Measuring cellular forces using bis-aliphatic hydrazone crosslinked stress-relaxing hydrogels

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.

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

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

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.

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

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

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.

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.

Measuring human mesenchymal stem cell remodeling in hydrogels with a step-change in elastic modulus

Human mesenchymal stem cells (hMSCs) are instrumental in the wound healing process. They migrate to wounds from their native niche in response to chemical signals released during the inflammatory phase of healing. At the wound, hMSCs downregulate inflammation and regulate tissue regeneration. Delivering additional hMSCs to wounds using cell-laden implantable hydrogels has the potential to improve healing outcomes and restart healing in chronic wounds. For these materials to be effective, cells must migrate from the scaffold into the native tissue. This requires cells to traverse a step-change in material properties at the implant-tissue interface. Migration of cells in material with highly varying properties is not well characterized. We measure 3D encapsulated hMSC migration and remodeling in a well-characterized hydrogel with a step-change in stiffness. This cell-degradable hydrogel is composed of 4-arm poly(ethylene glycol)-norbornene cross-linked with an enzymatically-degradable peptide. The scaffold is made with two halves of different stiffnesses separated by an interface where stiffness changes rapidly. We characterize changes in structure and rheology of the pericellular region using multiple particle tracking microrheology (MPT). MPT measures Brownian motion of embedded particles and relates it to material rheology. We measure more remodeling in the soft region of the hydrogel than the stiff region on day 1 post-encapsulation and similar remodeling everywhere on day 6. In the interface region, we measure hMSC-mediated remodeling along the interface and migration towards the stiff side of the scaffold. These results can improve materials designed for cell delivery from implants to a wound to enhance healing.

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.

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.

Connecting the Dynamics and Reactivity of Arylboronic Acids to Emergent and Stimuli-Responsive Material Properties

Over the past two decades, arylboronic acid-functionalized biomaterials have been used in a variety of sensing and stimuli-responsive scaffolds. Their diverse applications result from the diverse reactivity of arylboronic acids,...

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.

Engineering hydrogels for personalized disease modeling and regenerative medicine

Technological innovations and advances in scientific understanding have created an environment where data can be collected, analyzed, and interpreted at scale, ushering in the era of personalized medicine. The ability to isolate cells from individual patients offers tremendous promise if those cells can be used to generate functional tissue replacements or used in disease modeling to determine optimal treatment strategies. Here, we review recent progress in the use of hydrogels to create artificial cellular microenvironments for personalized tissue engineering and regenerative medicine applications, as well as to develop personalized disease models. We highlight engineering strategies to control stem cell fate through hydrogel design, and the use of hydrogels in combination with organoids, advanced imaging methods, and novel bioprinting techniques to generate functional tissues. We also discuss the use of hydrogels to study molecular mechanisms underlying diseases and to create personalized in vitro disease models to complement existing pre-clinical models. Continued progress in the development of engineered hydrogels, in combination with other emerging technologies, will be essential to realize the immense potential of personalized medicine. STATEMENT OF SIGNIFICANCE: In this review, we cover recent advances in hydrogel engineering strategies with applications in personalized medicine. Specifically, we focus on material systems to expand or control differentiation of patient-derived stem cells, and hydrogels to reprogram somatic cells to pluripotent states. We then review applications of hydrogels in developing personalized engineered tissues. We also highlight the use of hydrogel systems as personalized disease models, focusing on specific examples in fibrosis and cancer, and more broadly on drug screening strategies using patient-derived cells and hydrogels. We believe this review will be a valuable contribution to the Special Issue and the readership of Acta Biomaterialia will appreciate the comprehensive overview of the utility of hydrogels in the developing field of personalized medicine.

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.

Advances in Extracellular Matrix-Mimetic Hydrogels to Guide Stem Cell Fate

In the fields of regenerative medicine and tissue engineering, stem cells offer vast potential for treating or replacing diseased and damaged tissue. Much progress has been made in understanding stem cell biology, yielding protocols for directing stem cell differentiation toward the cell type of interest for a specific application. One particularly interesting and powerful signaling cue is the extracellular matrix (ECM) surrounding stem cells, a network of biopolymers that, along with cells, makes up what we define as a tissue. The composition, structure, biochemical features, and mechanical properties of the ECM are varied in different tissues and developmental stages, and serve to instruct stem cells toward a specific lineage. By understanding and recapitulating some of these ECM signaling cues through engineered ECM-mimicking hydrogels, stem cell fate can be directed in vitro. In this review, we will summarize recent advances in material systems to guide stem cell fate, highlighting innovative methods to capture ECM functionalities and how these material systems can be used to provide basic insight into stem cell biology or make progress toward therapeutic objectives.

