Vascular smooth muscle cell durotaxis depends on substrate stiffness gradient strength

Survivin modulates stiffness-induced vascular smooth muscle cell motility

Arterial stiffness is a contributor to cardiovascular diseases (CVDs) and is associated with the aberrant migration of vascular smooth muscle cells (VSMCs). However, the mechanisms driving VSMC migration in stiff environments remain unclear. We recently demonstrated that survivin is upregulated in mouse and human VSMCs cultured on stiff hydrogels, where it modulates stiffness-mediated cell proliferation. However, its role in stiffness-dependent VSMC migration remains unknown. To assess its impact on migration, we performed time-lapse microscopy on VSMCs seeded on fibronectin-coated soft and stiff hydrogels, mimicking the physiological stiffness of normal and diseased arteries. We observed that VSMC motility increased under stiff conditions, while pharmacologic or siRNA-mediated inhibition of survivin reduced stiffness-stimulated migration to rates similar to those observed under soft conditions. Further investigation revealed that cells on stiff hydrogels exhibited greater directional movement and robust lamellipodial protrusion compared to those on soft hydrogels. Interestingly, survivin-inhibited cells on stiff hydrogels showed reduced directional persistence and lamellipodial protrusion. We also found that survivin overexpression modestly increased cell motility and partially rescued the lack of directional persistence compared to green fluorescent protein (GFP)-expressing VSMCs on soft hydrogels. Mechanistically, stiffness- and survivin-dependent cell migration involves focal adhesion kinase (FAK) and actin dynamics, as stiffness increases phosphorylated FAK recruitment to focal adhesions and promotes actin organization and stress fiber formation-effects that are disrupted by survivin inhibition. In conclusion, our findings establish that mechanotransduction through a survivin-FAK-actin cascade converts extracellular matrix stiffness into stiffness-sensitive motility, suggesting that targeting this pathway may offer therapeutic strategies for CVD.

Mechanosensing alters platelet migration

Platelets have long been established as a safeguard of our vascular system. Recently, haptotactic platelet migration has been discovered as a part of the immune response. In addition, platelets exhibit mechanosensing properties, changing their behavior in response to the stiffness of the underlying substrate. However, the influence of substrate stiffness on platelet migration behavior remains elusive. Here, we investigated the migration of platelets on fibrinogen-coated polydimethylsiloxane (PDMS) substrates with different stiffnesses. Using phase-contrast and fluorescence microscopy as well as a deep-learning neural network, we tracked single migrating platelets and measured their migration distance and velocity. We found that platelets migrated on stiff PDMS substrates (E = 2 MPa), while they did not migrate on soft PDMS substrates (E = 5 kPa). Platelets migrated also on PDMS substrates with intermediate stiffness (E = 100 kPa), but their velocity and the fraction of migrating platelets were diminished compared to platelets on stiff PDMS substrates. The straightness of platelet migration, however, was not significantly influenced by substrate stiffness. We used scanning ion conductance microscopy (SICM) to image the three-dimensional shape of migrating platelets, finding that platelets on soft substrates did not show the polarization and shape change associated with migration. Furthermore, the fibrinogen density gradient, which is generated by migrating platelets, was reduced for platelets on soft substrates. Our work demonstrates that substrate stiffness, and thus platelet mechanosensing, influences platelet migration. Substrate stiffness for optimal platelet migration is quite high (>100 kPa) in comparison to other cell types, with possible implications on platelet behavior in inflammatory and injured tissue. STATEMENT OF SIGNIFICANCE: Platelets can feel and react to the stiffness of their surroundings - a process called mechanosensation. Additionally, platelets migrate via substrate-bound fibrinogen as part of the innate immune response during injury or inflammation. It has been shown that the migration of immune cells is influenced by the stiffness of the underlying substrate, but the effect of substrate stiffness on the migration of platelets has not yet been investigated. Using differently stiff substrates made from PDMS, we show that substrate stiffness affects platelet migration. Stiff substrates facilitate fast and frequent platelet migration with a strong platelet shape anisotropy and a strong fibrinogen removal while soft substrates inhibit platelet migration. These findings highlight the influence of the stiffness of the surrounding tissue on the platelet immune response, possibly enhancing platelet migration in inflamed tissue.

Chemo-mechanical model of cell polarization initiated by structural polarity

Cell polarization is crucial in most physiological functions. Living cells at the extracellular matrix (ECM) actively coordinate a polarized morphology to target the preferred signals. In particular, the initial heterogeneity of subcellular components, termed as structural polarity, has been discovered to mediate the early attachment and transmigration of cells in tumour metastasis. However, how heterogeneous cells initiate the early polarization remains incompletely discovered. Here, we establish a multiscale model of a cell to explore the chemo-mechanical mechanisms of cell polarization initiated by structural polarity. The two-dimensional vertex model of the cell is built with the main mechanical components of eukaryotic cells. The initial structural polarity of the modeled cell is introduced by seeding heterogeneous actin filaments at the cell cortex and quantified by the ratio of the filamentous forces at the vertices. Then, the structural polarity is integrated in the reaction-diffusion system of Rho GTPase (Cdc42) at the cell cortex to obtain the traction forces at the leading vertices. Finally, the modeled cell is actuated to spread under the traction forces and discovered to develop into a characteristic polarized morphology. The results indicate that the cell polarization is initiated and dynamically developed by structural polarity through the reaction-diffusion system of Cdc42. In addition, the bistability of Cdc42 activation at the cell cortex is defined and discovered to dominate the polarization status of the cell. Furthermore, biphasic (i.e., positive and negative) durotaxis of the cell is successfully modeled at an ECM with a stiffness gradient, and concluded to be codetermined by the chemo-mechanical coupling of the initial structural polarity and ECM stiffness gradient. The proposed multiscale model provides a quantitative way to probe cell polarization coupled with mechanical stimuli, biochemical reaction and cytoskeletal reorganization, and holds the potential to guide studies of cell polarization under multiple stimuli.

Mechanical network motifs as targets for mechanomedicine

The identification and analysis of network motifs has been widely used in the functional analysis of signaling components, disease discovery and other fields. The positive feedback loop (PFL) is a simple but important network motif. The formation of a PFL is regulated by mechanical cues such as substrate stiffness, fiber stretching and cell compression in the cell microenvironment. Here, we propose a new term, 'mechanical PFL', and analyze the mechanisms of mechanical PFLs at molecular, subcellular and cellular scales. More and more therapies are being targeted against mechanosignaling pathways at the experimental and preclinical stages, and exploring mechanical PFLs as potential mechanomedicine targets could be a new direction for disease treatment.

