Dimensionality and spreading influence MSC YAP/TAZ signaling in hydrogel environments

Mechanical confinement triggers spreading and migration of immobile cells by deforming nucleus

Cells in vivo are often subject to the challenge of spatial confinement from neighboring cells and extracellular matrix (ECM) that are usually adhesive and deformable. Here, we showed that confinement makes initially quiescent round cells on soft adhesive substrates spread and migrate, exhibiting a phenotype similar to that of cells on unconfined stiff substrates. Interestingly, the confinement-induced cell spreading and migration exist widely in many cell types, and depend on formins, cell contractility and endonuclear YAP-TEAD interaction. Finally, we demonstrated the nucleus is a mechanosensor independent of ECM rigidity, and its flattening alone is sufficient to trigger YAP nuclear translocation, assembly of focal adhesions and stress fibers, cell spreading and migration. Thus, our findings revealed a new inside-out mechanism through which the nucleus directly detects and responds to external mechanical confinement, and could have important implications for cell migration in crowded micro-environments during cancer metastasis, wound healing and embryonic development.

RGD peptide hydrogel downregulates mechanosignal YAP to inhibit postoperative scarring

OBJECTIVE

Glaucoma filtration failure may result from an overabundance of human Tenon's capsule fibroblasts (HTFs) forming a filtration tract scar. Conversely, the Yes-associated protein (YAP), a transcriptional activator of the Hippo signaling pathway, is a crucial matrix stiffness regulator of matrix production and fibroblast activation. With superior biocompatibility and biodegradability, RGD peptide hydrogels imitate the structure of real tissues' extracellular matrix (ECM). The purpose of this research was to determine whether down-regulating YAP expression via RGD peptide hydrogels may prevent HTFs activation and ECM protein secretion. Transforming growth factor-β2 (TGF-β2) was used to induce the activation of HTFs in a cellular model of scarring following glaucoma filtration surgery. Utilizing SD rats, a murine model of subconjunctival injury was established. The shape of collagen fibers was observed through Masson staining, and the expression of YAP and α-smooth muscle actin (α-SMA) was identified through immunohistochemistry. RGD peptide hydrogel was discovered to have anti-scarring properties in a mouse eye injury model, as well as the ability to lessen HTFs activation, YAP expression, cytosolic nucleus accumulation, and the expression of connective tissue growth factor (CTGF) and ECM proteins. The best concentration was found to be 1.0 weight percent among them. This concentration not only makes it easier to inject a drug subconjunctivally in vivo and maintain the filtration vesicle space in the conjunctiva, but it also inhibits the activation of fibroblasts into myofibroblasts and down-regulates the expression of the Hippo-YAP signaling pathway in Tenon's capsule fibroblasts.

STATEMENT OF SIGNIFICANCE

1. The homogenous reticular three-dimensional nanostructure that made up the interior structure of the 1.0 weight percent gel had good drug delivery characteristics for long-lasting controlled drug release. 2. RGD peptide hydrogel had a certain matrix hardness, which could mimic the normal connective tissue hardness under the conjunctiva. 3. RGD peptide hydrogels could prevented the development of rat conjunctival fibrosis. 4. RGD peptide hydrogel could inhibit the expression of YAP and its target gene CTGF, as well as α-SMA, ECM proteins in HTFs. 5. RGD peptide hydrogel has good biocompatibility, biodegradability, and stable mechanical properties, and can also be used as a promising carrier for the controlled release of drugs.

Matrix Stiffness Regulates Mechanotransduction and Vascular Network Formation of hiPSC-Derived Endothelial Progenitors Encapsulated in 3D Hydrogels

The mechanical properties of the extracellular matrix (ECM), particularly stiffness, regulate endothelial progenitor responses during vascular development, yet their behavior in physiologically compliant matrices (<1 kPa) remains underexplored. Using norbornene-modified hyaluronic acid (NorHA) hydrogels with tunable stiffness (190-884 Pa), we investigated how hydrogel stiffness influences cell morphology, endothelial maturation, mechanotransduction, and microvascular network formation in human induced pluripotent stem cell-derived endothelial progenitors (hiPSC-EPs). Our findings reveal a stiffness-dependent tradeoff between mechanotransduction and vascular network formation. At intermediate stiffness (551 Pa), cells exhibited the greatest increase in endothelial marker CD31 expression and Yes-associated protein (YAP)/ transcriptional coactivator with PDZ-binding motif (TAZ) nuclear translocation, indicating enhanced mechanotransduction and endothelial maturation. However, this did not translate to superior plexus formation. Instead, the most compliant matrix (190 Pa) supported greater vascular connectivity, characterized by longer branches (∼0.03/volume vs. 0.015 at 551 Pa) and enhanced actin remodeling. 3D cell contraction measurements revealed a 15.6-fold higher basal displacement in compliant hydrogels, suggesting that cell-generated forces and matrix deformability collectively drive vascular morphogenesis. Unlike prior studies focusing on pathological stiffness ranges (>10 kPa), our results emphasize that vascularization is not solely driven by the most mechanotransductive environment but rather by a balance of compliance, contractility, and cell-induced remodeling. These findings underscore the need to design hydrogels that provide sufficient mechanotransduction for endothelial maturation while maintaining compliance to support dynamic vascular morphogenesis. This work provides a mechanically tuned framework for optimizing microenvironments to balance endothelial differentiation and vascular network formation in tissue engineering and regenerative medicine.

In situ tailored confining microenvironment for lung cancer spheroids

The mechanical properties and physical confinement of the extracellular matrix (ECM) are crucial roles in regulating tumor growth and progression. Extensive efforts have been dedicated to replicating the physical characteristics of tumor tissue by developing two-dimensional (2D) and three-dimensional (3D) in vitro models. However, it remains a significant challenge to modulate the local microenvironment around the specific cells according to the culture progress. In this study, we develop a 3D culture platform for multicellular lung cancer spheroids using a gelatin-based hydrogel with adjustable density and stiffness. Then, by utilizing a two-photon mediated bioprinting technique, we construct 3D confining microstructures with micrometer accuracy to enclose the selected spheroids within the hydrogel matrix. Diverse transcriptional profilings of cells are observed in response to the increased ECM density and stiffness compared to the additional confining stress. In addition, changed confining stress can regulate the tumor cells with contrary impacts on the cell cycle-related pathways. Our model not only allows for modifications to the mechanical microenvironment of the overall matrix but also facilitates localized adjustments throughout the culture evolution. This approach serves as a valuable tool for investigating tumor progression and understanding cell-ECM interactions.

Interpenetrating Polymer Network Hydrogels with Tunable Viscoelasticity and Proteolytic Cleavability to Direct Stem Cells In Vitro

The dynamic nature of cellular microenvironments, regulated by the viscoelasticity and enzymatic cleavage of the extracellular matrix, remains challenging to emulate in engineered synthetic biomaterials. To address this, a novel platform of cell-instructive hydrogels is introduced, composed of two concurrently forming interpenetrating polymer networks (IPNs). These IPNs consist of the same basic building blocks - four-armed poly(ethylene glycol) and the sulfated glycosaminoglycan (sGAG) heparin - are cross-linked through either chemical or physical interactions, allowing for precise and selective tuning of the hydrogel's stiffness, viscoelasticity, and proteolytic cleavability. The studies of the individual and combined effects of these parameters on stem cell behavior revealed that human mesenchymal stem cells exhibited increased spreading and Yes-associated protein transcriptional activity in more viscoelastic and cleavable sGAG-IPN hydrogels. Furthermore, human induced pluripotent stem cell (iPSC) cysts displayed enhanced lumen formation, growth, and pluripotency maintenance when cultured in sGAG-IPN hydrogels with higher viscoelasticity. Inhibition studies emphasized the pivotal roles of actin dynamics and matrix metalloproteinase activity in iPSC cyst morphology, which varied with the viscoelastic properties of the hydrogels. Thus, the introduced sGAG-IPN hydrogel platform offers a powerful methodology for exogenous stem cell fate control.

Role of the cytoskeleton in cellular reprogramming: effects of biophysical and biochemical factors

The cytoskeleton plays a crucial role in regulating cellular behavior, acting as both a structural framework and a mediator of mechanical and biochemical signals that influence cell fate. In the context of cellular reprogramming, modifications to the cytoskeleton can have profound effects on lineage commitment and differentiation efficiency. This review explores the impact of mechanical forces such as substrate stiffness, topography, extracellular fluid viscosity, and cell seeding density on cytoskeletal organization and mechanotransduction pathways, including Rho/ROCK and YAP/TAZ signaling. Additionally, we examine the influence of biochemical agents that modulate cytoskeletal dynamics, such as actin and microtubule polymerization inhibitors, and their effects on stem cell differentiation. By understanding how cytoskeletal remodeling governs cellular identity, this review highlights potential strategies for improving reprogramming efficiency and directing cell fate by manipulating mechanical and biochemical cues.