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.

Investigations into the role of non-bond interaction on gelation mechanism of silk fibroin hydrogel

Silk fibroin hydrogel not only has biocompatibility, but also has environmental response ability. It plays an important role in the development of material. The gelation mechanism of silk fibroin hydrogel is very important to textile and medicine fields. The molecular dynamics simulation was used to discuss the structure and non-bond interaction of silk fibroin hydrogel. The results show that the non-bond interactions between silk fibroin molecules and water molecules have certain influence on the formation of silk fibroin hydrogel. According to the hydrogen bond analysis, the hydrogen bonds are mainly formed between random coil peptide fragments at the two ends of silk fibroin molecules and residues 252-254 are the key residues. The electrostatic and polar solvation interactions between silk fibroin molecules plays a major role in cross-linking of the coil segments of two silk fibroin molecules.

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.

Soft Materials by Design: Unconventional Polymer Networks Give Extreme Properties

Hydrogels are polymer networks infiltrated with water. Many biological hydrogels in animal bodies such as muscles, heart valves, cartilages, and tendons possess extreme mechanical properties including being extremely tough, strong, resilient, adhesive, and fatigue-resistant. These mechanical properties are also critical for hydrogels' diverse applications ranging from drug delivery, tissue engineering, medical implants, wound dressings, and contact lenses to sensors, actuators, electronic devices, optical devices, batteries, water harvesters, and soft robots. Whereas numerous hydrogels have been developed over the last few decades, a set of general principles that can rationally guide the design of hydrogels using different materials and fabrication methods for various applications remain a central need in the field of soft materials. This review is aimed at synergistically reporting: (i) general design principles for hydrogels to achieve extreme mechanical and physical properties, (ii) implementation strategies for the design principles using unconventional polymer networks, and (iii) future directions for the orthogonal design of hydrogels to achieve multiple combined mechanical, physical, chemical, and biological properties. Because these design principles and implementation strategies are based on generic polymer networks, they are also applicable to other soft materials including elastomers and organogels. Overall, the review will not only provide comprehensive and systematic guidelines on the rational design of soft materials, but also provoke interdisciplinary discussions on a fundamental question: why does nature select soft materials with unconventional polymer networks to constitute the major parts of animal bodies?

The nonequilibrium behaviors of covalent adaptable network polymers during the topology transition

Vitrimers with bond exchange reactions (BERs) are a class of covalent adaptable network (CAN) polymers at the forefront of recent polymer research. They exhibit malleable and self-healable behaviors and combine the advantages of easy processability of thermoplastics and excellent mechanical properties of thermosets. For thermally sensitive vitrimers, a molecular topology melting/frozen transition is triggered when the BERs are activated to rearrange the network architecture. Notable volume expansion and stress relaxation are accompanied, which can be used to identify the BER activation temperature and rate as well as to determine the malleability and interfacial welding kinetics of vitrimers. Existing works on vitrimers reveal the rate-dependent behaviors of the nonequilibrium network during the topology transition. However, it remains unclear what the quantitative relationship with heating rate is, and how it will affect the macroscopic stress relaxation. In this paper, we study the responses of an epoxy-based vitrimer subjected to a change in temperature and mechanical loading during the topology transition. Using thermal expansion tests, the thermal strain evolution is shown to depend on the temperature-changing rate, which reveals the nonequilibrium states with rate-dependent structural relaxation. The influences of structural relaxation on the stress relaxation behaviors are examined in both uniaxial tension and compression modes. Assisted by a theoretical model, the study reveals how to tune the material and thermal-temporal conditions to promote the contribution of BERs during the reprocessing of vitrimers.

Natural-Based Hydrogels for Tissue Engineering Applications

In the field of tissue engineering and regenerative medicine, hydrogels are used as biomaterials to support cell attachment and promote tissue regeneration due to their unique biomimetic characteristics. The use of natural-origin materials significantly influenced the origin and progress of the field due to their ability to mimic the native tissues' extracellular matrix and biocompatibility. However, the majority of these natural materials failed to provide satisfactory cues to guide cell differentiation toward the formation of new tissues. In addition, the integration of technological advances, such as 3D printing, microfluidics and nanotechnology, in tissue engineering has obsoleted the first generation of natural-origin hydrogels. During the last decade, a new generation of hydrogels has emerged to meet the specific tissue necessities, to be used with state-of-the-art techniques and to capitalize the intrinsic characteristics of natural-based materials. In this review, we briefly examine important hydrogel crosslinking mechanisms. Then, the latest developments in engineering natural-based hydrogels are investigated and major applications in the field of tissue engineering and regenerative medicine are highlighted. Finally, the current limitations, future challenges and opportunities in this field are discussed to encourage realistic developments for the clinical translation of tissue engineering strategies.