Polyzwitterion-grafted decellularized bovine intercostal arteries as new substitutes of small-diameter arteries for vascular regeneration

Coronary artery bypass grafting is acknowledged as a major clinical approach for treatment of severe coronary artery atherosclerotic heart disease. This procedure typically requires autologous small-diameter vascular grafts. However, the limited availability of the donor vessels and associated trauma during tissue harvest underscore the necessity for artificial arterial alternatives. Herein, decellularized bovine intercostal arteries were successfully fabricated with lengths ranging from 15 to 30 cm, which also closely match the inner diameters of human coronary arteries. These decellularized arterial grafts exhibited great promise following poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) grafting from the inner surface. Such surface modification endowed the decellularized arteries with superior mechanical strength, enhanced anticoagulant properties and improved biocompatibility, compared to the decellularized bovine intercostal arteries alone, or even those decellularized grafts modified with both heparin and vascular endothelial growth factor. After replacement of the carotid arteries in rabbits, all surface-modified vascular grafts have shown good patency within 30 days post-implantation. Notably, strong signal was observed after α-SMA immunofluorescence staining on the PMPC-grafted vessels, indicating significant potential for regenerating the vascular smooth muscle layer and thereby restoring full structures of the artery. Consequently, the decellularized bovine intercostal arteries surface modified by PMPC can emerge as a potent candidate for small-diameter artificial blood vessels, and have shown great promise to serve as viable substitutes of arterial autografts.

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.

From stress fiber to focal adhesion: a role of actin crosslinkers in force transmission

The contractile apparatus, stress fiber (SF), is connected to the cell adhesion machinery, focal adhesion (FA), at the termini of SF. The SF-FA complex is essential for various mechanical activities of cells, including cell adhesion to the extracellular matrix (ECM), ECM rigidity sensing, and cell migration. This mini-review highlights the importance of SF mechanics in these cellular activities. Actin-crosslinking proteins solidify SFs by attenuating myosin-driven flows of actin and myosin filaments within the SF. In the solidified SFs, viscous slippage between actin filaments in SFs and between the filaments and the surrounding cytosol is reduced, leading to efficient transmission of myosin-generated contractile force along the SFs. Hence, SF solidification via actin crosslinking ensures exertion of a large force to FAs, enabling FA maturation, ECM rigidity sensing and cell migration. We further discuss intracellular mechanisms for tuning crosslinker-modulated SF mechanics and the potential relationship between the aberrance of SF mechanics and pathology including cancer.

Decoding the "Fingerprint" of Implant Materials: Insights into the Foreign Body Reaction

Foreign body reaction (FBR) is a prevalent yet often overlooked pathological phenomenon, particularly within the field of biomedical implantation. The presence of FBR poses a heavy burden on both the medical and socioeconomic systems. This review seeks to elucidate the protein "fingerprint" of implant materials, which is generated by the physiochemical properties of the implant materials themselves. In this review, the activity of macrophages, the formation of foreign body giant cells (FBGCs), and the development of fibrosis capsules in the context of FBR are introduced. Additionally, the relationship between various implant materials and FBR is elucidated in detail, as is an overview of the existing approaches and technologies employed to alleviate FBR. Finally, the significance of implant components (metallic materials and non-metallic materials), surface CHEMISTRY (charge and wettability), and physical characteristics (topography, roughness, and stiffness) in establishing the protein "fingerprint" of implant materials is also well documented. In conclusion, this review aims to emphasize the importance of FBR on implant materials and provides the current perspectives and approaches in developing implant materials with anti-FBR properties.

Positive and negative durotaxis - mechanisms and emerging concepts

Cell migration is controlled by the coordinated action of cell adhesion, cytoskeletal dynamics, contractility and cell extrinsic cues. Integrins are the main adhesion receptors to ligands of the extracellular matrix (ECM), linking the actin cytoskeleton to the ECM and enabling cells to sense matrix rigidity and mount a directional cell migration response to stiffness gradients. Most models studied show preferred migration of single cells or cell clusters towards increasing rigidity. This is referred to as durotaxis, and since its initial discovery in 2000, technical advances and elegant computational models have provided molecular level details of stiffness sensing in cell migration. However, modeling has long predicted that, depending on cell intrinsic factors, such as the balance of cell adhesion molecules (clutches) and the motor proteins pulling on them, cells might also prefer adhesion to intermediate rigidity. Recently, experimental evidence has supported this notion and demonstrated the ability of cells to migrate towards lower rigidity, in a process called negative durotaxis. In this Review, we discuss the significant conceptual advances that have been made in our appreciation of cell plasticity and context dependency in stiffness-guided directional cell migration.

Fabrication of gradient hydrogels using a thermophoretic approach in microfluidics

The extracellular matrix presents spatially varying physical cues that can influence cell behavior in many processes. Physical gradients within hydrogels that mimic the heterogenous mechanical microenvironment are useful to study the impact of these cues on cellular responses. Therefore, simple and reliable techniques to create such gradient hydrogels are highly desirable. This work demonstrates the fabrication of stiffness gradient Gellan gum (GG) hydrogels by applying a temperature gradient across a microchannel containing hydrogel precursor solution. Thermophoretic migration of components within the precursor solution generates a concentration gradient that mirrors the temperature gradient profile, which translates into mechanical gradients upon crosslinking. Using this technique, GG hydrogels with stiffness gradients ranging from 20 to 90 kPa over 600µm are created, covering the elastic moduli typical of moderately hard to hard tissues. MC3T3 osteoblast cells are then cultured on these gradient substrates, which exhibit preferential migration and enhanced osteogenic potential toward the stiffest region on the gradient. Overall, the thermophoretic approach provides a non-toxic and effective method to create hydrogels with defined mechanical gradients at the micron scale suitable forin vitrobiological studies and potentially tissue engineering applications.

In silico analysis shows that dynamic changes in curvature guide cell migration over long distances

In vitro experiments have shown that cell scale curvatures influence cell migration; cells avoid convex hills and settle in concave valleys. However, it is not known whether dynamic changes in curvature can guide cell migration. This study extends a previous in-silico model to explore the effects over time of changing the substrate curvature on cell migration guidance. By simulating a dynamic surface curvature using traveling wave patterns, we investigate the influence of wave height and speed, and find that long-distance cell migration guidance can be achieved on specific wave patterns. We propose a mechanistic explanation of what we call dynamic curvotaxis and highlight those cellular features that may be involved. Our results open a new area of study for understanding cell mobility in dynamic environments, from single-cell in vitro experiments to multi-cellular in vivo mechanisms.

Durotaxis and negative durotaxis: where should cells go?

