A practical guide to hydrogels for cell culture

Design and validation of functionalized redox-responsive hydrogel beads for high-throughput screening of antibody-secreting mammalian cells

Antibody drugs play a vital role in diagnostics and therapy. However, producing antibodies from mammalian cells is challenging owing to cellular heterogeneity, which can be addressed by applying droplet-based microfluidic platforms for high-throughput screening (HTS). Here, we designed an integrated system based on disulfide-bonded redox-responsive hydrogel beads (redox-HBs), which were prepared through enzymatic hydrogelation, to compartmentalize, screen, select, retrieve, and recover selected Chinese hamster ovary (CHO) cells secreting high levels of antibodies. Moreover, redox-HBs were functionalized with protein G as an antibody-binding module to capture antibodies secreted from encapsulated cells. As proof-of-concept, cells co-producing immunoglobulin G (IgG) as the antibody and green fluorescent protein (GFP) as the reporter molecule, denoted as CHO(IgG/GFP), were encapsulated into functionalized redox-HBs. Additionally, antibody-secreting cells were labeled with protein L-conjugated horseradish peroxidase using a tyramide amplification system, enabling fluorescence staining of the antibody captured inside the beads. Redox-HBs were then applied to fluorescence-activated droplet sorting, and selected redox-HBs were degraded by reducing the disulfide bonds to recover the target cells. The results indicated the potential of the developed HTS platform for selecting a single cell viable for biopharmaceutical production.

Reconstitution of human tissue barrier function for precision and personalized medicine

Tissue barriers in a body, well known as tissue-to-tissue interfaces represented by endothelium of the blood vessels or epithelium of organs, are essential for maintaining physiological homeostasis by regulating molecular and cellular transports. It is crucial for predicting drug response to understand physiology of tissue barriers through which drugs are absorbed, distributed, metabolized and excreted. Since the FDA Modernization Act 2.0, which prompts the inception of alternative technologies for animal models, tissue barrier chips, one of the applications of organ-on-a-chip or microphysiological system (MPS), have only recently been utilized in the context of drug development. Recent advancements in stem cell technology have brightened the prospects for the application of tissue barrier chips in personalized medicine. In past decade, designing and engineering these microfluidic devices, and demonstrating the ability to reconstitute tissue functions were main focus of this field. However, the field is now advancing to the next level of challenges: validating their utility in drug evaluation and creating personalized models using patient-derived cells. In this review, we briefly introduce key design parameters to develop functional tissue barrier chip, explore the remarkable recent progress in the field of tissue barrier chips and discuss future perspectives on realizing personalized medicine through the utilization of tissue barrier chips.

Using Chemistry To Recreate the Complexity of the Extracellular Matrix: Guidelines for Supramolecular Hydrogel-Cell Interactions

Hydrogels have emerged as a promising class of extracellular matrix (ECM)-mimicking materials in regenerative medicine. Here, we briefly describe current state-of-the-art of ECM-mimicking hydrogels, ranging from natural to hybrid to completely synthetic versions, giving the prelude to the importance of supramolecular interactions to make true ECM mimics. The potential of supramolecular interactions to create ECM mimics for cell culture is illustrated through a focus on two different supramolecular hydrogel systems, both developed in our laboratories. We use some recent, significant findings to present important design principles underlying the cell-material interaction. To achieve cell spreading, we propose that slow molecular dynamics (monomer exchange within fibers) is crucial to ensure the robust incorporation of cell adhesion ligands within supramolecular fibers. Slow bulk dynamics (stress-relaxation─fiber rearrangements, τ1/2 ≈ 1000 s) is required to achieve cell spreading in soft gels (<1 kPa), while gel stiffness overrules dynamics in stiffer gels. Importantly, this resonates with the findings of others which specialize in different material types: cell spreading is impaired in case substrate relaxation occurs faster than clutch binding and focal adhesion lifetime. We conclude with discussing considerations and limitations of the supramolecular approach as well as provide a forward thinking perspective to further understand supramolecular hydrogel-cell interactions. Future work may utilize the presented guidelines underlying cell-material interactions to not only arrive at the next generation of ECM-mimicking hydrogels but also advance other fields, such as bioelectronics, opening up new opportunities for innovative applications.

The importance of 3D fibre architecture in cancer and implications for biomaterial model design

The need for improved prediction of clinical response is driving the development of cancer models with enhanced physiological relevance. A new concept of 'precision biomaterials' is emerging, encompassing patient-mimetic biomaterial models that seek to accurately detect, treat and model cancer by faithfully recapitulating key microenvironmental characteristics. Despite recent advances allowing tissue-mimetic stiffness and molecular composition to be replicated in vitro, approaches for reproducing the 3D fibre architectures found in tumour extracellular matrix (ECM) remain relatively unexplored. Although the precise influences of patient-specific fibre architecture are unclear, we summarize the known roles of tumour fibre architecture, underlining their implications in cell-matrix interactions and ultimately clinical outcome. We then explore the challenges in reproducing tissue-specific 3D fibre architecture(s) in vitro, highlighting relevant biomaterial fabrication techniques and their benefits and limitations. Finally, we discuss imaging and image analysis techniques (focussing on collagen I-optimized approaches) that could hold the key to mapping tumour-specific ECM into high-fidelity biomaterial models. We anticipate that an interdisciplinary approach, combining materials science, cancer research and image analysis, will elucidate the role of 3D fibre architecture in tumour development, leading to the next generation of patient-mimetic models for mechanistic studies and drug discovery.

Alginate-Chitosan Biodegradable and Biocompatible Based Hydrogel for Breast Cancer Immunotherapy and Diagnosis: A Comprehensive Review

Breast cancer is the most common type of cancer and the second leading cause of cancer-related mortality in females. There are many side effects due to chemotherapy and traditional surgery, like fatigue, loss of appetite, skin irritation, and drug resistance to cancer cells. Immunotherapy has become a hopeful approach toward cancer treatment, generating long-lasting immune responses in malignant tumor patients. Recently, hydrogel has received more attention toward cancer therapy due to its specific characteristics, such as decreased toxicity, fewer side effects, and better biocompatibility drug delivery to the particular tumor location. Researchers globally reported various investigations on hydrogel research for tumor diagnosis. The hydrogel-based multilayer platform with controlled nanostructure has received more attention for its antitumor effect. Chitosan and alginate play a leading role in the formation of the cross-link in a hydrogel. Also, they help in the stability of the hydrogel. This review discusses the properties, preparation, biocompatibility, and bioavailability of various research and clinical approaches of the multipolymer hydrogel made of alginate and chitosan for breast cancer treatment. With a focus on cases of breast cancer and the recovery rate, there is a need to find out the role of hydrogel in drug delivery for breast cancer treatment.

3D bioprinting of dense cellular structures within hydrogels with spatially controlled heterogeneity

Embedded bioprinting is an emerging technology for precise deposition of cell-laden or cell-only bioinks to construct tissue like structures. Bioink is extruded or transferred into a yield stress hydrogel or a microgel support bath allowing print needle motion during printing and providing temporal support for the printed construct. Although this technology has enabled creation of complex tissue structures, it remains a challenge to develop a support bath with user-defined extracellular mimetic cues and their spatial and temporal control. This is crucial to mimic the dynamic nature of the native tissue to better regenerate tissues and organs. To address this, we present a bioprinting approach involving printing of a photocurable viscous support layer and bioprinting of a cell-only or cell-laden bioink within this viscous layer followed by brief exposure to light to partially crosslink the support layer. This approach does not require shear thinning behavior and is suitable for a wide range of photocurable hydrogels to be used as a support. It enables multi-material printing to spatially control support hydrogel heterogeneity including temporal delivery of bioactive cues (e.g. growth factors), and precise patterning of dense multi-cellular structures within these hydrogel supports. Here, dense stem cell aggregates are printed within methacrylated hyaluronic acid-based hydrogels with patterned heterogeneity to spatially modulate human mesenchymal stem cell osteogenesis. This study has significant impactions on creating tissue interfaces (e.g. osteochondral tissue) in which spatial control of extracellular matrix properties for patterned stem cell differentiation is crucial.

