Stem cell migration and mechanotransduction on linear stiffness gradient hydrogels

Gradient Hydrogels-Overview of Techniques Demonstrating the Existence of a Gradient

Gradient hydrogels are promising future materials which could be usable in tissue engineering (scaffolds), pharmaceutical (drug delivery systems with controlled release) and many others related disciplines. These hydrogels exhibit a more complex inner (gradient) structure (e.g., concentration gradient) than simple isotropic hydrogel. Gradient-structured hydrogels could be beneficial in, for example, understanding intercellular interactions. The fabrication of gradient hydrogels has been relatively deeply explored, but a comprehensive description of the physico-chemical techniques demonstrating the existence of a gradient structure is still missing. Here, we summarize the state-of-the-art available experimental techniques applicable in proving and/or describing in physico-chemical terms the inner gradient structure of hydrogels. The aim of this paper is to give the reader an overview of the existing database of suitable techniques for characterizing gradient hydrogels.

Insights into the present and future of cartilage regeneration and joint repair

Knee osteoarthritis is the most common joint disease. It causes pain and suffering for affected patients and is the source of major economic costs for healthcare systems. Despite ongoing research, there is a lack of knowledge regarding disease mechanisms, biomarkers, and possible cures. Current treatments do not fulfill patients' long-term needs, and it often requires invasive surgical procedures with subsequent long periods of rehabilitation. Researchers and companies worldwide are working to find a suitable cell source to engineer or regenerate a functional and healthy articular cartilage tissue to implant in the damaged area. Potential cell sources to accomplish this goal include embryonic stem cells, mesenchymal stem cells, or induced pluripotent stem cells. The differentiation of stem cells into different tissue types is complex, and a suitable concentration range of specific growth factors is vital. The cellular microenvironment during early embryonic development provides crucial information regarding concentrations of signaling molecules and morphogen gradients as these are essential inducers for tissue development. Thus, morphogen gradients implemented in developmental protocols aimed to engineer functional cartilage tissue can potentially generate cells comparable to those within native cartilage. In this review, we have summarized the problems with current treatments, potential cell sources for cell therapy, reviewed the progress of new treatments within the regenerative cartilage field, and highlighted the importance of cell quality, characterization assays, and chemically defined protocols.

Elucidating the combinatorial effect of substrate stiffness and surface viscoelasticity on cellular phenotype

Cells maintain tensional homeostasis by monitoring the mechanics of their microenvironment. In order to understand this mechanotransduction phenomenon, hydrogel materials have been developed with either controllable linear elastic or viscoelastic properties. Native biological tissues, and biomaterials used for medical purposes, often have complex mechanical properties. However, due to the difficulty in completely decoupling the elastic and viscous components of hydrogel materials, the effect of complex composite materials on cellular responses has largely gone unreported. Here, we characterize a novel composite hydrogel system capable of decoupling and individually controlling both the bulk stiffness and surface viscoelasticity of the material by combining polyacrylamide (PA) gels with microgel thin films. By taking advantage of the high degree of control over stiffness offered by PA gels and viscoelasticity, in terms of surface loss tangent, of microgel thin films, it is possible to study the influence that bulk substrate stiffness and surface loss tangent have on complex fibroblast responses, including cellular and nuclear morphology and gene expression. This material system provides a facile method for investigating cellular responses to complex material mechanics with great precision and allows for a greater understanding of cellular mechanotransduction mechanisms than previously possible through current model material platforms.

Harder, better, faster, stronger: biochemistry and biophysics in the immunosurveillance concert

Antitumor immunosurveillance is triggered by immune cell recognition of characteristic biochemical signals on the surfaces of cancer cells. Recent data suggest that the mechanical properties of cancer cells influence the strength of these signals, with physically harder target cells (more rigid) eliciting better, faster, and stronger cytotoxic responses against metastasis. Using analogies to a certain electronic music duo, we argue that the biophysical properties of cancer cells and their environment can adjust the volume and tone of the antitumor immune response. We also consider the potential influence of biomechanics-based immunosurveillance in disease progression and posit that targeting the biophysical properties of cancer cells in concert with their biochemical features could increase the efficacy of immunotherapy.

Cell Adhesion and Migration on Thickness Gradient Bilayer Polymer Brush Surfaces: Effects of Properties of Polymeric Materials of the Underlayer

In the field of tissue engineering and biomaterials, controlling the surface properties and mechanical properties of scaffold materials is crucial and has attracted much attention. Here, two types of bilayer polymer brushes composed of a hydrophilic underlying layer and a cationic surface layer [made of poly(2-aminoethyl methacrylate)] with a thickness gradient were prepared by surface-initiated atom-transfer radical polymerization. To investigate the influence of the stiffness as a mechanical property of the polymer brush on cell behavior, the underlayer was prepared from either 2-methacryloyloxyethyl phosphorylcholine or oligo(ethylene glycol) methyl ether methacrylate, with the bilayers designated as gradient poly(2-methacryloyloxyethyl phosphorylcholine)-block-poly(2-aminoethyl methacrylate) [grad-pMbA] and gradient poly(oligo[ethylene glycol] methyl ether methacrylate)-block-poly(2-aminoethyl methacrylate) [grad-pEGbA], respectively. Characterization of these surfaces was performed by spectroscopic ellipsometry, X-ray reflectivity, and determination of the zeta potential, static contact angle, and force curve. These diblock copolymer brushes with a thickness gradient helped to distinguish the effects of the mechanical and surface properties of the brushes on cell behavior. The attachment and motility of L929 fibroblasts and epithelial MCF 10A cells on the fabricated brushes were then assessed. L929 cells had a round shape on the thin surface layer of grad-pMbA and spread well on thicker areas. In contrast, MCF 10A cells spread well in areas of any thickness of either grad-pMbA or grad-pEGbA. Single MCF 10A cells migrated randomly on grad-pMbA, whereas grouped cells started to climb up along the thickness gradient of grad-pMbA. In contrast, both single and grouped MCF 10A cells migrated randomly on grad-pEGbA. These thickness gradient diblock copolymer brushes are simple, reproducible, and reasonable platforms that can facilitate practical applications of biomaterials, for example, in tissue engineering and biomaterials.

Analysis of Three-Dimensional Cell Migration in Dopamine-Modified Poly(aspartic acid)-Based Hydrogels

Several types of promising cell-based therapies for tissue regeneration have been developing worldwide. However, for successful therapeutical application of cells in this field, appropriate scaffolds are also required. Recently, the research for suitable scaffolds has been focusing on polymer hydrogels due to their similarity to the extracellular matrix. The main limitation regarding amino acid-based hydrogels is their difficult and expensive preparation, which can be avoided by using poly(aspartamide) (PASP)-based hydrogels. PASP-based materials can be chemically modified with various bioactive molecules for the final application purpose. In this study, dopamine containing PASP-based scaffolds is investigated, since dopamine influences several cell biological processes, such as adhesion, migration, proliferation, and differentiation, according to the literature. Periodontal ligament cells (PDLCs) of neuroectodermal origin and SH-SY5Y neuroblastoma cell line were used for the in vitro experiments. The chemical structure of the polymers and hydrogels was proved by ¹H-NMR and FTIR spectroscopy. Scanning electron microscopical (SEM) images confirmed the suitable pore size range of the hydrogels for cell migration. Cell viability assay was carried out according to a standardized protocol using the WST-1 reagent. To visualize three-dimensional cell distribution in the hydrogel matrix, two-photon microscopy was used. According to our results, dopamine containing PASP gels can facilitate vertical cell penetration from the top of the hydrogel in the depth of around 4 cell layers (~150 μm). To quantify these observations, a detailed image analysis process was developed and firstly introduced in this paper.