Durotaxis and negative durotaxis are processes in which cell migration is directed by extracellular stiffness. Durotaxis is the tendency of cells to migrate toward stiffer areas, while negative durotaxis occurs when cells migrate toward regions with lower stiffness. The mechanisms of both processes are not yet fully understood. Additionally, the connection between durotaxis and negative durotaxis remains unclear. In this review, we compare the mechanisms underlying durotaxis and negative durotaxis, summarize the basic principles of both, discuss the possible reasons why some cell types exhibit durotaxis while others exhibit negative durotaxis, propose mechanisms of switching between these processes, and emphasize the challenges in the investigation of durotaxis and negative durotaxis.

Quantification of cell migration: metrics selection to model application

Cell migration plays an essential role in physiological and pathological states, such as immune response, tissue generation and tumor development. This phenomenon can occur spontaneously or it can be triggered by an external stimuli, including biochemical, mechanical, or electrical cues that induce or direct cells to migrate. The migratory response to these cues is foundational to several fields including neuroscience, cancer and regenerative medicine. Various platforms are available to qualitatively and quantitatively measure cell migration, making the measurements of cell motility straight-forward. Migratory behavior must be analyzed by multiple metrics and then models to connect the measurements to physiological meaning. This review will focus on describing and quantifying cell movement for individual cell migration.

Positive, negative and controlled durotaxis

Cell migration is a physical process central to life. Among others, it regulates embryogenesis, tissue regeneration, and tumor growth. Therefore, understanding and controlling cell migration represent fundamental challenges in science. Specifically, the ability of cells to follow stiffness gradients, known as durotaxis, is ubiquitous across most cell types. Even so, certain cells follow positive stiffness gradients while others move along negative gradients. How the physical mechanisms involved in cell migration work to enable a wide range of durotactic responses is still poorly understood. Here, we provide a mechanistic rationale for durotaxis by integrating stochastic clutch models for cell adhesion with an active gel theory of cell migration. We show that positive and negative durotaxis found across cell types are explained by asymmetries in the cell adhesion dynamics. We rationalize durotaxis by asymmetric mechanotransduction in the cell adhesion behavior that further polarizes the intracellular retrograde flow and the protruding velocity at the cell membrane. Our theoretical framework confirms previous experimental observations and explains positive and negative durotaxis. Moreover, we show how durotaxis can be engineered to manipulate cell migration, which has important implications in biology, medicine, and bioengineering.

Materials and extracellular matrix rigidity highlighted in tissue damages and diseases: Implication for biomaterials design and therapeutic targets

Rigidity (or stiffness) of materials and extracellular matrix has proven to be one of the most significant extracellular physicochemical cues that can control diverse cell behaviors, such as contractility, motility, and spreading, and the resultant pathophysiological phenomena. Many 2D materials engineered with tunable rigidity have enabled researchers to elucidate the roles of matrix biophysical cues in diverse cellular events, including migration, lineage specification, and mechanical memory. Moreover, the recent findings accumulated under 3D environments with viscoelastic and remodeling properties pointed to the importance of dynamically changing rigidity in cell fate control, tissue repair, and disease progression. Thus, here we aim to highlight the works related with material/matrix-rigidity-mediated cell and tissue behaviors, with a brief outlook into the studies on the effects of material/matrix rigidity on cell behaviors in 2D systems, further discussion of the events and considerations in tissue-mimicking 3D conditions, and then examination of the in vivo findings that concern material/matrix rigidity. The current discussion will help understand the material/matrix-rigidity-mediated biological phenomena and further leverage the concepts to find therapeutic targets and to design implantable materials for the treatment of damaged and diseased tissues.

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Discuss the cutting-edge findings on the role of matrix rigidity in dictating diverse cell behaviors.

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Underscore the dynamic matrix rigidity that interplays with cells, and the related pathophysiological phenomena.

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Illuminate the significance of matrix rigidity in clinically-relevant settings.

Photoresponsive Hydrogels for Studying Mechanotransduction of Cells

Hydrogels are important platform materials for in vitro cellular studies. Mechanistic studies on durotaxis, the directional movement of a cell affected by a spatial gradient of stiffness of the underlying substrate, requires materials such as polyacrylamide, polyethylene glycol, or PDMS, in which the stiffness can be controlled in a spatiotemporal manner. Here, we describe the synthesis of an o-nitrobenzyl-based photocleavable cross-linker and its incorporation into a polyacrylamide hydrogel to render it photoresponsive. Precise control over the physical properties of the gel allows observation of glioblastoma durotaxis under surface stiffness conditions relevant to the actual brain environment.

Stiffness-dependent active wetting enables optimal collective cell durotaxis

The directed migration of cellular clusters enables morphogenesis, wound healing, and collective cancer invasion. Gradients of substrate stiffness are known to direct the migration of cellular clusters in a process called collective durotaxis, but underlying mechanisms remain unclear. Here, we unveil a connection between collective durotaxis and the wetting properties of cellular clusters. We show that clusters of cancer cells dewet soft substrates and wet stiff ones. At intermediate stiffness, at the crossover from low to high wettability, clusters on uniform-stiffness substrates become maximally motile, and clusters on stiffness gradients exhibit optimal durotaxis. Durotactic velocity increases with cluster size, stiffness gradient, and actomyosin activity. We demonstrate this behavior on substrates coated with the cell-cell adhesion protein E-cadherin and then establish its generality on substrates coated with extracellular matrix. We develop a physical model of three-dimensional active wetting that explains this mode of collective durotaxis in terms of a balance between in-plane active traction and tissue contractility, and out-of-plane surface tension. Finally, we show that the distribution of cluster displacements has a heavy tail, with infrequent but large cellular hops that contribute to durotactic migration. Our study demonstrates a physical mechanism of collective durotaxis, through both cell-cell and cell-substrate adhesion ligands, based on the wetting properties of active droplets.

Substrate stiffness modulates migration and local intercellular membrane motion in pulmonary endothelial cell monolayers

The pulmonary artery endothelium forms a semipermeable barrier that limits macromolecular flux through intercellular junctions. This barrier is maintained by an intrinsic forward protrusion of the interacting membranes between adjacent cells. However, the dynamic interactions of these membranes have been incompletely quantified. Here, we present a novel technique to quantify the motion of the peripheral membrane of the cells, called paracellular morphological fluctuations (PMFs), and to assess the impact of substrate stiffness on PMFs. Substrate stiffness impacted large-length scale morphological changes such as cell size and motion. Cell size was larger on stiffer substrates, whereas the speed of cell movement was decreased on hydrogels with stiffness either larger or smaller than 1.25 kPa, consistent with cells approaching a jammed state. Pulmonary artery endothelial cells moved fastest on 1.25 kPa hydrogel, a stiffness consistent with a healthy pulmonary artery. Unlike these large-length scale morphological changes, the baseline of PMFs was largely insensitive to the substrate stiffness on which the cells were cultured. Activation of store-operated calcium channels using thapsigargin treatment triggered a transient increase in PMFs beyond the control treatment. However, in hypocalcemic conditions, such an increase in PMFs was absent on 1.25 kPa hydrogel but was present on 30 kPa hydrogel-a stiffness consistent with that of a hypertensive pulmonary artery. These findings indicate that 1) PMFs occur in cultured endothelial cell clusters, irrespective of the substrate stiffness; 2) PMFs increase in response to calcium influx through store-operated calcium entry channels; and 3) stiffer substrate promotes PMFs through a mechanism that does not require calcium influx.

