Stem Cell Therapy for Cartilage Repair in Osteoarthritis-A Hydrogel Focused Approach

Sem cells hold tremendous promise for the treatment of cartilage repair in osteoarthritis. In addition to their multipotency, stem cells possess immunomodulatory effects that can alleviate inflammation and enhance cartilage repair. However, the widely clinical application of stem cell therapy to cartilage repair and osteoarthritis has proven difficult due to challenges in large-scale production, viability maintenance in pathological tissue site and limited therapeutic biological activity. This review aims to provide a perspective from hydrogel-focused approach to address few key challenges in stem cell-based therapy for cartilage repair and highlight recent progress in advanced hydrogels, particularly microgels and dynamic hydrogels systems for improving stem cell survival, retention and regulation of stem cell fate. Finally, progress in hydrogel-assisted gene delivery and genome editing approaches for the development of next generation of stem cell therapy for cartilage repair in osteoarthritis are highlighted.

Soft overcomes the hard: Flexible materials adapt to cell adhesion to promote cell mechanotransduction

Cell behaviors and functions show distinct contrast in different mechanical microenvironment. Numerous materials with varied rigidity have been developed to mimic the interactions between cells and their surroundings. However, the conventional static materials cannot fully capture the dynamic alterations at the bio-interface, especially for the molecular motion and the local mechanical changes in nanoscale. As an alternative, flexible materials have great potential to sense and adapt to mechanical changes in such complex microenvironment. The flexible materials could promote the cellular mechanosensing by dynamically adjusting their local mechanics, topography and ligand presentation to adapt to intracellular force generation. This process enables the cells to exhibit comparable or even higher level of mechanotransduction and the downstream 'hard' phenotypes compared to the conventional stiff or rigid ones. Here, we highlight the relevant studies regarding the development of such adaptive materials to mediate cell behaviors across the rigidity limitation on soft substrates. The concept of 'soft overcomes the hard' will guide the future development and application of biological materials.

Viscoelasticity Acts as a Marker for Tumor Extracellular Matrix Characteristics

Biological materials such as extracellular matrix scaffolds, cancer cells, and tissues are often assumed to respond elastically for simplicity; the viscoelastic response is quite commonly ignored. Extracellular matrix mechanics including the viscoelasticity has turned out to be a key feature of cellular behavior and the entire shape and function of healthy and diseased tissues, such as cancer. The interference of cells with their local microenvironment and the interaction among different cell types relies both on the mechanical phenotype of each involved element. However, there is still not yet clearly understood how viscoelasticity alters the functional phenotype of the tumor extracellular matrix environment. Especially the biophysical technologies are still under ongoing improvement and further development. In addition, the effect of matrix mechanics in the progression of cancer is the subject of discussion. Hence, the topic of this review is especially attractive to collect the existing endeavors to characterize the viscoelastic features of tumor extracellular matrices and to briefly highlight the present frontiers in cancer progression and escape of cancers from therapy. Finally, this review article illustrates the importance of the tumor extracellular matrix mechano-phenotype, including the phenomenon viscoelasticity in identifying, characterizing, and treating specific cancer types.

A dysfunctional TRPV4-GSK3β pathway prevents osteoarthritic chondrocytes from sensing changes in extracellular matrix viscoelasticity

Changes in the composition and viscoelasticity of the extracellular matrix in load-bearing cartilage influence the proliferation and phenotypes of chondrocytes, and are associated with osteoarthritis. However, the underlying molecular mechanism is unknown. Here we show that the viscoelasticity of alginate hydrogels regulates cellular volume in healthy human chondrocytes (with faster stress relaxation allowing cell expansion and slower stress relaxation restricting it) but not in osteoarthritic chondrocytes. Cellular volume regulation in healthy chondrocytes was associated with changes in anabolic gene expression, in the secretion of multiple pro-inflammatory cytokines, and in the modulation of intracellular calcium regulated by the ion-channel protein transient receptor potential cation channel subfamily V member 4 (TRPV4), which controls the phosphorylation of glycogen synthase kinase 3β (GSK3β), an enzyme with pleiotropic effects in osteoarthritis. A dysfunctional TRPV4-GSK3β pathway in osteoarthritic chondrocytes rendered the cells unable to respond to environmental changes in viscoelasticity. Our findings suggest strategies for restoring chondrocyte homeostasis in osteoarthritis.

