Synthetic matrix metalloproteinase-sensitive hydrogels for the conduction of tissue regeneration: engineering cell-invasion characteristics

High-Throughput Screening of Thiol-ene Click Chemistries for Bone Adhesive Polymers

Metal surgical pins and screws are employed in millions of orthopedic surgical procedures every year worldwide, but their usability is limited in the case of complex, comminuted fractures or in surgeries on smaller bones. Therefore, replacing such implants with a bone adhesive material has long been considered an attractive option. However, synthesizing a biocompatible bone adhesive with a high bond strength that is simple to apply presents many challenges. To rapidly identify candidate polymers for a biocompatible bone adhesive, we employed a high-throughput screening strategy to assess human mesenchymal stromal cell (hMSC) adhesion toward a library of polymers synthesized via thiol-ene click chemistry. We chose thiol-ene click chemistry because multifunctional monomers can be rapidly cured via ultraviolet (UV) light while minimizing residual monomer, and it provides a scalable manufacturing process for candidate polymers identified from a high-throughput screen. This screening methodology identified a copolymer (1-S2-FT01) composed of the monomers 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO) and pentaerythritol tetrakis (3-mercaptopropionate) (PETMP), which supported highest hMSC adhesion across a library of 90 polymers. The identified copolymer (1-S2-FT01) exhibited favorable compressive and tensile properties compared to existing commercial bone adhesives and adhered to bone with adhesion strengths similar to commercially available bone glues such as Histoacryl. Furthermore, this cytocompatible polymer supported osteogenic differentiation of hMSCs and could adhere 3D porous polymer scaffolds to the bone tissue, making this polymer an ideal candidate as an alternative bone adhesive with broad utility in orthopedic surgery.

Delivery of Growth Factors to Enhance Bone Repair

The management of critical-sized bone defects caused by nonunion, trauma, infection, malignancy, pseudoarthrosis, and osteolysis poses complex reconstruction challenges for orthopedic surgeons. Current treatment modalities, including autograft, allograft, and distraction osteogenesis, are insufficient for the diverse range of pathology encountered in clinical practice, with significant complications associated with each. Therefore, there is significant interest in the development of delivery vehicles for growth factors to aid in bone repair in these settings. This article reviews innovative strategies for the management of critical-sized bone loss, including novel scaffolds designed for controlled release of rhBMP, bioengineered extracellular vesicles for delivery of intracellular signaling molecules, and advances in regional gene therapy for sustained signaling strategies. Improvement in the delivery of growth factors to areas of significant bone loss has the potential to revolutionize current treatment for this complex clinical challenge.

Rapid fabrication of physically robust hydrogels

Hydrogel materials show promise for diverse applications, particular as biocompatible materials due to their high water content. Despite advances in hydrogel technology in recent years, their application is often severely limited by inadequate mechanical properties and time-consuming fabrication processes. Here we report a rapid hydrogel preparation strategy that achieves the simultaneous photo-crosslinking and establishment of biomimetic soft-hard material interface microstructures. These biomimetic interfacial-bonding nanocomposite hydrogels are prepared within seconds and feature clearly separated phases but have a strongly bonded interface. Due to effective interphase load transfer, biomimetic interfacial-bonding nanocomposite gels achieve an ultrahigh toughness (138 MJ m-3) and exceptional tensile strength (15.31 MPa) while maintaining a structural stability that rivals or surpasses that of commonly used elastomer (non-hydrated) materials. Biomimetic interfacial-bonding nanocomposite gels can be fabricated into arbitrarily complex structures via three-dimensional printing with micrometre-level precision. Overall, this work presents a generalizable preparation strategy for hydrogel materials and acrylic elastomers that will foster potential advances in soft materials.

3D Printing of Noncytotoxic High-Resolution Microchannels in Bisphenol-A Ethoxylate Dimethacrylate Tissue-Mimicking Materials

The ability to create cell-laden fluidic models that mimic the geometries and physical properties of vascularized tissue would be extremely beneficial to the study of disease etiologies and future therapies, including in the case of cancer where there is increasing interest in studying alterations to the microvasculature. Engineered systems can present significant advantages over animal studies, alleviating challenges associated with variable complexity and control. Three-dimensional (3D)-printable tissue-mimicking hydrogels can offer an alternative, where control of the biophysical properties of the materials can be achieved. Hydrogel-based systems that can recreate complex 3D structures and channels with diameters <500 μm are challenging to produce. We present a noncytotoxic photo-responsive hydrogel that supports 3D printing of complex 3D structures with microchannels down to 150 μm in diameter. Fine tuning of the 3D-printing process has allowed the production of complex structures, where for demonstration purposes we present a helical channel with diameters between 250 and 370 μm around a central channel of 150 μm in diameter in materials with mechanical and acoustic properties that closely replicate those of tissue. The ability to control and accurately reproduce the complex features of the microvasculature has value across a wide range of biomedical applications, especially when the materials involved accurately mimic the physical properties of tissue. An approach that is additionally cell compatible provides a unique setup that can be exploited to study aspects of biomedical research with an unprecedented level of accuracy.

Hydrolytic hydrogels tune mesenchymal stem cell persistence and immunomodulation for enhanced diabetic cutaneous wound healing

Diabetes is associated with an altered global inflammatory state with impaired wound healing. Mesenchymal stem/stromal cells (MSC) are being explored for treatment of diabetic cutaneous wounds due to their regenerative properties. These cells are commonly delivered by injection, but the need to prolong the retention of MSC at sites of injury has spurred the development of biomaterial-based MSC delivery vehicles. However, controlling biomaterial degradation rates in vivo remains a therapeutic-limiting challenge. Here, we utilize hydrolytically degradable ester linkages to engineer synthetic hydrogels with tunable in vivo degradation kinetics for temporally controlled delivery of MSC. In vivo hydrogel degradation rate can be controlled by altering the ratio of ester to amide linkages in the hydrogel macromers. These hydrolytic hydrogels degrade at rates that enable unencumbered cutaneous wound healing, while enhancing the local persistence MSC compared to widely used protease-degradable hydrogels. Furthermore, hydrogel-based delivery of MSC modulates local immune responses and enhances cutaneous wound repair in diabetic mice. This study introduces a simple strategy for engineering tunable degradation modalities into synthetic biomaterials, overcoming a key barrier to their use as cell delivery vehicles.