Nanofiber curvature with Rho GTPase activity increases mouse embryonic fibroblast random migration velocity

Mechanotransduction arises from information encoded in the shape of materials such as curvature. It induces activation of small GTPase signaling affecting cell phenotypes including differentiation. We carried out a set of preliminary experiments to test the hypothesis that curvature (1/radius) would also affect cell motility due to signal pathway crosstalk. High molecular weight poly (methyl methacrylate) straight nanofibers were electrospun with curvature ranging from 41 to 1 μm-1 and collected on a passivated glass substrate. The fiber curvature increased mouse mesenchymal stem cell aspect ratio (P < 0.02) and decreased cell area (P < 0.01). Despite little effect on some motility patterns such as polarity and persistence, we found selected fiber curvatures can increase normalized random fibroblastic mouse embryonic cell (MEF) migration velocity close to 2.5 times compared with a flat surface (P < 0.001). A maximum in the velocity curve occurred near 2.5 μm-1 and may vary with the time since initiation of attachment to the surface (range of 0-20 h). In the middle range of fiber curvatures, the relative relationship to curvature was similar regardless of treatment with Rho-kinase inhibitor (Y27632) or cdc42 inhibitor (ML141), although it was decreased on most curvatures (P < 0.05). However, below a critical curvature threshold MEFs may not be able to distinguish shallow curvature from a flat surface, while still being affected by contact guidance. The preliminary data in this manuscript suggested the large low curvature fibers were interpreted in a manner similar to a non-curved surface. Thus, curvature is a biomaterial construct design parameter that should be considered when specific biological responses are desired. Statement of integration, innovation, and insight  Replacement of damaged or diseased tissues that cannot otherwise regenerate is transforming modern medicine. However, the extent to which we can rationally design materials to affect cellular outcomes remains low. Knowing the effect of material stiffness and diameter on stem cell differentiation, we investigated cell migration and signaling on fibrous scaffolds. By investigating diameters across orders of magnitude (50-2000 nm), we identified a velocity maximum of ~800 nm. Furthermore, the results suggest large fibers may not be interpreted by single cells as a curved surface. This work presents insight into the design of constructs for engineering tissues.

Perspectives on scaling production of adipose tissue for food applications

With rising global demand for food proteins and significant environmental impact associated with conventional animal agriculture, it is important to develop sustainable alternatives to supplement existing meat production. Since fat is an important contributor to meat flavor, recapitulating this component in meat alternatives such as plant based and cell cultured meats is important. Here, we discuss the topic of cell cultured or tissue engineered fat, growing adipocytes in vitro that could imbue meat alternatives with the complex flavor and aromas of animal meat. We outline potential paths for the large scale production of in vitro cultured fat, including adipogenic precursors during cell proliferation, methods to adipogenically differentiate cells at scale, as well as strategies for converting differentiated adipocytes into 3D cultured fat tissues. We showcase the maturation of knowledge and technology behind cell sourcing and scaled proliferation, while also highlighting that adipogenic differentiation and 3D adipose tissue formation at scale need further research. We also provide some potential solutions for achieving adipose cell differentiation and tissue formation at scale based on contemporary research and the state of the field.

Strategies for improving adipose-derived stem cells for tissue regeneration

Adipose-derived stem cells (ADSCs) have promising applications in tissue regeneration. Currently, there are only a few ADSC products that have been approved for clinical use. The clinical application of ADSCs still faces many challenges. Here, we review emerging strategies to improve the therapeutic efficacy of ADSCs in tissue regeneration. First, a great quantity of cells is often needed for the stem cell therapies, which requires the advanced cell expansion technologies. In addition cell-derived products are also required for the development of ‘cell-free’ therapies to overcome the drawbacks of cell-based therapies. Second, it is necessary to strengthen the regenerative functions of ADSCs, including viability, differentiation and paracrine ability, for the tissue repair and regeneration required for different physiological and pathophysiological conditions. Third, poor delivery efficiency also restricts the therapeutic effect of ADSCs. Effective methods to improve cell delivery include alleviating harsh microenvironments, enhancing targeting ability and prolonging cell retention. Moreover, we also point out some critical issues about the sources, effectiveness and safety of ADSCs. With these advanced strategies to improve the therapeutic efficacy of ADSCs, ADSC-based treatment holds great promise for clinical applications in tissue regeneration.

Optical coherence elastography for assessing the influence of intraocular pressure on elastic wave dispersion in the cornea

The cornea is a highly specialized organ that relies on its mechanical stiffness to maintain its aspheric geometry and refractive power, and corneal diseases such as keratoconus have been linked to abnormal tissue stiffness and biomechanics. Dynamic optical coherence elastography (OCE) is a clinically promising non-contact and non-destructive imaging technique that can provide measurements of corneal tissue stiffness directly in vivo. The method relies on the concepts of elastography where shear waves are generated and imaged within a tissue to obtain mechanical properties such as tissue stiffness. The accuracy of OCE-based measurements is ultimately dependent on the mathematical theories used to model wave behavior in the tissue of interest. In the cornea, elastic waves propagate as guided wave modes which are highly dispersive and can be mathematically complex to model. While recent groups have developed detailed theories for estimating corneal tissue properties from guided wave behavior, the effects of intraocular pressure (IOP)-induced prestress have not yet been considered. It is known that prestress alone can strongly influence wave behavior, in addition to the associated non-linear changes in tissue properties. This present study shows that failure to account for the effects of prestress may result in overestimations of the corneal shear moduli, particularly at high IOPs. We first examined the potential effects of IOP and IOP-induced prestress using a combination of approximate mathematical theories describing wave behavior in thin plates with observations made from data published in the OCE literature. Through wave dispersion analysis, we deduce that IOP introduces a tensile hoop stress and may also influence an elastic foundational effect that were observable in the low-frequency components of the dispersion curves. These effects were incorporated into recently developed models of wave behavior in nearly incompressible, transversely isotropic (NITI) materials. Fitting of the modified NITI model with ex vivo porcine corneal data demonstrated that incorporation of the effects of IOP resulted in reduced estimates of corneal shear moduli. We believe this demonstrates that overestimation of corneal stiffness occurs if IOP is not taken into consideration. Our work may be helpful in separating inherent corneal stiffness properties that are independent of IOP; changes in these properties and in IOP are distinct, clinically relevant issues that affect the cornea health.