A metamaterial scaffold beyond modulus limits: enhanced osteogenesis and angiogenesis of critical bone defects

Metallic scaffolds have shown promise in regenerating critical bone defects. However, limitations persist in achieving a modulus below 100 MPa due to insufficient strength. Consequently, the osteogenic impact of lower modulus and greater bone tissue strain ( > 1%) remains unclear. Here, we introduce a metamaterial scaffold that decouples strength and modulus through two-stage deformation. The scaffold facilitates an effective modulus of only 13 MPa, ensuring adaptability during bone regeneration. Followed by a stiff stage, it provides the necessary strength for load-bearing requirements. In vivo, the scaffold induces > 2% callus strain, upregulating calcium channels and HIF-1α to enhance osteogenesis and angiogenesis. 4-week histomorphology reveals a 44% and 498% increase in new bone fraction versus classic scaffolds with 500 MPa and 13 MPa modulus, respectively. This design transcends traditional modulus-matching paradigms, prioritizing bone tissue strain requirements. Its tunable mechanical properties also present promising implications for advancing osteogenesis mechanisms and addressing clinical challenges.

Measurement and Comparison of Hyaluronic Acid Hydrogel Mechanics Across Length Scales

Hydrogels are an important class of biomaterials that are being developed for use in medicine, such as in drug delivery and tissue engineering applications. To improve properties (e.g., injectability, nutrient transport, cell invasion), hydrogels are often processed as hydrogel microparticles (microgels) that can be used as suspensions or jammed into granular hydrogels. The mechanical properties of microgels are important across length scales, from macroscale bulk properties of granular assemblies to microscale interactions with cells; however, microgel mechanics are rarely reported due to challenges in their measurement. To address this, we report here a cost-effective, easy-to-use do-it-yourself (DIY) active feedback micropipette aspiration device to quantify the mechanics of individual microgels. Using norbornene-modified hyaluronic acid (NorHA) synthesized via an environmentally friendly, aqueous reaction as an exemplary hydrogel, we compare hydrogel mechanics across scales at various macromer concentrations. Hydrogels tested via uniaxial compression exhibit similar moduli values, trends of increasing modulus with increasing macromer concentration, and mechanical stability over time to the same formulations processed as microgels via batch emulsions (~170 μm) and tested via micropipette aspiration. Moduli range from ~50 to ~100 kPa as the NorHA macromer concentration increases from 3 wt% to 5 wt%. These findings are validated by testing with spherical nanoindentation, with similar moduli measured. Collectively, this work provides an accessible device that allows for the rapid testing of microgel mechanical properties, while also improving our understanding of hydrogel mechanics across scales for use in the design of microgels for biomedical applications.

Chemically defined and dynamic click hydrogels support hair cell differentiation in human inner ear organoids

The mechanical properties in the inner ear microenvironment play a key role in its patterning during embryonic development. To recapitulate inner ear development in vitro, three-dimensional tissue engineering strategies including the application of representative tissue models and scaffolds are of increasing interest. Human inner ear organoids are a promising model to recapitulate developmental processes; however, the current protocol requires Matrigel that contains ill-defined extracellular matrix components. Here, we implement an alternative, chemically defined, dynamic hydrogel to support the differentiation of human inner ear organoids. Specifically, thiol-norbornene and hydrazide-aldehyde click chemistries are used to fabricate inner ear organoid-laden, gelatin-based scaffolds. We identify optimal formulations to support hair cell development with comparable efficiency and fidelity to Matrigel-cultured organoids. These results suggest that the chemically defined hydrogel may serve as a viable alternative to Matrigel for inner ear tissue engineering.

The role of RAP2 in regulation of cell volume on bone marrow mesenchymal stem cell fate determination

The extracellular matrix guides cell behavior through mechanical properties, which plays a role in determining cell function and can even influence stem cell fate. Compared with adherent culture, the three-dimensional culture environment is closer to the growth conditions in vivo, but is limited by standardization of material properties and observation and measurement methods. Therefore, it is necessary to study the relationship among the three-dimensional morphological characteristics of cells, cytoskeleton, and stem cell differentiation under adherent culture conditions. Here, we control the cell volume by adjusting the cell density, microfilament cytoskeleton tension, and osmotic pressure of the culture environment, and analyze the cell morphological features and differentiation to the osteoblastic and adipogenic lineages. Based on the in vitro and in vivo results, we identify cell volume as the true reflection of the cytoskeleton tension under stress stimuli compared with cell spreading area. By adjusting cell volume, cytoskeletal tension and cell differentiation can be regulated without affecting cell spreading area. Further study shows that the Ras-related small GTPase RAP2 inhibits the activity of mechanical transducers Lamin A/C and YAP1, playing an important role in cell volume regulation of cell differentiation. In summary, our results support the close relationship between cell volume and cytoskeleton tension. The regulatory role of cell volume on cell differentiation is modulated, at least in part, by RAP2-related mechanosensitive pathways. Our insights into how cell volume regulates cell differentiation may build a bridge between two-dimensional and three-dimensional mechanical studies in cell biology.

Stiffness Regulates the Morphology and Stemness of Limbal Niche Cells Through Unique nYAP/cYAP Translocation

Purpose

To investigate the effect of matrix stiffness on the morphology and stem characters of maintenance and differentiation of limbal niche cells (LNCs) and the mechanisms involved.

Methods

Human LNCs were isolated, cultured, and identified based on published literature, and LNCs from passages 4 to 6 (P4-P6) were used in this study. They were coated with hydrogels of different concentrations to prepare matrices with different stiffnesses, and non-coated plate were used for the control group. Elastic modulus values were determined by atomic force microscopy (AFM). The expression of putative stem cell markers (SOX2, OCT4, PAX6) and fibrosis markers (α-SMA, COL1A1, S100A4) was analyzed by immunofluorescence and quantitative reverse-transcription PCR (RT-qPCR). The intracellular distribution and expression of Yes-associated protein (YAP) and drosophila mothers against decapentaplegic protein family members 2 and 3 (SMAD2/3) accordingly were analyzed using immunofluorescence and western blot.

Results

The elastic modulus values of plastic, low-concentration hydrogel-coated surfaces, and high-concentration hydrogel-coated surfaces were 3261.05 ± 172.78 MPa, 30.39 ± 5.84 kPa, and 6.99 ± 4.04 kPa, respectively; thus, they were referred to as the dish, stiff, and soft groups. Using an in vitro model to explore the effect of matrix stiffness on LNCs, we found that a soft substrate could activate YAP to change the morphology and elevate the stemness of LNCs, whereas activation of SMAD2/3 on a stiff substrate decreased nuclear YAP (nYAP) levels, leading to myofibroblast phenotype. Inhibition of SMAD2/3 on stiff substrates partially restored LNC stemness by promoting YAP nuclear translocation.

Conclusions

Our findings confirm that matrix stiffness regulates the stemness and differentiation of LNCs through the YAP/SMAD signaling pathway, indicating a potential strategy for the treatment of limbal stem cell deficiency based on LNCs.

Cell confinement by micropatterning induces phenotypic changes in cancer-associated fibroblasts

Recent advances in single-cell studies have revealed the vast transcriptomic heterogeneity of cancer-associated fibroblasts (CAFs), with each subset likely having unique roles in the tumor microenvironment. However, it is still unclear how different CAF subsets should be cultured in vitro to recapitulate their in vivo phenotype. The inherent plasticity of CAFs, or their ability to dynamically change their phenotype in response to different environmental stimuli, makes it highly challenging to induce and maintain a specific CAF state in vitro. In this study, we investigated how cell shape and confinement on two-dimensional culture substrates with different stiffnesses influence CAF transcriptomic profile and phenotype. Using micropatterning of polyacrylamide hydrogels to induce shape- and confinement-dependent changes in cell morphology, we observed that micropatterned CAFs exhibited phenotypic shifts towards more desmoplastic and inflammatory CAF subsets. Additionally, micropatterning enabled control over a range of CAF-specific markers and pathways. Lastly, we report how micropatterned and non-micropatterned CAFs respond differently to anti-cancer drugs, highlighting the importance of phenotype-oriented therapy that considers for CAF plasticity and regulatory networks. Control over CAF morphology offers a unique opportunity to establish highly robust CAF phenotypes in vitro, facilitating deeper understanding of CAF plasticity, heterogeneity, and development of novel therapeutic targets. STATEMENT OF SIGNIFICANCE: Cancer-associated fibroblasts (CAFs) are the dominant stromal cell type in many cancers, and recent studies have revealed that they are highly heterogeneous and comprise several subpopulations. It is still unclear how different subsets of CAFs should be cultured in vitro to recapitulate their in vivo phenotype. In this study, we investigated how cell shape and confinement affect CAF transcriptomic profile and phenotype. We report that micropatterned CAFs resemble desmoplastic and inflammatory CAF subsets observed in vivo and respond differently to anti-cancer drugs as compared to non-patterned CAFs. Control over CAF morphology enables the generation of highly robust CAF phenotypes in vitro, facilitating deeper understanding of CAF plasticity and heterogeneity.

