The cavity-to-cavity migration of leukaemic cells through 3D honey-combed hydrogels with adjustable internal dimension and stiffness

Modular formation of in vitro tumor models for oncological research/therapeutic drug screening

In recognition of the lethal nature of cancer, extensive efforts have been made to understand the mechanistic causation while identifying the effective therapy modality in hope to eradicate cancerous cells with minimal damage to healthy cells. In search of such effective therapeutics, establishing pathophysiologically relevant in vitro models would be of importance in empowering our capabilities of truly identifying those potent ones with significantly reduction of the preclinical periods for rapid translation. In this regard, wealthy progresses have been achieved over past decades in establishing various in vitro and in vivo tumor models. Ideally, the tumor models should maximally recapture the key pathophysiological attributes of their native counterparts. Many of the current models have demonstrated their utilities but also showed some noticeable limitations. This book chapter will briefly review some of the mainstream platforms for in vitro tumor models followed by detailed elaboration on the modular strategies to form in vitro tumor models with complex structures and spatial organization of cellular components. Clearly, with the ability to modulate the building modules it becomes a new trend to form in vitro tumor models following a bottom-up approach, which offers a high flexibility to satisfy the needs for pathophysiological study, anticancer drug screening or design of personalized treatment.

A nanoparticle reinforced microporous methacrylated silk fibroin hydrogel to promote bone regeneration

Natural polymer-based hydrogels have been widely applied in bone tissue engineering due to their excellent biocompatibility and outstanding ability of drug encapsulation. However, they have relatively weak mechanical properties and lack bioactivity. Hence, we developed a bioactive nanoparticle composite hydrogel by incorporating LAPONITE®, which is an osteo-inductive inorganic nanoparticle. The incorporation of the nanoparticle significantly enhanced its mechanical properties. In vitro evaluation indicated that the nanocomposite hydrogel could exhibit good biocompatibility. Besides, the nanocomposite hydrogel was proved to have excellent osteogenic ability with up-regulated expression of osteogenic markers such as type I collagen (COL-I), runt-related transcription factor-2 (Runx-2) and osteocalcin (OCN). Furthermore, the in vivo study confirmed that the composite nanocomposite hydrogel could significantly promote new bone formation, providing a prospective strategy for bone tissue regeneration.

Characterization and structure-property relationships of an injectable thiol-Michael addition hydrogel toward compatibility with glioblastoma therapy

Glioblastoma multiforme (GBM) is an aggressive primary brain cancer and although patients undergo surgery and chemoradiotherapy, residual cancer cells still migrate to healthy brain tissue and lead to tumor relapse after treatment. New therapeutic strategies are therefore urgently needed to better mitigate this tumor recurrence. To address this need, we envision after surgical removal of the tumor, implantable biomaterials in the resection cavity can treat or collect residual GBM cells for their subsequent eradication. To this end, we systematically characterized a poly(ethylene glycol)-based injectable hydrogel crosslinked via a thiol-Michael addition reaction by tuning its hydration level and aqueous NaHCO₃ concentration. The physical and chemical properties of the different formulations were investigated by assessing the strength and stability of the polymer networks and their swelling behavior. The hydrogel biocompatibility was assessed by performing in vitro cytotoxicity assays, immunoassays, and immunocytochemistry to monitor the reactivity of astrocytes cultured on the hydrogel surface over time. These characterization studies revealed key structure-property relationships. Furthermore, the results indicated hydrogels synthesized with 0.175 M NaHCO₃ and 50 wt% water content swelled the least, possessed a storage modulus that can withstand high intracranial pressures while avoiding a mechanical mismatch, had a sufficiently crosslinked polymer network, and did not degrade rapidly. This formulation was not cytotoxic to astrocytes and produced minimal immunogenic responses in vitro. These properties suggest this hydrogel formulation is the most optimal for implantation in the resection cavity and compatible toward GBM therapy. STATEMENT OF SIGNIFICANCE: Survival times for glioblastoma patients have not improved significantly over the last several decades, as cancer cells remain after conventional therapies and form secondary tumors. We characterized a biodegradable, injectable hydrogel to reveal structure-property relationships that can be tuned to conform the hydrogel toward glioblastoma therapy. Nine formulations were systematically characterized to optimize the hydrogel based on physical, chemical, and biological compatibility with the glioblastoma microenvironment. This hydrogel can potentially be used for adjuvant therapy to glioblastoma treatment, such as by providing a source of molecular release for therapeutic agents, which will be investigated in future work. The optimized formulation will be developed further to capture and eradicate glioblastoma cells with chemical and physical stimuli in future research.