Tuning Hydrogels by Mixing Dynamic Cross-Linkers: Enabling Cell-Instructive Hydrogels and Advanced Bioinks

Rational design of hydrogels that balance processability and extracellular matrix (ECM) biomimicry remains a challenge for tissue engineering and biofabrication. Hydrogels suitable for biofabrication techniques, yet tuneable to match the mechanical (static and dynamic) properties of native tissues remain elusive. Dynamic covalent hydrogels possessing shear-thinning/self-healing (processability) and time-dependent cross-links (mechanical properties) provide a potential solution, yet can be difficult to rationally control. Here, the straightforward modular mixing of dynamic cross-links with different timescales (hydrazone and oxime) is explored using rheology, self-healing tests, extrusion printing, and culture of primary human dermal fibroblasts. Maintaining a constant polymer content and cross-linker concentration, the stiffness and stress relaxation can be tuned across two orders of magnitude. All formulations demonstrate a similar flow profile after network rupture, allowing the separation of initial mechanical properties from flow behavior during printing. Furthermore, the self-healing nature of hydrogels with high hydrazone content enables recyclability of printed structures. Last, a distinct threshold for cell spreading and morphology is observed within this hydrogel series, even in multi-material constructs. Simple cross-linker mixing enables fine control and is of general interest for bioink development, targeting viscoelastic properties of specific cellular niches, and as an accessible and flexible platform for designing dynamic networks.

Optical coherence tomography

Optical coherence tomography (OCT) is a non-contact method for imaging the topological and internal microstructure of samples in three dimensions. OCT can be configured as a conventional microscope, as an ophthalmic scanner, or using endoscopes and small diameter catheters for accessing internal biological organs. In this Primer, we describe the principles underpinning the different instrument configurations that are tailored to distinct imaging applications and explain the origin of signal, based on light scattering and propagation. Although OCT has been used for imaging inanimate objects, we focus our discussion on biological and medical imaging. We examine the signal processing methods and algorithms that make OCT exquisitely sensitive to reflections as weak as just a few photons and that reveal functional information in addition to structure. Image processing, display and interpretation, which are all critical for effective biomedical imaging, are discussed in the context of specific applications. Finally, we consider image artifacts and limitations that commonly arise and reflect on future advances and opportunities.

Dual-Light-Triggered In Situ Structure and Function Regulation of Injectable Hydrogels for High-Efficient Anti-Infective Wound Therapy

Most injectable hydrogels used in biomedical engineering have unsatisfactory and untunable mechanical properties, making it difficult to match them with the mechanical strengths of different tissues and organs, which can cause a series of adverse consequences such as immune rejection and soft tissue contusion. In this contribution, dopamine-modified hyaluronic acid (HA-DA) is developed as the backbone for an injectable hydrogel using a catechol-Fe³⁺ coordination crosslinking strategy. Due to dynamic physical crosslinking, the hydrogel can be easily injected through a single syringe. Into the hydrogel, black phosphorous nanosheets loaded with a Zr-based porphyrinic metal-organic framework (PCN@BP) are introduced that could generate reactive oxygen species (ROS) under 660 nm laser irradiation, this promotes the oxidative coupling of dopamine in the presence of the ROS, introducing in situ chemical crosslinking into the hydrogel. A physical/chemical double-crosslinked hydrogel is obtained, effectively improving the hydrogel's mechanical properties, which are tuned in situ by adjusting the irradiation time to match the mechanical modulus of different biological tissues. Combining the excellent photothermal properties and photodynamic performance of the PCN@BP nanosheets yields effective sterilization under mild conditions (below 50 °C, low ROS production). The results show that this hydrogel is an excellent multifunctional wound dressing.

Recent advances of 3D hydrogel culture systems for mesenchymal stem cell-based therapy and cell behavior regulation

Mesenchymal stem cells (MSCs) have been increasingly recognized as resources for disease treatments and regenerative medicine. Meanwhile, the unique chemical and physical properties of hydrogels provide innate advantages to achieve...

Graphene-Oxide Porous Biopolymer Hybrids Enhance In Vitro Osteogenic Differentiation and Promote Ectopic Osteogenesis In Vivo

Over the years, natural-based scaffolds have presented impressive results for bone tissue engineering (BTE) application. Further, outstanding interactions have been observed during the interaction of graphene oxide (GO)-reinforced biomaterials with both specific cell cultures and injured bone during in vivo experimental conditions. This research hereby addresses the potential of fish gelatin/chitosan (GCs) hybrids reinforced with GO to support in vitro osteogenic differentiation and, further, to investigate its behavior when implanted ectopically. Standard GCs formulation was referenced against genipin (Gp) crosslinked blend and 0.5 wt.% additivated GO composite (GCsGp/GO 0.5 wt.%). Pre-osteoblasts were put in contact with these composites and induced to differentiate in vitro towards mature osteoblasts for 28 days. Specific bone makers were investigated by qPCR and immunolabeling. Next, CD1 mice models were used to assess de novo osteogenic potential by ectopic implantation in the subcutaneous dorsum pocket of the animals. After 4 weeks, alkaline phosphate (ALP) and calcium deposits together with collagen synthesis were investigated by biochemical analysis and histology, respectively. Further, ex vivo materials were studied after surgery regarding biomineralization and morphological changes by means of qualitative and quantitative methods. Furthermore, X-ray diffraction and Fourier-transform infrared spectroscopy underlined the newly fashioned material structuration by virtue of mineralized extracellular matrix. Specific bone markers determination stressed the osteogenic phenotype of the cells populating the material in vitro and successfully differentiated towards mature bone cells. In vivo results of specific histological staining assays highlighted collagen formation and calcium deposits, which were further validated by micro-CT. It was observed that the addition of 0.5 wt.% GO had an overall significant positive effect on both in vitro differentiation and in vivo bone cell recruitment in the subcutaneous region. These data support the GO bioactivity in osteogenesis mechanisms as being self-sufficient to elevate osteoblast differentiation and bone formation in ectopic sites while lacking the most common osteoinductive agents.

Cellular modulation by the mechanical cues from biomaterials for tissue engineering

Mechanical cues from the extracellular matrix (ECM) microenvironment are known to be significant in modulating the fate of stem cells to guide developmental processes and maintain bodily homeostasis. Tissue engineering has provided a promising approach to the repair or regeneration of damaged tissues. Scaffolds are fundamental in cell-based regenerative therapies. Developing artificial ECM that mimics the mechanical properties of native ECM would greatly help to guide cell functions and thus promote tissue regeneration. In this review, we introduce various mechanical cues provided by the ECM including elasticity, viscoelasticity, topography, and external stimuli, and their effects on cell behaviours. Meanwhile, we discuss the underlying principles and strategies to develop natural or synthetic biomaterials with different mechanical properties for cellular modulation, and explore the mechanism by which the mechanical cues from biomaterials regulate cell function toward tissue regeneration. We also discuss the challenges in multimodal mechanical modulation of cell behaviours and the interplay between mechanical cues and other microenvironmental factors.