Designing Pathway-Controlled Multicomponent Ultrashort Peptide Hydrogels with Diverse Functionalities at the Nanoscale for Directing Cellular Behavior

Tuning self-assembling pathways by implementing different external stimuli has been extensively studied, owing to their effective control over structural and mechanical properties. Consequently, multicomponent peptide hydrogels with high structural tunability and stimuli responsiveness are crucial in dictating cellular behavior. Herein, we have implemented both coassembly approach and pathway-dependent self-assembly to design nonequilibrium nanostructures to understand the thermodynamic and kinetic aspects of peptide self-assembly toward controlling cellular response. Our system involved an ultrashort peptide gelator and a hydrophilic surfactant which coassembled through different pathways, i.e., heat-cool and sonication methods with variable energy input. Interestingly, it was possible to access diverse structural and mechanical properties at the nanoscale in a single coassembled system. Further, the hydrophilic surfactant provided additional surface functionalities, thus creating an efficient hydrophilic matrix for cellular interaction. Such diverse functionalities in a single coassembled system could lead to the development of advanced scaffolds, with applications in various biomedical fields.

Wired for Success: Probing the Effect of Tissue-Engineered Neural Interface Substrates on Cell Viability

This study investigates the electrochemical behavior of GelMA-based hydrogels and their interactions with PC12 neural cells under electrical stimulation in the presence of conducting substrates. Focusing on indium tin oxide (ITO), platinum, and gold mylar substrates supporting conductive scaffolds composed of hydrogel, graphene oxide, and gold nanorods, we explored how the substrate materials affect scaffold conductivity and cell viability. We examined the impact of an optimized electrical stimulation protocol on the PC12 cell viability. According to our findings, substrate selection significantly influences conductive hydrogel behavior, affecting cell viability and proliferation as a result. In particular, the ITO substrates were found to provide the best support for cell viability with an average of at least three times higher metabolic activity compared to platinum and gold mylar substrates over a 7 day stimulation period. The study offers new insights into substrate selection as a platform for neural cell stimulation and underscores the critical role of substrate materials in optimizing the efficacy of neural interfaces for biomedical applications. In addition to extending existing work, this study provides a robust platform for future explorations aimed at tailoring the full potential of tissue-engineered neural interfaces.

BMSC loaded photo-crosslinked hyaluronic acid/collagen hydrogel incorporating FG4592 for enhanced cell proliferation and nucleus pulposus differentiation

Intervertebral disc degeneration arises from damage or degeneration of the nucleus pulposus (NP). In this study, we developed a photo-crosslinkable hydrogel incorporating FG4592 to support the growth and differentiation of bone-marrow-derived mesenchymal stem cells (BMSC). Initially, hyaluronic acid was modified with tyramine and combined with collagen to introduce riboflavin as a photo-crosslinker. This hydrogel transitioned from liquid to gel upon exposure to blue light in 3 min. The results showed that the hydrogel was biodegradable and had mechanical properties comparable to those of human NP tissues. Scanning electron microscopy after BMSC seeding in the hydrogel revealed an even distribution, and cells adhered to the collagen fibers in the hydrogel with minimal cell mortality. The effect of FG4592 on BMSC proliferation and differentiation was examined, revealing the capability of FG4592 to promote BMSC proliferation and direct differentiation resembling human NP cells. After cultivating BMSCs in the photo-crosslinked hydrogel, there was an upregulation in the expression of glycosaminoglycans, aggrecan, type II collagen, and keratin 19 proteins. Cross-species analyses of rat and human BMSCs revealed consistent results. For potential clinical applications, BMSC loaded with photo-crosslinked hydrogels can be injected into damaged intervertebral disc to facilitate NP regeneration.

Design approaches for 3D cell culture and 3D bioprinting platforms

The natural habitat of most cells consists of complex and disordered 3D microenvironments with spatiotemporally dynamic material properties. However, prevalent methods of in vitro culture study cells under poorly biomimetic 2D confinement or homogeneous conditions that often neglect critical topographical cues and mechanical stimuli. It has also become increasingly apparent that cells in a 3D conformation exhibit dramatically altered morphological and phenotypical states. In response, efforts toward designing biomaterial platforms for 3D cell culture have taken centerstage over the past few decades. Herein, we present a broad overview of biomaterials for 3D cell culture and 3D bioprinting, spanning both monolithic and granular systems. We first critically evaluate conventional monolithic hydrogel networks, with an emphasis on specific experimental requirements. Building on this, we document the recent emergence of microgel-based 3D growth media as a promising biomaterial platform enabling interrogation of cells within porous and granular scaffolds. We also explore how jammed microgel systems have been leveraged to spatially design and manipulate cellular structures using 3D bioprinting. The advent of these techniques heralds an unprecedented ability to experimentally model complex physiological niches, with important implications for tissue bioengineering and biomedical applications.

Gravity-Oriented Microfluidic Device for Biocompatible End-to-End Fabrication of Cell-Laden Microgels

Droplet microfluidics are extensively utilized to generate monodisperse cell-laden microgels in biomedical applications. However, maintaining cell viability is still challenging due to overexposure to harsh conditions in subsequent procedures that recover the microgels from the oil phase. Here, a gravity-oriented microfluidic device for end-to-end fabrication of cell-laden microgels is reported, which integrates dispersion, gelation, and extraction into a continuous workflow. This innovative on-chip extraction, driven by native buoyancy and kinetically facilitated by pseudosurfactant, exhibits 100% retrieval efficiency for microgels with a wide range of sizes and stiffnesses. The viability of encapsulated cells is perfectly maintained at ≈98% with minimal variations within and between batches. The end-to-end fabrication remarkably enhances the biocompatibility and practicality of microfluidics-based cell encapsulation and is promising to be compatible with various applications ranging from single-cell analysis to clinical therapy.

Dissecting TGF-β-induced glioblastoma invasion with engineered hyaluronic acid hydrogels

Glioma stem cells (GSCs) contribute to rapid cellular invasion in glioblastoma (GBM). Transforming growth factor-β (TGF-β) has been strongly implicated in supporting key GSC functions, including stemness, immunosuppression, and resistance. Although TGF-β is well-known as a driver of cancer invasion, how TGF-β supports the invasion of GSCs is not well understood. Progress in understanding mechanisms of TGF-β-driven invasion in GSC-derived tumors has been limited by an absence of three-dimensional (3D) culture systems that support TGF-β-stimulated invasion. Here, we show that 3D hyaluronic acid (HA) matrices can address this need. We perform bioinformatic analysis of human glioma datasets, which reveals progressive enrichment of TGF-β-related gene expression with increasingly aggressive glioma grade and GBM subtype. We then experimentally screen the invasion of a panel of human GSC spheroids through a set of 3D matrix systems, including collagen I, Matrigel, and HA, and find that only HA recapitulates TGF-β-induced invasion. We then show that GSCs differ in their ability to invade HA in a way that can be predicted from TGF-β receptor 2 expression and SMAD2 phosphorylation. GSC spheroid invasion depends strongly on the presence of RGD peptides on the HA backbone but is surprisingly independent of matrix metalloprotease degradability. Finally, we demonstrate that TGF-β stimulates invasion through SMAD-dependent signaling, consistent with recent observations that TGF-β/SMAD signals drive tumor microtube formation and invasion. Our work supports further development of HA as a matrix platform for dissecting contributions of TGF-β and other cytokines to GBM invasion and screening of cytokine-dependent invasion in human tumors.