Current hydrogel advances in physicochemical and biological response-driven biomedical application diversity

Hydrogel is a type of versatile platform with various biomedical applications after rational structure and functional design that leverages on material engineering to modulate its physicochemical properties (e.g., stiffness, pore size, viscoelasticity, microarchitecture, degradability, ligand presentation, stimulus-responsive properties, etc.) and influence cell signaling cascades and fate. In the past few decades, a plethora of pioneering studies have been implemented to explore the cell-hydrogel matrix interactions and figure out the underlying mechanisms, paving the way to the lab-to-clinic translation of hydrogel-based therapies. In this review, we first introduced the physicochemical properties of hydrogels and their fabrication approaches concisely. Subsequently, the comprehensive description and deep discussion were elucidated, wherein the influences of different hydrogels properties on cell behaviors and cellular signaling events were highlighted. These behaviors or events included integrin clustering, focal adhesion (FA) complex accumulation and activation, cytoskeleton rearrangement, protein cyto-nuclei shuttling and activation (e.g., Yes-associated protein (YAP), catenin, etc.), cellular compartment reorganization, gene expression, and further cell biology modulation (e.g., spreading, migration, proliferation, lineage commitment, etc.). Based on them, current in vitro and in vivo hydrogel applications that mainly covered diseases models, various cell delivery protocols for tissue regeneration and disease therapy, smart drug carrier, bioimaging, biosensor, and conductive wearable/implantable biodevices, etc. were further summarized and discussed. More significantly, the clinical translation potential and trials of hydrogels were presented, accompanied with which the remaining challenges and future perspectives in this field were emphasized. Collectively, the comprehensive and deep insights in this review will shed light on the design principles of new biomedical hydrogels to understand and modulate cellular processes, which are available for providing significant indications for future hydrogel design and serving for a broad range of biomedical applications.

Controlled domain gels with a biomimetic gradient environment for osteochondral tissue regeneration

In order to repair an osteochondral defect, it is critical to advance a bi-lineage constructive scaffold with gradient transition. In this study, we developed a simple and straightforward approach for fabricating a multi-domain gel scaffold through the assembly/disassembly of low-molecular-weight gels (LMWGs) inside a stable PEGDA network by photopolymerization. The versatility of this technology enabled to vary biological, topological, and mechanical properties through material selection and to generate a chondrogenic-osteogenic gradient transition. The multi-domain gel exhibited an increasing stiffness gradient along the longitudinal direction from the cartilage layer at approximately 20 kPa to the bone layer at approximately 300 kPa as well as spatial variation at the gradient interface. Moreover, the transitional layer with a condensed structure and intermediate stiffness prevented delamination of the contrasting layers and maintained microenvironmental differences in the upper and lower layers. The in vitro results indicated that each domain had an individual capacity to spatially control the differentiation of MSCs toward osteoblastic lineage and chondrocytic lineage. This was mainly because the mechanical and topographical cues from the respective domains played an important role in modulating the Rho-ROCK signaling pathway, whereas the blockage of ROCK signals significantly impaired domain-modulated osteogenesis and enhanced chondrogenesis. Additionally, the quantity and quality of osteochondral repair were evaluated at 4 and 8 weeks through histological analysis and micro-computed tomography (micro-CT). The results indicated that the multi-domain gels distinctly improved the regeneration of subchondral bone and cartilage tissues. Overall, the outcomes of this study can motivate future bioinspired gradient and heterogeneity strategies for osteochondral tissue regeneration. STATEMENT OF SIGNIFICANCE: The regeneration of osteochondral defects remains a major challenge due to the complexity of osteochondral structure and the limited self-repair capacity of cartilage. The gradient design to mimic the transition between the calcified cartilage and the subchondral bone plate as well as the zones of cartilage is an effective strategy. In this study, controlled multi-domain gels were fabricated through the assembly/disassembly of low-molecular-weight gels inside a stable PEGDA network by photopolymerization. The prepared multi-domain gels showed a chondrogenic-osteogenic gradient transition, which decreased the possibility of delamination and stimulated osteochondral tissue regeneration in vivo. Overall, our study promotes new strategies of bioinspired gradients and heterogeneities for more challenging applications.