Using Stereochemistry to Control Mechanical Properties in Thiol-Yne Click-Hydrogels

The stereochemistry of polymers has a profound impact on their mechanical properties. While this has been observed in thermoplastics, studies on how stereochemistry affects the bulk properties of swollen networks, such as hydrogels, are limited. Typically, changing the stiffness of a hydrogel is achieved at the cost of changing another parameter, that in turn affects the physical properties of the material and ultimately influences the cellular response. Herein, we report that by manipulating the stereochemistry of a double bond, formed in situ during gelation, materials with diverse mechanical properties but comparable physical properties can be obtained. Click-hydrogels that possess a high % trans content are stiffer than their high % cis analogues by almost a factor of 3. Human mesenchymal stem cells acted as a substrate stiffness cell reporter demonstrating the potential of these platforms to study mechanotransduction without the influence of other external factors.

Bioink Platform Utilizing Dual-Stage Crosslinking of Hyaluronic Acid Tailored for Chondrogenic Differentiation of Mesenchymal Stromal Cells

3D bioprinting often involves application of highly concentrated polymeric bioinks to enable fabrication of stable cell-hydrogel constructs, although poor cell survival, compromised stem cell differentiation, and an inhomogeneous distribution of newly produced extracellular matrix (ECM) are frequently observed. Therefore, this study presents a bioink platform using a new versatile dual-stage crosslinking approach based on thiolated hyaluronic acid (HA-SH), which not only provides stand-alone 3D printability but also facilitates effective chondrogenic differentiation of mesenchymal stromal cells. A range of HA-SH with different molecular weights is synthesized and crosslinked with acrylated (PEG-diacryl) and allylated (PEG-diallyl) polyethylene glycol in a two-step reaction scheme. The initial Michael addition is used to achieve ink printability, followed by UV-mediated thiol-ene reaction to stabilize the printed bioink for long-term cell culture. Bioinks with high molecular weight HA-SH (>200 kDa) require comparably low polymer content to facilitate bioprinting. This leads to superior quality of cartilaginous constructs which possess a coherent ECM and a strongly increased stiffness of long-term cultured constructs. The dual-stage system may serve as an example to design platforms using two independent crosslinking reactions at one functional group, which allows adjusting printability as well as material and biological properties of bioinks.

Mechanics of 3D Cell-Hydrogel Interactions: Experiments, Models, and Mechanisms

Hydrogels are highly water-swollen molecular networks that are ideal platforms to create tissue mimetics owing to their vast and tunable properties. As such, hydrogels are promising cell-delivery vehicles for applications in tissue engineering and have also emerged as an important base for ex vivo models to study healthy and pathophysiological events in a carefully controlled three-dimensional environment. Cells are readily encapsulated in hydrogels resulting in a plethora of biochemical and mechanical communication mechanisms, which recapitulates the natural cell and extracellular matrix interaction in tissues. These interactions are complex, with multiple events that are invariably coupled and spanning multiple length and time scales. To study and identify the underlying mechanisms involved, an integrated experimental and computational approach is ideally needed. This review discusses the state of our knowledge on cell-hydrogel interactions, with a focus on mechanics and transport, and in this context, highlights recent advancements in experiments, mathematical and computational modeling. The review begins with a background on the thermodynamics and physics fundamentals that govern hydrogel mechanics and transport. The review focuses on two main classes of hydrogels, described as semiflexible polymer networks that represent physically cross-linked fibrous hydrogels and flexible polymer networks representing the chemically cross-linked synthetic and natural hydrogels. In this review, we highlight five main cell-hydrogel interactions that involve key cellular functions related to communication, mechanosensing, migration, growth, and tissue deposition and elaboration. For each of these cellular functions, recent experiments and the most up to date modeling strategies are discussed and then followed by a summary of how to tune hydrogel properties to achieve a desired functional cellular outcome. We conclude with a summary linking these advancements and make the case for the need to integrate experiments and modeling to advance our fundamental understanding of cell-matrix interactions that will ultimately help identify new therapeutic approaches and enable successful tissue engineering.