Uniform sized cancer spheroids production using hydrogel-based droplet microfluidics: a review

Three-dimensional (3D) cell culture models have been extensively utilized in various mechanistic studies as well as for drug development studies as superior in vitro platforms than conventional two-dimensional (2D) cell culture models. This is especially the case in cancer biology, where 3D cancer models, such as spheroids or organoids, have been utilized extensively to understand the mechanisms of cancer development. Recently, many sophisticated 3D models such as organ-on-a-chip models are emerging as advanced in vitro models that can more accurately mimic the in vivo tissue functions. Despite such advancements, spheroids are still considered as a powerful 3D cancer model due to the relatively simple structure and compatibility with existing laboratory instruments, and also can provide orders of magnitude higher throughput than complex in vitro models, an extremely important aspects for drug development. However, creating well-defined spheroids remain challenging, both in terms of throughputs in generation as well as reproducibility in size and shape that can make it challenging for drug testing applications. In the past decades, droplet microfluidics utilizing hydrogels have been highlighted due to their potentials. Importantly, core-shell structured gel droplets can avoid spheroid-to-spheroid adhesion that can cause large variations in assays while also enabling long-term cultivation of spheroids with higher uniformity by protecting the core organoid area from external environment while the outer porous gel layer still allows nutrient exchange. Hence, core-shell gel droplet-based spheroid formation can improve the predictivity and reproducibility of drug screening assays. This review paper will focus on droplet microfluidics-based technologies for cancer spheroid production using various gel materials and structures. In addition, we will discuss emerging technologies that have the potential to advance the production of spheroids, prospects of such technologies, and remaining challenges.

The Plasma Membrane and Mechanoregulation in Cells

Cells inhabit a mechanical microenvironment that they continuously sense and adapt to. The plasma membrane (PM), serving as the boundary of the cell, plays a pivotal role in this process of adaptation. In this Review, we begin by examining well-studied processes where mechanoregulation proves significant. Specifically, we highlight examples from the immune system and stem cells, besides discussing processes involving fibroblasts and other cell types. Subsequently, we discuss the common molecular players that facilitate the sensing of the mechanical signal and transform it into a chemical response covering integrins YAP/TAZ and Piezo. We then review how this understanding of molecular elements is leveraged in drug discovery and tissue engineering alongside a discussion of the methodologies used to measure mechanical properties. Focusing on the processes of endocytosis, we discuss how cells may respond to altered membrane mechanics using endo- and exocytosis. Through the process of depleting/adding the membrane area, these could also impact membrane mechanics. We compare pathways from studies illustrating the involvement of endocytosis in mechanoregulation, including clathrin-mediated endocytosis (CME) and the CLIC/GEEC (CG) pathway as central examples. Lastly, we review studies on cell-cell fusion during myogenesis, the mechanical integrity of muscle fibers, and the reported and anticipated roles of various molecular players and processes like endocytosis, thereby emphasizing the significance of mechanoregulation at the PM.

Enzyme-loaded rod-like microgel shapes: a step towards the creation of shape-specific microreactors for blood detoxification purposes

Rapid removal of toxic substances is crucial to restore the normal functions of our body and ensure survival. Due to their high substrate specificity and catalytic efficiency, enzymes are unique candidates to deplete toxic compounds. While enzymes display several limitations including low stability and high immunogenicity, these can be overcome by entrapping them in a diverse range of carriers. The resulting micro/nanoreactors shield the enzymes from their surroundings, preventing their misfolding or denaturation thus allowing them to conduct their function. The micro/nanoreactors must circulate in the blood stream for extended periods of time to ensure complete depletion of the toxic agents. Surprisingly, while it is widely acknowledged that non-spherical carriers exhibit longer residence time in the bloodstream than their spherical counterparts, so far, all the reported micro/nanoreactors have been assembled with a spherical architecture. Herein, we address this important issue by pioneering the first shape-specific microreactors. We use UV-assisted punching to create rod-like microgel shapes with dimensions of 8 μm × 1 μm × 2 μm and demonstrate their biocompatibility by conducting hemolysis and cell viability assays with a macrophage and an endothelial cell line. Upon encapsulation of the model enzyme β-lactamase, the successful fabrication of rod-shaped microreactors is demonstrated by their ability to convert the yellow nitrocefin substrate into its hydrolyzed product.

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

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

Self-Degrading Multifunctional PEG-Based Hydrogels-Tailormade Substrates for Cell Culture

The use of PEG-based hydrogels as cell culture matrix to mimic the natural extracellular matrix (ECM) has been realized using a range of well-defined, tunable, and dynamic scaffolds, although they require cell adhesion ligands such as RGDS-peptide (Arg-Gly-Asp-Ser) to promote cell adhesion. Herein the synthesis of ionic and degradable hydrogels is demonstrated for cell culture by crosslinking [PEG-SH]4 with the zwitterionic crosslinker N,N-bis(acryloxyethyl)-N-methyl-N-(3-sulfopropyl) ammonium betaine (BMSAB) and the cationic crosslinker N,N-bis(acryloxyethyl)-N,N-dimethyl-1-ammonium iodide (BDMAI). Depending on the amount of ionic crosslinker used in gel formation, the hydrogels show tunable gelation time and stiffness. At the same time, the ionic groups act as catalysts for hydrolytic degradation, thereby allowing to define a stability window. The latter could be tailored in a straightforward manner by introducing the non-degradable crosslinker tri(ethylene glycol) divinyl ether. In addition, both ionic crosslinkers favor cell attachment in comparison to the pristine PEG hydrogels. The degradation is examined by swelling behavior, rheology, and fluorescence correlation spectroscopy indicating degradation kinetics depending on diffusion of incorporated fluorescent molecules.

Hyaluronic acid-based hydrogels with tunable mechanics improved structural and contractile properties of cells

Extracellular matrix (ECM) regulates cellular responses through mechanotransduction. The standard approach of in vitro culturing on plastic surfaces overlooks this phenomenon, so there is a need for biocompatible materials that exhibit adjustable mechanical and structural properties, promote cell adhesion and proliferation at low cost and for use in 2D or 3D cell cultures. This study presents a new tunable hydrogel system prepared from high-molecular hyaluronic acid (HA), Bovine serum albumin (BSA), and gelatin cross-linked using EDC/NHS. Hydrogels with Young's moduli (E) ranging from subunit to units of kilopascals were prepared by gradually increasing HA and BSA concentrations. Concentrated high-molecular HA network led to stiffer hydrogel with lower cluster size and swelling capacity. Medium and oxygen diffusion capability of all hydrogels showed they are suitable for 3D cell cultures. Mechanical and structural changes of mouse embryonic fibroblasts (MEFs) on hydrogels were compared with cells on standard cultivation surfaces. Experiments showed that hydrogels have suitable mechanical and cell adhesion capabilities, resulting in structural changes of actin filaments. Lastly, applying hydrogel for a more complex HL-1 cell line revealed improved mechanical and electrophysiological contractile properties.

Engineered Bio-Based Hydrogels for Cancer Immunotherapy

Immunotherapy represents a revolutionary paradigm in cancer management, showcasing its potential to impede tumor metastasis and recurrence. Nonetheless, challenges including limited therapeutic efficacy and severe immune-related side effects are frequently encountered, especially in solid tumors. Hydrogels, a class of versatile materials featuring well-hydrated structures widely used in biomedicine, offer a promising platform for encapsulating and releasing small molecule drugs, biomacromolecules, and cells in a controlled manner. Immunomodulatory hydrogels present a unique capability for augmenting immune activation and mitigating systemic toxicity through encapsulation of multiple components and localized administration. Notably, hydrogels based on biopolymers have gained significant interest owing to their biocompatibility, environmental friendliness, and ease of production. This review delves into the recent advances in bio-based hydrogels in cancer immunotherapy and synergistic combinatorial approaches, highlighting their diverse applications. It is anticipated that this review will guide the rational design of hydrogels in the field of cancer immunotherapy, fostering clinical translation and ultimately benefiting patients.