Understanding and Regulating Cell-Matrix Interactions Using Hydrogels of Designable Mechanical Properties

Similar to natural tissues, hydrogels contain abundant water, so they are considered as promising biomaterials for studying the influence of the mechanical properties of extracellular matrices (ECM) on various cell functions. In recent years, the growing research on cellular mechanical response has revealed that many cell functions, including cell spreading, migration, tumorigenesis and differentiation, are related to the mechanical properties of ECM. Therefore, how cells sense and respond to the extracellular mechanical environment has gained considerable attention. In these studies, hydrogels are widely used as the in vitro model system. Hydrogels of tunable stiffness, viscoelasticity, degradability, plasticity, and dynamical properties have been engineered to reveal how cells respond to specific mechanical features. In this review, we summarize recent process in this research direction and specifically focus on the influence of the mechanical properties of the ECM on cell functions, how cells sense and respond to the extracellular mechanical environment, and approaches to adjusting the stiffness of hydrogels.

Hybrid gelatin/oxidized chondroitin sulfate hydrogels incorporating bioactive glass nanoparticles with enhanced mechanical properties, mineralization, and osteogenic differentiation

Biopolymer based hydrogels are characteristic of their biocompatibility and capability of mimicking extracellular matrix structure to support cellular behavior. However, these hydrogels suffer from low mechanical properties, uncontrolled degradation, and insufficient osteogenic activity, which limits their applications in bone regeneration. In this study, we developed hybrid gelatin (Gel)/oxidized chondroitin sulfate (OCS) hydrogels that incorporated mesoporous bioactive glass nanoparticles (MBGNs) as bioactive fillers for bone regeneration. Gel-OCS hydrogels could be self-crosslinked in situ under physiological conditions in the presence of borax. The incorporation of MBGNs enhanced the crosslinking and accelerated the gelation. The gelation time decreased with increasing the concentration of MBGNs added. Incorporation of MBGNs in the hydrogels significantly improved the mechanical properties in terms of enhanced storage modulus and compressive strength. The injectability of the hydrogels was not significantly affected by the MBGN incorporation. Also, the proliferation and osteogenic differentiation of rat bone marrow mesenchymal stem cells in vitro and rat cranial defect restoration in vivo were significantly promoted by the hydrogels in the presence of MBGNs. The hybrid Gel-OCS/MBGN hydrogels show promising potential as injectable biomaterials or scaffolds for bone regeneration/repair applications given their tunable degradation and gelation behavior as well as favorable mechanical behavior and osteogenic activities.

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In situ self-crosslinking of hybrid Gel-OCS/MBGN hydrogels.

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Hybrid hydrogels are porous, injectable and bioadhesive.

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Incorporation of MBGNs enhances mechanical and mineralization properties of hydrogels.

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Osteogenic differentiation of BMSCs enhanced after incorporating MBGNs into hydrogels.

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The presence of MBGNs enhances in vivo rat cranial defect restoration.

Mechanical behavior of bioactive poly(ethylene glycol) diacrylate matrices for biomedical application

The biomedical applications of physically entangled polymeric hydrogels are generally limited due to their weak mechanical properties, rapid swelling and dissolution in physiologically relevant environment. Chemical crosslinking helps stabilizing hydrogel structure and enhancing mechanical properties, thereby allowing a higher stability in phisiological environment. In this context, it is known that the mechanical properties of the hydrogel are affected by both the molecular weight (MW) of the starting polymer and the concentration of the crosslinker. Here, our aim was to assess the influence of polymer MW and concentration in the precursor solution on the mechanical features of the final hydrogel and their influence on cells-material interaction. In detail, 3D synthetic matrices based on poly(ethylene glycol) diacrylate (PEGDA) at two molecular weights (PEG 700 and PEG 3400) and at three different concentrations (10, 20, 40 w/v %), which were photopolymerized using darocour as an initiator, were studied. Then, infrared and swelling analyses, along with a comprehensive mechanical characterization of the obtained hydrogels (i.e. oscillatory shear and confined compression tests), were performed. Finally, to evaluate the influence of the mechanical features on the biological behaviour, the hydrogels were characterized in terms of cell adhesion percentage and cell viability after functionalizing the substrates with RGD peptide at three different concentrations. Results have demonstrated that both the Young's modulus (E) in compression and the elastic modulus (G') in shear of the hydrogels increase with increasing polymer precursor concentration. E decreased as MW increased, and the differences are more relevant for more concentrated hydrogels. On the contrary, G' appears to increase with increasing PEGDA MW and in particular for the lowest polymer precursor concentration. The biological results have demonstrated that cells cultured for longer times seem to prefer PEG 3400 hydrogels with a larger mesh size structure that posses higher viscoelastic properties in shear.