Designing Microgels for Cell Culture and Controlled Assembly of Tissue Microenvironments

Micron-sized hydrogels, termed microgels, are emerging as multifunctional platforms that can recapitulate tissue heterogeneity in engineered cell microenvironments. The microgels can function as either individual cell culture units or can be assembled into larger scaffolds. In this manner, individual microgels can be customized for single or multi-cell co-culture applications, or heterogeneous populations can be used as building blocks to create microporous assembled scaffolds that more closely mimic tissue heterogeneities. The inherent versatility of these materials allows user-defined control of the microenvironments, from the order of singly encapsulated cells to entire three-dimensional cell scaffolds. These hydrogel scaffolds are promising for moving towards personalized medicine approaches and recapitulating the multifaceted microenvironments that exist in vivo.

Microrheological characterization of covalent adaptable hydrogel degradation in response to temporal pH changes that mimic the gastrointestinal tract

Covalent adaptable hydrogels (CAHs) reversibly adapt their structure in response to external stimuli, emerging as a new platform for biological applications. Due to the unique and complex nature of these materials, a characterization technique is needed to measure the rheology of these CAHs in biological processes. μ2rheology, microrheology in a microfluidic device, is a technique that can fully characterize real-time CAH degradation in a changing environment, such as the pH environment of the GI tract. This characterization will enable design and tailoring of these materials for controlled and targeted oral drug delivery. Using μ2rheology, we can exchange the fluid environment without sample loss and measure the change in CAH rheological properties. We show degradation kinetics and material property evolution are independent of degradation history. However, the initial cross-link density at each pH exchange can be decreased by degradation history which decreases the time for the CAH to degrade to the gel-sol transition. These results indicate that CAH degradation can be tuned by changing the initial material properties by varying polymer concentration and ratio of functional groups. We also show that μ2rheology will enable the design of new dynamic materials for targeted drug delivery by enabling these materials to be characterized and tailored in vitro.

Toward Stimuli-Responsive Dynamic Thermosets through Continuous Development and Improvements in Covalent Adaptable Networks (CANs)

Covalent adaptable networks (CANs), unlike typical thermosets or other covalently crosslinked networks, possess a unique, often dormant ability to activate one or more forms of stimuli-responsive, dynamic covalent chemistries as a means to transition their behavior from that of a viscoelastic solid to a material with fluid-like plastic flow. Upon application of a stimulus, such as light or other irradiation, temperature, or even a distinct chemical signal, the CAN responds by transforming to a state of temporal plasticity through activation of either reversible addition or reversible bond exchange, either of which allows the material to essentially re-equilibrate to an altered set of conditions that are distinct from those in which the original covalently crosslinked network is formed, often simultaneously enabling a new and distinct shape, function, and characteristics. As such, CANs span the divide between thermosets and thermoplastics, thus offering unprecedented possibilities for innovation in polymer and materials science. Without attempting to comprehensively review the literature, recent developments in CANs are discussed here with an emphasis on the most effective dynamic chemistries that render these materials to be stimuli responsive, enabling features that make CANs more broadly applicable.

3D Extracellular Matrix Mimics: Fundamental Concepts and Role of Materials Chemistry to Influence Stem Cell Fate

Synthetic 3D extracellular matrices (ECMs) find application in cell studies, regenerative medicine, and drug discovery. While cells cultured in a monolayer may exhibit unnatural behavior and develop very different phenotypes and genotypes than in vivo, great efforts in materials chemistry have been devoted to reproducing in vitro behavior in in vivo cell microenvironments. This requires fine-tuning the biochemical and structural actors in synthetic ECMs. This review will present the fundamentals of the ECM, cover the chemical and structural features of the scaffolds used to generate ECM mimics, discuss the nature of the signaling biomolecules required and exploited to generate bioresponsive cell microenvironments able to induce a specific cell fate, and highlight the synthetic strategies involved in creating functional 3D ECM mimics.

Viscoelasticity of hydrazone crosslinked poly(ethylene glycol) hydrogels directs chondrocyte morphology during mechanical deformation

Adaptable dynamic covalent crosslinks temporally modulate the biophysical transmission of physiologically relevant compressive strains to encapsulated chondrocytes for cartilage tissue engineering.