Directed cell migration towards softer environments

How cells sense tissue stiffness to guide cell migration is a fundamental question in development, fibrosis and cancer. Although durotaxis-cell migration towards increasing substrate stiffness-is well established, it remains unknown whether individual cells can migrate towards softer environments. Here, using microfabricated stiffness gradients, we describe the directed migration of U-251MG glioma cells towards less stiff regions. This 'negative durotaxis' does not coincide with changes in canonical mechanosensitive signalling or actomyosin contractility. Instead, as predicted by the motor-clutch-based model, migration occurs towards areas of 'optimal stiffness', where cells can generate maximal traction. In agreement with this model, negative durotaxis is selectively disrupted and even reversed by the partial inhibition of actomyosin contractility. Conversely, positive durotaxis can be switched to negative by lowering the optimal stiffness by the downregulation of talin-a key clutch component. Our results identify the molecular mechanism driving context-dependent positive or negative durotaxis, determined by a cell's contractile and adhesive machinery.

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

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

Visible-Light Stiffness Patterning of GelMA Hydrogels Towards In Vitro Scar Tissue Models

Variations in mechanical properties of the extracellular matrix occurs in various processes, such as tissue fibrosis. The impact of changes in tissue stiffness on cell behaviour are studied in vitro using various types of biomaterials and methods. Stiffness patterning of hydrogel scaffolds, through the use of stiffness gradients for instance, allows the modelling and studying of cellular responses to fibrotic mechanisms. Gelatine methacryloyl (GelMA) has been used extensively in tissue engineering for its inherent biocompatibility and the ability to precisely tune its mechanical properties. Visible light is now increasingly employed for crosslinking GelMA hydrogels as it enables improved cell survival when performing cell encapsulation. We report here, the photopatterning of mechanical properties of GelMA hydrogels with visible light and eosin Y as the photoinitiator using physical photomasks and projection with a digital micromirror device. Using both methods, binary hydrogels with areas of different stiffnesses and hydrogels with stiffness gradients were fabricated. Their mechanical properties were characterised using force indentation with atomic force microscopy, which showed the efficiency of both methods to spatially pattern the elastic modulus of GelMA according to the photomask or the projected pattern. Crosslinking through projection was also used to build constructs with complex shapes. Overall, this work shows the feasibility of patterning the stiffness of GelMA scaffolds, in the range from healthy to pathological stiffness, with visible light. Consequently, this method could be used to build in vitro models of healthy and fibrotic tissue and study the cellular behaviours involved at the interface between the two.

Extracellular Matrix Stiffness in Lung Health and Disease

The extracellular matrix (ECM) provides structural support and imparts a wide variety of environmental cues to cells. In the past decade, a growing body of work revealed that the mechanical properties of the ECM, commonly known as matrix stiffness, regulate the fundamental cellular processes of the lung. There is growing appreciation that mechanical interplays between cells and associated ECM are essential to maintain lung homeostasis. Dysregulation of ECM-derived mechanical signaling via altered mechanosensing and mechanotransduction pathways is associated with many common lung diseases. Matrix stiffening is a hallmark of lung fibrosis. The stiffened ECM is not merely a sequelae of lung fibrosis but can actively drive the progression of fibrotic lung disease. In this article, we provide a comprehensive view on the role of matrix stiffness in lung health and disease. We begin by summarizing the effects of matrix stiffness on the function and behavior of various lung cell types and on regulation of biomolecule activity and key physiological processes, including host immune response and cellular metabolism. We discuss the potential mechanisms by which cells probe matrix stiffness and convert mechanical signals to regulate gene expression. We highlight the factors that govern matrix stiffness and outline the role of matrix stiffness in lung development and the pathogenesis of pulmonary fibrosis, pulmonary hypertension, asthma, chronic obstructive pulmonary disease (COPD), and lung cancer. We envision targeting of deleterious matrix mechanical cues for treatment of fibrotic lung disease. Advances in technologies for matrix stiffness measurements and design of stiffness-tunable matrix substrates are also explored. © 2022 American Physiological Society. Compr Physiol 12:3523-3558, 2022.

The Extracellular Matrix Stiffening: A Trigger of Prostate Cancer Progression and Castration Resistance?

Despite advancements made in diagnosis and treatment, prostate cancer remains the second most diagnosed cancer among men worldwide in 2020, and the first in North America and Europe. Patients with localized disease usually respond well to first-line treatments, however, up to 30% develop castration-resistant prostate cancer (CRPC), which is often metastatic, making this stage of the disease incurable and ultimately fatal. Over the last years, interest has grown into the extracellular matrix (ECM) stiffening as an important mediator of diseases, including cancers. While this process is increasingly well-characterized in breast cancer, a similar in-depth look at ECM stiffening remains lacking for prostate cancer. In this review, we scrutinize the current state of literature regarding ECM stiffening in prostate cancer and its potential association with disease progression and castration resistance.

Amoeboid-like migration ensures correct horizontal cell layer formation in the developing vertebrate retina

Migration of cells in the developing brain is integral for the establishment of neural circuits and function of the central nervous system. While migration modes during which neurons employ predetermined directional guidance of either preexisting neuronal processes or underlying cells have been well explored, less is known about how cells featuring multipolar morphology migrate in the dense environment of the developing brain. To address this, we here investigated multipolar migration of horizontal cells in the zebrafish retina. We found that these cells feature several hallmarks of amoeboid-like migration that enable them to tailor their movements to the spatial constraints of the crowded retina. These hallmarks include cell and nuclear shape changes, as well as persistent rearward polarization of stable F-actin. Interference with the organization of the developing retina by changing nuclear properties or overall tissue architecture hampers efficient horizontal cell migration and layer formation showing that cell-tissue interplay is crucial for this process. In view of the high proportion of multipolar migration phenomena observed in brain development, the here uncovered amoeboid-like migration mode might be conserved in other areas of the developing nervous system.