Colloidal crystals of compliant microgel beads to study cell migration and mechanosensitivity in 3D

Tissues are defined not only by their biochemical composition, but also by their distinct mechanical properties. It is now widely accepted that cells sense their mechanical environment and respond to it. However, studying the effects of mechanics in in vitro 3D environments is challenging since current 3D hydrogel assays convolve mechanics with gel porosity and adhesion. Here, we present novel colloidal crystals as modular 3D scaffolds where these parameters are principally decoupled by using monodisperse, protein-coated PAAm microgel beads as building blocks, so that variable stiffness regions can be achieved within one 3D colloidal crystal. Characterization of the colloidal crystal and oxygen diffusion simulations suggested the suitability of the scaffold to support cell survival and growth. This was confirmed by live-cell imaging and fibroblast culture over a period of four days. Moreover, we demonstrate unambiguous durotactic fibroblast migration and mechanosensitive neurite outgrowth of dorsal root ganglion neurons in 3D. This modular approach of assembling 3D scaffolds from mechanically and biochemically well-defined building blocks allows the spatial patterning of stiffness decoupled from porosity and adhesion sites in principle and provides a platform to investigate mechanosensitivity in 3D environments approximating tissues in vitro.

Electrospun nanofibers for the fabrication of engineered vascular grafts

Attention has recently increased in the application of electrospun fibers because of their putative capability to create nanoscale platforms toward tissue engineering. To some extent, electrospun fibers are applicable to the extracellular matrix by providing a three-dimensional microenvironment in which cells could easily acquire definite functional shape and maintain the cell-to-cell connection. It is noteworthy to declare that placement in different electrospun substrates with appropriate physicochemical properties enables cells to promote their bioactivities, dynamics growth and differentiation, leading to suitable restorative effects. This review paper aims to highlight the application of biomaterials in engineered vascular grafts by using electrospun nanofibers to promote angiogenesis and neovascularization.

Variations of Elastic Modulus and Cell Volume with Temperature for Cortical Neurons

Neurons change their growth dynamics and mechanical properties in response to external stimuli such as stiffness of the local microenvironment, ambient temperature, and biochemical or geometrical guidance cues. Here we use combined atomic force microscopy (AFM) and fluorescence microscopy experiments to investigate the relationship between external temperature, soma volume, and elastic modulus for cortical neurons. We measure how changes in ambient temperature affect the volume and the mechanical properties of neuronal cells at both the bulk (elastic modulus) and local (elasticity maps) levels. The experimental data demonstrate that both the volume and the elastic modulus of the neuron soma vary with changes in temperature. Our results show a decrease by a factor of 2 in the soma elastic modulus as the ambient temperature increases from room (25 °C) to physiological (37 °C) temperature, while the volume of the soma increases by a factor of 1.3 during the same temperature sweep. Using high-resolution AFM force mapping, we measure the temperature-induced variations within different regions of the elasticity maps (low and high values of elastic modulus) and correlate these variations with the dynamics of cytoskeleton components and molecular motors. We quantify the change in soma volume with temperature and propose a simple theoretical model that relates this change with variations in soma elastic modulus. These results have significant implications for understanding neuronal development and functions, as ambient temperature, cytoskeletal dynamics, and cellular volume may change with variations in physiological conditions, for example, during tissue compression and infections in vivo as well as during cell manipulation and tissue regeneration ex vivo.

Development of solvent-casting particulate leaching (SCPL) polymer scaffolds as improved three-dimensional supports to mimic the bone marrow niche

The need for new approaches to investigate ex vivo the causes and effects of tumor and to achieve improved cancer treatments and medical therapies is particularly urgent for malignant pathologies such as lymphomas and leukemias, whose tissue initiator cells interact with the stroma creating a three-dimensional (3D) protective environment that conventional mono- and bi-dimensional (2D) models are not able to simulate realistically. The solvent-casting particulate leaching (SCPL) technique, that is already a standard method to produce polymer-based scaffolds for bone tissue repair, is proposed here to fabricate innovative 3D porous structures to mimic the bone marrow niche in vitro. Two different polymers, namely a rigid polymethyl methacrylate (PMMA) and a flexible polyurethane (PU), were evaluated to the purpose, whereas NaCl, in the form of common salt table, resulted to be an efficient porogen. The adoption of an appropriate polymer-to-salt ratio, experimentally defined as 1:4 for both PMMA and PU, gave place to a rich and interconnected porosity, ranging between 82.1 vol% and 91.3 vol%, and the choice of admixing fine-grained or coarse-grained salt powders allowed to control the final pore size. The mechanical properties under compression load were affected both by the polymer matrix and by the scaffold's architecture, with values of the elastic modulus indicatively varying between 29 kPa and 1283 kPa. Preliminary tests performed with human stromal HS-5 cells co-cultured with leukemic cells allowed us to conclude that stromal cells grown associated to the supports keep their well-known protective and pro-survival effect on cancer cells, indicating that these devices can be very useful to mimic the bone marrow microenvironment and therefore to assess the efficacy of novel therapies in pre-clinical studies.