Using Stereochemistry to Control Mechanical Properties in Thiol-Yne Click-Hydrogels

The stereochemistry of polymers has a profound impact on their mechanical properties. While this has been observed in thermoplastics, studies on how stereochemistry affects the bulk properties of swollen networks, such as hydrogels, are limited. Typically, changing the stiffness of a hydrogel is achieved at the cost of changing another parameter, that in turn affects the physical properties of the material and ultimately influences the cellular response. Herein, we report that by manipulating the stereochemistry of a double bond, formed in situ during gelation, materials with diverse mechanical properties but comparable physical properties can be obtained. Click-hydrogels that possess a high % trans content are stiffer than their high % cis analogues by almost a factor of 3. Human mesenchymal stem cells acted as a substrate stiffness cell reporter demonstrating the potential of these platforms to study mechanotransduction without the influence of other external factors.

Click-functionalized hydrogel design for mechanobiology investigations

The advancement of click-functionalized hydrogels in recent years has coincided with rapid growth in the fields of mechanobiology, tissue engineering, and regenerative medicine. Click chemistries represent a group of reactions that possess high reactivity and specificity, are cytocompatible, and generally proceed under physiologic conditions. Most notably, the high level of tunability afforded by these reactions enables the design of user-controlled and tissue-mimicking hydrogels in which the influence of important physical and biochemical cues on normal and aberrant cellular behaviors can be independently assessed. Several critical tissue properties, including stiffness, viscoelasticity, and biomolecule presentation, are known to regulate cell mechanobiology in the context of development, wound repair, and disease. However, many questions still remain about how the individual and combined effects of these instructive properties regulate the cellular and molecular mechanisms governing physiologic and pathologic processes. In this review, we discuss several click chemistries that have been adopted to design dynamic and instructive hydrogels for mechanobiology investigations. We also chart a path forward for how click hydrogels can help reveal important insights about complex tissue microenvironments.

Dual Functionalization of Gelatin for Orthogonal and Dynamic Hydrogel Cross-Linking

Gelatin-based hydrogels are widely used in biomedical fields because of their abundance of bioactive motifs that support cell adhesion and matrix remodeling. Although inherently bioactive, unmodified gelatin exhibits temperature-dependent rheology and solubilizes at body temperature, making it unstable for three-dimensional (3D) cell culture. Therefore, the addition of chemically reactive motifs is required to render gelatin-based hydrogels with highly controllable cross-linking kinetics and tunable mechanical properties that are critical for 3D cell culture. This article provides a series of methods toward establishing orthogonally cross-linked gelatin-based hydrogels for dynamic 3D cell culture. In particular, we prepared dually functionalized gelatin macromers amenable for sequential, orthogonal covalent cross-linking. Central to this material platform is the synthesis of norbornene-functionalized gelatin (GelNB), which forms covalently cross-linked hydrogels via orthogonal thiol-norbornene click cross-linking. Using GelNB as the starting material, we further detail the methods for synthesizing gelatin macromers susceptible to hydroxyphenylacetic acid (HPA) dimerization (i.e., GelNB-HPA) and hydrazone bonding (i.e., GelNB-CH) for on-demand matrix stiffening. Finally, we outline the protocol for synthesizing a gelatin macromer capable of adjusting hydrogel stress relaxation via boronate ester bonding (i.e., GelNB-BA). The combination of these orthogonal chemistries affords a wide range of gelatin-based hydrogels as biomimetic matrices in tissue engineering and regenerative medicine applications.