Computational modeling reveals a vital role for proximity-driven additive and synergistic cell-cell interactions in increasing cancer invasiveness

Solid-tumor cell invasion typically occurs by collective migration of attached cell-cohorts, yet we show here that indirect cell-interactions through the substrate can also drive invasiveness. We have previously shown that well-spaced, invasive cancer cells push-into and indent gels to depths of 10 µm, while closely adjacent, non-contacting cancer cells may reach up to 18 µm, potentially relying on cell-cell interactions through the gel-substrate. To test that, we developed finite element models of indenting cells, using experimental gel mechanics, cell mechanostructure, and force magnitudes. We show that under 50-350 nN of combined traction and normal forces, a stiff nucleus-region is essential in facilitating 5-10 µm single-cell indentations, while uniformly soft cells attain 1.6-fold smaller indentations. We observe that indentation depths of cells in close proximity (0.5-50 µm distance) increase relative to well-spaced cells, due to additive, continuum mechanics-driven contributions. Specifically, 2-3 cells applying 220 nN normal forces gained up to 3% in depth, which interestingly increased to 7.8% when two cells, 10 µm apart, applied unequal force-magnitudes (i.e., 220 and 350 nN). Such additive, energy-free contributions can reduce cell mechanical energy -output required for invasiveness, yet the experimentally observed 10-18 µm depths likely necessitate synergistic, mechanobiological changes, which may be mechanically triggered. We note that nucleus stiffening or cytoplasm softening by 25-50% increased indentation depths by only 1-7%, while depths increase nearly linearly with force-magnitude even to two-fold levels. Hence, cell-proximity triggered, synergistic and additive cell-interactions through the substrate can drive collective cancer-cell invasiveness, even without direct cell-cell interactions. STATEMENT OF SIGNIFICANCE: Metastatic cancer invasion typically occurs collectively in attached cell-cohorts. We have previously shown increased invasiveness in closely adjacent cancer cells that are able to push-into and indent soft-gels more deeply than single, well-spaced cells. Using finite element models, we reveal mechanisms of cell-proximity driven invasiveness, demonstrating an important role for the stiff nucleus. Cell-proximity can additively induce small increase in indentation depth via continuum mechanics contributions, especially when adjacent cells apply unequal forces, and without requiring increased cell-mechanical-energy-output. Concurrently, proximity-triggered synergistic interactions that produce changes in cell mechanics or capacity for increased force-levels can facilitate deep invasive-indentations. Thus, we reveal concurrent additive and synergistic mechanisms to drive collective cancer-cell invasiveness even without direct cell-cell interactions.

Regulation of cell attachment, spreading, and migration by hydrogel substrates with independently tunable mesh size

Hydrogels are widely used as substrates to investigate interactions between cells and their microenvironment as they mimic many attributes of the extracellular matrix. The stiffness of hydrogels is an important property that is known to regulate cell behavior. Beside stiffness, cells also respond to structural cues such as mesh size. However, since the mesh size of hydrogel is intrinsically coupled to its stiffness, its role in regulating cell behavior has never been independently investigated. Here, we report a hydrogel system whose mesh size and stiffness can be independently controlled. Cell behavior, including spreading, migration, and formation of focal adhesions is significantly altered on hydrogels with different mesh sizes but with the same stiffness. At the transcriptional level, hydrogel mesh size affects cellular mechanotransduction by regulating nuclear translocation of yes-associated protein. These findings demonstrate that the mesh size of a hydrogel plays an important role in cell-substrate interactions. STATEMENT OF SIGNIFICANCE: Hydrogels are ideal platforms with which to investigate interactions between cells and their microenvironment as they mimic many physical properties of the extracellular matrix. However, the mesh size of hydrogels is intrinsically coupled to their stiffness, making it challenging to investigate the contribution of mesh size to cell behavior. In this work, we use hydrogel-on-glass substrates with defined thicknesses whose stiffness and mesh size can be independently tuned. We use these substrates to isolate the effects of mesh size on cell behavior, including attachment, spreading, migration, focal adhesion formation and YAP localization in the nucleus. Our results show that mesh size has significant, yet often overlooked, effects, on cell behavior, and contribute to a further understanding of cell-substrate interactions.

Multiscale mechanobiology: Coupling models of adhesion kinetics and nonlinear tissue mechanics

The mechanical behavior of tissues at the macroscale is tightly coupled to cellular activity at the microscale. Dermal wound healing is a prominent example of a complex system in which multiscale mechanics regulate restoration of tissue form and function. In cutaneous wound healing, a fibrin matrix is populated by fibroblasts migrating in from a surrounding tissue made mostly out of collagen. Fibroblasts both respond to mechanical cues, such as fiber alignment and stiffness, as well as exert active stresses needed for wound closure. Here, we develop a multiscale model with a two-way coupling between a microscale cell adhesion model and a macroscale tissue mechanics model. Starting from the well-known model of adhesion kinetics proposed by Bell, we extend the formulation to account for nonlinear mechanics of fibrin and collagen and show how this nonlinear response naturally captures stretch-driven mechanosensing. We then embed the new nonlinear adhesion model into a custom finite element implementation of tissue mechanical equilibrium. Strains and stresses at the tissue level are coupled with the solution of the microscale adhesion model at each integration point of the finite element mesh. In addition, solution of the adhesion model is coupled with the active contractile stress of the cell population. The multiscale model successfully captures the mechanical response of biopolymer fibers and gels, contractile stresses generated by fibroblasts, and stress-strain contours observed during wound healing. We anticipate that this framework will not only increase our understanding of how mechanical cues guide cellular behavior in cutaneous wound healing, but will also be helpful in the study of mechanobiology, growth, and remodeling in other tissues.

Cell type-specific orientation and migration responses for a microgrooved surface with shallow grooves

BACKGROUND

Directional cell migration due to mechanosensing for in vivo microenvironment, such as microgrooved surfaces, is an essential process in tissue growth and repair in both normal and pathological states. Cell migration responses on the microgrooved surfaces might be reflected by the cell type difference, which is deeply involved in cellular physiological functions. Although the responses are implicated in focal adhesions (FAs) of cells, limited information is available about cell migration behavior on the microgrooved surfaces whose dimensions are comparable with the size of FAs.

OBJECTIVE

In the present study, we investigated the cell orientation and migration behavior of normal vascular smooth muscle cells (VSMCs) and cervical cancer HeLa cells on the microgrooved surface.