Effects of Calcium Chloride Crosslinking Solution Concentration on the Long-Term Cell Viability of 16HBE14o- Human Bronchial Cells Embedded in Alginate-Based Hydrogels

In this preliminary study, the long-term effects of calcium chloride crosslinking concentration on viability of 16HBE14o- human bronchial epithelial cells embedded in alginate-extracellular matrix (ECM) or alginate-methylcellulose-ECM hydrogels have been investigated. There is currently a limited understanding regarding the effects of crosslinking solution concentration on lung epithelial cells embedded in hydrogel. Furthermore, the effects of calcium chloride concentration in crosslinking solutions on other cell types have not been reported regarding whether the addition of viscosity and stiffness tuning agents such as methylcellulose will alter the responses of cells to changes in calcium chloride concentration in crosslinking solutions. While there were no significant effects of calcium chloride concentration on cell viability in alginate-ECM hydrogels, there is a decrease in cell viability in alginate-methylcellulose-ECM hydrogels crosslinked with 300 mM calcium chloride crosslinking solution. These findings have implications in the maintenance of 16HBE14o- 3D cultures with respect to the gelation of alginate with high concentrations of ionic crosslinking solution.

Hydrogel Elastic Energy: A Stressor Triggering an Adaptive Stress-Mediated Cell Response

The crosstalk between the cells and the extracellular matrix (ECM) is bidirectional and consists of a pushing/pulling stretch exerted by the cells and a mechanical resistance counteracted by the surrounding microenvironment. It is widely recognized that the stiffness of the ECM, its viscoelasticity, and its overall deformation are the most important traits influencing the response of the cells. Here these three parameters are combined into a concept of elastic energy, which in biological terms represents the mechanical feedback that cells perceive when the ECM is deformed. It is shown that elastic energy is a stress factor that influences the response of cells in three-dimensional (3D) cultures. Strikingly, the higher the elastic energy of the matrix and thus the mechanical feedback, the higher the stress state of the cells, which correlates with the formation of G3BP-mediated stress granules. This condition is associated with an increase in alkaline phosphatase (ALP) activity but a decrease in gene expression and is mediated by the nuclear translocation of Yes-associated protein (YAP). This work supports the importance of considering the elastic energy as mechano-controller in regulating cellular stress state in 3D cultures.

Extracellular Peptide-Ligand Dimerization Actuator Receptor Design for Reversible and Spatially Dosed 3D Cell-Material Communication

Transmembrane receptors that endow mammalian cells with the ability to sense and respond to biomaterial-bound ligands will prove instrumental in bridging the fields of synthetic biology and biomaterials. Materials formed with thiol-norbornene chemistry are amenable to thiol-peptide patterning, and this study reports the rational design of synthetic receptors that reversibly activate cellular responses based on peptide-ligand recognition. This transmembrane receptor platform, termed Extracellular Peptide-ligand Dimerization Actuator (EPDA), consists of stimulatory or inhibitory receptor pairs that come together upon extracellular peptide dimer binding with corresponding monobody receptors. Intracellularly, Stimulatory EPDAs phosphorylate a substrate that merges two protein halves, whereas Inhibitory EPDAs revert split proteins back to their unmerged, inactive state via substrate dephosphorylation. To identify ligand-receptor pairs, over 2000 candidate monobodies were built in silico using PETEI, a novel computational algorithm we developed. The top 30 monobodies based on predicted peptide binding affinity were tested experimentally, and monobodies that induced the highest change in protein merging (green fluorescent protein, GFP) were incorporated in the final EPDA receptor design. In soluble form, stimulatory peptides induce intracellular GFP merging in a time- and concentration-dependent manner, and varying levels of green fluorescence were observed based on stimulatory and inhibitory peptide-ligand dosing. EPDA-programmed cells encapsulated in thiol-norbornene hydrogels patterned with stimulatory and inhibitory domains exhibited 3D activation or deactivation based on their location within peptide-patterned hydrogels. EPDA receptors can recognize a myriad of peptide-ligands bound to 3D materials, can reversibly induce cellular responses beyond fluorescence, and are widely applicable in biological research and regenerative medicine.

The impact of mechanical tuning on the printability of decellularized amniotic membrane bioinks for cell-laden bioprinting of soft tissue constructs

Decellularized extracellular matrix (dECM) bioinks hold significant potential in the 3D bioprinting of tissue-engineered constructs (TECs). While 3D bioprinting allows for the creation of custom-designed TECs, the development of bioinks based solely on dAM, without the inclusion of supporting agents or chemical modifications, remains underexplored. In this study, we present the concentration-dependent printability and rheological properties of dAM bioinks, along with an analysis of their in vitro cellular responses. Our findings demonstrate that increasing dAM concentrations, within the range of 1 to 3% w/v, enhances the mechanical moduli of the bioinks, enabling the 3D printing of flat structures with superior shape fidelity. In vitro assays reveal high cell viability across all dAM bioink formulations; however, at 3% w/v, the bioink tends to impede fibroblast proliferation, resulting in round cell morphology. We propose that bioinks containing 2% w/v dAM strike an optimal balance, providing fine-resolved features and a supportive microenvironment for fibroblasts, promoting elongated spindle-like morphology and enhanced proliferation. These results underscore the importance of dAM concentration in regulating the properties and performance of bioinks, particularly regarding cell viability and morphology, for the successful 3D bioprinting of soft tissues.

Altered Mechanobiology of PDAC Cells with Acquired Chemoresistance to Gemcitabine and Paclitaxel

Background: Pancreatic ductal adenocarcinoma acquired resistance to chemotherapy poses a major limitation to patient survival. Despite understanding some biological mechanisms of chemoresistance, much about those mechanisms remains to be uncovered. Mechanobiology, which studies the physical properties of cells, holds promise as a potential target for addressing the challenges of chemoresistance in PDAC. Therefore, we, here in an initial step, assessed the altered mechanobiology of PDAC cells with acquired chemoresistance to gemcitabine and paclitaxel. Methods: Five PDAC cell lines and six stably resistant subclones were assessed for force generation on elastic micropillar arrays. Those measurements of mechanical phenotype were complemented by single-cell motility and invasion in 3D collagen-based matrix assays. Further, the nuclear translocation of Yes-associated protein (YAP), as a measure of active mechanical status, was compared, and biomarkers of the epithelial-to-mesenchymal transition (EMT) were evaluated using RT-qPCR. Results: The PDAC cells with acquired chemoresistance exert higher traction forces than their parental/wild-type (WT) cells. In 2D, single-cell motility was altered for all the chemoresistant cells, with a cell-type specific pattern. In 3D, the spheroids of the chemoresistant PDAC cells were able to invade the matrix and remodel collagen more than their WT clones. However, YAP nuclear translocation and EMT were not significantly altered in relation to changes in other physical parameters. Conclusions: This is the first study to investigate and report on the altered mechanobiological features of PDAC cells that have acquired chemoresistance. A better understanding of mechanical features could help in identifying future targets to overcome chemoresistance in PDAC.

Spatial regulation of hydrogel polymerization reaction using ultrasound-driven streaming vortex

Ultrasound is gaining attention as an alternative tool to regulate chemical processes due to its advantages such as high cost-effectiveness, rapid response, and contact-free operation. Previous studies have demonstrated that acoustic bubble cavitation can generate energy to synthesize functional materials. In this study, we introduce a method to control the spatial distribution of physical and chemical properties of hydrogels by using an ultrasound-mediated particle manipulation technique. We developed a surface acoustic wave device that can localize micro-hydrogel particles, which are formed during gelation, in a hydrogel solution. The hydrogel fabricated with the application of surface acoustic waves exhibited gradients in mechanical, mass transport, and structural properties. We demonstrated that the gel having the property gradients could be utilized as a cell-culture substrate dictating cellular shapes, which is beneficial for interfacial tissue engineering. The acoustic method and fabricated hydrogels with property gradients can be applied to design flexible polymeric materials for soft robotics, biomedical sensors, or bioelectronics applications.