GelMA hydrogel as a scaffold to enhance the survival and differentiation of human induced lateral ganglionic eminence precursor cells

Cell reprogramming holds enormous potential to revolutionize our understanding of neurological and neurodevelopmental disorders, as well as enhance drug discovery and regenerative medicine. We have developed a direct cell reprogramming technology that allows us to generate lineage-specific neural cells. To extend our technology, we have investigated the incorporation of directly reprogrammed human lateral ganglionic eminence precursor cells (hiLGEPs) in a 3-dimensional (3D) matrix. Hydrogels are one of the most promising bio-scaffolds for 3D cell culture, providing cells with a supportive environment to adhere, proliferate, and differentiate. In particular, gelatin methacryloyl (GelMA) hydrogels have been used for a variety of 3D biomedical applications due to their biocompatibility, enzymatic cleavage, cell adhesion and tunable physical characteristics. This study therefore investigated the effect of GelMA hydrogel encapsulation on the survival and differentiation of hiLGEPs, both in vitro and following ex vivo transplantation into a quinolinic acid (QA) lesion rat organotypic slice culture model. We demonstrate, for the first time, that the encapsulation of hiLGEPs in GelMA hydrogel significantly enhances the survival and generation of DARPP32+ striatal neurons both in vitro and following ex vivo transplant. Furthermore, GelMA-encapsulated hiLGEPs were predominantly located away from the reactive astrocyte network that forms following QA lesioning, suggesting GelMA provides a protective barrier for cells in regions of inflammatory activation. Overall, these results indicate that GelMA hydrogel has the potential to act as a 3D bio-scaffold to augment the viability and differentiation of hiLGEPs for research and translation of pharmaceutical development and regenerative medicine.

Review: 3D cell models for organ-on-a-chip applications

Two-dimensional (2D) cultures do not fully reflect the human organs' physiology and the real effectiveness of the used therapy. Therefore, three-dimensional (3D) models are increasingly used in bioanalytical science. Organ-on-a-chip systems are used to obtain cellular in vitro models, better reflecting the human body's in vivo characteristics and allowing us to obtain more reliable results than standard preclinical models. Such 3D models can be used to understand the behavior of tissues/organs in response to selected biophysical and biochemical factors, pathological conditions (the mechanisms of their formation), drug screening, or inter-organ interactions. This review characterizes 3D models obtained in microfluidic systems. These include spheroids/aggregates, hydrogel cultures, multilayers, organoids, or cultures on biomaterials. Next, the methods of formation of different 3D cultures in Organ-on-a-chip systems are presented, and examples of such Organ-on-a-chip systems are discussed. Finally, current applications of 3D cell-on-a-chip systems and future perspectives are covered.

A Protein-Adsorbent Hydrogel with Tunable Stiffness for Tissue Culture Demonstrates Matrix-Dependent Stiffness Responses

Although tissue culture plastic has been widely employed for cell culture, the rigidity of plastic is not physiologic. Softer hydrogels used to culture cells have not been widely adopted in part because coupling chemistries are required to covalently capture extracellular matrix (ECM) proteins and support cell adhesion. To create an in vitro system with tunable stiffnesses that readily adsorbs ECM proteins for cell culture, we present a novel hydrophobic hydrogel system via chemically converting hydroxyl residues on the dextran backbone to methacrylate groups, thereby transforming non-protein adhesive, hydrophilic dextran to highly protein adsorbent substrates. Increasing methacrylate functionality increases the hydrophobicity in the resulting hydrogels and enhances ECM protein adsorption without additional chemical reactions. These hydrophobic hydrogels permit facile and tunable modulation of substrate stiffness independent of hydrophobicity or ECM coatings. Using this approach, we show that substrate stiffness and ECM adsorption work together to affect cell morphology and proliferation, but the strengths of these effects vary in different cell types. Furthermore, we reveal that stiffness mediated differentiation of dermal fibroblasts into myofibroblasts is modulated by the substrate ECM. Our material system demonstrates remarkable simplicity and flexibility to tune ECM coatings and substrate stiffness and study their effects on cell function.

3D Hydrogel Culture System Recapitulates Key Tuberculosis Phenotypes and Demonstrates Pyrazinamide Efficacy

The mortality caused by tuberculosis (TB) infections is a global concern, and there is a need to improve understanding of the disease. Current in vitro infection models to study the disease have limitations such as short investigation durations and divergent transcriptional signatures. This study aims to overcome these limitations by developing a 3D collagen culture system that mimics the biomechanical and extracellular matrix (ECM) of lung microenvironment (collagen fibers, stiffness comparable to in vivo conditions) as the infection primarily manifests in the lungs. The system incorporates Mycobacterium tuberculosis (Mtb) infected human THP-1 or primary monocytes/macrophages. Dual RNA sequencing reveals higher mammalian gene expression similarity with patient samples than 2D macrophage infections. Similarly, bacterial gene expression more accurately recapitulates in vivo gene expression patterns compared to bacteria in 2D infection models. Key phenotypes observed in humans, such as foamy macrophages and mycobacterial cords, are reproduced in the model. This biomaterial system overcomes challenges associated with traditional platforms by modulating immune cells and closely mimicking in vivo infection conditions, including showing efficacy with clinically relevant concentrations of anti-TB drug pyrazinamide, not seen in any other in vitro infection model, making it reliable and readily adoptable for tuberculosis studies and drug screening.

A 3D bioprinted hydrogel gut-on-chip with integrated electrodes for transepithelial electrical resistance (TEER) measurements

Conventional gut-on-chip (GOC) models typically represent the epithelial layer of the gut tissue, neglecting other important components such as the stromal compartment and the extracellular matrix (ECM) that play crucial roles in maintaining intestinal barrier integrity and function. These models often employ hard, flat porous membranes for cell culture, thus failing to recapitulate the soft environment and complex 3D architecture of the intestinal mucosa. Alternatively, hydrogels have been recently introduced in GOCs as ECM analogs to support the co-culture of intestinal cells inin vivo-like configurations, and thus opening new opportunities in the organ-on-chip field. In this work, we present an innovative GOC device that includes a 3D bioprinted hydrogel channel replicating the intestinal villi architecture containing both the epithelial and stromal compartments of the gut mucosa. The bioprinted hydrogels successfully support both the encapsulation of fibroblasts and their co-culture with intestinal epithelial cells under physiological flow conditions. Moreover, we successfully integrated electrodes into the microfluidic system to monitor the barrier formation in real time via transepithelial electrical resistance measurements.

Injectable Hydrogels: A Paradigm Tailored with Design, Characterization, and Multifaceted Approaches

Biomaterials denoting self-healing and versatile structural integrity are highly curious in the biomedicine segment. The injectable and/or printable 3D printing technology is explored in a few decades back, which can alter their dimensions temporarily under shear stress, showing potential healing/recovery tendency with patient-specific intervention toward the development of personalized medicine. Thus, self-healing injectable hydrogels (IHs) are stunning toward developing a paradigm for tissue regeneration. This review comprises the designing of IHs, rheological characterization and stability, several benchmark consequences for self-healing IHs, their translation into tissue regeneration of specific types, applications of IHs in biomedical such as anticancer and immunomodulation, wound healing and tissue/bone regeneration, antimicrobial potentials, drugs, gene and vaccine delivery, ocular delivery, 3D printing, cosmeceuticals, and photothermal therapy as well as in other allied avenues like agriculture, aerospace, electronic/electrical industries, coating approaches, patents associated with therapeutic/nontherapeutic avenues, and numerous futuristic challenges and solutions.