Photoredox Decarboxylative C(sp3)-N Coupling of α-Diazoacetates with Alkyl N-Hydroxyphthalimide Esters for Diversified Synthesis of Functionalized N-Alkyl Hydrazones

Herein we report a metal-free photocatalytic coupling reaction for the synthesis of structurally and functionally diverse N-alkyl hydrazones from α-diazoacetates and N-alkyl hydroxyphthalimide esters. By employing Rose Bengal as a photocatalyst with yellow LEDs irradiation, over 60 N-alkyl hydrazones were synthesized. Fluorescence quenching analysis and deuterium incorporation experiments reveal that Hantzsch ester serves as both an electron donor and proton source for the reaction. This strategy offers a simple retrosynthetic disconnection for conventionally inaccessible C(sp³)-rich N-alkyl hydrazones.

Hydrogels Based on Schiff Base Linkages for Biomedical Applications

Schiff base, an important family of reaction in click chemistry, has received significant attention in the formation of self-healing hydrogels in recent years. Schiff base reversibly reacts even in mild conditions, which allows hydrogels with self-healing ability to recover their structures and functions after damages. Moreover, pH-sensitivity of the Schiff base offers the hydrogels response to biologically relevant stimuli. Different types of Schiff base can provide the hydrogels with tunable mechanical properties and chemical stabilities. In this review, we summarized the design and preparation of hydrogels based on various types of Schiff base linkages, as well as the biomedical applications of hydrogels in drug delivery, tissue regeneration, wound healing, tissue adhesives, bioprinting, and biosensors.

Microrheological characterization of covalent adaptable hydrogels for applications in oral delivery

The feasibility of a covalent adaptable hydrogel (CAH) as an oral delivery platform is explored using μ2rheology, microrheology in a microfluidic device. CAH degradation is initiated by physiologically relevant pHs, including incubation at a single pH and consecutively at different pHs. At a single pH, we determine CAH degradation can be tuned by changing the pH, which can be exploited for controlled release. We calculate the critical relaxation exponent, which defines the gel-sol transition and is independent of the degradation pH. We mimic the changing pH environment through part of the gastrointestinal tract (pH 4.3 to 7.4 or pH 7.4 to 4.3) in our microfluidic device. We determine that dynamic material property evolution is consistent with degradation at a single pH. However, the time scale of degradation is reduced by the history of degradation. These investigations inform the design of this material as a new vehicle for targeted delivery.

A Feedback Loop between Hypoxia and Matrix Stress Relaxation Increases Oxygen-Axis Migration and Metastasis in Sarcoma

Upregulation of collagen matrix crosslinking directly increases its ability to relieve stress under the constant strain imposed by solid tumor, a matrix property termed stress relaxation. However, it is unknown how rapid stress relaxation in response to increased strain impacts disease progression in a hypoxic environment. Previously, it has been demonstrated that hypoxia-induced expression of the crosslinker procollagen-lysine, 2-oxoglutarate 5-dioxygenase 2 (PLOD2), in sarcomas has resulted in increased lung metastasis. Here, we show that short stress relaxation times led to increased cell migration along a hypoxic gradient in 3D collagen matrices, and rapid stress relaxation upregulated PLOD2 expression via TGFβ-SMAD2 signaling, forming a feedback loop between hypoxia and the matrix. Inhibition of this pathway led to a decrease in migration along the hypoxic gradients. In vivo, sarcoma primed in a hypoxic matrix with short stress relaxation time enhanced collagen fiber size and tumor density and increased lung metastasis. High expression of PLOD2 correlated with decreased overall survival in patients with sarcoma. Using a patient-derived sarcoma cell line, we developed a predictive platform for future personalized studies and therapeutics. Overall, these data show that the interplay between hypoxia and matrix stress relaxation amplifies PLOD2, which in turn accelerates sarcoma cell motility and metastasis. SIGNIFICANCE: These findings demonstrate that mechanical (stress relaxation) and chemical (hypoxia) properties of the tumor microenvironment jointly accelerate sarcoma motility and metastasis via increased expression of collagen matrix crosslinker PLOD2.

Supramolecular and dynamic covalent hydrogel scaffolds: from gelation chemistry to enhanced cell retention and cartilage regeneration

This review highlights the most recent progress in gelation strategies of biomedical supramolecular and dynamic covalent crosslinking hydrogels and their applications for enhancing cell retention and cartilage regeneration.

Bio Mimicking of Extracellular Matrix

Biomaterials play a critical role in regenerative strategies such as stem cell-based therapies and tissue engineering, aiming to replace, remodel, regenerate, or support damaged tissues and organs. The design of appropriate three-dimensional (3D) scaffolds is crucial for generating bio-inspired replacement tissues. These scaffolds are primarily composed of degradable or non-degradable biomaterials and can be employed as cells, growth factors, or drug carriers. Naturally derived and synthetic biomaterials have been widely used for these purposes, but the ideal biomaterial remains to be found. Researchers from diversified fields have attempted to design and fabricate novel biomaterials, aiming to find novel theranostic approaches for tissue engineering and regenerative medicine. Since no single biomaterial has been found to possess all the necessary characteristics for an ideal performance, over the years scientists have tried to develop composite biomaterials that complement and combine the beneficial properties of multiple materials into a superior matrix. Herein, we highlight the structural features and performance of various biomaterials and their application in regenerative medicine and for enhanced tissue engineering approaches.