Reversible dynamic mechanics of hydrogels for regulation of cellular behavior

The mechanical properties of the native extracellular matrix play a key role in regulating cell behavior during developmental, healing and homeostatic processes. Since these properties change over time, it may be valuable to have the capacity to dynamically vary the mechanical properties of engineered hydrogels used in tissue engineering strategies to better mimic the dynamic mechanical behavior of native extracellular matrix. However, in situ repeatedly reversible dynamic tuning of hydrogel mechanics is still limited. In this study, we have engineered a hydrogel system with reversible dynamic mechanics using a dual-crosslinkable alginate hydrogel. The effect of reversible mechanical signals on encapsulated stem cells in dynamically tunable hydrogels has been demonstrated. In situ stiffening of hydrogels decreases cell spreading and proliferation, and subsequent softening of hydrogels gives way to an increase in cell spreading and proliferation. The hydrogel stiffening and softening, and resulting cellular responses are repeatedly reversible. This hydrogel system provides a promising platform for investigating the effect of repeatedly reversible changes in extracellular matrix mechanics on cell behaviors. STATEMENT OF SIGNIFICANCE: Since the mechanical properties of native extracellular matrix (ECM) change over time during development, healing and homeostatic processes, it may be valuable to have the capacity to dynamically control the mechanics of biomaterials used in tissue engineering and regenerative medicine applications to better mimic this behavior. Unlike previously reported biomaterials whose mechanical properties can be changed by the user only a limited number of times, this system provides the capacity to induce unlimited alterations to the mechanical properties of an engineered ECM for 3D cell culture. This study presents a strategy for on-demand dynamic and reversible control of materials' mechanics by single and dual-crosslinking mechanisms using oxidized and methacrylated alginates. By demonstrating direct changes in encapsulated human mesenchymal stem cell morphology, proliferation and chondrogenic differentiation in response to multiple different dynamic changes in hydrogel mechanics, we have established a repeatedly reversible 3D cellular mechanosensing system. This system provides a powerful platform tool with a wide range of stiffness tunability to investigate the role of dynamic mechanics on cellular mechanosensing and behavioral responses.

Articular cartilage and osteochondral tissue engineering techniques: Recent advances and challenges

In spite of the considerable achievements in the field of regenerative medicine in the past several decades, osteochondral defect regeneration remains a challenging issue among diseases in the musculoskeletal system because of the spatial complexity of osteochondral units in composition, structure and functions. In order to repair the hierarchical tissue involving different layers of articular cartilage, cartilage-bone interface and subchondral bone, traditional clinical treatments including palliative and reparative methods have showed certain improvement in pain relief and defect filling. It is the development of tissue engineering that has provided more promising results in regenerating neo-tissues with comparable compositional, structural and functional characteristics to the native osteochondral tissues. Here in this review, some basic knowledge of the osteochondral units including the anatomical structure and composition, the defect classification and clinical treatments will be first introduced. Then we will highlight the recent progress in osteochondral tissue engineering from perspectives of scaffold design, cell encapsulation and signaling factor incorporation including bioreactor application. Clinical products for osteochondral defect repair will be analyzed and summarized later. Moreover, we will discuss the current obstacles and future directions to regenerate the damaged osteochondral tissues.

Contact guidance via heterogeneity of substrate elasticity

Contact guidance, the widely-known phenomenon of cell alignment, is an essential step in the organization of adherent cells. This guidance is known to occur by, amongst other things, anisotropic features in the environment including elastic heterogeneity. To understand the origins of this guidance we employed a novel statistical thermodynamics framework, which recognises the non-thermal fluctuations in the cellular response, for modelling the response of the cells seeded on substrates with alternating soft and stiff stripes. Consistent with observations, the modelling framework predicts the existence of three regimes of cell guidance: (i) in regime I for stripe widths much larger than the cell size guidance is primarily entropic; (ii) for stripe widths on the order of the cell size in regime II guidance is biochemically mediated and accompanied by changes to the cell morphology while (iii) in regime III for stripe widths much less than the cell size there is no guidance as cells cannot sense the substrate heterogeneity. Guidance in regimes I and II is due to "molli-avoidance" with cells primarily residing on the stiff stripes. While the molli-avoidance tendency is not lost with decreasing density of collagen coating the substrate, the reduced focal adhesion formation with decreasing collagen density tends to inhibit contact guidance. Our results provide clear physical insights into the interplay between cell mechano-sensitivity and substrate elastic heterogeneity that ultimately leads to the contact guidance of cells in heterogeneous tissues. STATEMENT OF SIGNIFICANCE: Cellular morphology and organization play a crucial role in the micro-architecture of tissues and dictates their biological and mechanical functioning. Despite the importance of cellular organization in all facets of tissue biology, the fundamental question of how a cell organizes itself in an anisotropic environment is still poorly understood. We employ a novel statistical thermodynamics framework which recognises the non-thermal fluctuations in the cellular response to investigate cell guidance on substrates with alternating soft and stiff stripes. The propensity of cells to primarily reside on stiff stripes results in strong guidance when the period of the stripes is larger than the cell size. For smaller stripe periods, cells sense a homogeneous substrate and guidance is lost.

Self-renewal or quiescence? Orchestrating the fate of mesenchymal stem cells by matrix viscoelasticity via PI3K/Akt-CDK1 pathway

To control the fate of mesenchymal stem cells (MSCs) in a 3D environment by adjusting the mechanical parameters of MSC-loading scaffolds, is one of the hot topics in the field of regenerative biomaterials. However, a thorough understanding of the relevant MSCs behaviors affected by viscoelasticity, a dynamic physical parameter of scaffolds, is still lacking. Herein, we established an alginate hydrogel system with constant stiffness and tunable stress relaxation rate, which is a key parameter for the viscoelastic property of material. MSCs were cultured inside three groups of alginate hydrogels with various stress relaxation rates, and then RNA-seq analysis of cells was performed. Results showed that the change of stress relaxation rates of hydrogels regulated the most of the different expression genes of MSCs, which were enriched in cell proliferation-related pathways. MSCs cultured in hydrogels with fast stress relaxation rate presented a high self-renewal proliferation profile via activating phosphatidylinositol 3- kinase (PI3K)/protein kinase B (Akt) pathway. In contrast, a slow stress relaxation rate of hydrogels induced MSCs to enter a reversible quiescence state due to the weakened PI3K/Akt activation. Combined with a further finite element analysis, we speculated that the quiescence of MSCs could be served as a positive strategy for MSCs to deal with the matrix with a low deformation to keep stemness. Based on the results, we identified that stress relaxation rate of hydrogel was a potential physical factor of hydrogel to regulate the self-renewal or quiescence of MSCs. Thus, our findings provide a significant guiding principle for the design of MSCs-encapsulated biomaterials.

Modelling the central nervous system: tissue engineering of the cellular microenvironment

With the increasing prevalence of neurodegenerative diseases, improved models of the central nervous system (CNS) will improve our understanding of neurophysiology and pathogenesis, whilst enabling exploration of novel therapeutics. Studies of brain physiology have largely been carried out using in vivo models, ex vivo brain slices or primary cell culture from rodents. Whilst these models have provided great insight into complex interactions between brain cell types, key differences remain between human and rodent brains, such as degree of cortical complexity. Unfortunately, comparative models of human brain tissue are lacking. The development of induced Pluripotent Stem Cells (iPSCs) has accelerated advancement within the field of in vitro tissue modelling. However, despite generating accurate cellular representations of cortical development and disease, two-dimensional (2D) iPSC-derived cultures lack an entire dimension of environmental information on structure, migration, polarity, neuronal circuitry and spatiotemporal organisation of cells. As such, researchers look to tissue engineering in order to develop advanced biomaterials and culture systems capable of providing necessary cues for guiding cell fates, to construct in vitro model systems with increased biological relevance. This review highlights experimental methods for engineering of in vitro culture systems to recapitulate the complexity of the CNS with consideration given to previously unexploited biophysical cues within the cellular microenvironment.