Thiol-ene cross-linked alginate hydrogel encapsulation modulates the extracellular matrix of kidney organoids by reducing abnormal type 1a1 collagen deposition

Differentiated kidney organoids from induced pluripotent stem cells hold promise as a treatment for patients with kidney diseases. Before these organoids can be translated to the clinic, shortcomings regarding their cellular and extracellular compositions, and their developmental plateau need to be overcome. We performed a proteomic analysis on kidney organoids cultured for a prolonged culture time and we found a specific change in the extracellular matrix composition with increased expression of types 1a1, 2 and 6a1 collagen. Such an excessive accumulation of specific collagen types is a hallmark of renal fibrosis that causes a life-threatening pathological condition by compromising key functions of the human kidney. Here we hypothesized the need for a three-dimensional environment to grow the kidney organoids, which could better mimic the in vivo surroundings of the developing kidney than standard culture on an air-liquid interface. Encapsulating organoids for four days in a soft, thiol-ene cross-linked alginate hydrogel resulted in decreased type 1a1 collagen expression. Furthermore, the encapsulation did not result in any changes of organoid structural morphology. Using a biomaterial to modulate collagen expression allows for a prolonged kidney organoid culture in vitro and a reduction of abnormal type 1a1 collagen expression bringing kidney organoids closer to clinical application.

Tuning Viscoelasticity in Alginate Hydrogels for 3D Cell Culture Studies

Physical properties of the extracellular matrix (ECM) affect cell behaviors ranging from cell adhesion and migration to differentiation and gene expression, a process known as mechanotransduction. While most studies have focused on the impact of ECM stiffness, using linearly elastic materials such as polyacrylamide gels as cell culture substrates, biological tissues and ECMs are viscoelastic, which means they exhibit time-dependent mechanical responses and dissipate mechanical energy. Recent studies have revealed ECM viscoelasticity, independent of stiffness, as a critical physical parameter regulating cellular processes. These studies have used biomaterials with tunable viscoelasticity as cell-culture substrates, with alginate hydrogels being one of the most commonly used systems. Here, we detail the protocols for three approaches to modulating viscoelasticity in alginate hydrogels for 2D and 3D cell culture studies, as well as the testing of their mechanical properties. Viscoelasticity in alginate hydrogels can be tuned by varying the molecular weight of the alginate polymer, changing the type of crosslinker-ionic versus covalent-or by grafting short poly(ethylene-glycol) (PEG) chains to the alginate polymer. As these approaches are based on commercially available products and simple chemistries, these protocols should be accessible for scientists in the cell biology and bioengineering communities. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Tuning viscoelasticity by varying alginate molecular weight Basic Protocol 2: Tuning viscoelasticity with ionic versus covalent crosslinking Basic Protocol 3: Tuning viscoelasticity by adding PEG spacers to alginate chains Support Protocol 1: Testing mechanical properties of alginate hydrogels Support Protocol 2: Conjugating cell-adhesion peptide RGD to alginate.

Designing Hydrogels for 3D Cell Culture Using Dynamic Covalent Crosslinking

Designing simple biomaterials to replicate the biochemical and mechanical properties of tissues is an ongoing challenge in tissue engineering. For several decades, new biomaterials have been engineered using cytocompatible chemical reactions and spontaneous ligations via click chemistries to generate scaffolds and water swollen polymer networks, known as hydrogels, with tunable properties. However, most of these materials are static in nature, providing only macroscopic tunability of the scaffold mechanics, and do not reflect the dynamic environment of natural extracellular microenvironment. For more complex applications such as organoids or co-culture systems, there remain opportunities to investigate cells that locally remodel and change the physicochemical properties within the matrices. In this review, advanced biomaterials where dynamic covalent chemistry is used to produce stable 3D cell culture models and high-resolution constructs for both in vitro and in vivo applications, are discussed. The implications of dynamic covalent chemistry on viscoelastic properties of in vitro models are summarized, case studies in 3D cell culture are critically analyzed, and opportunities to further improve the performance of biomaterials for 3D tissue engineering are discussed.