MMP-responsive nanomaterials

Matrix metalloproteinases (MMP) are enzymes that degrade the extracellular matrix and regulate essential normal cell behaviors. Inhibition of these enzymes has been a strategy for anti-cancer therapy since the 1990s, but with limited success. A new type of MMP-targeting strategy exploits the innate selective hydrolytic activity and consequent catalytic signal amplification of the proteinases, rather than inhibiting it. Using nanomaterials, the enzymatic chemical reaction can trigger the temporal and spatial activation of the anti-cancer effects, amplify the associated response, and cause mechanical damage or report on cancer cells. We analyzed nearly 60 literature studies that incorporate chemical design strategies that lead to spatial, temporal, and mechanical control of the anti-cancer effect through four modes of action: nanomaterial shrinkage, induced aggregation, formation of cytotoxic nanofibers, and activation by de-PEGylation. From the literature analysis, we derived chemical design guidelines to control and enhance MMP activation of nanomaterials of various chemical compositions (peptide, lipid, polymer, inorganic). Finally, the review includes a guide on how multiple characteristics of the nanomaterial, such as substrate modification, supramolecular structure, and electrostatic charge should be collectively considered for the targeted MMP to result in optimal kinetics of enzyme action on the nanomaterial, which allow access to amplification and additional levels of spatial, temporal, and mechanical control of the response. Although this review focuses on the design strategies of MMP-responsive nanomaterials in cancer applications, these guidelines are expected to be generalizable to systems that target MMP for treatment or detection of cancer and other diseases, as well as other enzyme-responsive nanomaterials.

Bioactive Materials for Bone Regeneration: Biomolecules and Delivery Systems

Novel tissue regeneration strategies are constantly being developed worldwide. Research on bone regeneration is noteworthy, as many promising new approaches have been documented with novel strategies currently under investigation. Innovative biomaterials that allow the coordinated and well-controlled repair of bone fractures and bone loss are being designed to reduce the need for autologous or allogeneic bone grafts eventually. The current engineering technologies permit the construction of synthetic, complex, biomimetic biomaterials with properties nearly as good as those of natural bone with good biocompatibility. To ensure that all these requirements meet, bioactive molecules are coupled to structural scaffolding constituents to form a final product with the desired physical, chemical, and biological properties. Bioactive molecules that have been used to promote bone regeneration include protein growth factors, peptides, amino acids, hormones, lipids, and flavonoids. Various strategies have been adapted to investigate the coupling of bioactive molecules with scaffolding materials to sustain activity and allow controlled release. The current manuscript is a thorough survey of the strategies that have been exploited for the delivery of biomolecules for bone regeneration purposes, from choosing the bioactive molecule to selecting the optimal strategy to synthesize the scaffold and assessing the advantages and disadvantages of various delivery strategies.

Development of a bioactive tunable hyaluronic-protein bioconjugate hydrogel for tissue regenerative applications

Every year, there are approximately 500 000 peripheral nerve injury (PNI) procedures due to trauma in the US alone. Autologous and acellular nerve grafts are among current clinical repair options; however, they are limited largely by the high costs associated with donor nerve tissue harvesting and implant processing, respectively. Therefore, there is a clinical need for an off-the-shelf nerve graft that can recapitulate the native microenvironment of the nerve. In our previous work, we created a hydrogel scaffold that incorporates mechanical and biological cues that mimic the peripheral nerve microenvironment using chemically modified hyaluronic acid (HA). However, with our previous work, the degradation profile and cell adhesivity was not ideal for tissue regeneration, in particular, peripheral nerve regeneration. To improve our previous hydrogel, HA was conjugated with fibrinogen using Michael-addition to assist in cell adhesion and hydrogel degradability. The addition of the fibrinogen linker was found to contribute to faster scaffold degradation via active enzymatic breakdown, compared to HA alone. Additionally, cell count and metabolic activity was significantly higher on HA conjugated fibrinogen compared previous hydrogel formulations. This manuscript discusses the various techniques deployed to characterize our new modified HA fibrinogen chemistry physically, mechanically, and biologically. This work addresses the aforementioned concerns by incorporating controllable degradability and increased cell adhesivity while maintaining incorporation of hyaluronic acid, paving the pathway for use in a variety of applications as a multi-purpose tissue engineering platform.

Multifunctional silk vinyl sulfone-based hydrogel scaffolds for dynamic material-cell interactions

Biochemical and mechanical interactions between cells and the surrounding extracellular matrix influence cell behavior and fate. Mimicking these features in vitro has prompted the design and development of biomaterials, with continuing efforts to improve tailorable systems that also incorporate dynamic chemical functionalities. The majority of these chemistries have been incorporated into synthetic biomaterials, here we focus on modifications of silk protein with dynamic features achieved via enzymatic, "click", and photo-chemistries. The one-pot synthesis of vinyl sulfone modified silk (SilkVS) can be tuned to manipulate the degree of functionalization. The resultant modified protein-based material undergoes three different gelation mechanisms, enzymatic, "click", and light-induced, to generate hydrogels for in vitro cell culture. Further, the versatility of this chemical functionality is exploited to mimic cell-ECM interactions via the incorporation of bioactive peptides and proteins or by altering the mechanical properties of the material to guide cell behavior. SilkVS is well-suited for use in in vitro culture, providing a natural protein with both tunable biochemistry and mechanics.

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

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

Tunable enzymatically degradable hydrogels for controlled cargo release with dynamic mechanical properties

Here, we designed enzymatically degradable hydrogels with tunable mesh sizes and crosslinking points to evaluate the effectiveness of network structure estimations in predicting dynamic mechanical properties and cargo retention or release. Poly(ethylene glycol) (PEG) hydrogels were prepared through a thiol-ene click reaction between four- or eight-arm PEG functionalized with vinyl sulfone and cysteine residues of collagenase-degradable peptides to create well-defined, homogenous, and robust materials with a range of mesh sizes estimated from the elasticity theory or Flory-Rehner theory. Time-dependent changes in mechanical properties associated with hydrogel degradation, i.e., dynamics of storage modulus, which is determined by the relationship between the hydrogel mesh and enzyme sizes, were characterized. The shear modulus G' decreased by enzyme addition, and the degradation rate decreased with the initial crosslinking density of the hydrogel. The degradation rate could also be controlled with the reactivity of peptide sequences against collagenase. With these findings, the retention and release of FITC-dextran were successfully controlled by tuning the mesh size and degradability of the hydrogel. This report provides useful insights for designing hydrogels as cell scaffolds or functional molecular delivery matrices with tunable dynamic mechanical properties and the resulting release of loaded drugs or proteins.

Organoid co-culture model of the human endometrium in a fully synthetic extracellular matrix enables the study of epithelial-stromal crosstalk

BACKGROUND

The human endometrium undergoes recurring cycles of growth, differentiation, and breakdown in response to sex hormones. Dysregulation of epithelial-stromal communication during hormone-mediated signaling may be linked to myriad gynecological disorders for which treatments remain inadequate. Here, we describe a completely defined, synthetic extracellular matrix that enables co-culture of human endometrial epithelial and stromal cells in a manner that captures healthy and disease states across a simulated menstrual cycle.

METHODS

We parsed cycle-dependent endometrial integrin expression and matrix composition to define candidate cell-matrix interaction cues for inclusion in a polyethylene glycol (PEG)-based hydrogel crosslinked with matrix metalloproteinase-labile peptides. We semi-empirically screened a parameter space of biophysical and molecular features representative of the endometrium to define compositions suitable for hormone-driven expansion and differentiation of epithelial organoids, stromal cells, and co-cultures of the two cell types.