METHOD

The surface comprises shallow grooves with 2-μm width and approximately 150-nm depth, which indicates the same order of magnitude as that of the horizontal and vertical size of FAs, respectively. Moreover, VSMCs presenting well-aligned actin stress fibers with mature FAs revealed marked cell elongation and directional migration on the grooves; however, HeLa cells with nonoriented F-actin with smaller FAs did not. Furthermore, atomic force microscopy live cell imaging revealed that the internal force of the actin stress fibers was significantly higher in VSMCs than that in HeLa cells, and the increase or decrease in the cytoskeletal forces improved or diminished the sensing ability for shallow grooves, respectively.

RESULTS

The results strongly indicated that directional cell migration with contact guidance responses should be modulated by cell type-specific cytoskeletal arrangements and intracellular traction forces. The differences in cell type-specific orientation and migration responses can be emphasized on the microgrooves as large as the horizontal and vertical size of FAs.

CONCLUSION

The microgoove structure in the size range of the FA protein complex is a powerful tool to clarify subtle differences in the intracellular force-dependent substrate mechanosensing.

Neuromechanobiology: An Expanding Field Driven by the Force of Greater Focus

The brain processes information by transmitting signals through highly connected and dynamic networks of neurons. Neurons use specific cellular structures, including axons, dendrites and synapses, and specific molecules, including cell adhesion molecules, ion channels and chemical receptors to form, maintain and communicate among cells in the networks. These cellular and molecular processes take place in environments rich of mechanical cues, thus offering ample opportunities for mechanical regulation of neural development and function. Recent studies have suggested the importance of mechanical cues and their potential regulatory roles in the development and maintenance of these neuronal structures. Also suggested are the importance of mechanical cues and their potential regulatory roles in the interaction and function of molecules mediating the interneuronal communications. In this review, the current understanding is integrated and promising future directions of neuromechanobiology are suggested at the cellular and molecular levels. Several neuronal processes where mechanics likely plays a role are examined and how forces affect ligand binding, conformational change, and signal induction of molecules key to these neuronal processes are indicated, especially at the synapse. The disease relevance of neuromechanobiology as well as therapies and engineering solutions to neurological disorders stemmed from this emergent field of study are also discussed.

Macrophage Phenotypic Changes on FN-Coated Physical Gradient Hydrogels

The chemical and physical properties are two crucial cues when designing tissue engineering scaffold to mimic living tissue. Macrophages, the major players in the immune response, react rapidly to microenvironmental signals, including gradients of physical or chemical cues. Spatiotemporal gradients can modulate cell behavior, such as polarization, proliferation, and adhesion. Here, we studied macrophage phenotypic changes on untreated and fibronectin (FN)-coated methacrylated gellan gum with varying stiffnesses. The compressive moduli of hydrogel with different stiffnesses ranged from ∼5 to 30 kPa. Fibronectin was chemically attached to the substrate to facilitate macrophage proliferation, adhesion, and polarization. Classically (M1) and alternatively (M2) activated macrophages were cultured on both untreated and FN-coated gels. FN-coated substrates elevated cell numbers and enhanced macrophage spreading. The urea/nitrite ratio indicated that untreated rigid substrates shifted both polarizations toward a more proinflammatory phenotype. FN-coated substrates had no impact on M1 polarization. In contrast, FN-coated stiffer gels polarized M2 cells toward an anti-proinflammatory state based on arginine activity and CD206 expression. In addition, macrophage polarization on the softer gel was not influenced by the neighboring cells cultured on the stiffer side of the gel. Using mechanical gradients to control macrophage polarization can be a useful tool in ensuring a proper healing response and for tissue engineering.

Effects of substrate stiffness on mast cell migration

Mast cells (MCs) play important roles in multiple pathologies, including fibrosis; however, their behaviors in different extracellular matrix (ECM) environments have not been fully elucidated. Accordingly, in this study, the migration of MCs on substrates with different stiffnesses was investigated using time-lapse video microscopy. Our results showed that MCs could appear in round, spindle, and star-like shapes; spindle-shaped cells accounted for 80-90 % of the total observed cells. The migration speed of round cells was significantly lower than that of cells with other shapes. Interestingly, spindle-shaped MCs migrated in a jiggling and wiggling motion between protrusions. The persistence index of MC migration was slightly higher on stiffer substrates. Moreover, we found that there was an intermediate optimal stiffness at which the migration efficiency was the highest. These findings may help to improve our understanding of MC-induced pathologies and the roles of MC migration in the immune system.

Progress in the mechanical modulation of cell functions in tissue engineering

In mammals, mechanics at multiple stages-nucleus to cell to ECM-underlie multiple physiological and pathological functions from its development to reproduction to death. Under this inspiration, substantial research has established the role of multiple aspects of mechanics in regulating fundamental cellular processes, including spreading, migration, growth, proliferation, and differentiation. However, our understanding of how these mechanical mechanisms are orchestrated or tuned at different stages to maintain or restore the healthy environment at the tissue or organ level remains largely a mystery. Over the past few decades, research in the mechanical manipulation of the surrounding environment-known as substrate or matrix or scaffold on which, or within which, cells are seeded-has been exceptionally enriched in the field of tissue engineering and regenerative medicine. To do so, traditional tissue engineering aims at recapitulating key mechanical milestones of native ECM into a substrate for guiding the cell fate and functions towards specific tissue regeneration. Despite tremendous progress, a big puzzle that remains is how the cells compute a host of mechanical cues, such as stiffness (elasticity), viscoelasticity, plasticity, non-linear elasticity, anisotropy, mechanical forces, and mechanical memory, into many biological functions in a cooperative, controlled, and safe manner. High throughput understanding of key cellular decisions as well as associated mechanosensitive downstream signaling pathway(s) for executing these decisions in response to mechanical cues, solo or combined, is essential to address this issue. While many reports have been made towards the progress and understanding of mechanical cues-particularly, substrate bulk stiffness and viscoelasticity-in regulating the cellular responses, a complete picture of mechanical cues is lacking. This review highlights a comprehensive view on the mechanical cues that are linked to modulate many cellular functions and consequent tissue functionality. For a very basic understanding, a brief discussion of the key mechanical players of ECM and the principle of mechanotransduction process is outlined. In addition, this review gathers together the most important data on the stiffness of various cells and ECM components as well as various tissues/organs and proposes an associated link from the mechanical perspective that is not yet reported. Finally, beyond addressing the challenges involved in tuning the interplaying mechanical cues in an independent manner, emerging advances in designing biomaterials for tissue engineering are also explored.