Stepwise Stiffening/Softening of and Cell Recovery from Reversibly Formulated Hydrogel Interpenetrating Networks

Biomechanical contributions of the extracellular matrix underpin cell growth and proliferation, differentiation, signal transduction, and other fate decisions. As such, biomaterials whose mechanics can be spatiotemporally altered- particularly in a reversible manner- are extremely valuable for studying these mechanobiological phenomena. Herein, a poly(ethylene glycol) (PEG)-based hydrogel model consisting of two interpenetrating step-growth networks is introduced that are independently formed via largely orthogonal bioorthogonal chemistries and sequentially degraded with distinct recombinant sortases, affording reversibly tunable stiffness ranges that span healthy and diseased soft tissues (e.g., 500 Pa-6 kPa) alongside terminal cell recovery for pooled and/or single-cell analysis in a near "biologically invisible" manner. Spatiotemporal control of gelation within the primary supporting network is achieved via mask-based and two-photon lithography; these stiffened patterned regions can be subsequently returned to the original soft state following sortase-based secondary network degradation. Using this approach, the effects of 4D-triggered network mechanical changes on human mesenchymal stem cell morphology and Hippo signaling, as well as Caco-2 colorectal cancer cell mechanomemory using transcriptomics and metabolic assays are investigated. This platform is expected to be of broad utility for studying and directing mechanobiological phenomena, patterned cell fate, and disease resolution in softer matrices.

Revealing Early Spatial Patterns of Cellular Responsivity in Fiber-Reinforced Microenvironments

Fiber-reinforcement approaches have been used to replace aligned tissues with engineered constructs after injury or surgical resection, strengthening soft biomaterial scaffolds and replicating anisotropic, load-bearing properties. However, most studies focus on the macroscale aspects of these scaffolds, rarely considering the cell-biomaterial interactions that govern remodeling and extracellular matrix organization toward aligned neo-tissues. As initial cell-biomaterial responses within fiber-reinforced microenvironments likely influence the long-term efficacy of repair and regeneration strategies, here we elucidate the roles of spatial orientation, substrate stiffness, and matrix remodeling on early cell-fiber interactions. Bovine mesenchymal stromal cells (MSCs) were cultured in soft fibrin gels reinforced with a stiff 100 µm polyglycolide-co-caprolactone fiber. Gel stiffness and remodeling capacity were modulated by fibrinogen concentration and aprotinin treatment, respectively. MSCs were imaged at 3 days and evaluated for morphology, mechanoresponsiveness (nuclear Yes-associated protein [YAP] localization), and spatial features including distance and angle deviation from fiber. Within these constructs, morphological conformity decreased as a function of distance from fiber. However, these correlations were weak (R2 = 0.01043 for conformity and R2 = 0.05542 for nuclear YAP localization), illustrating cellular heterogeneity within fiber-enforced microenvironments. To better assess cell-fiber interactions, we applied machine-learning strategies to our heterogeneous dataset of cell-shape and mechanoresponsive parameters. Principal component analysis (PCA) was used to project 23 input parameters (not including distance) onto 5 principal components (PCs), followed by agglomerative hierarchical clustering to classify cells into 3 groups. These clusters exhibited distinct levels of morpho-mechanoresponse (combination of morphological conformity and YAP signaling) and were classified as high response (HR), medium response (MR), and low response (LR) clusters. Cluster distribution varied spatially, with most cells (61%) closest to the fiber (0-75 µm) belonging to the HR cluster, and most cells (55%) furthest from the fiber (225-300 µm) belonging to the LR cluster. Modulation of gel stiffness and fibrin remodeling showed differential effects for HR cells, with stiffness influencing the level of mechanoresponse and remodeling capacity influencing the location of responding cells. Together, these novel findings demonstrate early trends in cellular patterning of the fiber-reinforced microenvironment, showing how spatial orientation, substrate biophysical properties, and matrix remodeling may guide the amplitude and localization of cellular mechanoresponses. These trends may guide approaches to optimize the design of microscale scaffold architecture and substrate properties for enhancing organized tissue assembly at the macroscale.

Tumor spheroid elasticity estimation using mechano-microscopy combined with a conditional generative adversarial network

BACKGROUND AND OBJECTIVES

Techniques for imaging the mechanical properties of cells are needed to study how cell mechanics influence cell function and disease progression. Mechano-microscopy (a high-resolution variant of compression optical coherence elastography) generates elasticity images of a sample undergoing compression from the phase difference between optical coherence microscopy (OCM) B-scans. However, the existing mechano-microscopy signal processing chain (referred to as the algebraic method) assumes the sample stress is uniaxial and axially uniform, such that violation of these assumptions reduces the accuracy and precision of elasticity images. Furthermore, it does not account for prior information regarding the sample geometry or mechanical property distribution. In this study, we investigate the feasibility of training a conditional generative adversarial network (cGAN) to generate elasticity images from phase difference images of samples containing a cell spheroid embedded in a hydrogel.

METHODS

To construct the cGAN training and simulated test sets, we generated 30,000 artificial elasticity images using a parametric model and computed the corresponding phase difference images using finite element analysis to simulate compression applied to the artificial samples. We also imaged real MCF7 breast tumor spheroids embedded in hydrogel using mechano-microscopy to construct the experimental test set and evaluated the cGAN using the algebraic elasticity images and co-registered OCM and confocal fluorescence microscopy (CFM) images.

RESULTS

Comparison with the simulated test set ground truth elasticity images shows the cGAN produces a lower root mean square error (median: 3.47 kPa, 95 % confidence interval (CI) [3.41, 3.52]) than the algebraic method (median: 4.91 kPa, 95 % CI [4.85, 4.97]). For the experimental test set, the cGAN elasticity images contain features resembling stiff nuclei at locations corresponding to nuclei seen in the algebraic elasticity, OCM, and CFM images. Furthermore, the cGAN elasticity images are higher resolution and more robust to noise than the algebraic elasticity images.

CONCLUSIONS

The cGAN elasticity images exhibit better accuracy, spatial resolution, sensitivity, and robustness to noise than the algebraic elasticity images for both simulated and real experimental data.

A Scalable Method to Fabricate 2D Hydrogel Substrates for Mechanobiology Studies with Independent Tuning of Adhesiveness and Stiffness

Mechanical signals from the extracellular matrix are crucial in guiding cellular behavior. Two-dimensional hydrogel substrates for cell cultures serve as exceptional tools for mechanobiology studies because they mimic the biomechanical and adhesive characteristics of natural environments. However, the interdisciplinary knowledge required to synthetize and manipulate these biomaterials typically restricts their widespread use in biological laboratories, which may not have the material science expertise or specialized instrumentation. To address this, we propose a scalable method that requires minimal setup to produce 2D hydrogel substrates with independent modulation of the rigidity and adhesiveness within the range typical of natural tissues. In this method, norbornene-terminated 8-arm polyethylene glycol is stoichiometrically functionalized with RGD peptides and crosslinked with a di-cysteine terminated peptide via a thiol-ene click reaction. Since the synthesis process significantly influences the final properties of the hydrogels, we provide a detailed description of the chemical procedure to ensure reproducibility and high throughput results. We demonstrate examples of cell mechanosignaling by monitoring the activation state of the mechanoeffector proteins YAP/TAZ. This method effectively dissects the influence of biophysical and adhesive cues on cell behavior. We believe that our procedure will be easily adopted by other cell biology laboratories, improving its accessibility and practical application.

Injectable hydrogels for bone regeneration with tunable degradability via peptide chirality modification

The degradability of hydrogels plays a pivotal role in bone regeneration, yet its precise effects on the bone repair process remain poorly understood. Traditional studies have been limited by the use of hydrogels with insufficient variation in degradation properties for thorough comparative analysis. Addressing this gap, our study introduces the development of matrix metalloproteinase (MMP)-responsive hydrogels engineered with a tunable degradation rate, specifically designed for bone regeneration applications. These innovative hydrogels are synthesized by integrating MMP-sensitive peptides, which exhibit chirality-transferred amino acids, with norbornene (NB)-modified 8-arm polyethylene glycol (PEG) macromers to form the hydrogel network. The degradation behavior of these hydrogels is manipulated through the chirality of the incorporated peptides, resulting in the classification into L, LD, and D hydrogels. Remarkably, the L hydrogel variant shows a significantly enhanced degradation rate, both in vitro and in vivo, which in turn fosters bone regeneration by promoting cell migration and upregulating osteogenic gene expression. This research highlights the fundamental role of hydrogel degradability in bone repair and lays the groundwork for the advancement of degradable hydrogel technologies for bone regeneration, offering new insights and potential for future biomaterials development.