FINDINGS

Each cell type exhibited characteristic morphological and molecular responses to hormone changes when co-encapsulated in hydrogels tuned to a stiffness regime similar to the native tissue and functionalized with a collagen-derived adhesion peptide (GFOGER) and a fibronectin-derived peptide (PHSRN-K-RGD). Analysis of cell-cell crosstalk during interleukin 1B (IL1B)-induced inflammation revealed dysregulation of epithelial proliferation mediated by stromal cells.

CONCLUSIONS

Altogether, we demonstrate the development of a fully synthetic matrix to sustain the dynamic changes of the endometrial microenvironment and support its applications to understand menstrual health and endometriotic diseases.

FUNDING

This work was supported by The John and Karine Begg Foundation, the Manton Foundation, and NIH U01 (EB029132).

Mimicking Molecular Pathways in the Design of Smart Hydrogels for the Design of Vascularized Engineered Tissues

Biomaterials are pivotal in supporting and guiding vascularization for therapeutic applications. To design effective, bioactive biomaterials, understanding the cellular and molecular processes involved in angiogenesis and vasculogenesis is crucial. Biomaterial platforms can replicate the interactions between cells, the ECM, and the signaling molecules that trigger blood vessel formation. Hydrogels, with their soft and hydrated properties resembling natural tissues, are widely utilized; particularly synthetic hydrogels, known for their bio-inertness and precise control over cell-material interactions, are utilized. Naturally derived and synthetic hydrogel bases are tailored with specific mechanical properties, controlled for biodegradation, and enhanced for cell adhesion, appropriate biochemical signaling, and architectural features that facilitate the assembly and tubulogenesis of vascular cells. This comprehensive review showcases the latest advancements in hydrogel materials and innovative design modifications aimed at effectively guiding and supporting vascularization processes. Furthermore, by leveraging this knowledge, researchers can advance biomaterial design, which will enable precise support and guidance of vascularization processes and ultimately enhance tissue functionality and therapeutic outcomes.

Advances in Gelatin Bioinks to Optimize Bioprinted Cell Functions

Gelatin is a widely utilized bioprinting biomaterial due to its cell-adhesive and enzymatically cleavable properties, which improve cell adhesion and growth. Gelatin is often covalently cross-linked to stabilize bioprinted structures, yet the covalently cross-linked matrix is unable to recapitulate the dynamic microenvironment of the natural extracellular matrix (ECM), thereby limiting the functions of bioprinted cells. To some extent, a double network bioink can provide a more ECM-mimetic, bioprinted niche for cell growth. More recently, gelatin matrices are being designed using reversible cross-linking methods that can emulate the dynamic mechanical properties of the ECM. This review analyzes the progress in developing gelatin bioink formulations for 3D cell culture, and critically analyzes the bioprinting and cross-linking techniques, with a focus on strategies to optimize the functions of bioprinted cells. This review discusses new cross-linking chemistries that recapitulate the viscoelastic, stress-relaxing microenvironment of the ECM, and enable advanced cell functions, yet are less explored in engineering the gelatin bioink. Finally, this work presents the perspective on the areas of future research and argues that the next generation of gelatin bioinks should be designed by considering cell-matrix interactions, and bioprinted constructs should be validated against currently established 3D cell culture standards to achieve improved therapeutic outcomes.

Impact of Inter- and Intra-Donor Variability by Age on the Gel-to-Tissue Transition in MMP-Sensitive PEG Hydrogels for Cartilage Regeneration

Matrix metalloproteinase (MMP)-sensitive hydrogels are promising for cartilage tissue engineering due to cell-mediated control over hydrogel degradation. However, any variability in MMP, tissue inhibitors of matrix metalloproteinase (TIMP), and/or extracellular matrix (ECM) production among donors will impact neotissue formation in the hydrogels. The goal for this study was to investigate the impact of inter- and intra-donor variability on the hydrogel-to-tissue transition. Transforming growth factor β3 was tethered into the hydrogel to maintain the chondrogenic phenotype and support neocartilage production, allowing the use of chemically defined medium. Bovine chondrocytes were isolated from two donor groups, skeletally immature juvenile and skeletally mature adult donors (inter-donor variability) and three donors within each group (intra-donor group variability). While the hydrogel supported neocartilaginous growth by all donors, donor age impacted MMP, TIMP, and ECM synthesis rates. Of the MMPs and TIMPs studied, MMP-1 and TIMP-1 were the most abundantly produced by all donors. Adult chondrocytes secreted higher levels of MMPs, which was accompanied by higher production of TIMPs. Juvenile chondrocytes exhibited more rapid ECM growth. By day 29, juvenile chondrocytes had surpassed the gel-to-tissue transition. On the contrary, the adult donors had a percolated polymer network indicating that despite higher levels of MMPs the gel-to-transition had not yet been achieved. The intra-donor group variability of MMP, TIMP, and ECM production was higher in adult chondrocytes but did not impact the extent of the gel-to-tissue transition. In summary, age-dependent inter-donor variations in MMPs and TIMPs significantly impact the timing of the gel-to-tissue transition in MMP-sensitive hydrogels.

In vitro strategies for mimicking dynamic cell-ECM reciprocity in 3D culture models

The extracellular microenvironment regulates cell decisions through the accurate presentation at the cell surface of a complex array of biochemical and biophysical signals that are mediated by the structure and composition of the extracellular matrix (ECM). On the one hand, the cells actively remodel the ECM, which on the other hand affects cell functions. This cell-ECM dynamic reciprocity is central in regulating and controlling morphogenetic and histogenetic processes. Misregulation within the extracellular space can cause aberrant bidirectional interactions between cells and ECM, resulting in dysfunctional tissues and pathological states. Therefore, tissue engineering approaches, aiming at reproducing organs and tissues in vitro, should realistically recapitulate the native cell-microenvironment crosstalk that is central for the correct functionality of tissue-engineered constructs. In this review, we will describe the most updated bioengineering approaches to recapitulate the native cell microenvironment and reproduce functional tissues and organs in vitro. We have highlighted the limitations of the use of exogenous scaffolds in recapitulating the regulatory/instructive and signal repository role of the native cell microenvironment. By contrast, strategies to reproduce human tissues and organs by inducing cells to synthetize their own ECM acting as a provisional scaffold to control and guide further tissue development and maturation hold the potential to allow the engineering of fully functional histologically competent three-dimensional (3D) tissues.

Systematic d-Amino Acid Substitutions to Control Peptide and Hydrogel Degradation in Cellular Microenvironments

Enzymatically degradable peptides are commonly used as linkers within hydrogels for biological applications; however, controlling the degradation of these engineered peptides with different contexts and cell types can prove challenging. In this work, we systematically examined the substitution of d-amino acids (D-AAs) for different l-amino acids in a peptide sequence commonly utilized in enzymatically degradable hydrogels (VPMS↓MRGG) to create peptide linkers with a range of different degradation times, in solution and in hydrogels, and investigated the cytocompatibility of these materials. We found that increasing the number of D-AA substitutions increased the resistance to enzymatic degradation both for free peptide and peptide-linked hydrogels; yet, this trend also was accompanied by increased cytotoxicity in cell culture. This work demonstrates the utility of D-AA-modified peptide sequences to create tunable biomaterials platforms tempered by considerations of cytotoxicity, where careful selection and optimization of different peptide designs is needed for specific biological applications.