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

Cartilage tissue engineering strategies often rely on hydrogels with fixed covalent crosslinks for chondrocyte encapsulation, yet the resulting material properties are largely elastic and can impede matrix deposition. To address this limitation, hydrazone crosslinked poly(ethylene glycol) hydrogels were formulated to achieve tunable viscoelastic properties and to study how chondrocyte proliferation and matrix deposition vary with the time-dependent material properties of covalent adaptable networks. Hydrazone equilibrium differences were leveraged to produce average stress relaxation times from hours (4.01 × 103 s) to months (2.78 × 106 s) by varying the percentage of alkyl-hydrazone (aHz) and benzyl-hydrazone (bHz) crosslinks. Swelling behavior and degradation associated with adaptability were characterized to quantify temporal network changes that can influence the behavior of encapsulated chondrocytes. After four weeks, mass swelling ratios varied from 36 ± 3 to 17 ± 0.4 and polymer retention ranged from 46 ± 4% to 92 ± 5%, with higher aHz content leading to loss of network connectivity with time. Hydrogels were formulated near the Flory-Stockmayer bHz percolation threshold (17% bHz) to investigate chondrocyte response to distinct levels of covalent architecture adaptability. Four weeks post-encapsulation, formulations with average relaxation times of 3 days (2.6 × 105s) revealed increased cellularity and an interconnected articular cartilage-specific matrix. Chondrocytes embedded in this adaptable formulation (22% bHz) deposited 190 ± 30% more collagen and 140 ± 20% more sulfated glycosaminoglycans compared to the 100% bHz control, which constrained matrix deposition to pericellular space. Collectively, these findings indicate that incorporating highly adaptable aHz crosslinks enhanced regenerative outcomes. However, connected networks containing more stable bHz bonds were required to achieve the highest quality neocartilaginous tissue. STATEMENT OF SIGNIFICANCE: Covalently crosslinked hydrogels provide robust mechanical support for cartilage tissue engineering applications in articulating joints. However, these materials traditionally demonstrate purely elastic responses to deformation despite the dynamic viscoelastic properties of native cartilage tissue. Here, we present hydrazone poly(ethylene glycol) hydrogels with tunable viscoelastic properties and study covalent adaptable networks for cartilage tissue engineering. Using hydrazone equilibrium and Flory-Stockmayer theory we identified average relaxation times leading to enhanced regenerative outcomes and showed that extracellular matrix deposition was biphasic as a function of the hydrazone covalent adaptability. We also showed that the incorporation of highly adaptable covalent crosslinks could improve cellularity of neotissue, but that a percolating network of more stable bonds was required to maintain scaffold integrity and form the highest quality neocartilaginous tissue.

Designer hydrogels: Shedding light on the physical chemistry of the pancreatic cancer microenvironment

Pancreatic ductal adenocarcinoma (PDAC) is currently the third leading cause of cancer mortality in the United States, with a 5-year survival of ∼8%. PDAC is characterized by a dense and hypo-vascularized stroma consisting of proliferating cancer cells, cancer-associated fibroblasts, macrophages and immune cells, as well as excess matrices including collagens, fibronectin, and hyaluronic acid. In addition, PDAC has increased interstitial pressures and a hypoxic/acidic tumor microenvironment (TME) that impedes drug delivery and blocks cancer-directed immune mechanisms. In spite of increasing options in targeted therapy, PDAC has mostly remained treatment recalcitrant. Owing to its critical roles on governing PDAC progression and treatment outcome, TME and its interplay with the cancer cells are increasingly studied. In particular, three-dimensional (3D) hydrogels derived from or inspired by components in the TME are progressively developed. When properly designed, these hydrogels (e.g., Matrigel, collagen gel, hyaluronic acid-based, and semi-synthetic hydrogels) can provide pathophysiologically relevant compositions, conditions, and contexts for supporting PDAC cell fate processes. This review summarizes recent efforts in using 3D hydrogels for fundamental studies on cell-matrix or cell-cell interactions in PDAC.