Adaptable hydrogel with reversible linkages for regenerative medicine: Dynamic mechanical microenvironment for cells

Hydrogels are three-dimensional platforms that serve as substitutes for native extracellular matrix. These materials are starting to play important roles in regenerative medicine because of their similarities to native matrix in water content and flexibility. It would be very advantagoues for researchers to be able to regulate cell behavior and fate with specific hydrogels that have tunable mechanical properties as biophysical cues. Recent developments in dynamic chemistry have yielded designs of adaptable hydrogels that mimic dynamic nature of extracellular matrix. The current review provides a comprehensive overview for adaptable hydrogel in regenerative medicine as follows. First, we outline strategies to design adaptable hydrogel network with reversible linkages according to previous findings in supramolecular chemistry and dynamic covalent chemistry. Next, we describe the mechanism of dynamic mechanical microenvironment influence cell behaviors and fate, including how stress relaxation influences on cell behavior and how mechanosignals regulate matrix remodeling. Finally, we highlight techniques such as bioprinting which utilize adaptable hydrogel in regenerative medicine. We conclude by discussing the limitations and challenges for adaptable hydrogel, and we present perspectives for future studies.

Mechanobiological Interactions between Dynamic Compressive Loading and Viscoelasticity on Chondrocytes in Hydrazone Covalent Adaptable Networks for Cartilage Tissue Engineering

Mechanobiological cues influence chondrocyte biosynthesis and are often used in tissue engineering applications to improve the repair of articular cartilage in load-bearing joints. In this work, the biophysical effects of an applied dynamic compression on chondrocytes encapsulated in viscoelastic hydrazone covalent adaptable networks (CANs) is explored. Here, hydrazone CANs exhibit viscoelastic loss tangents ranging from (9.03 ± 0.01) 10-4 to (1.67 ± 0.09) 10-3 based on the molar percentages of alkyl-hydrazone and benzyl-hydrazone crosslinks. Notably, viscoelastic alkyl-hydrazone crosslinks improve articular cartilage specific gene expression showing higher SOX9 expression in free swelling hydrogels and dynamic compression reduces hypertrophic chondrocyte markers (COL10A1, MMP13) in hydrazone CANs. Interestingly, dynamic compression also improves matrix biosynthesis in elastic benzyl-hydrazone controls but reduces biosynthesis in viscoelastic alkyl-hydrazone CANs. Additionally, intermediate levels of viscoelastic adaptability demonstrate the highest levels of matrix biosynthesis in hydrazone CANs, demonstrating on average 70 ± 4 µg of sulfated glycosaminoglycans per day and 31 ± 3 µg of collagen per day over one month in dynamic compression bioreactors. Collectively, the results herein demonstrate the role of matrix adaptability and viscoelasticity on chondrocytes in hydrazone CANs during dynamic compression, which may prove useful for tissue engineering applications in load-bearing joints.

Biomedical Applications of Electrospun Graphene Oxide

Graphene oxide (GO) has broad potential in the biomedical sector. The oxygen-abundant nature of GO means the material is hydrophilic and readily dispersible in water. GO has also been known to improve cell proliferation, drug loading, and antimicrobial properties of composites. Electrospun composites likewise have great potential for biomedical applications because they are generally biocompatible and bioresorbable, possess low immune rejection risk, and can mimic the structure of the extracellular matrix. In the current review, GO-containing electrospun composites for tissue engineering applications are described in detail. In addition, electrospun GO-containing materials for their use in drug and gene delivery, wound healing, and biomaterials/medical devices have been examined. Good biocompatibility and anionic-exchange properties of GO make it an ideal candidate for drug and gene delivery systems. Drug/gene delivery applications for electrospun GO composites are described with a number of examples. Various systems using electrospun GO-containing therapeutics have been compared for their potential uses in cancer therapy. Micro- to nanosized electrospun fibers for wound healing applications and antimicrobial applications are explained in detail. Applications of various GO-containing electrospun composite materials for medical device applications are listed. It is concluded that the electrospun GO materials will find a broad range of biomedical applications such as cardiac patches, medical device coatings, sensors, and triboelectric nanogenerators for motion sensing and biosensing.