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.

Protease-degradable hydrogels with multifunctional biomimetic peptides for bone tissue engineering

Mimicking bone extracellular matrix (ECM) is paramount to develop novel biomaterials for bone tissue engineering. In this regard, the combination of integrin-binding ligands together with osteogenic peptides represents a powerful approach to recapitulate the healing microenvironment of bone. In the present work, we designed polyethylene glycol (PEG)-based hydrogels functionalized with cell instructive multifunctional biomimetic peptides (either with cyclic RGD-DWIVA or cyclic RGD-cyclic DWIVA) and cross-linked with matrix metalloproteinases (MMPs)-degradable sequences to enable dynamic enzymatic biodegradation and cell spreading and differentiation. The analysis of the intrinsic properties of the hydrogel revealed relevant mechanical properties, porosity, swelling and degradability to engineer hydrogels for bone tissue engineering. Moreover, the engineered hydrogels were able to promote human mesenchymal stem cells (MSCs) spreading and significantly improve their osteogenic differentiation. Thus, these novel hydrogels could be a promising candidate for applications in bone tissue engineering, such as acellular systems to be implanted and regenerate bone or in stem cells therapy.

A 3D Perfusable Platform for In Vitro Culture of Patient Derived Xenografts

Many advanced cancer models, such as patient-derived xenografts (PDXs), offer significant benefits in their preservation of the native tumor's heterogeneity and susceptibility to treatments, but face significant barriers to use in their reliance on a rodent host for propagation and screening. PDXs remain difficult to implement in vitro, particularly in configurations that enable both detailed cellular analysis and high-throughput screening (HTS). Complex multilineage co-cultures with stromal fibroblasts, endothelium, and other cellular and structural components of the tumor microenvironment (TME) further complicate ex vivo implementation. Herein, the culture of multiple prostate cancer (PCa)-derived PDX models as 3D clusters within engineered biomimetic hydrogel matrices, in a HTS-compatible multiwell microfluidic format, alongside bone marrow-derived stromal cells and a perfused endothelial channel. Polymeric hydrogel matrices are customized for each cell type, enabling cell survival in vitro and facile imaging across all conditions. PCa PDXs demonstrate unique morphologies and reliance on TME partners, retention of known phenotype, and expected sensitivity or resistance to standard PCa therapeutics. This novel integration of technologies provides a fully human model, and expands the information to be gathered from each specimen, while avoiding the time and labor involved with animal-based testing.

Multisized Photoannealable Microgels Regulate Cell Spreading, Aggregation, and Macrophage Phenotype through Microporous Void Space

Microgels are an emerging platform for in vitro models and guiding cell fate due to their inherent porosity and tunability. This work describes a light-based technique for rapidly annealing microgels across a range of diameters. Utilizing 8-arm poly(ethylene) glycol-vinyl sulfone, the number of arms available for crosslinking, functionalization, and annealing is stoichiometrically controlled. Small and large microgels are fabricated to explore how microgel diameter impacts void space and the role of porosity on cell spreading, cell aggregation, and macrophage polarization. Mesenchymal stromal cells spread rapidly in both formulations, yet the smaller microgels permit a higher cell density. When seeded with macrophages, the smaller microgels promote an M1 phenotype, while larger microgels promote an M2 phenotype. As another application, the inherent porosity of annealed microgels is leveraged to induce cell aggregation. Finally, the microgels are implanted to examine how different size microgels influence endogenous cell invasion and macrophage polarization. The use of ultraviolet light allows for microgels to be noninvasively injected into a desired mold or wound defect before annealing, and microgels of different properties combined to create a heterogeneous scaffold. This approach is clinically relevant given its tunability and fast annealing time.

Methacrylated Fibrinogen Hydrogels for 3D Cell Culture and Delivery

Methacrylation was performed on fibrinogen to design a new biomedical hydrogel for 3D cell culture or as a biodegradable delivery matrix for in vivo implantation. The methacrylation of denatured fibrinogen in solution was performed using methacrylic anhydride (MAA). The extent of fibrinogen methacrylation was quantified by proton NMR and controlled using stochiometric quantities of MAA during the reaction. The methacrylated fibrinogen (FibMA) hydrogels were formed by light-activated free-radical polymerization in the presence of macromolecular cross-linking polymers made from acrylated poly(ethylene glycol) (PEG). The biocompatibility and biodegradability of the FibMA hydrogels were characterized by in vitro assays and in vivo implantation experiments using quantitative magnetic resonance imaging (MRI) of the implant volume. The FibMA supported the growth and metabolic activity of human dermal fibroblasts in both 2D and 3D cultures. The methacrylation did not alter important biological attributes of the fibrinogen, including the ability to support cell adhesion and 3D cell culture, as well as to undergo proteolysis. Animal experiments confirmed the biodegradability of the FibMA for potential use as a scaffold in tissue engineering, as a bioink for 3D printing, or as a biodegradable matrix for in vivo sustained delivery of bioactive factors. STATEMENT OF SIGNIFICANCE: : This paper describes methacrylated fibrinogen (FibMA) and the formation of a biomedical hydrogel from FibMA for cell culture and other biomedical applications. Inspired from methacrylated gelatin (GelMA), the FibMA is made from blood-derived fibrinogen which is more suitable for clinical use. Sharing similar properties to other hydrogels made from methacrylated proteins, the FibMA has yet to be reported in the literature. In this manuscript, we provide the methodology to produce the FibMA hydrogels, we document the mechanical versatility of this new biomaterial, and we show the biocompatibility using 3D cell culture studies and in vivo implantations.

Simple Technique for Microscopic Evaluation of Active Cellular Invasion into 3D Hydrogel Constructs

Materials that are evaluated for bioengineering purposes are carefully tested to evaluate cellular interactions with respect to biocompatibility and in some cases cell differentiation. A key perspective that is often considered is the ability for decellularized synthetic or natural based matrices to facilitate cell migration or tissue ingrowth. Current methods of measuring cell migration range from simple scratch assays to Boyden chamber inserts and fluorescent imaging of seeded spheroids. Many of these methods require tissue processing for histological analysis and fixing and staining for imaging, which can be difficult and dependent on the stability of the hydrogel subject. Herein we present a simple platform that can be manufactured using 3D printing and easily applied to in vitro cell culture, allowing the researcher to image live cellular migration into a cellular materials. We found this to be an adaptable, cheap, and replicable technique to evaluate cellular interaction that has applications in the research and development of hydrogels for tissue engineering purposes.