Hydrazone-Linkage-Based Self-Healing and Injectable Xanthan-Poly(ethylene glycol) Hydrogels for Controlled Drug Release and 3D Cell Culture

Polymeric hydrogels have been extensively explored for controlled drug-delivery applications, but there is an increasing demand for smart drug delivery combined with tunable physicochemical attributes and tissue engineering potential. In this work, novel xanthan-poly(ethylene glycol) (PEG) hydrogels were developed by cross-linking polysaccharide, oxidized xanthan, and 8-arm PEG hydrazine through dynamic, pH-responsive, and biodegradable hydrazone linkages. Aqueous solutions (pH 6.5) of oxidized xanthan and PEG hydrazine were mixed together at 37 °C to obtain hydrogels within minutes, and the formation of hydrazone linkages was ascertained using Fourier transform infrared spectroscopy. Fabrication of xanthan-PEG hydrogels using hydrazone linkages has not been reported previously. The 3% hydrogels exhibited the storage modulus of 194 Pa, which increased to 770 Pa for 5% hydrogels. When subjected to alternating cycles of varying strains of 1 and 800% (5 cycles), hydrogels demonstrated instant recovery each time the extreme strain was relieved, thus suggesting excellent self-healing capabilities. Doxorubicin (DOX), chemotherapeutic agent, was loaded onto hydrogels, and release studies were carried out at pH 5.5 (tumoral) and 7.4 (physiological). The cumulative release from 3, 4, and 5% hydrogels at pH 5.5 was 81.06, 61.98, and 41.67%, whereas the release at pH 7.4 was 47.43, 37.01, and 35.34% at 30 days. MTT assay showed that oxidized xanthan and PEG hydrazine are not toxic to mammalian cells (NIH-3T3), as the cell viabilities were found to be 84.66 and 102% for concentrations up to 1 mg/mL. The live/dead assay with encapsulated NIH-3T3 cells showed no significant dead cell population, suggesting excellent compatibility of hydrogels in 2D and 3D culture. DOX-loaded hydrogels exhibited cytotoxicity against A549 cells when exposed to media released from hydrogels. Overall, hydrogels developed in this work may have potential applications in drug delivery and 3D cell culture for cell delivery.

Achieving Controlled Biomolecule-Biomaterial Conjugation

The conjugation of biomolecules can impart materials with the bioactivity necessary to modulate specific cell behaviors. While the biological roles of particular polypeptide, oligonucleotide, and glycan structures have been extensively reviewed, along with the influence of attachment on material structure and function, the key role played by the conjugation strategy in determining activity is often overlooked. In this review, we focus on the chemistry of biomolecule conjugation and provide a comprehensive overview of the key strategies for achieving controlled biomaterial functionalization. No universal method exists to provide optimal attachment, and here we will discuss both the relative advantages and disadvantages of each technique. In doing so, we highlight the importance of carefully considering the impact and suitability of a particular technique during biomaterial design.

About Chemical Strategies to Fabricate Cell-Instructive Biointerfaces with Static and Dynamic Complexity

Properly functioning cell-instructive biointerfaces are critical for healthy integration of biomedical devices in the body and serve as decisive tools for the advancement of our understanding of fundamental cell biological phenomena. Studies are reviewed that use covalent chemistries to fabricate cell-instructive biointerfaces. These types of biointerfaces typically result in a static presentation of predefined cell-instructive cues. Chemically defined, but dynamic cell-instructive biointerfaces introduce spatiotemporal control over cell-instructive cues and present another type of biointerface, which promises a more biomimetic way to guide cell behavior. Therefore, strategies that offer control over the lateral sorting of ligands, the availability and molecular structure of bioactive ligands, and strategies that offer the ability to induce physical, chemical and mechanical changes in situ are reviewed. Specific attention is paid to state-of-the-art studies on dynamic, cell-instructive 3D materials. Future work is expected to further deepen our understanding of molecular and cellular biological processes investigating cell-type specific responses and the translational steps toward targeted in vivo applications.

Recent Progress in Developing Injectable Matrices for Enhancing Cell Delivery and Tissue Regeneration

Biomaterials are key factors in regenerative medicine. Matrices used for cell delivery are especially important, as they provide support to transplanted cells that is essential for promoting cell survival, retention, and desirable phenotypes. Injectable matrices have become promising and attractive due to their minimum invasiveness and ease of use. Conventional injectable matrices mostly use hydrogel precursor solutions that form solid, cell-laden hydrogel scaffolds in situ. However, these materials are associated with challenges in biocompatibility, shear-induced cell death, lack of control over cellular phenotype, lack of macroporosity and remodeling, and relatively weak mechanical strength. This Progress Report provides a brief overview of recent progress in developing injectable matrices to overcome the limitations of conventional in situ hydrogels. Biocompatible chemistry and shear-thinning hydrogels have been introduced to promote cell survival and retention. Emerging investigations of the effects of matrix properties on cellular function in 3D provide important guidelines for promoting desirable cellular phenotypes. Moreover, several novel approaches are combining injectability with macroporosity to achieve macroporous, injectable matrices for cell delivery.