Measuring cell proliferation in bioprinting research

Tissue-like constructs, intended for application in tissue engineering and regenerative medicine, can be produced by three-dimensional (3D) bioprinting of cells in hydrogels. It is essential that the viability and proliferation of the encapsulated cells can be reliably determined. Methods currently used to evaluate cell proliferation, such as quantification of DNA and measurement of metabolic activity, have been developed for application in 2D cultures and might not be suitable for bioinks. In this study, human fibroblasts were either cast or printed in gelatin methacryloyl (GelMA) or sodium alginate hydrogels and cell proliferation was assessed by AlamarBlue, PicoGreen and visual cell counts. Comparison of data extrapolated from standard curves generated from 2D cultures and 3D hydrogels showed potential inaccuracies. Moreover, there were pronounced discrepancies in cell numbers obtained from these assays; the different bioinks strongly influenced the outcomes. Overall, the results indicate that more than one method should be applied for better assessment of cell proliferation in bioinks.

Polyethylene glycol hydrogel coatings for protection of electroactive bacteria against chemical shocks

Loss of bioelectrochemical activity in low resource environments or from chemical toxin exposure is a significant limitation in microbial electrochemical cells (MxCs), necessitating the development of materials that can stabilize and protect electroactive biofilms. Here, polyethylene glycol (PEG) hydrogels were designed as protective coatings over anodic biofilms, and the effect of the hydrogel coatings on biofilm viability under oligotrophic conditions and ammonia-N (NH4+-N) shocks was investigated. Hydrogel deposition occurred through polymerization of PEG divinyl sulfone and PEG tetrathiol precursor molecules, generating crosslinked PEG coatings with long-term hydrolytic stability between pH values of 3 and 10. Simultaneous monitoring of coated and uncoated electrodes co-located within the same MxC anode chamber confirmed that the hydrogel did not compromise biofilm viability, while the coated anode sustained nearly a 4 × higher current density (0.44 A/m2) compared to the uncoated anode (0.12 A/m2) under oligotrophic conditions. Chemical interactions between NH4+-N and PEG hydrogels revealed that the hydrogels provided a diffusive barrier to NH4+-N transport. This enabled PEG-coated biofilms to generate higher current densities during NH4+-N shocks and faster recovery afterwards. These results indicate that PEG-based coatings can expand the non-ideal chemical environments that electroactive biofilms can reliably operate in.

Injectable and Self-Curing Single-Component Hydrogel for Stem Cell Encapsulation and In Vivo Bone Regeneration

An ideal hydrogel for stem cell therapy would be injectable and efficiently promote stem cell proliferation and differentiation in body. Herein, an injectable, single-component hydrogel with hyaluronic acid (HA) modified with phenylboronic acid (PBA) and spermidine (SM) is introduced. The resulting HAps (HA-PBA-SM) hydrogel is based on the reversible crosslinking between the diol and the ionized PBA, which is stabilized by the SM. It has a shear-thinning property, enabling its injection through a syringe to form a stable hydrogel inside the body. In addition, HAps hydrogel undergoes a post-injection "self-curing," which stiffens the hydrogel over time. This property allows the HAps hydrogel to meet the physical requirements for stem cell therapy in rigid tissues, such as bone, while maintaining injectability. The hydrogel enabled favorable proliferation of human mesenchymal stem cells (hMSCs) and promoted their differentiation and mineralization. After the injection of hMSCs-containing HAps into a rat femoral defect model, efficient osteogenic differentiation of hMSCs and bone regeneration is observed. The study demonstrates that simple cationic modification of PBA-based hydrogel enabled efficient gelation with shear-thinning and self-curing properties, and it would be highly useful for stem cell therapy and in vivo bone regeneration.

Ultra-low content physio-chemically crosslinked gelatin hydrogel improves encapsulated 3D cell culture

Gelatin-based hydrogels are extensively used for 3D cell culture, bioprinting, and tissue engineering due to their cell-adhesive nature and tunable physio-chemical properties. Gelatin hydrogels for 3D cell culture are often developed using high-gelatin content (frequently 10-15 % w/v) to ensure fast gelation and improved stability. While highly stable, such matrices restrict the growth of encapsulated cells due to creating a dense, restrictive environment around the encapsulated cells. Hydrogels with lower polymer content are known to improve 3D cell growth, yet fabrication of ultra-low concentration gelatin hydrogels is challenging while ensuring fast gelation and stability. Here, we demonstrate that physical gelation and photo-crosslinking in gelatin results in a fast-gelling hydrogel at a remarkably low gelatin concentration of 1 % w/v (GelPhy/Photo). The GelPhy/Photo hydrogel was highly stable, allowed uniform 3D distribution of cells, and significantly improved the spreading of encapsulated 3T3 fibroblast cells. Moreover, human cholangiocarcinoma (HuCCT-1) cells encapsulated in 1 % GelPhy/Photo matrix grew and self-assembled into epithelial cysts with lumen, which could not be achieved in a traditional high-concentration gelatin hydrogel. These findings pave the way to significantly improve existing gelatin hydrogels for 3D cell culture applications.

Combinatorial Microgels for 3D ECM Screening and Heterogeneous Microenvironmental Culture of Primary Human Hepatic Stellate Cells

Nonalcoholic fatty liver disease affects 30% of the United States population and its progression can lead to nonalcoholic steatohepatitis (NASH), and increased risks for cirrhosis and hepatocellular carcinoma. NASH is characterized by a highly heterogeneous liver microenvironment created by the fibrotic activity of hepatic stellate cells (HSCs). While HSCs have been widely studied in 2D, further advancements in physiologically relevant 3D culture platforms for the in vitro modeling of these heterogeneous environments are needed. In this study, the use of stiffness-variable, extracellular matrix (ECM) protein-conjugated polyethylene glycol microgels as 3D cell culture scaffolds to modulate HSC activation is demonstrated. These microgels as a high throughput ECM screening system to identify HSC matrix remodeling and metabolic activities in distinct heterogeneous microenvironmental conditions are further employed. The 6 kPa fibronectin microgels are shown to significantly increase HSC matrix remodeling and metabolic activities in single or multiple-component microenvironments. Overall, heterogeneous microenvironments consisting of multiple distinct ECM microgels promoted a decrease in HSC matrix remodeling and metabolic activities compared to homogeneous microenvironments. The study envisions this ECM screening platform being adapted to a broad number of cell types to aid the identification of ECM microenvironments that best recapitulate the desired phenotype, differentiation, or drug efficacy.

3D Models of Sarcomas: The Next-generation Tool for Personalized Medicine

Sarcoma is a complex and heterogeneous cancer that has been difficult to study in vitro. While two-dimensional (2D) cell cultures and mouse models have been the dominant research tools, three-dimensional (3D) culture systems such as organoids have emerged as promising alternatives. In this review, we discuss recent developments in sarcoma organoid culture, with a focus on their potential as tools for drug screening and biobanking. We also highlight the ways in which sarcoma organoids have been used to investigate the mechanisms of gene regulation, drug resistance, metastasis, and immune interactions. Sarcoma organoids have shown to retain characteristics of in vivo biology within an in vitro system, making them a more representative model for sarcoma research. Our review suggests that sarcoma organoids offer a potential path forward for translational research in this field and may provide a platform for developing personalized therapies for sarcoma patients.

Biomaterial engineering for cell transplantation

The current paradigm of medicine is mostly designed to block or prevent pathological events. Once the disease-led tissue damage occurs, the limited endogenous regeneration may lead to depletion or loss of function for cells in the tissues. Cell therapy is rapidly evolving and influencing the field of medicine, where in some instances attempts to address cell loss in the body. Due to their biological function, engineerability, and their responsiveness to stimuli, cells are ideal candidates for therapeutic applications in many cases. Such promise is yet to be fully obtained as delivery of cells that functionally integrate with the desired tissues upon transplantation is still a topic of scientific research and development. Main known impediments for cell therapy include mechanical insults, cell viability, host's immune response, and lack of required nutrients for the transplanted cells. These challenges could be divided into three different steps: 1) Prior to, 2) during the and 3) after the transplantation procedure. In this review, we attempt to briefly summarize published approaches employing biomaterials to mitigate the above technical challenges. Biomaterials are offering an engineerable platform that could be tuned for different classes of cell transplantation to potentially enhance and lengthen the pharmacodynamics of cell therapies.