Gelation Kinetics and Mechanical Properties of Thiol-Tetrazole Methylsulfone Hydrogels Designed for Cell Encapsulation

Hydrogel precursors that crosslink within minutes are essential for the development of cell encapsulation matrices and their implementation in automated systems. Such timescales allow sufficient mixing of cells and hydrogel precursors under low shear forces and the achievement of homogeneous networks and cell distributions in the 3D cell culture. The previous work showed that the thiol-tetrazole methylsulfone (TzMS) reaction crosslinks star-poly(ethylene glycol) (PEG) hydrogels within minutes at around physiological pH and can be accelerated or slowed down with small pH changes. The resulting hydrogels are cytocompatible and stable in cell culture conditions. Here, the gelation kinetics and mechanical properties of PEG-based hydrogels formed by thiol-TzMS crosslinking as a function of buffer, crosslinker structure and degree of TzMS functionality are reported. Crosslinkers of different architecture, length and chemical nature (PEG versus peptide) are tested, and degree of TzMS functionality is modified by inclusion of RGD cell-adhesive ligand, all at concentration ranges typically used in cell culture. These studies corroborate that thiol/PEG-4TzMS hydrogels show gelation times and stiffnesses that are suitable for 3D cell encapsulation and tunable through changes in hydrogel composition. The results of this study guide formulation of encapsulating hydrogels for manual and automated 3D cell culture.

Hydrogel-based microenvironment engineering of haematopoietic stem cells

Haematopoietic Stem cells (HSCs) have the potential for self-renewal and multilineage differentiation, and their behaviours are finely tuned by the microenvironment. HSC transplantation (HSCT) is widely used in the treatment of haematologic malignancies while limited by the quantity of available HSCs. With the development of tissue engineering, hydrogels have been deployed to mimic the HSC microenvironment in vitro. Engineered hydrogels influence HSC behaviour by regulating mechanical strength, extracellular matrix microstructure, cellular ligands and cytokines, cell-cell interaction, and oxygen concentration, which ultimately facilitate the acquisition of sufficient HSCs. Here, we review recent advances in the application of hydrogel-based microenvironment engineering of HSCs, and provide future perspectives on challenges in basic research and clinical practice.

Mimicking the neural stem cell niche: An engineer’s view of cell: material interactions

Neural stem cells have attracted attention in recent years to treat neurodegeneration. There are two neurogenic regions in the brain where neural stem cells reside, one of which is called the subventricular zone (SVZ). The SVZ niche is a complicated microenvironment providing cues to regulate self-renewal and differentiation while maintaining the neural stem cell’s pool. Many scientists have spent years understanding the cellular and structural characteristics of the SVZ niche, both in homeostasis and pathological conditions. On the other hand, engineers focus primarily on designing platforms using the knowledge they acquire to understand the effect of individual factors on neural stem cell fate decisions. This review provides a general overview of what we know about the components of the SVZ niche, including the residing cells, extracellular matrix (ECM), growth factors, their interactions, and SVZ niche changes during aging and neurodegenerative diseases. Furthermore, an overview will be given on the biomaterials used to mimic neurogenic niche microenvironments and the design considerations applied to add bioactivity while meeting the structural requirements. Finally, it will discuss the potential gaps in mimicking the microenvironment.

Matrices Activated with Messenger RNA

Over two decades of preclinical and clinical experience have confirmed that gene therapy-activated matrices are potent tools for sustained gene modulation at the implantation area. Matrices activated with messenger RNA (mRNA) are the latest development in the area, and they promise an ideal combination of efficiency and safety. Indeed, implanted mRNA-activated matrices allow a sustained delivery of mRNA and the continuous production of therapeutic proteins in situ. In addition, they are particularly interesting to generate proteins acting on intracellular targets, as the translated protein can directly exert its therapeutic function. Still, mRNA-activated matrices are incipient technologies with a limited number of published records, and much is still to be understood before their successful implementation. Indeed, the design parameters of mRNA-activated matrices are crucial for their performance, as they affect mRNA stability, device immunogenicity, translation efficiency, and the duration of the therapy. Critical design factors include matrix composition and its mesh size, mRNA chemical modification and sequence, and the characteristics of the nanocarriers used for mRNA delivery. This review aims to provide some background relevant to these technologies and to summarize both the design space for mRNA-activated matrices and the current knowledge regarding their pharmaceutical performance. Furthermore, we will discuss potential applications of mRNA-activated matrices, mainly focusing on tissue engineering and immunomodulation.

Osteoregeneration of Critical-Size Defects Using Hydroxyapatite-Chitosan and Silver-Chitosan Nanocomposites

Bone is a natural nanocomposite composed of proteins and minerals that can regenerate itself. However, there are conditions in which this process is impaired, such as extensive bone defects and infections of the bone or surrounding tissue. This study evaluates the osteoregenerative capacity of bone grafting materials in animals with induced bone defects. Colloidal chitosan dispersion nanocomposites, nanohydroxyapatite−chitosan (NHAP-Q) and nanosilver−chitosan (AgNP-Q), were synthesized and characterized. Non-critical-size defects in Wistar rats were used to evaluate the material’s biocompatibility, and critical-size defects in the calvarias of guinea pigs were used to evaluate the regenerative capacity of the bones. Moreover, the toxicity of the nanocomposites was evaluated in the heart, liver, spleen, kidneys, and skin. Histological, radiographic, and electron microscopy tests were also performed. The results showed that neither material produced pathological changes. Radiographic examination showed a significant reduction in defects (75.1% for NHAP-Q and 79.3% for AgNP-Q), angiogenesis, and trabecular formation. A toxicological assessment of all the organs did not show changes in the ultrastructure of tissues, and the distribution of silver was different for different organs (spleen > skin > heart > kidney > liver). The results suggest that both materials are highly biocompatible, and AgNP-Q achieved similar bone regeneration to that reported with autologous bone. The main research outcome of the present study was the combination of two types of NPs to enhance antimicrobial and osteoregeneration activities. These colloidal chitosan dispersions show promise as future biomaterials in the medical field for applications in fast-healing fractures, including broken bones in the oral cavity and hip replacement infections.

Encapsulation of Gold-Based Anticancer Agents in Protease-Degradable Peptide Nanofilaments Enhances Their Potency

We investigated the use of amphiphilic, protease-cleavable peptides as encapsulation moieties for hydrophobic metallodrugs, in order to enhance their bioavailability and consequent activity. Two hydrophobic, gold-containing anticancer agents varying in aromatic ligand distribution (Au(I)-N-heterocyclic carbene compounds 1 and 2) were investigated. These were encapsulated into amphiphilic decapeptides that form soluble filamentous structures with hydrophobic cores, varying supramolecular packing arrangements and surface charge. Peptide sequence strongly dictates the supramolecular packing within the aromatic core, which in turn dictates drug loading. Anionic peptide filaments can effectively load 1, and to a lesser extent 2, while their cationic counterparts could not, collectively demonstrating that loading efficiency is dictated by both aromatic and electrostatic (mis)matching between drug and peptide. Peptide nanofilaments were nontoxic to cancerous and noncancerous cells. By contrast, those loaded with 1 and 2 displayed enhanced cytotoxicity in comparison to 1 and 2 alone, when exposed to Caki-1 and MDA-MB-231 cancerous cell lines, while no cytotoxicity was observed in noncancerous lung fibroblasts, IMR-90. We propose that the enhanced in vitro activity results from the enhanced proteolytic activity in the vicinity of the cancer cells, thereby breaking the filaments into drug-bound peptide fragments that are taken up by these cells, resulting in enhanced cytotoxicity toward cancer cells.