Amyloids Are Novel Cell-Adhesive Matrices

Amyloids are highly ordered peptide/protein aggregates traditionally associated with multiple human diseases including neurodegenerative disorders. However, recent studies suggest that amyloids can also perform several biological functions in organisms varying from bacteria to mammals. In many lower organisms, amyloid fibrils function as adhesives due to their unique surface topography. Recently, amyloid fibrils have been shown to support attachment and spreading of mammalian cells by interacting with the cell membrane and by cell adhesion machinery activation. Moreover, similar to cellular responses on natural extracellular matrices (ECMs), mammalian cells on amyloid surfaces also use integrin machinery for spreading, migration, and differentiation. This has led to the development of biocompatible and implantable amyloid-based hydrogels that could induce lineage-specific differentiation of stem cells. In this chapter, based on adhesion of both lower organisms and mammalian cells on amyloid nanofibrils, we posit that amyloids could have functioned as a primitive extracellular matrix in primordial earth.

Stress relaxing hyaluronic acid-collagen hydrogels promote cell spreading, fiber remodeling, and focal adhesion formation in 3D cell culture

The physical and architectural cues of the extracellular matrix (ECM) play a critical role in regulating important cellular functions such as spreading, migration, proliferation, and differentiation. Natural ECM is a complex viscoelastic scaffold composed of various distinct components that are often organized into a fibrillar microstructure. Hydrogels are frequently used as synthetic ECMs for 3D cell culture, but are typically elastic, due to covalent crosslinking, and non-fibrillar. Recent work has revealed the importance of stress relaxation in viscoelastic hydrogels in regulating biological processes such as spreading and differentiation, but these studies all utilize synthetic ECM hydrogels that are non-fibrillar. Key mechanotransduction events, such as focal adhesion formation, have only been observed in fibrillar networks in 3D culture to date. Here we present an interpenetrating network (IPN) hydrogel system based on HA crosslinked with dynamic covalent bonds and collagen I that captures the viscoelasticity and fibrillarity of ECM in tissues. The IPN hydrogels exhibit two distinct processes in stress relaxation, one from collagen and the other from HA crosslinking dynamics. Stress relaxation in the IPN hydrogels can be tuned by modulating HA crosslinker affinity, molecular weight of the HA, or HA concentration. Faster relaxation in the IPN hydrogels promotes cell spreading, fiber remodeling, and focal adhesion (FA) formation - behaviors often inhibited in other hydrogel-based materials in 3D culture. This study presents a new, broadly adaptable materials platform for mimicking key ECM features of viscoelasticity and fibrillarity in hydrogels for 3D cell culture and sheds light on how these mechanical and structural cues regulate cell behavior.

Functional and Biomimetic Materials for Engineering of the Three-Dimensional Cell Microenvironment

The cell microenvironment has emerged as a key determinant of cell behavior and function in development, physiology, and pathophysiology. The extracellular matrix (ECM) within the cell microenvironment serves not only as a structural foundation for cells but also as a source of three-dimensional (3D) biochemical and biophysical cues that trigger and regulate cell behaviors. Increasing evidence suggests that the 3D character of the microenvironment is required for development of many critical cell responses observed in vivo, fueling a surge in the development of functional and biomimetic materials for engineering the 3D cell microenvironment. Progress in the design of such materials has improved control of cell behaviors in 3D and advanced the fields of tissue regeneration, in vitro tissue models, large-scale cell differentiation, immunotherapy, and gene therapy. However, the field is still in its infancy, and discoveries about the nature of cell-microenvironment interactions continue to overturn much early progress in the field. Key challenges continue to be dissecting the roles of chemistry, structure, mechanics, and electrophysiology in the cell microenvironment, and understanding and harnessing the roles of periodicity and drift in these factors. This review encapsulates where recent advances appear to leave the ever-shifting state of the art, and it highlights areas in which substantial potential and uncertainty remain.