Double-Network Organohydrogels Toughened by Solvent Exchange

Double-network hydrogels based on calcium alginate are extensively exploited. Unfortunately, their low strength and unstable constitution to open environments limit their application potential. Herein, a new type of double-network organohydrogel (OHG) is proposed. By solvent exchange, a stable physical network is established based on dimethyl sulfoxide (DMSO)-alginate in the presence of a polyacrylamide network. The DMSO content endows tunable mechanical properties, with a maximum tensile strength of ≈1.7 MPa. Importantly, the OHG shows much better environmental stability compared to the conventional double-network hydrogels. Due to the reversible association of hydrogen bonds, the OHG possesses some unique properties, including free-shapeability, shape-memory, and self-adhesion, that offers several promising ways to utilize alginate-based gels for wide applications.

An Assessment of the Mechanophysical and Hormonal Impact on Human Endometrial Epithelium Mechanics and Receptivity

The endometrial epithelium and underlying stroma undergo profound changes to support and limit embryo adhesion and invasion, which occur in the secretory phase of the menstrual cycle during the window of implantation. This coincides with a peak in progesterone and estradiol production. We hypothesized that the interplay between hormone-induced changes in the mechanical properties of the endometrial epithelium and stroma supports this process. To study it, we used hormone-responsive endometrial adenocarcinoma-derived Ishikawa cells growing on substrates of different stiffness. We showed that Ishikawa monolayers on soft substrates are more tightly clustered and uniform than on stiff substrates. Probing for mechanical alterations, we found accelerated stress-relaxation after apical nanoindentation in hormone-stimulated monolayers on stiff substrates. Traction force microscopy furthermore revealed an increased number of foci with high traction in the presence of estradiol and progesterone on soft substrates. The detection of single cells and small cell clusters positive for the intermediate filament protein vimentin and the progesterone receptor further underscored monolayer heterogeneity. Finally, adhesion assays with trophoblast-derived AC-1M-88 spheroids were used to examine the effects of substrate stiffness and steroid hormones on endometrial receptivity. We conclude that the extracellular matrix and hormones act together to determine mechanical properties and, ultimately, embryo implantation.

Human Cell-Derived Matrix Composite Hydrogels with Diverse Composition for Use in Vasculature-on-chip Models

Microphysiological and organ-on-chip platforms seek to address critical gaps in human disease models and drug development that underlie poor rates of clinical success for novel interventions. While the fabrication technology and model cells used to synthesize organs-on-chip have advanced considerably, most platforms rely on animal-derived or synthetic extracellular matrix as a cell substrate, limiting mimicry of human physiology and precluding use in modeling diseases in which matrix dynamics play a role in pathogenesis. Here, the development of human cell-derived matrix (hCDM) composite hydrogels for use in 3D microphysiologic models of the vasculature is reported. hCDM composite hydrogels are derived from human donor fibroblasts and maintain a complex milieu of basement membrane, proteoglycans, and nonfibrillar matrix components. The use of hCDM composite hydrogels as 2D and 3D cell culture substrates is demonstrated, and hCDM composite hydrogels are patterned to form engineered human microvessels. Interestingly, hCDM composite hydrogels are enriched in proteins associated with vascular morphogenesis as determined by mass spectrometry, and functional analysis demonstrates proangiogenic signatures in human endothelial cells cultured in these hydrogels. In conclusion, this study suggests that human donor-derived hCDM composite hydrogels could address technical gaps in human organs-on-chip development and serve as substrates to promote vascularization.

A Self-Assembled 3D Model Demonstrates How Stiffness Educates Tumor Cell Phenotypes and Therapy Resistance in Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) is characterized by a dense and stiff extracellular matrix (ECM) associated with tumor progression and therapy resistance. To further the understanding of how stiffening of the tumor microenvironment (TME) contributes to aggressiveness, a three-dimensional (3D) self-assembling hydrogel disease model is developed based on peptide amphiphiles (PAs, PA-E3Y) designed to tailor stiffness. The model displays nanofibrous architectures reminiscent of native TME and enables the study of the invasive behavior of PDAC cells. Enhanced tuneability of stiffness is demonstrated by interacting thermally annealed aqueous solutions of PA-E3Y (PA-E3Yh) with divalent cations to create hydrogels with mechanical properties and ultrastructure similar to native tumor ECM. It is shown that stiffening of PA-E3Yh hydrogels to levels found in PDAC induces ECM deposition, promotes epithelial-to-mesenchymal transition (EMT), enriches CD133+/CXCR4+ cancer stem cells (CSCs), and subsequently enhances drug resistance. The findings reveal how a stiff 3D environment renders PDAC cells more aggressive and therefore more faithfully recapitulates in vivo tumors.

Adhesive Sulfabetaine Polymer Hydrogels for the Sandwich Cell Culture

Sandwich culture systems are techniques that cultivate cells by sandwiching them between the top and bottom substrates. Since the substrates can be separated, the system is expected to be applied to the construct layering of patterned cells and to the isolation of stacked cells. In this study, we prepared hydrogels composed of zwitterionic sulfabetaine polymers, poly[2-(2-(methacryloyloxyethyl)dimethylammonio)ethyl-1-sulfate] (PZBMA). The ZBMA homopolymers have been shown to form aggregates in aqueous solutions due to their intermolecular interactions. The water content of the PZBMA hydrogels in water was ∼70% regardless of N,N'-methylenebis(acrylamide), BIS, content as the cross-linker. The results indicated that the intermolecular interaction contributed more to the swelling behaviors than the chemical cross-linker. However, PZBMA hydrogels with 0.1 mol % BIS showed not only high elongation (∼850%) properties but also high adhesiveness and self-healing properties. When this PZBMA hydrogel was impregnated with collagen and subjected to sandwich culture using Madin-Darby canine kidney (MDCK) cells, a three-dimensional morphology of MDCK cell aggregates was constructed. Such a sulfabetaine hydrogel is expected to be developed for regenerative medicine.

Outlook and opportunities for engineered environments of breast cancer dormancy

Dormant, disseminated breast cancer cells resist treatment and may relapse into malignant metastases after decades of quiescence. Identifying how and why these dormant breast cancer cells are triggered into outgrowth is a key unsolved step in treating latent, metastatic breast cancer. However, our understanding of breast cancer dormancy in vivo is limited by technical challenges and ethical concerns with triggering the activation of dormant breast cancer. In vitro models avoid many of these challenges by simulating breast cancer dormancy and activation in well-controlled, bench-top conditions, creating opportunities for fundamental insights into breast cancer biology that complement what can be achieved through animal and clinical studies. In this review, we address clinical and preclinical approaches to treating breast cancer dormancy, how precisely controlled artificial environments reveal key interactions that regulate breast cancer dormancy, and how future generations of biomaterials could answer further questions about breast cancer dormancy.

Successful topical treatment of human biofilms using multiple antibiotic elution from a collagen-rich hydrogel

Chronic non-healing wounds significantly strain modern healthcare systems, affecting 1-2% of the population in developed countries with costs ranging between $28.1 and $96.8 billion annually. Additionally, it has been established that chronic wounds resulting from comorbidities, such as peripheral vascular disease and diabetes mellitus, tend to be polymicrobial in nature. Treatment of polymicrobial chronic wounds with oral and IV antibiotics can result in antimicrobial resistance, leading to more difficult-to-treat wounds. Ideally, chronic ulcers would be topically treated with antibiotic combinations tailored to the microbiome of a patient's wound. We have previously shown that a topical collagen-rich hydrogel (cHG) can elute single antibiotics to inhibit bacterial growth in a manner that is nontoxic to mammalian cells. Here, we analyzed the microbiology of cultures taken from human patients diagnosed with diabetes mellitus suffering from chronic wounds present for more than 6 weeks. Additionally, we examined the safety of the elution of multiple antibiotics from collagen-rich hydrogel in mammalian cells in vivo. Finally, we aimed to create tailored combinations of antibiotics impregnated into cHG to successfully target and treat infections and eradicate biofilms cultured from human chronic diabetic wound tissue. We found that the majority of human chronic wounds in our study were polymicrobial in nature. The elution of multiple antibiotics from cHG was well-tolerated in mammalian cells, making it a potential topical treatment of the polymicrobial chronic wound. Finally, combinations of antibiotics tailored to each patient's microbiome eluted from a collagen-rich hydrogel successfully treated bacterial cultures isolated from patient samples via an in vitro assay.