Squid Suckerin-Spider Silk Fusion Protein Hydrogel for Delivery of Mesenchymal Stem Cell Secretome to Chronic Wounds

Chronic wounds are non-healing wounds characterized by a prolonged inflammation phase. Excessive inflammation leads to elevated protease levels and consequently to a decrease in growth factors at wound sites. Stem cell secretome therapy has been identified as a treatment strategy to modulate the microenvironment of chronic wounds via supplementation with anti-inflammatory/growth factors. However, there is a need to develop better secretome delivery systems that are able to encapsulate the secretome without denaturation, in a sustained manner, and that are fully biocompatible. To address this gap, a recombinant squid suckerin-spider silk fusion protein is developed with cell-adhesion motifs capable of thermal gelation at physiological temperatures to form hydrogels for encapsulation and subsequent release of the stem cell secretome. Freeze-thaw treatment of the protein hydrogel results in a modified porous cryogel that maintains slow degradation and sustained secretome release. Chronic wounds of diabetic mice treated with the secretome-laden cryogel display increased wound closure, presence of endothelial cells, granulation wound tissue thickness, and reduced inflammation with no fibrotic scar formation. Overall, these in vivo indicators of wound healing demonstrate that the fusion protein hydrogel displays remarkable potential as a delivery system for secretome-assisted chronic wound healing.

3D-bioprinted, phototunable hydrogel models for studying adventitial fibroblast activation in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease of the lung vasculature, characterized by elevated pulmonary blood pressure, remodeling of the pulmonary arteries, and ultimately right ventricular failure. Therapeutic interventions for PAH are limited in part by the lack ofin vitroscreening platforms that accurately reproduce dynamic arterial wall mechanical properties. Here we present a 3D-bioprinted model of the pulmonary arterial adventitia comprised of a phototunable poly(ethylene glycol) alpha methacrylate (PEG-αMA)-based hydrogel and primary human pulmonary artery adventitia fibroblasts (HPAAFs). This unique biomaterial emulates PAH pathogenesisin vitrothrough a two-step polymerization reaction. First, PEG-αMA macromer was crosslinked off-stoichiometry by 3D bioprinting an acidic bioink solution into a basic gelatin support bath initiating a base-catalyzed thiol-ene reaction with synthetic and biodegradable crosslinkers. Then, matrix stiffening was induced by photoinitiated homopolymerization of unreacted αMA end groups. A design of experiments approach produced a hydrogel platform that exhibited an initial elastic modulus (E) within the range of healthy pulmonary arterial tissue (E= 4.7 ± 0.09 kPa) that was stiffened to the pathologic range of hypertensive tissue (E= 12.8 ± 0.47 kPa) and supported cellular proliferation over time. A higher percentage of HPAAFs cultured in stiffened hydrogels expressed the fibrotic marker alpha-smooth muscle actin than cells in soft hydrogels (88 ± 2% versus 65 ± 4%). Likewise, a greater percentage of HPAAFs were positive for the proliferation marker 5-ethynyl-2'-deoxyuridine (EdU) in stiffened models (66 ± 6%) compared to soft (39 ± 6%). These results demonstrate that 3D-bioprinted, phototunable models of pulmonary artery adventitia are a tool that enable investigation of fibrotic pathogenesisin vitro.

Engineered hydrogels for mechanobiology

Cells' local mechanical environment can be as important in guiding cellular responses as many well-characterized biochemical cues. Hydrogels that mimic the native extracellular matrix can provide these mechanical cues to encapsulated cells, allowing for the study of their impact on cellular behaviours. Moreover, by harnessing cellular responses to mechanical cues, hydrogels can be used to create tissues in vitro for regenerative medicine applications and for disease modelling. This Primer outlines the importance and challenges of creating hydrogels that mimic the mechanical and biological properties of the native extracellular matrix. The design of hydrogels for mechanobiology studies is discussed, including appropriate choice of cross-linking chemistry and strategies to tailor hydrogel mechanical cues. Techniques for characterizing hydrogels are explained, highlighting methods used to analyze cell behaviour. Example applications for studying fundamental mechanobiological processes and regenerative therapies are provided, along with a discussion of the limitations of hydrogels as mimetics of the native extracellular matrix. The article ends with an outlook for the field, focusing on emerging technologies that will enable new insights into mechanobiology and its role in tissue homeostasis and disease.

Loading and Release of Quercetin from Contact-Drawn Polyvinyl Alcohol Fiber Scaffolds

Polymeric drug releasing systems have numerous applications for the treatment of chronic diseases and traumatic injuries. In this study, a simple, cost-effective, and scalable method for dry spinning of crosslinked polyvinyl alcohol (PVA) fibers is presented. This method utilizes an entangled solution of PVA to form liquid bridges that are drawn into rapidly drying fibers through extensional flow. The fibers are crosslinked by a one-pot reaction in which glyoxal is introduced to the PVA solution prior to contact drawing. Failure analysis of fiber formation is used to understand the interplay of polymer concentration, glyoxal concentration, and crosslinking time to identify appropriate formulations for the production of glyoxal-crosslinked PVA fibers. The small molecule quercetin (an anti-inflammatory plant flavonoid) can be added to the one-pot reaction and is shown to be incorporated into the fibers in a concentration-dependent manner. Upon rehydration in an aqueous medium, the glyoxal-crosslinked PVA fiber scaffolds retain their morphology and slowly degrade, as measured over the course of 10 days. As the scaffolds degrade, they release the loaded quercetin, reaching a cumulative release of 56 ± 6% of the loaded drug after 10 days. The bioactivity of the released quercetin is verified by combining quercetin-loaded fibers with contact-drawn polyethylene oxide-type I collagen (PEO-Col) fibers and monitoring the growth of PC12 cells on the fibers. PC12 cells readily attach to the PEO-Col fibers and display increased nerve growth factor-induced elongation and neurite formation in the presence of quercetin-loaded PVA fibers relative to substrates formed from only PEO-Col fibers or PEO-Col and PVA fibers without quercetin.