Mechanical and biological behavior of double network hydrogels reinforced with alginate versus gellan gum

Alginate and gellan gum have both been used by researchers as reinforcing networks to create tough and biocompatible polyethylene glycol (PEG) based double network (DN) hydrogels; however, the relative advantages and disadvantages of each approach are not understood. This study directly compares the mechanical and biological properties of polyethylene glycol di-methacrylate (PEGDMA) hybrid DN hydrogels reinforced with either gellan gum or sodium alginate using PEGDMA concentrations from 10 to 20 wt% and reinforcing network concentrations of 1 and 2 wt%. The findings demonstrate that gellan gum reinforcement is more effective at increasing the strength, stiffness, and toughness of PEGDMA DN hydrogels. In contrast, alginate reinforcement yields DN hydrogels with greater stretchability compared to gellan gum reinforced PEGDMA. Furthermore, separate measurements of toughness via unnotched work of rupture testing and notched fracture toughness testing showed a strong correlation of these two properties for a single reinforcing network type, but not across the two types of reinforcing networks. This suggests that additional notched fracture toughness experiments are important for understanding the full mechanical response when comparing different tough DN hydrogel systems. Regarding the biological response, after conjugation of matrix protein to the surface of both materials robust cell attachment and spreading was supported with higher yes associated protein (YAP) nuclear expression observed in populations adhering to the stiffer gellan gum-PEGDMA material. This study provides valuable insights regarding how to design double network hydrogels for specific property requirements, e.g., for use in biomedical devices, as scaffolding for tissue engineering, or in soft robotic applications.

Gel microspheres enhance the stemness of ADSCs by regulating cell-ECM interaction

The gel microsphere culture system (GMCS) showed various advantages for mesenchymal stem cell (MSC) expansion and delivery, such as high specific surface area, small and regular shape, extensive adjustability, and biomimetic properties. Although various technologies and materials have been developed to promote the development of gel microspheres, the differences in the biological status of MSCs between the GMCS and the traditional Petri dish culture system (PDCS) are still unknown, hindering gel microspheres from becoming a culture system as widely used as petri dishes. In the previous study, an excellent "all-in-one" GMCS has been established for the expansion of human adipose-derived MSCs (hADSCs), which showed convenient cell culture operation. Here, we performed transcriptome and proteome sequencing on hADSCs cultured on the "all-in-one" GMCS and the PDCS. We found that hADSCs cultured in the GMCS kept in an undifferentiation status with a high stemness index, whose transcriptome profile is closer to the adipose progenitor cells (APCs) in vivo than those cultured in the PDCS. Further, the high stemness status of hADSCs in the GMCS was maintained through regulating cell-ECM interaction. For application, bilayer scaffolds were constructed by osteo- and chondro-differentiation of hADSCs cultured in the GMCS and the PDCS. The effect of osteochondral regeneration of the bilayer scaffolds in the GMCS group was better than that in the PDCS group. This study revealed the high stemness and excellent functionality of MSCs cultured in the GMCS, which promoted the application of gel microspheres in cell culture and tissue regeneration.

Enhanced hepatotoxicity assessment through encapsulated HepG2 spheroids in gelatin hydrogel matrices: Bridging the gap from 2D to 3D culture

Conventional 2D drug screening often fails to accurately predict clinical outcomes. We present an innovative approach to improve hepatotoxicity assessment by encapsulating HepG2 spheroids in gelatin hydrogel matrices with different mechanical properties. Encapsulated spheroids exhibit sustained liver-specific functionality, enhanced expression of drug-metabolizing enzymes, and increased drug sensitivity compared to 2D cultures. The platform detects critical variations in drug response, with significant differences in IC50 values between 2D and spheroid cultures ranging from 1.3-fold to > 13-fold, particularly for acetaminophen. Furthermore, drug-metabolizing enzyme expression varies across hydrogel concentrations, suggesting a role for matrix mechanical properties in modulating hepatocyte function. This novel spheroid-hydrogel platform offers a transformative approach to hepatotoxicity assessment, providing increased sensitivity, improved prediction, and a more physiologically relevant environment. The use of such advanced in vitro models can accelerate drug development, reduce animal testing, and contribute to improved patient safety and clinical outcomes.

Functionalization of Alginate Hydrogels with a Multifunctional Peptide Supports Mesenchymal Stem Cell Adhesion and Reduces Bacterial Colonization

Hydrogels with cell adhesive moieties stand out as promising materials to enhance tissue healing and regeneration. Nonetheless, bacterial infections of the implants represent an unmet major concern. In the present work, we developed an alginate hydrogel modified with a multifunctional peptide containing the RGD cell adhesive motif in combination with an antibacterial peptide derived from the 1-11 region of lactoferrin (LF). The RGD-LF branched peptide was successfully anchored to the alginate backbone by carbodiimide chemistry, as demonstrated by 1H NMR and fluorescence measurements. The functionalized hydrogel presented desirable physicochemical properties (porosity, swelling and rheological behavior) to develop biomaterials for tissue engineering. The viability of mesenchymal stem cells (MSCs) on the peptide-functionalized hydrogels was excellent, with values higher than 85 % at day 1, and higher than 95 % after 14 days in culture. Moreover, the biological characterization demonstrated the ability of the hydrogels to significantly enhance ALP activity of MSCs as well as to decrease bacterial colonization of both Gram-positive and Gram-negative models. Such results prove the potential of the functionalized hydrogels as novel biomaterials for tissue engineering, simultaneously displaying cell adhesive activity and the capacity to prevent bacterial contamination, a dual bioactivity commonly not found for these types of hydrogels.

Droplet-based microfluidics for engineering shape-controlled hydrogels with stiffness gradient

Current biofabrication strategies are limited in their ability to replicate native shape-to-function relationships, that are dependent on adequate biomimicry of macroscale shape as well as size and microscale spatial heterogeneity, within cell-laden hydrogels. In this study, a novel diffusion-based microfluidics platform is presented that meets these needs in a two-step process. In the first step, a hydrogel-precursor solution is dispersed into a continuous oil phase within the microfluidics tubing. By adjusting the dispersed and oil phase flow rates, the physical architecture of hydrogel-precursor phases can be adjusted to generate spherical and plug-like structures, as well as continuous meter-long hydrogel-precursor phases (up to 1.75 m). The second step involves the controlled introduction a small molecule-containing aqueous phase through a T-shaped tube connector to enable controlled small molecule diffusion across the interface of the aqueous phase and hydrogel-precursor. Application of this system is demonstrated by diffusing co-initiator sodium persulfate (SPS) into hydrogel-precursor solutions, where the controlled SPS diffusion into the hydrogel-precursor and subsequent photo-polymerization allows for the formation of unique radial stiffness patterns across the shape- and size-controlled hydrogels, as well as allowing the formation of hollow hydrogels with controllable internal architectures. Mesenchymal stromal cells are successfully encapsulated within hollow hydrogels and hydrogels containing radial stiffness gradient and found to respond to the heterogeneity in stiffness through the yes-associated protein mechano-regulator. Finally, breast cancer cells are found to phenotypically switch in response to stiffness gradients, causing a shift in their ability to aggregate, which may have implications for metastasis. The diffusion-based microfluidics thus finds application mimicking native shape-to-function relationship in the context of tissue engineering and provides a platform to further study the roles of micro- and macroscale architectural features that exist within native tissues.

Insights gained from computational modeling of YAP/TAZ signaling for cellular mechanotransduction

YAP/TAZ signaling pathway is regulated by a multiplicity of feedback loops, crosstalk with other pathways, and both mechanical and biochemical stimuli. Computational modeling serves as a powerful tool to unravel how these different factors can regulate YAP/TAZ, emphasizing biophysical modeling as an indispensable tool for deciphering mechanotransduction and its regulation of cell fate. We provide a critical review of the current state-of-the-art of computational models focused on YAP/TAZ signaling.