Cell Type Dependent Suppression of Inflammatory Mediators by Myocardin Related Transcription Factors

Myocardin related transcription factors (MRTFs: MYOCD/myocardin, MRTF-A, and MRTF-B) play a key role in smooth muscle cell differentiation by activating contractile genes. In atherosclerosis, MRTF levels change, and most notable is a fall of MYOCD. Previous work described anti-inflammatory properties of MRTF-A and MYOCD, occurring through RelA binding, suggesting that MYOCD reduction could contribute to vascular inflammation. Recent studies have muddled this picture showing that MRTFs may show both anti- and pro-inflammatory properties, but the basis of these discrepancies remain unclear. Moreover, the impact of MRTFs on inflammatory signaling pathways in tissues relevant to human arterial disease is uncertain. The current work aimed to address these issues. RNA-sequencing after forced expression of myocardin in human coronary artery smooth muscle cells (hCASMCs) showed reduction of pro-inflammatory transcripts, including CCL2, CXCL8, IL6, and IL1B. Side-by-side comparison of MYOCD, MRTF-A, and MRTF-B in hCASMCs, showed that the anti-inflammatory impact was shared among MRTFs. Correlation analyses using human arterial transcriptomic datasets revealed negative correlations between MYOCD, MRTFA, and SRF, on the one hand, and the inflammatory transcripts, on the other. A pro-inflammatory drive from lipopolysaccharide, did not change the size of the suppressive effect of MRTF-A in hCASMCs on either mRNA or protein levels. To examine cell type-dependence, we compared the anti-inflammatory impact in hCASMCs, with that in human bladder SMCs, in endothelial cells, and in monocytes (THP-1 cells). Surprisingly, little anti-inflammatory activity was seen in endothelial cells and monocytes, and in bladder SMCs, MRTF-A was pro-inflammatory. CXCL8, IL6, and IL1B were increased by the MRTF-SRF inhibitor CCG-1423 and by MRTF-A silencing in hCASMCs, but depolymerization of actin, known to inhibit MRTF activity, had no stimulatory effect, an exception being IL1B. Co-immunoprecipitation supported binding of MRTF-A to RelA, supporting sequestration of this important pro-inflammatory mediator as a mechanism. Dexamethasone treatment and silencing of RelA (by 76 ± 1%) however only eliminated a fraction of the MRTF-A effect (≈25%), suggesting mechanisms beyond RelA binding. Indeed, SRF silencing suggested that MRTF-A suppression of IL1B and CXCL8 depends on SRF. This work thus supports an anti-inflammatory impact of MRTF-SRF signaling in hCASMCs and in intact human arteries, but not in several other cell types.

Nanoscale Surface Topography Reduces Focal Adhesions and Cell Stiffness by Enhancing Integrin Endocytosis

Both substrate stiffness and surface topography regulate cell behavior through mechanotransduction signaling pathways. Such intertwined effects suggest that engineered surface topographies might substitute or cancel the effects of substrate stiffness in biomedical applications. However, the mechanisms by which cells recognize topographical features are not fully understood. Here we demonstrate that the presence of nanotopography drastically alters cell behavior such that neurons and stem cells cultured on rigid glass substrates behave as if they were on soft hydrogels. With atomic force microscopy, we show that rigid nanotopography resembles the effects of soft hydrogels in reducing cell stiffness and membrane tension. Further, we reveal that nanotopography reduces focal adhesions and cell stiffness by enhancing the endocytosis and the subsequent removal of integrin receptors. This mechanistic understanding will support the rational design of nanotopography that directs cells on rigid materials to behave as if they were on soft ones.

Interrogating cardiac muscle cell mechanobiology on stiffness gradient hydrogels

Extracellular matrix (ECM) remodeling is a major facet of cardiac development and disease, yet our understanding of cardiomyocyte mechanotransduction remains limited. To enhance our understanding of cardiomyocyte mechanosensation, we studied stiffness-driven changes to cell morphology and mechanomarker expression in H9C2 cells and neonatal rat cardiomyocytes (NRCMs). Linear stiffness gradient polyacrylamide hydrogels (2-33 kPa) coated with ECM proteins including Collagen I (Col), Fibronectin (Fn) or Laminin (Ln) were used to represent necrotic, healthy, and infarcted cardiac tissue on a continuous stiffness gradient. Cell size, cell shape and nuclear size were found to be mechanosensitive in H9C2 cells, as was the expression or nuclear translocalization of the mechanomarkers Lamin-A, YAP, and MRTF-A. Minor differences were observed between the different ECM coatings, with the same overarching stiffness-dependent trends being observed across Col, Fn and Ln coated hydrogels. Inhibition of mechanotransduction in H9C2 cells using blebbistatin or Y27632 resulted in disruptions to cell shape, nuclear shape, and nuclear size, however, trends in cell size and mechanomarker expression were not significantly attenuated. Mechanosensation in NRCMs was much less marked, with no significant changes in cell morphology being detected, although YAP did become increasingly nuclear localized with increasing stiffness. In α-actinin positive cells, striations formed with regular structure and frequency at all stiffnesses for Col and Fn coated hydrogels, but not Ln coated gels. In this study, we used our stiffness gradient hydrogels to comprehensively map the relationship between ECM stiffness and cardiac cell phenotype and found that less mature H9C2 cardiac cells are more sensitive to ECM changes than the more developed neonatal cardiomyocytes.

Nisin-based antibacterial and antiadhesive layer-by-layer coatings

Some removable medical devices such as catheters and cardiovascular biomaterials require antiadhesive properties towards both prokaryotic and eukaryotic cells in order to prevent the tissues from infections upon implantation and, from alteration upon removal. In order to inhibit cell adhesion, we developed ultrathin hydrated Layer-by-Layer (LbL) coatings composed of biocompatible polyelectrolytes, namely chondroitin sulfate A (CSA) and poly-l-lysine (PLL). The coatings were crosslinked with genipin (GnP), a natural and biocompatible crosslinking agent, to increase their resistance against environmental changes. In order to confer antibacterial activity to the coatings, we proceeded to the electrostatically-driven immobilization of nisin Z, an antimicrobial peptide (AMP) active against gram-positive bacteria. The nisin-enriched coatings had a significantly increased anti-proliferative impact on fibroblasts, as well as a strong contact-killing activity against Staphylococcus aureus in the short and long term.

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.

Keloid fibroblasts have elevated and dysfunctional mechanotransduction signaling that is independent of TGF-β

BACKGROUND

Fibroblasts found in keloid tissues are known to present an altered sensitivity to microenvironmental stimuli. However, the impact of changes in extracellular matrix stiffness on phenotypes of normal fibroblasts (NFs) and keloid fibroblasts (KFs) is poorly understood.