Advances in Tumor Organoids for the Evaluation of Drugs: A Bibliographic Review

Tumor organoids are defined as self-organized three-dimensional assemblies of heterogeneous cell types derived from patient samples that mimic the key histopathological, genetic, and phenotypic characteristics of the original tumor. This technology is proposed as an ideal candidate for the evaluation of possible therapies against cancer, presenting advantages over other models which are currently used. However, there are no reports in the literature that relate the techniques and material development of tumor organoids or that emphasize in the physicochemical and biological properties of materials that intent to biomimicry the tumor extracellular matrix. There is also little information regarding the tools to identify the correspondence of native tumors and tumoral organoids (tumoroids). Moreover, this paper relates the advantages of organoids compared to other models for drug evaluation. A growing interest in tumoral organoids has arisen from 2009 to the present, aimed at standardizing the process of obtaining organoids, which more accurately resemble patient-derived tumor tissue. Likewise, it was found that the characteristics to consider for the development of organoids, and therapeutic responses of them, are cell morphology, physiology, the interaction between cells, the composition of the cellular matrix, and the genetic, phenotypic, and epigenetic characteristics. Currently, organoids have been used for the evaluation of drugs for brain, lung, and colon tumors, among others. In the future, tumor organoids will become closer to being considered a better model for studying cancer in clinical practice, as they can accurately mimic the characteristics of tumors, in turn ensuring that the therapeutic response aligns with the clinical response of patients.

Tunable PEG Hydrogels for Discerning Differential Tumor Cell Response to Biomechanical Cues

Increased extracellular matrix (ECM) density in the tumor microenvironment has been shown to influence aspects of tumor progression such as proliferation and invasion. Increased matrix density means cells experience not only increased mechanical properties, but also a higher density of bioactive sites. Traditional in vitro ECM models like Matrigel and collagen do not allow these properties to be investigated independently. In this work, a poly(ethylene glycol)-based scaffold is used which modifies with integrin-binding sites for cell attachment and matrix metalloproteinase 2 and 9 sensitive sites for enzyme-mediated degradation. The polymer backbone density and binding site concentration are independently tuned and the effect each of these properties and their interaction have on the proliferation, invasion, and focal complex formation of two different tumor cell lines is evaluated. It is seen that the cell line of epithelial origin (Hs 578T, triple negative breast cancer) proliferates more, invades less, and forms more mature focal complexes in response to an increase in matrix adhesion sites. Conversely, the cell line of mesenchymal origin (HT1080, fibrosarcoma) proliferates more in 2D culture but less in 3D culture, invades less, and forms more mature focal complexes in response to an increase in matrix stiffness.

Sterilization of Polymeric Implants: Challenges and Opportunities

Degradable and environmentally responsive polymers have been actively developed for drug delivery and regenerative medicine applications, yet inadequate consideration of their compatibility with terminal sterilization presents notable barriers to clinical translation. This Review discusses industry-established terminal sterilization methods and aseptic processing and contrasts them with innovative approaches aimed at preserving the integrity of polymeric implants. Regulatory guidelines, fiscal considerations, and potential pitfalls are discussed to encourage early integration of sterility regulatory considerations in material designs.

Fluorescent Peptomer Substrates for Differential Degradation by Metalloproteases

Proteases, especially MMPs, are attractive biomarkers given their central role in both physiological and pathological processes. Distinguishing MMP activity with degradable substrates, however, is a difficult task due to overlapping substrate specificity profiles. Here, we developed a system of peptomers (peptide-peptoid hybrids) to probe the impact of non-natural residues on MMP specificity for an MMP peptide consensus sequence. Peptoids are non-natural, N-substituted glycines with a large side-chain diversity. Given the presence of a hallmark proline residue in the P3 position of MMP consensus sequences, we hypothesized that peptoids may offer N-substituted alternatives to generate differential interactions with MMPs. To investigate this hypothesis, peptomer substrates were exposed to five different MMPs, as well as bacterial collagenase, and monitored by fluorescence resonance energy transfer and liquid chromatography-mass spectrometry to determine the rate of cleavage and the composition of degraded fragments, respectively. We found that peptoid residues are well tolerated in the P3 and P3' substrate sites and that the identity of the peptoid in these sites displays a moderate influence on the rate of cleavage. However, peptoid residues were even better tolerated in the P1 substrate site where activity was more strongly correlated with side-chain identity than side-chain position. All MMPs explored demonstrated similar trends in specificity for the peptomers but exhibited different degrees of variability in proteolytic rate. These kinetic profiles served as "fingerprints" for the proteases and yielded separation by multivariate data analysis. To further demonstrate the practical application of this tunability in degradation kinetics, peptomer substrates were tethered into hydrogels and released over distinct timescales. Overall, this work represents a significant step toward the design of probes that maximize differential MMP behavior and presents design rules to tune degradation kinetics with peptoid substitutions, which has promising implications for diagnostic and prognostic applications using array-based sensors.

Silylated biomolecules: Versatile components for bioinks

Physical hydrogels prepared from natural biopolymers are the most popular components for bioinks. However, to improve the mechanical properties of the network, in particular its durability for long-lasting tissue engineering applications or its stiffness for bone/cartilage applications, covalent chemical hydrogels have to be considered. For that purpose, biorthogonal reactions are required to allow the inclusion of living cells within the bioink reservoir before the 3D printing procedure. Interestingly, such reactions also unlock the possibility to further multifunctionalize the network, adding bioactive moieties to tune the biological properties of the resulting printed biomaterial. Surprisingly, compared to the huge number of studies disclosing novel bioink compositions, no extensive efforts have been made by the scientific community to develop new chemical reactions meeting the requirements of both cell encapsulation, chemical orthogonality and versatile enough to be applied to a wide range of molecular components, including fragile biomolecules. That could be explained by the domination of acrylate photocrosslinking in the bioprinting field. On the other hand, proceeding chemoselectively and allowing the polymerization of any type of silylated molecules, the sol-gel inorganic polymerization was used as a crosslinking reaction to prepare hydrogels. Recent development of this strategy includes the optimization of biocompatible catalytic conditions and the silylation of highly attractive biomolecules such as amino acids, bioactive peptides, proteins and oligosaccharides. When one combines the simplicity and the versatility of the process, with the ease of functionalization of any type of relevant silylated molecules that can be combined in an infinite manner, it was obvious that a family of bioinks could emerge quickly. This review presents the sol-gel process in biocompatible conditions and the various classes of relevant silylated molecules that can be used as bioink components. The preparation of hydrogels and the kinetic considerations of the sol-gel chemistry which at least allowed cell encapsulation and extrusion-based bioprinting are discussed.

Hydrogel: A Potential Material for Bone Tissue Engineering Repairing the Segmental Mandibular Defect

Free flap surgery is currently the only successful method used by surgeons to reconstruct critical-sized defects of the jaw, and is commonly used in patients who have had bony lesions excised due to oral cancer, trauma, infection or necrosis. However, donor site morbidity remains a significant flaw of this strategy. Various biomaterials have been under investigation in search of a suitable alternative for segmental mandibular defect reconstruction. Hydrogels are group of biomaterials that have shown their potential in various tissue engineering applications, including bone regeneration, both through in vitro and in vivo pre-clinical animal trials. This review discusses different types of hydrogels, their fabrication techniques, 3D printing, their potential for bone regeneration, outcomes, and the limitations of various hydrogels in preclinical models for bone tissue engineering. This review also proposes a modified technique utilizing the potential of hydrogels combined with scaffolds and cells for efficient reconstruction of mandibular segmental defects.