Triple click chemistry for crosslinking, stiffening, and annealing of gelatin-based microgels

Microgels are spherical hydrogels with physicochemical properties ideal for many biomedical applications. For example, microgels can be used as individual carriers for suspension cell culture or jammed/annealed into granular hydrogels with micron-scale pores highly permissive to molecular transport and cell proliferation/migration. Conventionally, laborious optimization processes are often needed to create microgels with different moduli, sizes, and compositions. This work presents a new microgel and granular hydrogel preparation workflow using gelatin-norbornene-carbohydrazide (GelNB-CH). As a gelatin-derived macromer, GelNB-CH presents cell adhesive and degradable motifs while being amenable to three orthogonal click chemistries, namely the thiol-norbornene photo-click reaction, hydrazone bonding, and the inverse electron demand Diels-Alder (iEDDA) click reaction. The thiol-norbornene photo-click reaction (with thiol-bearing crosslinkers) and hydrazone bonding (with aldehyde-bearing crosslinkers) were used to crosslink the microgels and to realize on-demand microgel stiffening, respectively. The tetrazine-norbornene iEDDA click reaction (with tetrazine-bearing crosslinkers) was used to anneal microgels into granular hydrogels. In addition to materials development, we demonstrated the value of the triple-click chemistry granular hydrogels via culturing human mesenchymal stem cells and pancreatic cancer cells.

Voxelated bioprinting of modular double-network bio-ink droplets

Analogous of pixels to two-dimensional pictures, voxels-in the form of either small cubes or spheres-are the basic building blocks of three-dimensional objects. However, precise manipulation of viscoelastic bio-ink voxels in three-dimensional space represents a grand challenge in both soft matter science and biomanufacturing. Here, we present a voxelated bioprinting technology that enables the digital assembly of interpenetrating double-network hydrogel droplets made of polyacrylamide/alginate-based or hyaluronic acid/alginate-based polymers. The hydrogels are crosslinked via additive-free and biofriendly click reaction between a pair of stoichiometrically matched polymers carrying norbornene and tetrazine groups, respectively. We develop theoretical frameworks to describe the crosslinking kinetics and stiffness of the hydrogels, and construct a diagram-of-state to delineate their mechanical properties. Multi-channel print nozzles are developed to allow on-demand mixing of highly viscoelastic bio-inks without significantly impairing cell viability. Further, we showcase the distinctive capability of voxelated bioprinting by creating highly complex three-dimensional structures such as a hollow sphere composed of interconnected yet distinguishable hydrogel particles. Finally, we validate the cytocompatibility and in vivo stability of the printed double-network scaffolds through cell encapsulation and animal transplantation.

Unveiling the Intricate Connection: Cell Volume as a Key Regulator of Mechanotransduction

The volumes of living cells undergo dynamic changes to maintain the cells' structural and functional integrity in many physiological processes. Minor fluctuations in cell volume can serve as intrinsic signals that play a crucial role in cell fate determination during mechanotransduction. In this review, we discuss the variability of cell volume and its role in vivo, along with an overview of the mechanisms governing cell volume regulation. Additionally, we provide insights into the current approaches used to control cell volume in vitro. Furthermore, we summarize the biological implications of cell volume regulation and discuss recent advances in understanding the fundamental relationship between cell volume and mechanotransduction. Finally, we delve into the potential underlying mechanisms, including intracellular macromolecular crowding and cellular mechanics, that govern the global regulation of cell fate in response to changes in cell volume. By exploring the intricate interplay between cell volume and mechanotransduction, we underscore the importance of considering cell volume as a fundamental signaling cue to unravel the basic principles of mechanotransduction. Additionally, we propose future research directions that can extend our current understanding of cell volume in mechanotransduction. Overall, this review highlights the significance of considering cell volume as a fundamental signal in understanding the basic principles in mechanotransduction and points out the possibility of controlling cell volume to control cell fate, mitigate disease-related damage, and facilitate the healing of damaged tissues.

Enhanced osteogenesis of mesenchymal stem cells encapsulated in injectable microporous hydrogel

Delivery of therapeutic stem cells to treat bone tissue damage is a promising strategy that faces many hurdles to clinical translation. Among them is the design of a delivery vehicle which promotes desired cell behavior for new bone formation. In this work, we describe the use of an injectable microporous hydrogel, made of crosslinked gelatin microgels, for the encapsulation and delivery of human mesenchymal stem cells (MSCs) and compared it to a traditional nonporous injectable hydrogel. MSCs encapsulated in the microporous hydrogel showed rapid cell spreading with direct cell-cell connections whereas the MSCs in the nonporous hydrogel were entrapped by the surrounding polymer mesh and isolated from each other. On a per-cell basis, encapsulation in microporous hydrogel induced a 4 × increase in alkaline phosphatase (ALP) activity and calcium mineral deposition in comparison to nonporous hydrogel, as measured by ALP and calcium assays, which indicates more robust osteogenic differentiation. RNA-seq confirmed the upregulation of the genes and pathways that are associated with cell spreading and cell-cell connections, as well as the osteogenesis in the microporous hydrogel. These results demonstrate that microgel-based injectable hydrogels can be useful tools for therapeutic cell delivery for bone tissue repair.

Matrix Resistance Toward Proteolytic Cleavage Controls Contractility-Dependent Migration Modes During Angiogenic Sprouting

Tissue homeostasis and disease states rely on the formation of new blood vessels through angiogenic sprouting, which is tightly regulated by the properties of the surrounding extracellular matrix. While physical cues, such as matrix stiffness or degradability, have evolved as major regulators of cell function in tissue microenvironments, it remains unknown whether and how physical cues regulate endothelial cell migration during angiogenesis. To investigate this, a biomimetic model of angiogenic sprouting inside a tunable synthetic hydrogel is created. It is shown that endothelial cells sense the resistance of the surrounding matrix toward proteolytic cleavage and respond by adjusting their migration phenotype. The resistance cells encounter is impacted by the number of covalent matrix crosslinks, crosslink degradability, and the proteolytic activity of cells. When matrix resistance is high, cells switch from a collective to an actomyosin contractility-dependent single cellular migration mode. This switch in collectivity is accompanied by a major reorganization of the actin cytoskeleton, where stress fibers are no longer visible, and F-actin aggregates in large punctate clusters. Matrix resistance is identified as a previously unknown regulator of angiogenic sprouting and, thus, provides a mechanism by which the physical properties of the matrix impact cell migration modes through cytoskeletal remodeling.

Bioengineered Hydrogels Recapitulate Fibroblast Heterogeneity in Cancer

Recently mapped transcriptomic landscapes reveal the extent of heterogeneity in cancer-associated fibroblasts (CAFs) beyond previously established single-gene markers. Functional analyses of individual CAF subsets within the tumor microenvironment are critical to develop more accurate CAF-targeting therapeutic strategies. However, there is a lack of robust preclinical models that reflect this heterogeneity in vitro. In this study, single-cell RNA sequencing datasets acquired from head and neck squamous cell carcinoma tissues to predict microenvironmental and cellular features governing individual CAF subsets are leveraged. Some of these features are then incorporated into a tunable hyaluronan-based hydrogel system to culture patient-derived CAFs. Control over hydrogel degradability and integrin adhesiveness enabled derivation of the predominant myofibroblastic and inflammatory CAF subsets, as shown through changes in cell morphology and transcriptomic profiles. Last, using these hydrogel-cultured CAFs, microtubule dynamics are identified, but not actomyosin contractility, as a key mediator of CAF plasticity. The recapitulation of CAF heterogeneity in vitro using defined hydrogels presents unique opportunities for advancing the understanding of CAF biology and evaluation of CAF-targeting therapeutics.

Bioinspired and Photo-Clickable Thiol-Ene Bioinks for the Extrusion Bioprinting of Mechanically Tunable 3D Skin Models

Bioinks play a fundamental role in skin bioprinting, dictating the printing fidelity, cell response, and function of bioprinted 3D constructs. However, the range of bioinks that support skin cells' function and aid in the bioprinting of 3D skin equivalents with tailorable properties and customized shapes is still limited. In this study, we describe a bioinspired design strategy for bioengineering double crosslinked pectin-based bioinks that recapitulate the mechanical properties and the presentation of cell-adhesive ligands and protease-sensitive domains of the dermal extracellular matrix, supporting the bioprinting of bilayer 3D skin models. Methacrylate-modified pectin was used as a base biomaterial enabling hydrogel formation via either chain-growth or step-growth photopolymerization and providing independent control over bioink rheology, as well as the mechanical and biochemical cues of cell environment. By tuning the concentrations of crosslinker and polymer in bioink formulation, dermal constructs were bioprinted with a physiologically relevant range of stiffnesses that resulted in strikingly site-specific differences in the morphology and spreading of dermal fibroblasts. We also demonstrated that the developed thiol-ene photo-clickable bioinks allow for the bioprinting of skin models of varying shapes that support dermis and epidermis reconstruction. Overall, the engineered bioinks expand the range of printable biomaterials for the extrusion bioprinting of 3D cell-laden hydrogels and provide a versatile platform to study the impact of material cues on cell fate, offering potential for in vitro skin modeling.