Establishing Quartz Crystal Microbalance with Dissipation (QCM-D) Coupled with Spectroscopic Ellipsometry (SE) as an Advantageous Technique for the Characterization of Ultra-Thin Film Hydrogels

Despite the considerable significance of utilizing ultra-thin film (utf) hydrogels as multipurpose platforms for biomedical applications, there is still an important lack of adequate characterization techniques suitable for such materials. In this Perspective, the use of quartz crystal microbalance with dissipation (QCM-D) coupled with spectral ellipsometry (SE) is presented as a potential tool for the complete characterization of utf-hydrogels due to its nanometric sensitivity and high versatility. Herein, the fundaments for utf-hydrogel characterization are settled down as far as the QCM-D/SE response is explored under a wide range of different in operando wet working conditions measurements such as temperature or liquid media, among others. Therefore, the design of measuring protocols capable to perform is proposed and compiled, for the first time, complete and precise characterization of the cross-link density, thickness variations (swelling ratio determination), stability analyses, and mechanical studies (including the simultaneous generation of stress-strain curves and the evaluation of the viscoelastic; leading to the final determination of the Poisson's ratio) under different in operando conditions. Finally, the future challenges and implications for the general characterization of soft-thin films are discussed.

Antimicrobial Peptide- and Dentin Matrix-Functionalized Hydrogel for Vital Pulp Therapy via Synergistic Bacteriostasis, Immunomodulation, and Dentinogenesis

The preservation of vital pulps is crucial for maintaining the physiological functions of teeth; however, vital pulp therapy (VPT) of pulpitis teeth remains a substantial challenge due to uncontrolled infection, excessive inflammation, and limited regenerative potential. Current pulp capping agents have restricted effects in the infectious and inflammatory microenvironment. To address this, a multifunctional hydrogel (TGH/DM) with antibacterial, immunomodulatory, and mineralization-promoting effects is designed. The antimicrobial peptide (AMP) and demineralized dentin matrix are incorporated into the hydrogel, achieving sustainable delivery of AMP and a cocktail of growth factors. In vitro results show that TGH/DM could kill endodontic microbiota, ameliorate inflammatory responses of human dental pulp stem cells (hDPSCs), and prompt odontogenic differentiation of inflammatory hDPSCs via activation of peroxisome proliferator-activated receptor gamma. In vivo results suggest that TGH/DM is capable of inducing M2 phenotype transformation of macrophages in mice and fostering the regeneration of the dentin-pulp complex in inflamed pulps of beagle dogs. Overall, this study first proposes the synergistic regulation of AMP and tissue-specific extracellular matrix for the treatment of pulpitis, and the advanced hydrogel provides a facile and effective way for VPT.

A gel-coated air-liquid-interface culture system with tunable substrate stiffness matching healthy and diseased lung tissues

Since its invention in the late 1980s, the air-liquid-interface (ALI) culture system has been the standard in vitro model for studying human airway biology and pulmonary diseases. However, in a conventional ALI system, cells are cultured on a porous plastic membrane that is much stiffer than human airway tissues. Here, we develop a gel-ALI culture system by simply coating the plastic membrane with a thin layer of hydrogel with tunable stiffness matching that of healthy and fibrotic airway tissues. We determine the optimum gel thickness that does not impair the transport of nutrients and biomolecules essential to cell growth. We show that the gel-ALI system allows human bronchial epithelial cells (HBECs) to proliferate and differentiate into pseudostratified epithelium. Furthermore, we discover that HBECs migrate significantly faster on hydrogel substrates with stiffness matching that of fibrotic lung tissues, highlighting the importance of mechanical cues in human airway remodeling. The developed gel-ALI system provides a facile approach to studying the effects of mechanical cues in human airway biology and in modeling pulmonary diseases.NEW & NOTEWORTHY In a conventional ALI system, cells are cultured on a plastic membrane that is much stiffer than human airway tissues. We develop a gel-ALI system by coating the plastic membrane with a thin layer of hydrogel with tunable stiffness matching that of healthy and fibrotic airway tissues. We discover that human bronchial epithelial cells migrate significantly faster on hydrogel substrates with pathological stiffness, highlighting the importance of mechanical cues in human airway remodeling.

Mechanotransduction and epigenetic modulations of chromatin: Role of mechanical signals in gene regulation

Mechanical forces may be generated within a cell due to tissue stiffness, cytoskeletal reorganization, and the changes (even subtle) in the cell's physical surroundings. These changes of forces impose a mechanical tension within the intracellular protein network (both cytosolic and nuclear). Mechanical tension could be released by a series of protein-protein interactions often facilitated by membrane lipids, lectins and sugar molecules and thus generate a type of signal to drive cellular processes, including cell differentiation, polarity, growth, adhesion, movement, and survival. Recent experimental data have accentuated the molecular mechanism of this mechanical signal transduction pathway, dubbed mechanotransduction. Mechanosensitive proteins in the cell's plasma membrane discern the physical forces and channel the information to the cell interior. Cells respond to the message by altering their cytoskeletal arrangement and directly transmitting the signal to the nucleus through the connection of the cytoskeleton and nucleoskeleton before the information despatched to the nucleus by biochemical signaling pathways. Nuclear transmission of the force leads to the activation of chromatin modifiers and modulation of the epigenetic landscape, inducing chromatin reorganization and gene expression regulation; by the time chemical messengers (transcription factors) arrive into the nucleus. While significant research has been done on the role of mechanotransduction in tumor development and cancer progression/metastasis, the mechanistic basis of force-activated carcinogenesis is still enigmatic. Here, in this review, we have discussed the various cues and molecular connections to better comprehend the cellular mechanotransduction pathway, and we also explored the detailed role of some of the multiple players (proteins and macromolecular complexes) involved in mechanotransduction. Thus, we have described an avenue: how mechanical stress directs the epigenetic modifiers to modulate the epigenome of the cells and how aberrant stress leads to the cancer phenotype.

Antimicrobial research of carbohydrate polymer- and protein-based hydrogels as reservoirs for the generation of reactive oxygen species: A review

Reactive oxygen species (ROS) play an important role in biological milieu. Recently, the rapid growth in our understanding of ROS and their promise in antibacterial applications has generated tremendous interest in the combination of ROS generators with bulk hydrogels. Hydrogels represent promising supporters for ROS generators and can locally confine the nanoscale distribution of ROS generators whilst also promoting cellular integration via biomaterial-cell interactions. This review highlights recent efforts and progress in developing hydrogels derived from biological macromolecules with embedded ROS generators with a focus on antimicrobial applications. Initially, an overview of passive and active antibacterial hydrogels is provided to show the significance of proper hydrogel selection and design. These are followed by an in-depth discussion of the various approaches for ROS generation in hydrogels. The structural engineering and fabrication of ROS-laden hydrogels are given with a focus on their biomedical applications in therapeutics and diagnosis. Additionally, we discuss how a compromise needs to be sought between ROS generation and removal for maximizing the efficacy of therapeutic treatment. Finally, the current challenges and potential routes toward commercialization in this rapidly evolving field are discussed, focusing on the potential translation of laboratory research outcomes to real-world clinical outcomes.