A high-throughput 3D bioprinted cancer cell migration and invasion model with versatile and broad biological applicability

Understanding the underlying mechanisms of migration and metastasis is a key focus of cancer research. There is an urgent need to develop in vitro 3D tumor models that can mimic physiological cell-cell and cell-extracellular matrix interactions, with high reproducibility and that are suitable for high throughput (HTP) drug screening. Here, we developed a HTP 3D bioprinted migration model using a bespoke drop-on-demand bioprinting platform. This HTP platform coupled with tunable hydrogel systems enables (i) the rapid encapsulation of cancer cells within in vivo tumor mimicking matrices, (ii) in situ and real-time measurement of cell movement, (iii) detailed molecular analysis for the study of mechanisms underlying cell migration and invasion, and (iv) the identification of novel therapeutic options. This work demonstrates that this HTP 3D bioprinted cell migration platform has broad applications across quantitative cell and cancer biology as well as drug screening.

Static and photoresponsive dynamic materials to dissect physical regulation of cellular functions

Recent progress in mechanobiology has highlighted the importance of physical cues, such as mechanics, geometry (size), topography, and porosity, in the determination of cellular activities and fates, in addition to biochemical factors derived from their surroundings. In this review, we will first provide an overview of how such fundamental insights are identified by synchronizing the hierarchical nature of biological systems and static materials with tunable physical cues. Thereafter, we will explain the photoresponsive dynamic biomaterials to dissect the spatiotemporal aspects of the dependence of biological functions on physical cues.

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.

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.

Biomimetic Hydrogels in the Study of Cancer Mechanobiology: Overview, Biomedical Applications, and Future Perspectives

Hydrogels are biocompatible polymers that are tunable to the system under study, allowing them to be widely used in medicine, bioprinting, tissue engineering, and biomechanics. Hydrogels are used to mimic the three-dimensional microenvironment of tissues, which is essential to understanding cell–cell interactions and intracellular signaling pathways (e.g., proliferation, apoptosis, growth, and survival). Emerging evidence suggests that the malignant properties of cancer cells depend on mechanical cues that arise from changes in their microenvironment. These mechanobiological cues include stiffness, shear stress, and pressure, and have an impact on cancer proliferation and invasion. The hydrogels can be tuned to simulate these mechanobiological tissue properties. Although interest in and research on the biomedical applications of hydrogels has increased in the past 25 years, there is still much to learn about the development of biomimetic hydrogels and their potential applications in biomedical and clinical settings. This review highlights the application of hydrogels in developing pre-clinical cancer models and their potential for translation to human disease with a focus on reviewing the utility of such models in studying glioblastoma progression.

Simple Preparation of Injectable Hydrogels with Phase-Separated Structures That Can Encapsulate Live Cells

Injectable hydrogels are biomaterials that can be administered minimally invasively in liquid form and are considered promising artificial extracellular matrix (ECM) materials. However, ordinary injectable hydrogels are synthesized from water-soluble molecules to ensure injectability, resulting in non-phase-separated structures, making them structurally different from natural ECMs with phase-separated insoluble structural proteins, such as collagen and elastin. Here, we propose a simple material design approach to impart phase-separated structures to injectable hydrogels by adding inorganic salts. Injecting a gelling solution of mutually cross-linkable tetra-arm poly(ethylene glycol)s with potassium sulfate at optimal concentrations results in the formation of a hydrogel with phase-separated structures in situ. These phase-separated structures provide up to an 8-fold increase in fracture toughness while allowing the encapsulation of live mouse chondrogenic cells without compromising their proliferative activity. Our findings highlight that the concentration of inorganic salts is an important design parameter in injectable hydrogels for artificial ECMs.

Matrix Metalloproteases from Adipose Tissue-Derived Stromal Cells Are Spatiotemporally Regulated by Hydrogel Mechanics in a 3D Microenvironment

Adipose tissue-derived stromal cells (ASCs) are of interest in tissue engineering and regenerative medicine (TERM) due to their easy acquisition, multipotency, and secretion of a host of factors that promote regeneration. Retention of ASCs in or around lesions is poor following direct administration. Therefore, for TERM applications, ASCs can be ‘immobilized’ via their incorporation into hydrogels such as gelatine methacryloyl (GelMA). Tweaking GelMA concentration is a common approach to approximate the mechanical properties found in organs or tissues that need repair. Distinct hydrogel mechanics influence the ability of a cell to spread, migrate, proliferate, and secrete trophic factors. Mesenchymal cells such as ASCs are potent remodellers of the extracellular matrix (ECM). Not only do ASCs deposit components, they also secrete matrix metalloproteases (MMPs) which degrade ECM. In this work, we investigated if GelMA polymer concentration influenced the expression of active MMPs by ASCs. In addition, MMPs’ presence was interrogated with regard to ASCs morphology and changes in hydrogel ultrastructure. For this, immortalised ASCs were embedded in 5%, 10%, and 15% (w/v) GelMA hydrogels, photopolymerised and cultured for 14 d. Zymography in situ indicated that MMPs had a variable, hydrogel concentration-dependent influence on ASCs-secreted MMPs. In 5% GelMA, ASCs showed a high and sustained expression of MMPs, while, in 10% and 15% GelMA, such expression was almost null. ASCs morphology based on F-actin staining showed that increasing GelMA concentrations inhibit their spreading. Scanning electron microscopy (SEM) showed that hydrogel ultrastructure in terms of pore density, pore size, and percentage porosity were not consistently influenced by cells. Interestingly, changes in ultrastructural parameters were detected also in cell-free materials, albeit without a clear trend. We conclude that hydrogel concentration and its underlying mechanics influenced MMP expression by ASCs. The exact MMPs that respond to these mechanical cues should be defined in follow-up experiments.

Gelatin Methacryloyl Hydrogels for Musculoskeletal Tissue Regeneration

Musculoskeletal disorders are a significant burden on the global economy and public health. Hydrogels have significant potential for enhancing the repair of damaged and injured musculoskeletal tissues as cell or drug delivery systems. Hydrogels have unique physicochemical properties which make them promising platforms for controlling cell functions. Gelatin methacryloyl (GelMA) hydrogel in particular has been extensively investigated as a promising biomaterial due to its tuneable and beneficial properties and has been widely used in different biomedical applications. In this review, a detailed overview of GelMA synthesis, hydrogel design and applications in regenerative medicine is provided. After summarising recent progress in hydrogels more broadly, we highlight recent advances of GelMA hydrogels in the emerging fields of musculoskeletal drug delivery, involving therapeutic drugs (e.g., growth factors, antimicrobial molecules, immunomodulatory drugs and cells), delivery approaches (e.g., single-, dual-release system), and material design (e.g., addition of organic or inorganic materials, 3D printing). The review concludes with future perspectives and associated challenges for developing local drug delivery for musculoskeletal applications.

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.