OBJECTIVES

Investigation the impact of matrix stiffness on NFs and KFs mainly via detecting yes-associated protein (YAP) expression.

METHODS

We used fibronectin-coated polyacrylamide hydrogel substrates with a range from physiological to pathological stiffness values with or without TGF-β (fibrogenic inducer). Atomic force microscopy was used to measure the stiffness of fibroblasts. Cellular mechanoresponses were screened by immunocytochemistry, Western blot and Luminex assay.

RESULTS

KFs are stiffer than NFs with greater expression of α-SMA. In NFs, YAP nuclear translocation was induced by increasing matrix stiffness as well as by stimulation with TGF-β. In contrast, KFs showed higher baseline levels of nuclear YAP that was not responsive to matrix stiffness or TGF-β. TGF-β1 induced p-SMAD3 in both KFs and NFs, demonstrating the pathway was functional and not hyperactivated in KFs. Moreover, blebbistatin suppressed α-SMA expression and cellular stiffness in KFs, linking the elevated YAP signaling to keloid phenotype.

CONCLUSIONS

These data suggest that whilst normal skin fibroblasts respond to matrix stiffness in vitro, keloid fibroblasts have elevated activation of mechanotransduction signaling insensitive to the microenvironment. This elevated signaling appears linked to the expression of α-SMA, suggesting a direct link to disease pathogenesis. These findings suggest changes to keloid fibroblast phenotype related to mechanotransduction contribute to disease and may be a useful therapeutic target.

Regulation of the Muscarinic M3 Receptor by Myocardin-Related Transcription Factors

Myocardin-related transcription factors (MRTFs: myocardin/MYOCD, MRTF-A/MRTFA, and MRTF-B/MRTFB) are co-factors of serum response factor (SRF) that activate the smooth muscle cell (SMC) gene program and that play roles in cardiovascular development and mechanobiology. Gain and loss of function experiments have defined the SMC gene program under control of MRTFs, yet full understanding of their impact is lacking. In the present study, we tested the hypothesis that the muscarinic M₃ receptor (CHRM3) is regulated by MRTFs together with SRF. Forced expression of MYOCD (8d) in human coronary artery (SMC) followed by RNA-sequencing showed increased levels of M₂, M₃, and M₅ receptors (CHRM2: 2-fold, CHRM3: 16-fold, and CHRM5: 2-fold). The effect of MYOCD on M₃ was confirmed by RT-qPCR using both coronary artery and urinary bladder SMCs, and correlation analyses using human transcriptomic datasets suggested that M₃ may also be regulated by MRTF-B. Head-to-head comparisons of MYOCD, MRTF-A and MRTF-B, argued that while all MRTFs are effective, MRTF-B is the most powerful transactivator of CHRM3, causing a 600-fold increase at 120h. Accordingly, MRTF-B conferred responsiveness to the muscarinic agonist carbachol in Ca²⁺ imaging experiments. M₃ was suppressed on treatment with the MRTF-SRF inhibitor CCG-1423 using SMCs transduced with either MRTF-A or MRTF-B and using intact mouse esophagus in culture (by 92±2%). Moreover, silencing of SRF with a short hairpin reduced CHRM3 (by >60%) in parallel with α-actin (ACTA2). Tamoxifen inducible knockout of Srf in smooth muscle reduced Srf (by 54±4%) and Chrm3 (by 41±6%) in the urinary bladder at 10days, but Srf was much less reduced or unchanged in aorta, ileum, colon, trachea, and esophagus. Longer induction (21d) further accentuated the reduction of Chrm3 in the bladder and ileum, but no change was seen in the aorta. Single cell RNA-sequencing revealed that Mrtfb dominates in ECs, while Myocd dominates in SMCs, raising the possibility that Chrm3 may be driven by Mrtfb-Srf in the endothelium and by Myocd-Srf in SMCs. These findings define a novel transcriptional control mechanism for muscarinic M₃ receptors in human cells, and in mice, that could be targeted for therapy.

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.

Fabrication approaches for high-throughput and biomimetic disease modeling

There is often a tradeoff between in vitro disease modeling platforms that capture pathophysiologic complexity and those that are amenable to high-throughput fabrication and analysis. However, this divide is closing through the application of a handful of fabrication approaches-parallel fabrication, automation, and flow-driven assembly-to design sophisticated cellular and biomaterial systems. The purpose of this review is to highlight methods for the fabrication of high-throughput biomaterial-based platforms and showcase examples that demonstrate their utility over a range of throughput and complexity. We conclude with a discussion of future considerations for the continued development of higher-throughput in vitro platforms that capture the appropriate level of biological complexity for the desired application. STATEMENT OF SIGNIFICANCE: There is a pressing need for new biomedical tools to study and understand disease. These platforms should mimic the complex properties of the body while also permitting investigation of many combinations of cells, extracellular cues, and/or therapeutics in high-throughput. This review summarizes emerging strategies to fabricate biomimetic disease models that bridge the gap between complex tissue-mimicking microenvironments and high-throughput screens for personalized medicine.

An Automated Cell Tracking Approach With Multi-Bernoulli Filtering and Ant Colony Labor Division

In this article, we take as inspiration the labor division into scouts and workers in an ant colony and propose a novel approach for automated cell tracking in the framework of multi-Bernoulli random finite sets. To approximate the Bernoulli parameter sets, we first define an existence probability of an ant colony as well as its discrete density distribution. During foraging, the behavior of scouts is modeled as a chaotic movement to produce a set of potential candidates. Afterwards, a group of workers, i.e., a worker ant colony, is recruited for each candidate, which then embark on gathering heuristic information in a self-organized way. Finally, the pheromone field is formed by the corresponding worker ant colony, from which the Bernoulli parameter is derived and the state of the cell is estimated accordingly to be associated with the existing tracks. Performance comparisons with other previous approaches are conducted on both simulated and real cell image sequences and show the superiority of this algorithm.

Click-functionalized hydrogel design for mechanobiology investigations

The advancement of click-functionalized hydrogels in recent years has coincided with rapid growth in the fields of mechanobiology, tissue engineering, and regenerative medicine. Click chemistries represent a group of reactions that possess high reactivity and specificity, are cytocompatible, and generally proceed under physiologic conditions. Most notably, the high level of tunability afforded by these reactions enables the design of user-controlled and tissue-mimicking hydrogels in which the influence of important physical and biochemical cues on normal and aberrant cellular behaviors can be independently assessed. Several critical tissue properties, including stiffness, viscoelasticity, and biomolecule presentation, are known to regulate cell mechanobiology in the context of development, wound repair, and disease. However, many questions still remain about how the individual and combined effects of these instructive properties regulate the cellular and molecular mechanisms governing physiologic and pathologic processes. In this review, we discuss several click chemistries that have been adopted to design dynamic and instructive hydrogels for mechanobiology investigations. We also chart a path forward for how click hydrogels can help reveal important insights about complex tissue microenvironments.

Durotaxis

In this Primer, Sunyer and Trepat introduce durotaxis, the mode of migration by which cells follow gradients of extracellular matrix stiffness.