Regulation of collective cell polarity and migration using dynamically adhesive micropatterned substrates

Collective cell migration is a fundamental biological process in which groups of cells move together in a coordinated manner, and it is essential for tissue development and wound repair. However, the underlying mechanisms that orchestrate directionality in collectively migrating cells remain poorly understood. In this study, we employed dynamically adhesive micropatterned substrates to investigate the role of adhesive cues in directing epithelial migration. Our findings demonstrate that epithelial cells collectively polarize in response to asymmetric patterns of extracellular matrix (ECM), and the degree of polarization depends on the degree of asymmetry and requires calcium-dependent cell-cell adhesion. When released from the micropatterns, epithelial cells collectively migrate according to the direction of pre-established polarity, and cohesive migration specifically requires E-cadherin-containing adherens junctions. Finally, disruption of the microtubule network blocks collective polarization and functionally inhibits directed migration. Together, these results indicate that adhesive cues from the ECM guide collective epithelial polarity and migration, and this response depends on adherens junctions and microtubules. STATEMENT OF SIGNIFICANCE: This study employs a dynamically adhesive micropatterning platform to investigate the role of adhesive cues in directing the polarity and directional migration of epithelial cells. The findings demonstrate how asymmetric tissue geometry influences the collective directionality in simple epithelia and that this response is mediated by adherens junctions and the microtubule network. This work provides new insight into fundamental cellular processes involved in wound healing and has important implications for biomaterial and scaffold design.

High-Throughput Methods in the Discovery and Study of Biomaterials and Materiobiology

The complex interaction of cells with biomaterials (i.e., materiobiology) plays an increasingly pivotal role in the development of novel implants, biomedical devices, and tissue engineering scaffolds to treat diseases, aid in the restoration of bodily functions, construct healthy tissues, or regenerate diseased ones. However, the conventional approaches are incapable of screening the huge amount of potential material parameter combinations to identify the optimal cell responses and involve a combination of serendipity and many series of trial-and-error experiments. For advanced tissue engineering and regenerative medicine, highly efficient and complex bioanalysis platforms are expected to explore the complex interaction of cells with biomaterials using combinatorial approaches that offer desired complex microenvironments during healing, development, and homeostasis. In this review, we first introduce materiobiology and its high-throughput screening (HTS). Then we present an in-depth of the recent progress of 2D/3D HTS platforms (i.e., gradient and microarray) in the principle, preparation, screening for materiobiology, and combination with other advanced technologies. The Compendium for Biomaterial Transcriptomics and high content imaging, computational simulations, and their translation toward commercial and clinical uses are highlighted. In the final section, current challenges and future perspectives are discussed. High-throughput experimentation within the field of materiobiology enables the elucidation of the relationships between biomaterial properties and biological behavior and thereby serves as a potential tool for accelerating the development of high-performance biomaterials.

Surface Wettability of a Natural Rubber Composite under Stretching: A Model to Predict Cell Survival

We report the stress-strain effect of a stretchable natural rubber (NR)-calcium phosphate composite on the surface wettability (SW) using an innovative approach coupling a uniaxial tensile micromachine, goniometer, and microscope. In situ contact angle measurements in real time were performed during mechanical tension. Our results show that SW is guided by the stress-strain relationship with two different characteristics, depending on the static or dynamic experiments. The results evidenced the limits of the classical theory of wetting. Furthermore, based on the mechanically tunable SW of the system associated with the cytocompatibility of the NR composite, we have modeled such a system for application as a cell support. From the experimental surface energy value, our proposed 3D modeling numerical simulation predicted a window of opportunities for cell-NR survival under mechanical stimuli. The presented data and the thermodynamics-based theoretical approach enable not only accurate correlation of SW with mechanical properties of the NR composite but also provide huge potential for future cell supportability in view of tissue engineering.

Biphasic response of T cell activation to substrate stiffness

T cell activation is sensitive to the mechanical properties of an activating substrate. However, there are also contrasting results on how substrate stiffness affects T cell activation, including differences between T cells of mouse and human origin. Towards reconciling these differences, this report examines the response of primary human T cells to polyacrylamide gels with stiffness between 5 and 110 kPa presenting activating antibodies to CD3 and CD28. T cell proliferation and IL-2 secretion exhibited a biphasic functional response to substrate stiffness, which can be shifted by changing density of activating antibodies and abrogated by inhibition of cellular contractility. T cell morphology was modulated by stiffness at early time points. RNA-seq indicates that T cells show differing monotonic trends in upregulated genes and pathways towards both ends of the stiffness spectrum. These studies provide a framework of T cell mechanosensing and suggest an effect of ligand density that may reconcile different, contrasting patterns of stiffness sensing seen in previous studies.

Hydrogel Models with Stiffness Gradients for Interrogating Pancreatic Cancer Cell Fate

Pancreatic ductal adenocarcinoma (PDAC) is the most common type of pancreatic cancer and has seen only modest improvements in patient survival rate over the past few decades. PDAC is highly aggressive and resistant to chemotherapy, owing to the presence of a dense and hypovascularized fibrotic tissue, which is composed of stromal cells and extracellular matrices. Increase deposition and crosslinking of matrices by stromal cells lead to a heterogeneous microenvironment that aids in PDAC development. In the past decade, various hydrogel-based, in vitro tumor models have been developed to mimic and recapitulate aspects of the tumor microenvironment in PDAC. Advances in hydrogel chemistry and engineering should provide a venue for discovering new insights regarding how matrix properties govern PDAC cell growth, migration, invasion, and drug resistance. These engineered hydrogels are ideal for understanding how variation in matrix properties contributes to the progressiveness of cancer cells, including durotaxis, the directional migration of cells in response to a stiffness gradient. This review surveys the various hydrogel-based, in vitro tumor models and the methods to generate gradient stiffness for studying migration and other cancer cell fate processes in PDAC.

Extracellular matrix in multicellular aggregates acts as a pressure sensor controlling cell proliferation and motility

Imposed deformations play an important role in morphogenesis and tissue homeostasis, both in normal and pathological conditions. To perceive mechanical perturbations of different types and magnitudes, tissues need appropriate detectors, with a compliance that matches the perturbation amplitude. By comparing results of selective osmotic compressions of CT26 mouse cells within multicellular aggregates and global aggregate compressions, we show that global compressions have a strong impact on the aggregates growth and internal cell motility, while selective compressions of same magnitude have almost no effect. Both compressions alter the volume of individual cells in the same way over a shor-timescale, but, by draining the water out of the extracellular matrix, the global one imposes a residual compressive mechanical stress on the cells over a long-timescale, while the selective one does not. We conclude that the extracellular matrix is as a sensor that mechanically regulates cell proliferation and migration in a 3D environment.