Bottom-Up Extrusion-Based Biofabrication of the Osteoid Niche

Bone regeneration remains a clinical challenge given the transplantation incidence rate and the associated economic burden. Bottom-up osteoid tissue engineering has the potential to offer an alternative approach to current clinical solutions that suffer from various drawbacks. In this paper, deposition-based bioprinting is exploited while the effect is explored of both the crosslinking mechanism (gelatin methacryloyl (GelMA) versus gelatin norbornene (DS 91) crosslinked with thiolated gelatin (GelNBSH)) and the degree of substitution (GelNBSH versus norbornene-norbornene-modified gelatin (DS 169) crosslinked with thiolated gelatin (GelNBNBSH)) on the presented biophysical cues as well as on the osteogenic differentiation. The incorporation of tris(2-carboxyethyl)phosphine (TCEP) to the step-growth inks allows the production of reproducible and biocompatible scaffolds based on thiol-ene chemistry. Dental pulp stem cell encapsulation in GelNBNBSH biofabricated constructs shows a favorable response due to the combination of its stress relaxation and substrate rigidity (bulk compressive modulus of 11-30 kPa) as reflected by a sevenfold increase in calcium production compared to the tissue engineering standard GelMA. This work is the first to exploit a controlled biocompatible and cell-interactive thiolated macromolecular crosslinker (GelSH + TCEP) allowing the extrusion-based biofabrication of low concentration (5 w/v%) modified osteogenic gelatin-based inks (GelNBNBSH + TCEP).

Reversible Host-Guest Crosslinks in Supramolecular Hydrogels for On-Demand Mechanical Stimulation of Human Mesenchymal Stem Cells

Stem cells are regulated not only by biochemical signals but also by biophysical properties of extracellular matrix (ECM). The ECM is constantly monitored and remodeled because the fate of stem cells can be misdirected when the mechanical interaction between cells and ECM is imbalanced. A well-defined ECM model for bone marrow-derived human mesenchymal stem cells (hMSCs) based on supramolecular hydrogels containing reversible host-guest crosslinks is fabricated. The stiffness (Young's modulus E) of the hydrogels can be switched reversibly by altering the concentration of non-cytotoxic, free guest molecules dissolved in the culture medium. Fine-adjustment of substrate stiffness enables the authors to determine the critical stiffness level E* at which hMSCs turn the mechano-sensory machinery on or off. Next, the substrate stiffness across E* is switched and the dynamic adaptation characteristics such as morphology, traction force, and YAP/TAZ signaling of hMSCs are monitored. These data demonstrate the instantaneous switching of traction force, which is followed by YAP/TAZ signaling and morphological adaptation. Periodical switching of the substrate stiffness across E* proves that frequent applications of mechanical stimuli drastically suppress hMSC proliferation. Mechanical stimulation across E* level using dynamic hydrogels is a promising strategy for the on-demand control of hMSC transcription and proliferation.

Engineering transcriptional regulation for cell-based therapies

A major aim in the field of synthetic biology is developing tools capable of responding to user-defined inputs by activating therapeutically relevant cellular functions. Gene transcription and regulation in response to external stimuli are some of the most powerful and versatile of these cellular functions being explored. Motivated by the success of chimeric antigen receptor (CAR) T-cell therapies, transmembrane receptor-based platforms have been embraced for their ability to sense extracellular ligands and to subsequently activate intracellular signal transduction. The integration of transmembrane receptors with transcriptional activation platforms has not yet achieved its full potential. Transient expression of plasmid DNA is often used to explore gene regulation platforms in vitro. However, applications capable of targeting therapeutically relevant endogenous or stably integrated genes are more clinically relevant. Gene regulation may allow for engineered cells to traffic into tissues of interest and secrete functional proteins into the extracellular space or to differentiate into functional cells. Transmembrane receptors that regulate transcription have the potential to revolutionize cell therapies in a myriad of applications, including cancer treatment and regenerative medicine. In this review, we will examine current engineering approaches to control transcription in mammalian cells with an emphasis on systems that can be selectively activated in response to extracellular signals. We will also speculate on the potential therapeutic applications of these technologies and examine promising approaches to expand their capabilities and tighten the control of gene regulation in cellular therapies.

In Vivo Osteogenic and Angiogenic Properties of a 3D-Printed Isosorbide-Based Gyroid Scaffold Manufactured via Digital Light Processing

INTRODUCTION

Osteogenic and angiogenic properties of synthetic bone grafts play a crucial role in the restoration of bone defects. Angiogenesis is recognised for its support in bone regeneration, particularly in larger defects. The objective of this study is to evaluate the new bone formation and neovascularisation of a 3D-printed isosorbide-based novel CSMA-2 polymer in biomimetic gyroid structures.

METHODS

The gyroid scaffolds were fabricated by 3D printing CSMA-2 polymers with different hydroxyapatite (HA) filler concentrations using the digital light processing (DLP) method. A small animal subcutaneous model and a rat calvaria critical-size defect model were performed to analyse tissue compatibility, angiogenesis, and new bone formation.

RESULTS

The in vivo results showed good biocompatibility of the 3D-printed gyroid scaffolds with no visible prolonged inflammatory reaction. Blood vessels were found to infiltrate the pores from day 7 of the implantation. New bone formation was confirmed with positive MT staining and BMP-2 expression, particularly on scaffolds with 10% HA. Bone volume was significantly higher in the CSMA-2 10HA group compared to the sham control group.

DISCUSSION AND CONCLUSIONS

The results of the subcutaneous model demonstrated a favourable tissue response, including angiogenesis and fibrous tissue, indicative of the early wound healing process. The results from the critical-size defect model showcased new bone formation, as confirmed by micro-CT imaging and immunohistochemistry. The combination of CSMA-2 as the 3D printing material and the gyroid as the 3D structure was found to support essential events in bone healing, specifically angiogenesis and osteogenesis.

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.

The Role of Mechanotransduction in Contact Inhibition of Locomotion and Proliferation

Contact inhibition (CI) represents a crucial tumor-suppressive mechanism responsible for controlling the unbridled growth of cells, thus preventing the formation of cancerous tissues. CI can be further categorized into two distinct yet interrelated components: CI of locomotion (CIL) and CI of proliferation (CIP). These two components of CI have historically been viewed as separate processes, but emerging research suggests that they may be regulated by both distinct and shared pathways. Specifically, recent studies have indicated that both CIP and CIL utilize mechanotransduction pathways, a process that involves cells sensing and responding to mechanical forces. This review article describes the role of mechanotransduction in CI, shedding light on how mechanical forces regulate CIL and CIP. Emphasis is placed on filamin A (FLNA)-mediated mechanotransduction, elucidating how FLNA senses mechanical forces and translates them into crucial biochemical signals that regulate cell locomotion and proliferation. In addition to FLNA, trans-acting factors (TAFs), which are proteins or regulatory RNAs capable of directly or indirectly binding to specific DNA sequences in distant genes to regulate gene expression, emerge as sensitive players in both the mechanotransduction and signaling pathways of CI. This article presents methods for identifying these TAF proteins and profiling the associated changes in chromatin structure, offering valuable insights into CI and other biological functions mediated by mechanotransduction. Finally, it addresses unanswered research questions in these fields and delineates their possible future directions.

Enhancement of lacrimal gland cell function by decellularized lacrimal gland derived hydrogel

Sustainable treatment of aqueous deficient dry eye (ADDE) represents an unmet medical need and therefore requires new curative and regenerative approaches based on appropriatein vitromodels. Tissue specific hydrogels retain the individual biochemical composition of the extracellular matrix and thus promote the inherent cell´s physiological function. Hence, we created a decellularized lacrimal gland (LG) hydrogel (dLG-HG) meeting the requirements for a bioink as the basis of a LG model with potential forin vitroADDE studies. Varying hydrolysis durations were compared to obtain dLG-HG with best possible physical and ultrastructural properties while preserving the original biochemical composition. A particular focus was placed on dLG-HG´s impact on viability and functionality of LG associated cell types with relevance for a futurein vitromodel in comparison to the unspecific single component hydrogel collagen type-I (Col) and the common cell culture substrate Matrigel. Proliferation of LG epithelial cells (EpC), LG mesenchymal stem cells, and endothelial cells cultured on dLG-HG was enhanced compared to culture on Matrigel. Most importantly with respect to a functionalin vitromodel, the secretion capacity of EpC cultured on dLG-HG was higher than that of EpC cultured on Col or Matrigel. In addition to these promising cell related properties, a rapid matrix metalloproteinase-dependent biodegradation was observed, which on the one hand suggests a lively cell-matrix interaction, but on the other hand limits the cultivation period. Concluding, dLG-HG possesses decisive properties for the tissue engineering of a LGin vitromodel such as cytocompatibility and promotion of secretion, making it superior to unspecific cell culture substrates. However, deceleration of biodegradation should be addressed in future experiments.