Integration of electrotaxis and durotaxis in cancer cells: Subtle nonlinear responses to electromechanical coupling cues

Cells in living organisms live in multiphysics-coupled environments. There is growing evidence indicating that both exogenous electric field (EEF) and extracellular stiffness gradient (ESG) can regulate directional movement of cells, which are known as electrotaxis and durotaxis, respectively. How single cells respond to the ubiquitous electromechanical coupling cues, however, remains mysterious. Using microfluidic chip-based methodology and finite element-based electromechanical coupling design strategies, we develope an electromechanical coupling microchip system, enabling us to quantitatively investigate polarization and directional migration governed by EEF and ESG at the single cell level. It is revealed that both of electrotaxis and durotaxis nonlinearly depend on the physiological EEF and ESG, respectively. Specific combinations of EEF and ESG can subtly modify the polarization states of single cells and thus induce hyperpolarization and depolarization. Cells can integrate electrotaxis and durotaxis in response to multi-cue microenvironments via subtle mechanisms involving cooperation and competition during cellular electrosensing and mechanosensing. The work offers a platform for quantifying migration and polarization of cells driven by electromechanical cues, which is essential not only for elucidating physiological and pathological processes like embryo development, and invasion and metastasis of cancer cells, but for manipulating cell behaviors in a controllable and programmable fashion.

Modulating immune microenvironment during bone repair using biomaterials: Focusing on the role of macrophages

Bone is a self-regenerative tissue that can repair small defects and fractures. In large defects, bone tissue is unable to provide nutrients and oxygen for repair, and autologous grafting is used as the gold standard. As an alternative method, the bone tissue regeneration approach uses osteoconductive biomaterials to overcome bone graft disadvantages. However, biomaterials are considered as foreign components that can stimulate host immune responses. Although traditional principles have been aimed to minimize immune reactions, the design of biomaterials has steadily shifted toward creating an immunomodulatory microenvironment to harness immune cells and responses to repair damaged tissue. Among immune cells, macrophages secrete various immunomodulatory mediators and crosstalk with bone-forming cells and play key roles in bone tissue engineering. Macrophage polarization toward M1 and M2 subtypes mediate pro-inflammatory and anti-inflammatory responses, respectively, which are crucial for bone repairing at different stages. This review provides an overview of the crosstalk between various immune cells and biomaterials, macrophage polarization, and the effect of physicochemical properties of biomaterials on the immune responses, especially macrophages, in bone tissue engineering.

Mesenchymal Stem Cells Resist Mechanical Confinement through the Activation of the Cortex during Cell Division

The mechanical properties of the natural extracellular matrix (ECM) change extensively, but these specific properties provide a relatively stable environment for resident cells. Although the effect of matrix stiffness on cell functions has been widely studied, the molecular mechanism was still not fully understood. Matrix stiffening is a common phenomenon in tissue damaging processes. To explore the effect of the increase in local matrix stiffness on cell behaviors, a three-dimensional (3D) cell culture system with a tunable modulus but constant other physical parameters was constructed by the alginate hydrogel with different molecular weights and cross-linking degrees. By using this culture system, the transcriptome response of mesenchymal stem cells (MSCs) to matrix stiffness was explored. Furthermore, a finite element model was developed to simulate the interaction between cells and the matrix. Results revealed that the increased matrix stiffness promoted the proliferation-related signaling of MSCs, and this process depended on the increased cortex tension caused by the activation of RAS and myosin II.

Electrotaxis-on-Chip to Quantify Neutrophil Migration Towards Electrochemical Gradients

Electric fields are generated in vivo in a variety of physiologic and pathologic settings, including wound healing and immune response to injuries to epithelial barriers (e.g. lung pneumocytes). Immune cells are known to migrate towards both chemical (chemotaxis), physical (mechanotaxis) and electric stimuli (electrotaxis). Electrotaxis is the guided migration of cells along electric fields, and has previously been reported in T-cells and cancer cells. However, there remains a need for engineering tools with high spatial and temporal resolution to quantify EF guided migration. Here we report the development of an electrotaxis-on-chip (ETOC) platform that enables the quantification of dHL-60 cell, a model neutrophil-like cell line, migration toward both electrical and chemoattractant gradients. Neutrophils are the most abundant white blood cells and set the stage for the magnitude of the immune response. Therefore, developing engineering tools to direct neutrophil migration patterns has applications in both infectious disease and inflammatory disorders. The ETOC developed in this study has embedded electrodes and four migration zones connected to a central cell-loading chamber with migration channels [10 µm X 10 µm]. This device enables both parallel and competing chemoattractant and electric fields. We use our novel ETOC platform to investigate dHL-60 cell migration in three biologically relevant conditions: 1) in a DC electric field; 2) parallel chemical gradient and electric fields; and 3) perpendicular chemical gradient and electric field. In this study we used differentiated leukemia cancer cells (dHL60 cells), an accepted model for human peripheral blood neutrophils. We first quantified effects of electric field intensities (0.4V/cm-1V/cm) on dHL-60 cell electrotaxis. Our results show optimal migration at 0.6 V/cm. In the second scenario, we tested whether it was possible to increase dHL-60 cell migration to a bacterial signal [N-formylated peptides (fMLP)] by adding a parallel electric field. Our results show that there was significant increase (6-fold increase) in dHL60 migration toward fMLP and cathode of DC electric field (0.6V/cm, n=4, p-value<0.005) vs. fMLP alone. Finally, we evaluated whether we could decrease or re-direct dHL-60 cell migration away from an inflammatory signal [leukotriene B4 (LTB4)]. The perpendicular electric field significantly decreased migration (2.9-fold decrease) of dHL60s toward LTB4vs. LTB4 alone. Our microfluidic device enabled us to quantify single-cell electrotaxis velocity (7.9 µm/min ± 3.6). The magnitude and direction of the electric field can be more precisely and quickly changed than most other guidance cues such as chemical cues in clinical investigation. A better understanding of EF guided cell migration will enable the development of new EF-based treatments to precisely direct immune cell migration for wound care, infection, and other inflammatory disorders.

Engineering the Cellular Microenvironment of Post-infarct Myocardium on a Chip

Myocardial infarctions are one of the most common forms of cardiac injury and death worldwide. Infarctions cause immediate necrosis in a localized region of the myocardium, which is followed by a repair process with inflammatory, proliferative, and maturation phases. This repair process culminates in the formation of scar tissue, which often leads to heart failure in the months or years after the initial injury. In each reparative phase, the infarct microenvironment is characterized by distinct biochemical, physical, and mechanical features, such as inflammatory cytokine production, localized hypoxia, and tissue stiffening, which likely each contribute to physiological and pathological tissue remodeling by mechanisms that are incompletely understood. Traditionally, simplified two-dimensional cell culture systems or animal models have been implemented to elucidate basic pathophysiological mechanisms or predict drug responses following myocardial infarction. However, these conventional approaches offer limited spatiotemporal control over relevant features of the post-infarct cellular microenvironment. To address these gaps, Organ on a Chip models of post-infarct myocardium have recently emerged as new paradigms for dissecting the highly complex, heterogeneous, and dynamic post-infarct microenvironment. In this review, we describe recent Organ on a Chip models of post-infarct myocardium, including their limitations and future opportunities in disease modeling and drug screening.