Covalently adaptable elastin-like protein - hyaluronic acid (ELP - HA) hybrid hydrogels with secondary thermoresponsive crosslinking for injectable stem cell delivery

Shear-thinning, self-healing hydrogels are promising vehicles for therapeutic cargo delivery due to their ability to be injected using minimally invasive surgical procedures. We present an injectable hydrogel using a novel combination of dynamic covalent crosslinking with thermoresponsive engineered proteins. Ex situ at room temperature, rapid gelation occurs through dynamic covalent hydrazone bonds by simply mixing two components: hydrazine-modified elastin-like protein (ELP) and aldehyde-modified hyaluronic acid. This hydrogel provides significant mechanical protection to encapsulated human mesenchymal stem cells during syringe needle injection and rapidly recovers after injection to retain the cells homogeneously within a 3D environment. In situ, the ELP undergoes a thermal phase transition, as confirmed by Coherent anti-Stokes Raman scattering microscopy observation of dense ELP thermal aggregates. The formation of the secondary network reinforces the hydrogel and results in a 10-fold slower erosion rate compared to a control hydrogel without secondary thermal crosslinking. This improved structural integrity enables cell culture for three weeks post injection, and encapsulated cells maintain their ability to differentiate into multiple lineages, including chondrogenic, adipogenic, and osteogenic cell types. Together, these data demonstrate the promising potential of ELP-HA hydrogels for injectable stem cell transplantation and tissue regeneration.

Elastin-like protein-hyaluronic acid (ELP-HA) hydrogels with decoupled mechanical and biochemical cues for cartilage regeneration

Hyaluronic acid (HA) is a major component of cartilage extracellular matrix and is an attractive material for use as 3D injectable matrices for cartilage regeneration. While previous studies have shown the promise of HA-based hydrogels to support cell-based cartilage formation, varying HA concentration generally led to simultaneous changes in both biochemical cues and stiffness. How cells respond to the change of biochemical content of HA remains largely unknown. Here we report an adaptable elastin-like protein-hyaluronic acid (ELP-HA) hydrogel platform using dynamic covalent chemistry, which allows variation of HA concentration without affecting matrix stiffness. ELP-HA hydrogels were created through dynamic hydrazone bonds via the reaction between hydrazine-modified ELP (ELP-HYD) and aldehyde-modified HA (HA-ALD). By tuning the stoichiometric ratio of aldehyde groups to hydrazine groups while maintaining ELP-HYD concentration constant, hydrogels with variable HA concentration (1.5%, 3%, or 5%) (w/v) were fabricated with comparable stiffness. To evaluate the effects of HA concentration on cell-based cartilage regeneration, chondrocytes were encapsulated within ELP-HA hydrogels with varying HA concentration. Increasing HA concentration led to a dose-dependent increase in cartilage-marker gene expression and enhanced sGAG deposition while minimizing undesirable fibrocartilage phenotype. The use of adaptable protein hydrogels formed via dynamic covalent chemistry may be broadly applicable as 3D scaffolds with decoupled niche properties to guide other desirable cell fates and tissue repair.

Mixed Reversible Covalent Crosslink Kinetics Enable Precise, Hierarchical Mechanical Tuning of Hydrogel Networks

Hydrogels play a central role in a number of medical applications and new research aims to engineer their mechanical properties to improve their capacity to mimic the functional dynamics of native tissues. This study shows hierarchical mechanical tuning of hydrogel networks by utilizing mixtures of kinetically distinct reversible covalent crosslinks. A methodology is described to precisely tune stress relaxation in PEG networks formed from mixtures of two different phenylboronic acid derivatives with unique diol complexation rates, 4-carboxyphenylboronic acid, and o-aminomethylphenylboronic acid. Gel relaxation time and the mechanical response to dynamic shear are exquisitely controlled by the relative concentrations of the phenylboronic acid derivatives. The differences observed in the crossover frequencies corresponding to pK_a differences in the phenylboronic acid derivatives directly connect the molecular kinetics of the reversible crosslinks to the macroscopic dynamic mechanical behavior. Mechanical tuning by mixing reversible covalent crosslinking kinetics is found to be independent of other attributes of network architecture, such as molecular weight between crosslinks.

A silk fibroin hydrogel with reversible sol–gel transition

Herein, we prepare a novel silk fibroin hydrogel with a reversible thixotropic gel–sol transition triggered by a facile cycled shearing and resting procedure.

Engineering the Dynamic Properties of Protein Networks through Sequence Variation

The dynamic behavior of macromolecular networks dominates the mechanical properties of soft materials and influences biological processes at multiple length scales. In hydrogels prepared from self-assembling artificial proteins, stress relaxation and energy dissipation arise from the transient character of physical network junctions. Here we show that subtle changes in sequence can be used to program the relaxation behavior of end-linked networks of engineered coiled-coil proteins. Single-site substitutions in the coiled-coil domains caused shifts in relaxation time over 5 orders of magnitude as demonstrated by dynamic oscillatory shear rheometry and stress relaxation measurements. Networks with multiple relaxation time scales were also engineered. This work demonstrates how time-dependent mechanical responses of macromolecular materials can be encoded in genetic information.