The Golgi microtubules regulate single cell durotaxis

Current understandings on cell motility and directionality rely heavily on accumulated investigations of the adhesion-actin cytoskeleton-actomyosin contractility cycles, while microtubules have been understudied in this context. Durotaxis, the ability of cells to migrate up gradients of substrate stiffness, plays a critical part in development and disease. Here, we identify the pivotal role of Golgi microtubules in durotactic migration of single cells. Using high-throughput analysis of microtubule plus ends/focal adhesion interactions, we uncover that these non-centrosomal microtubules actively impart leading edge focal adhesion (FA) dynamics. Furthermore, we designed a new system where islands of higher stiffness were patterned within RGD peptide coated polyacrylamide gels. We revealed that the positioning of the Golgi apparatus is responsive to external mechanical cues and that the Golgi-nucleus axis aligns with the stiffness gradient in durotaxis. Together, our work unveils the cytoskeletal underpinning for single cell durotaxis. We propose a model in which the Golgi-nucleus axis serves both as a compass and as a steering wheel for durotactic migration, dictating cell directionality through the interaction between non-centrosomal microtubules and the FA dynamics.

Polyethersulfone/MWCNT nanocomposite scaffold for endometrial cell culture: preparation, characterization, andin vitroinvestigation

Endometrial cell culture is a method for investigating physiological or pathological conditions or simulatingin vivoconditions for embryo culture. The natural function of the endometrium depends on a polarized epithelium and 3D stromal compartments. The polymer-based scaffolds of simple polyethersulfone (PES), laser scratched PES (PES-LS), and multiwall carbon nanotubes (MWCNT) composited PES (PES-MWCNT) were prepared and used for bovine endometrial cells (bECs) culture. For better investigation of the relationship between physical structure and cell growth behavior, the surface morphologies of the scaffolds were evaluated by atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM) techniques. Three synthesized membranes (PES, PES-LS, and PES-MWCNT) were evaluated for the cell morphology, viability and, doubling time. Results showed acceptable physical and chemical fabrication of the polymers with no significant differences in the proportions of live cells to primary cultured cells, dead to live cells, and the cell doubling time among groups during the experiment (P > 0.05). Total cell count (live and dead cells) was significantly different on Day 2 among types of polymers. The results showed the comparable potential of the PES-MWCNT membrane for the bECs culture.

Method for Investigating Fibroblast Durotaxis

Durotaxis is the phenomena of directed cell migration driven by gradients of extracellular matrix stiffness. Durotaxis has been recently involved in the development of fibrosis by promoting the recruitment of pathological fibroblasts to areas of established fibrosis, thus amplifying the fibrotic response. Here, we describe the fabrication of mechanically patterned hydrogels that can be used to investigate molecular mechanisms controlling durotaxis of fibroblasts and other cells with mechanosensing properties. This method effectively creates a stiffness gradient of 275 Pa/μm, mimicking the natural spatial stiffness variations we observed in fibrotic tissues from mouse models of fibrosis and human fibrotic diseases.

Mechanical and Physical Regulation of Fibroblast-Myofibroblast Transition: From Cellular Mechanoresponse to Tissue Pathology

Fibroblasts are cells present throughout the human body that are primarily responsible for the production and maintenance of the extracellular matrix (ECM) within the tissues. They have the capability to modify the mechanical properties of the ECM within the tissue and transition into myofibroblasts, a cell type that is associated with the development of fibrotic tissue through an acute increase of cell density and protein deposition. This transition from fibroblast to myofibroblast-a well-known cellular hallmark of the pathological state of tissues-and the environmental stimuli that can induce this transition have received a lot of attention, for example in the contexts of asthma and cardiac fibrosis. Recent efforts in understanding how cells sense their physical environment at the micro- and nano-scales have ushered in a new appreciation that the substrates on which the cells adhere provide not only passive influence, but also active stimulus that can affect fibroblast activation. These studies suggest that mechanical interactions at the cell-substrate interface play a key role in regulating this phenotype transition by changing the mechanical and morphological properties of the cells. Here, we briefly summarize the reported chemical and physical cues regulating fibroblast phenotype. We then argue that a better understanding of how cells mechanically interact with the substrate (mechanosensing) and how this influences cell behaviors (mechanotransduction) using well-defined platforms that decouple the physical stimuli from the chemical ones can provide a powerful tool to control the balance between physiological tissue regeneration and pathological fibrotic response.

Physical traits of cancer

The role of the physical microenvironment in tumor development, progression, metastasis, and treatment is gaining appreciation. The emerging multidisciplinary field of the physical sciences of cancer is now embraced by engineers, physicists, cell biologists, developmental biologists, tumor biologists, and oncologists attempting to understand how physical parameters and processes affect cancer progression and treatment. Discoveries in this field are starting to be translated into new therapeutic strategies for cancer. In this Review, we propose four physical traits of tumors that contribute to tumor progression and treatment resistance: (i) elevated solid stresses (compression and tension), (ii) elevated interstitial fluid pressure, (iii) altered material properties (for example, increased tissue stiffness, which historically has been used to detect cancer by palpation), and (iv) altered physical microarchitecture. After defining these physical traits, we discuss their causes, consequences, and how they complement the biological hallmarks of cancer.

Durotaxis: The Hard Path from In Vitro to In Vivo

Durotaxis, the process by which cells follow gradients of extracellular mechanical stiffness, has been proposed as a mechanism driving directed migration. Despite the lack of evidence for its existence in vivo, durotaxis has become an active field of research, focusing on the mechanism by which cells respond to mechanical stimuli from the environment. In this review, we describe the technical and conceptual advances in the study of durotaxis in vitro, discuss to what extent the evidence suggests durotaxis may occur in vivo, and emphasize the urgent need for in vivo demonstration of durotaxis.

Role of Hydrogels in Bone Tissue Engineering: How Properties Shape Regeneration

Bone defect that resulted from trauma, tumors, and other reasons is believed as a common clinical problem, which exists mainly in post-traumatic healing. Additionally, autologous/allogeneic transplantation, bone tissue engineering attracts increasing attention due to the existing problem of the limited donor. The applications of biomaterials can be considered as a rising and promising strategy for bone regeneration. Especially, hydrogel is featured with hydrophilic characteristic, good biocompatibility, and porous structure, which shows unique properties for bone regeneration. The main properties of hydrogel such as surface property, adhesive property, mechanical property, porosity, and degradation property, generally present influences on the migration, proliferation, and differentiation of mesenchymal stem cells exclusively or in combination, which consequently affect the regeneration of bones. This review mainly focuses on the theme: "how properties of hydrogel shape bone regeneration." Moreover, the latest progress achieved in the above mentioned direction is further discussed. Despite the fascinating advances researchers have made, certain potential challenges continue to exist in the research field, which need to be addressed for accelerating the clinical translation of hydrogel in bone regeneration.