Advanced materials technologies to unravel mechanobiological phenomena

Advancements in materials-driven mechanobiology have yielded significant progress. Mechanobiology explores how cellular and tissue mechanics impact development, physiology, and disease, where extracellular matrix (ECM) dynamically interacts with cells. Biomaterial-based platforms emulate synthetic ECMs, offering precise control over cellular behaviors by adjusting mechanical properties. Recent technological advances enable in vitro models replicating active mechanical stimuli in vivo. These models manipulate cellular mechanics even at a subcellular level. In this review we discuss recent material-based mechanomodulatory studies in mechanobiology. We highlight the endeavors to mimic the dynamic properties of native ECM during pathophysiological processes like cellular homeostasis, lineage specification, development, aging, and disease progression. These insights may inform the design of accurate in vitro mechanomodulatory platforms that replicate ECM mechanics.

Serial Passaging Affects Stromal Cell Mechanosensitivity on Hyaluronic Acid Hydrogels

There is a tremendous interest in developing hydrogels as tunable in vitro cell culture platforms to study cell response to mechanical cues in a controlled manner. However, little is known about how common cell culture techniques, such as serial expansion on tissue culture plastic, affect subsequent cell behavior when cultured on hydrogels. In this work, a methacrylated hyaluronic acid hydrogel platform is leveraged to study stromal cell mechanotransduction. Hydrogels are first formed through thiol-Michael addition to model normal soft tissue (e.g., lung) stiffness (E ≈ 1 kPa). Secondary cross-linking via radical photopolymerization of unconsumed methacrylates allows matching of early- (E ≈ 6 kPa) and late-stage fibrotic tissue (E ≈ 50 kPa). Early passage (P1) human bone marrow mesenchymal stromal cells (hMSCs) display increased spreading, myocardin-related transcription factor-A (MRTF-A) nuclear localization, and focal adhesion size with increasing hydrogel stiffness. However, late passage (P5) hMSCs show reduced sensitivity to substrate mechanics with lower MRTF-A nuclear translocation and smaller focal adhesions on stiffer hydrogels compared to early passage hMSCs. Similar trends are observed in an immortalized human lung fibroblast line. Overall, this work highlights the implications of standard cell culture practices on investigating cell response to mechanical signals using in vitro hydrogel models.

Nuclear deformation regulates YAP dynamics in cancer associated fibroblasts

Cells cultured on stiff 2D substrates exert high intracellular force, resulting in mechanical deformation of their nuclei. This nuclear deformation (ND) plays a crucial role in the transport of Yes Associated Protein (YAP) from the cytoplasm to the nucleus. However, cells in vivo are in soft 3D environment with potentially much lower intracellular forces. Whether and how cells may deform their nuclei in 3D for YAP localization remains unclear. Here, by culturing human colon cancer associated fibroblasts (CAFs) on 2D, 2.5D, and 3D substrates, we differentiated the effects of stiffness, force, and ND on YAP localization. We found that nuclear translocation of YAP depends on the degree of ND irrespective of dimensionality, stiffness and total force. ND induced by the perinuclear force, not the total force, and nuclear membrane curvature correlate strongly with YAP activation. Immunostained slices of human tumors further supported the association between ND and YAP nuclear localization, suggesting ND as a potential biomarker for YAP activation in tumors. Additionally, we conducted quantitative analysis of the force dynamics of CAFs on 2D substrates to construct a stochastic model of YAP kinetics. This model revealed that the probability of YAP nuclear translocation, as well as the residence time in the nucleus follow a power law. This study provides valuable insights into the regulatory mechanisms governing YAP dynamics and highlights the significance of threshold activation in YAP localization. STATEMENT OF SIGNIFICANCE: Yes Associated Protein (YAP), a transcription cofactor, has been identified as one of the drivers of cancer progression. High tumor stiffness is attributed to driving YAP to the nucleus, wherein it activates pro-metastatic genes. Here we show, using cancer associated fibroblasts, that YAP translocation to the nucleus depends on the degree of nuclear deformation, irrespective of stiffness. We also identified that perinuclear force induced membrane curvature correlates strongly with YAP nuclear transport. A novel stochastic model of YAP kinetics unveiled a power law relationship between the activation threshold and persistence time of YAP in the nucleus. Overall, this study provides novel insights into the regulatory mechanisms governing YAP dynamics and the probability of activation that is of immense clinical significance.

3D printed scaffolds based on hyaluronic acid bioinks for tissue engineering: a review

Hyaluronic acid (HA) is widely distributed in human connective tissue, and its unique biological and physicochemical properties and ability to facilitate biological structure repair make it a promising candidate for three-dimensional (3D) bioprinting in the field of tissue regeneration and biomedical engineering. Moreover, HA is an ideal raw material for bioinks in tissue engineering because of its histocompatibility, non-immunogenicity, biodegradability, anti-inflammatory properties, anti-angiogenic properties, and modifiability. Tissue engineering is a multidisciplinary field focusing on in vitro reconstructions of mammalian tissues, such as cartilage tissue engineering, neural tissue engineering, skin tissue engineering, and other areas that require further clinical applications. In this review, we first describe the modification methods, cross-linking methods, and bioprinting strategies for HA and its derivatives as bioinks and then critically discuss the strengths, shortcomings, and feasibility of each method. Subsequently, we reviewed the practical clinical applications and outcomes of HA bioink in 3D bioprinting. Finally, we describe the challenges and opportunities in the development of HA bioink to provide further research references and insights.

3D Volumetric Mechanosensation of MCF7 Breast Cancer Spheroids in a Linear Stiffness Gradient GelAGE

The tumor microenvironment presents spatiotemporal shifts in biomechanical properties with cancer progression. Hydrogel biomaterials like GelAGE offer the stiffness tuneability to recapitulate dynamic changes in tumor tissues by altering photo-energy exposures. Here, a tuneable hydrogel with spatiotemporal control of stiffness and mesh-network is developed. The volume of MCF7 spheroids encapsulated in a linear stiffness gradient demonstrates an inverse relationship with stiffness (p < 0.0001). As spheroids are exposed to increased crosslinking (stiffer) and greater mechanical confinement, spheroid stiffness increases. Protein expression (TRPV4, β1 integrin, E-cadherin, and F-actin) decreases with increasing stiffness while showing strong correlations to spheroid volume (r2  > 0.9). To further investigate the role of volume, MCF7 spheroids are grown in a soft matrix for 5 days prior to a second polymerisation which presents a stiffness gradient to equally expanded spheroids. Despite being exposed to variable stiffness, these spheroids show even protein expression, confirming volume as a key regulator. Overall, this work showcases the versatility of GelAGE and demonstrates volume expansion as a key regulator of 3D mechanosensation in MCF7 breast cancer spheroids. This platform has the potential to further investigation into the role of stiffness and dimensionality in 3D spheroid culture for other types of cancers and diseases.

Engineered hydrogel reveals contribution of matrix mechanics to esophageal adenocarcinoma and identifies matrix-activated therapeutic targets

Increased extracellular matrix (ECM) stiffness has been implicated in esophageal adenocarcinoma (EAC) progression, metastasis, and resistance to therapy. However, the underlying protumorigenic pathways are yet to be defined. Additional work is needed to develop physiologically relevant in vitro 3D culture models that better recapitulate the human tumor microenvironment and can be used to dissect the contributions of matrix stiffness to EAC pathogenesis. Here, we describe a modular, tumor ECM-mimetic hydrogel platform with tunable mechanical properties, defined presentation of cell-adhesive ligands, and protease-dependent degradation that supports robust in vitro growth and expansion of patient-derived EAC 3D organoids (EAC PDOs). Hydrogel mechanical properties control EAC PDO formation, growth, proliferation, and activation of tumor-associated pathways that elicit stem-like properties in the cancer cells, as highlighted through in vitro and in vivo environments. We also demonstrate that the engineered hydrogel serves as a platform for identifying potential therapeutic targets to disrupt the contribution of protumorigenic matrix mechanics in EAC. Together, these studies show that an engineered PDO culture platform can be used to elucidate underlying matrix-mediated mechanisms of EAC and inform the development of therapeutics that target ECM stiffness in EAC.