Engineered Synthetic Matrices for Human Intestinal Organoid Culture and Therapeutic Delivery

Human intestinal organoids (HIOs) derived from pluripotent stem cells or adult stem cell biopsies represent a powerful platform to study human development, drug testing, and disease modeling in vitro, and serve as a cell source for tissue regeneration and therapeutic advances in vivo. Synthetic hydrogels can be engineered to serve as analogs of the extracellular matrix to support HIO growth and differentiation. These hydrogels allow for tuning the mechanical and biochemical properties of the matrix, offering an advantage over biologically derived hydrogels such as Matrigel. Human intestinal organoids have been used for repopulating transplantable intestinal grafts and for in vivo delivery to an injured intestinal site. The use of synthetic hydrogels for in vitro culture and for in vivo delivery is expected to significantly increase the relevance of human intestinal organoids for drug screening, disease modeling, and therapeutic applications.

Thermo-sensitive hydroxybutyl chitosan/diatom biosilica hydrogel with immune microenvironment regulatory for chronic wound healing

This study proposes a chronic wound therapeutic strategy based on extracellular matrix (ECM) biomimetics and immune regulation. The hydroxybutyl chitosan/diatom biosilica hydrogel (H/D) which can regulate the immune microenvironment, is prepared from hydroxybutyl chitosan (HBC) as matrix to construct the bionic ECM and diatom biosilica (DB) as structural active unit. The hierarchical porous structure of DB provides strong anchoring interface effect to enhance the mechanical strength of hydrogel, while maintaining its favorable temperature phase transition behavior, improving the material's fit to the wound and convenience of clinical use. Silicates released from DB in H/D accelerate the transition of wounds from inflammation to proliferation and remodeling. In cellular and diabetic rat models, H/D reduces inflammation (induces conversion of M1-type macrophages to M2-type), induces angiogenesis (1.96-fold of control), promotes fibroblast proliferation (180.36 % of control), collagen deposition, keratinocyte migration (47.34 % more than control), and re-epithelialization. This study validates a possible biological mechanism for H/D bioactive hydrogel-mediated regulation of the immune microenvironment and provides a simple synergistic dressing strategy.

Injectable microcapillary network hydrogels engineered by liquid-liquid phase separation for stem cell transplantation

Injectable hydrogels are promising carriers for cell delivery in regenerative medicine. However, injectable hydrogels composed of crosslinked polymer networks are often non-microporous and prevent biological communication with host tissues through signals, nutrients, oxygen, and cells, thereby limiting graft survival and tissue integration. Here we report injectable hydrogels with liquid-liquid phase separation-induced microcapillary networks (μCN) as stem cell-delivering scaffolds. The molecular modification of gelatin with hydrogen bonding moieties induced liquid-liquid phase separation when mixed with unmodified gelatin to form μCN structures in the hydrogels. Through spatiotemporally controlled covalent crosslinking and dissolution processes, porous μCN structures were formed in the hydrogels, which can enhance mass transport and cellular activity. The encapsulation of cells with injectable μCN hydrogels improved cellular spreading, migration, and proliferation. Transplantation of mesenchymal stem cells with injectable μCN hydrogels enhanced graft survival and recovered hindlimb ischemia by enhancing material-tissue communication with biological signals and cells through μCN. This facile approach may serve as an advanced scaffold for improving stem cell transplantation therapies in regenerative medicine.

Fibroblasts and Endothelial Cells in Three-Dimensional Models: A New Tool for Addressing the Pathogenesis of Systemic Sclerosis as a Prototype of Fibrotic Vasculopathies

Two-dimensional in vitro cultures have represented a milestone in biomedical and pharmacological research. However, they cannot replicate the architecture and interactions of in vivo tissues. Moreover, ethical issues regarding the use of animals have triggered strategies alternative to animal models. The development of three-dimensional (3D) models offers a relevant tool to investigate disease pathogenesis and treatment, modeling in vitro the in vivo environment. We aimed to develop a dynamic 3D in vitro model for culturing human endothelial cells (ECs) and skin fibroblasts, simulating the structure of the tissues mainly affected in systemic sclerosis (SSc), a prototypical autoimmune fibrotic vasculopathy. Dermal fibroblasts and umbilical vein ECs grown in scaffold or hydrogel, respectively, were housed in bioreactors under flow. Fibroblasts formed a tissue-like texture with the deposition of a new extracellular matrix (ECM) and ECs assembled tube-shaped structures with cell polarization. The fine-tuned dynamic modular system allowing 3D fibroblast/EC culture connection represents a valuable model of the in vivo interplay between the main players in fibrotic vasculopathy as SSc. This model can lead to a more accurate study of the disease's pathogenesis, avoiding the use of animals, and to the development of novel therapies, possibly resulting in improved patient management.

Enhanced bone regeneration in rat calvarial defects through BMP2 release from engineered poly(ethylene glycol) hydrogels

The clinical standard therapy for large bone defects, typically addressed through autograft or allograft donor tissue, faces significant limitations. Tissue engineering offers a promising alternative strategy for the regeneration of substantial bone lesions. In this study, we harnessed poly(ethylene glycol) (PEG)-based hydrogels, optimizing critical parameters including stiffness, incorporation of arginine-glycine-aspartic acid (RGD) cell adhesion motifs, degradability, and the release of BMP2 to promote bone formation. In vitro we demonstrated that human bone marrow derived stromal cell (hBMSC) proliferation and spreading strongly correlates with hydrogel stiffness and adhesion to RGD peptide motifs. Moreover, the incorporation of the osteogenic growth factor BMP2 into the hydrogels enabled sustained release, effectively inducing bone regeneration in encapsulated progenitor cells. When used in vivo to treat calvarial defects in rats, we showed that hydrogels of low and intermediate stiffness optimally facilitated cell migration, proliferation, and differentiation promoting the efficient repair of bone defects. Our comprehensive in vitro and in vivo findings collectively suggest that the developed hydrogels hold significant promise for clinical translation for bone repair and regeneration by delivering sustained and controlled stimuli from active signaling molecules.

Synthesis and Photopatterning of Synthetic Thiol-Norbornene Hydrogels

Hydrogels are a class of soft biomaterials and the material of choice for a myriad of biomedical applications due to their biocompatibility and highly tunable mechanical and biochemical properties. Specifically, light-mediated thiol-norbornene click reactions between norbornene-modified macromers and di-thiolated crosslinkers can be used to form base hydrogels amenable to spatial biochemical modifications via subsequent light reactions between pendant norbornenes in the hydrogel network and thiolated peptides. Macromers derived from natural sources (e.g., hyaluronic acid, gelatin, alginate) can cause off-target cell signaling, and this has motivated the use of synthetic macromers such as poly(ethylene glycol) (PEG). In this study, commercially available 8-arm norbornene-modified PEG (PEG-Nor) macromers were reacted with di-thiolated crosslinkers (dithiothreitol, DTT) to form synthetic hydrogels. By varying the PEG-Nor weight percent or DTT concentration, hydrogels with a stiffness range of 3.3 kPa-31.3 kPa were formed. Pendant norbornene groups in these hydrogels were used for secondary reactions to either increase hydrogel stiffness (by reacting with DTT) or to tether mono-thiolated peptides to the hydrogel network. Peptide functionalization has no effect on bulk hydrogel mechanics, and this confirms that mechanical and biochemical signals can be independently controlled. Using photomasks, thiolated peptides can also be photopatterned onto base hydrogels, and mesenchymal stem cells (MSCs) attach and spread on RGD-functionalized PEG-Nor hydrogels. MSCs encapsulated in PEG-Nor hydrogels are also highly viable, demonstrating the ability of this platform to form biocompatible hydrogels for 2D and 3D cell culture with user-defined mechanical and biochemical properties.