Structurally Dynamic Hydrogels for Biomedical Applications: Pursuing a Fine Balance between Macroscopic Stability and Microscopic Dynamics

Owing to their unique chemical and physical properties, hydrogels are attracting increasing attention in both basic and translational biomedical studies. Although the classical hydrogels with static networks have been widely reported for decades, a growing number of recent studies have shown that structurally dynamic hydrogels can better mimic the dynamics and functions of natural extracellular matrix (ECM) in soft tissues. These synthetic materials with defined compositions can recapitulate key chemical and biophysical properties of living tissues, providing an important means to understanding the mechanisms by which cells sense and remodel their surrounding microenvironments. This review begins with the overall expectation and design principles of dynamic hydrogels. We then highlight recent progress in the fabrication strategies of dynamic hydrogels including both degradation-dependent and degradation-independent approaches, followed by their unique properties and use in biomedical applications such as regenerative medicine, drug delivery, and 3D culture. Finally, challenges and emerging trends in the development and application of dynamic hydrogels are discussed.

Mechanotransduction assays for neural regeneration strategies: A focus on glial cells

Glial cells are mechanosensitive, and thus, engineered systems have taken a step forward to design mechanotransduction platforms in order to impart diverse mechanical stresses to cells. Mechanical strain encountered in the central nervous system can arise from diverse mechanisms, such as tissue reorganization, fluid flow, and axon growth, as well as pathological events including axon swelling or mechanical trauma. Biomechanical relevance of the in vitro mechanical testing requires to be placed in line with the physiological and mechanical changes in central nervous tissues that occur during the progression of neurodegenerative diseases. Mechanotransduction signaling utilized by glial cells and the recent approaches intended to model altered microenvironment adapted to pathological context are discussed in this review. New insights in systems merging substrate's stiffness and topography should be considered for further glial mechanotransduction studies, while testing platforms for drug discoveries promise great advancements in pharmacotherapy. Potential leads and strategies for clinical outcomes are expected to be developed following the exploration of these glial mechanosensitive signaling pathways.

Engineering in vitro human neural tissue analogs by 3D bioprinting and electrostimulation

There is a fundamental need for clinically relevant, reproducible, and standardized in vitro human neural tissue models, not least of all to study heterogenic and complex human-specific neurological (such as neuropsychiatric) disorders. Construction of three-dimensional (3D) bioprinted neural tissues from native human-derived stem cells (e.g., neural stem cells) and human pluripotent stem cells (e.g., induced pluripotent) in particular is appreciably impacting research and conceivably clinical translation. Given the ability to artificially and favorably regulate a cell's survival and behavior by manipulating its biophysical environment, careful consideration of the printing technique, supporting biomaterial and specific exogenously delivered stimuli, is both required and advantageous. By doing so, there exists an opportunity, more than ever before, to engineer advanced and precise tissue analogs that closely recapitulate the morphological and functional elements of natural tissues (healthy or diseased). Importantly, the application of electrical stimulation as a method of enhancing printed tissue development in vitro, including neuritogenesis, synaptogenesis, and cellular maturation, has the added advantage of modeling both traditional and new stimulation platforms, toward improved understanding of efficacy and innovative electroceutical development and application.

Mechanical Regulation of Transcription: Recent Advances

Mechanotransduction is the ability of a cell to sense mechanical cues from its microenvironment and convert them into biochemical signals to elicit adaptive transcriptional and other cellular responses. Here, we describe recent advances in the field of mechanical regulation of transcription, highlight mechanical regulation of the epigenome as a key novel aspect of mechanotransduction, and describe recent technological advances that could further elucidate the link between mechanical stimuli and gene expression. In this review, we emphasize the importance of mechanotransduction as one of the governing principles of cancer progression, underscoring the need to conduct further studies of the molecular mechanisms involved in sensing mechanical cues and coordinating transcriptional responses.

Speckle-dependent accuracy in phase-sensitive optical coherence tomography

Phase-sensitive optical coherence tomography (OCT) is used to measure motion in a range of techniques, such as Doppler OCT and optical coherence elastography (OCE). In phase-sensitive OCT, motion is typically estimated using a model of the OCT signal derived from a single reflector. However, this approach is not representative of turbid samples, such as tissue, which exhibit speckle. In this study, for the first time, we demonstrate, through theory and experiment that speckle significantly lowers the accuracy of phase-sensitive OCT in a manner not accounted for by the OCT signal-to-noise ratio (SNR). We describe how the inaccuracy in speckle reduces phase difference sensitivity and introduce a new metric, speckle brightness, to quantify the amount of constructive interference at a given location in an OCT image. Experimental measurements show an almost three-fold degradation in sensitivity between regions of high and low speckle brightness at a constant OCT SNR. Finally, we apply these new results in compression OCE to demonstrate a ten-fold improvement in strain sensitivity, and a five-fold improvement in contrast-to-noise by incorporating independent speckle realizations. Our results show that speckle introduces a limit to the accuracy of phase-sensitive OCT and that speckle brightness should be considered to avoid erroneous interpretation of experimental data.

Multiwell Combinatorial Hydrogel Array for High-Throughput Analysis of Cell-ECM Interactions

Biophysical cues in the extracellular matrix (ECM) regulate cell behavior in a complex, nonlinear, and interdependent manner. To quantify these important regulatory relationships and gain a comprehensive understanding of mechanotransduction, there is a need for high-throughput matrix platforms that enable parallel culture and analysis of cells in various matrix conditions. Here we describe a multiwell hyaluronic acid (HA) platform in which cells are cultured on combinatorial arrays of hydrogels spanning a range of elasticities and adhesivities. Our strategy utilizes orthogonal photopatterning of stiffness and adhesivity gradients, with the stiffness gradient implemented by a programmable light illumination system. The resulting platform allows individual treatment and analysis of each matrix environment while eliminating contributions of haptotaxis and durotaxis. In human mesenchymal stem cells, our platform recapitulates expected relationships between matrix stiffness, adhesivity, and cell mechanosensing. We further applied the platform to show that as integrin ligand density falls, cell adhesion and migration depend more strongly on CD44-mediated interactions with the HA backbone. We anticipate that our system could bear great value for mechanistic discovery and screening where matrix mechanics and adhesivity are expected to influence phenotype.

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 and mechanism of YAP/TAZ in the mechanical microenvironment of stem cells (Review)

Stem cells receive cues from their physical and mechanical microenvironment via mechanosensing and mechanotransduction. These cues affect proliferation, self‑renewal and differentiation into specific cell fates. A growing body of evidence suggests that yes‑associated protein (YAP) and transcriptional coactivator with PDZ‑binding motif (TAZ) mechanotransduction is key for driving stem cell behavior and regeneration via the Hippo and other signaling pathways. YAP/TAZ receive a range of physical cues, including extracellular matrix stiffness, cell geometry, flow shear stress and mechanical forces in the cytoskeleton, and translate them into cell‑specific transcriptional programs. However, the mechanism by which mechanical signals regulate YAP/TAZ activity in stem cells is not fully understand. The present review summarizes the current knowledge of the mechanisms involved in YAP/TAZ regulation on the physical and mechanical microenvironment, as well as its potential effects on stem cell differentiation.