Engineering of thermoresponsive gels as a fake metastatic niche

Chemoattraction through the CXCR4-CXCL12 axis has been shown to be an important mechanism to direct circulating tumor cells toward distant sites. The objective of this work was to prepare a fake metastatic niche made up of a gel loaded with CXCL12. The gel is designed to create a steep concentration gradient of the chemokine in the proximity of the site of administration/injection, aimed to divert and capture circulating CXCR4⁺ tumor cells. To this aim, different thermoresponsive gels based on methylcellulose (MC) or poloxamers, loaded with CXCL12, with or without hyaluronic acid (HA) were designed and their mechanical properties correlated with the ability to attract and capture in vitro CXCR4⁺ cells. Results of in vitro cell studies showed that all prepared gels induced CEM tumor cell migration whereas only gels based on MC embedded with CXCL12 are able to capture them.

Solvent-Assisted Micromolding of Biohybrid Hydrogels to Maintain Human Hematopoietic Stem and Progenitor Cells Ex Vivo

Array-format cell-culture carriers providing tunable matrix cues are instrumental in current cell biology and bioengineering. A new solvent-assisted demolding approach for the fabrication of microcavity arrays with very small feature sizes down to single-cell level (3 µm) of very soft biohybrid glycosaminoglycan-poly(ethylene glycol) hydrogels (down to a shear modulus of 1 kPa) is reported. It is further shown that independent additional options of localized conjugation of adhesion ligand peptides, presentation of growth factors through complexation to gel-based glycosaminoglycans, and secondary gel deposition for 3D cell embedding enable a versatile customization of the hydrogel microcavity arrays for cell culture studies. As a proof of concept, cell-instructive hydrogel compartment arrays are used to analyze the response of human hematopoietic stem and progenitor cells to defined biomolecular and spatial cues.

Fabrication of Inverted Colloidal Crystal Poly(ethylene glycol) Scaffold: A Three-dimensional Cell Culture Platform for Liver Tissue Engineering

The ability to maintain hepatocyte function in vitro, for the purpose of testing xenobiotics' cytotoxicity, studying virus infection and developing drugs targeted at the liver, requires a platform in which cells receive proper biochemical and mechanical cues. Recent liver tissue engineering systems have employed three-dimensional (3D) scaffolds composed of synthetic or natural hydrogels, given their high water retention and their ability to provide the mechanical stimuli needed by the cells. There has been growing interest in the inverted colloidal crystal (ICC) scaffold, a recent development, which allows high spatial organization, homotypic and heterotypic cell interaction, as well as cell-extracellular matrix (ECM) interaction. Herein, we describe a protocol to fabricate the ICC scaffold using poly (ethylene glycol) diacrylate (PEGDA) and the particle leaching method. Briefly, a lattice is made from microsphere particles, after which a pre-polymer solution is added, properly polymerized, and the particles are then removed, or leached, using an organic solvent (e.g., tetrahydrofuran). The dissolution of the lattice results in a highly porous scaffold with controlled pore sizes and interconnectivities that allow media to reach cells more easily. This unique structure allows high surface area for the cells to adhere to as well as easy communication between pores, and the ability to coat the PEGDA ICC scaffold with proteins also shows a marked effect on cell performance. We analyze the morphology of the scaffold as well as the hepatocarcinoma cell (Huh-7.5) behavior in terms of viability and function to explore the effect of ICC structure and ECM coatings. Overall, this paper provides a detailed protocol of an emerging scaffold that has wide applications in tissue engineering, especially liver tissue engineering.

A Nanoprinted Model of Interstitial Cancer Migration Reveals a Link between Cell Deformability and Proliferation

Metastatic progression of tumors requires the coordinated dissemination of cancerous cells through interstitial tissues and their replication in distant body locations. Despite their importance in cancer treatment decisions, key factors, such as cell shape adaptation and the role it plays in dense tissue invasion by cancerous cells, are not well understood. Here, we employ a 3D electrohydrodynamic nanoprinting technology to generate vertical arrays of topographical pores that mimic interstitial tissue resistance to the mesenchymal migration of cancerous cells, in order to determine the effect of nuclear size, cell deformability, and cell-to-substrate adhesion on tissue invasion efficiency. The high spatial and temporal resolution of our analysis demonstrates that the ability of cells to deform depends on the cell cycle phase, peaks immediately after mitosis, and is key to the invasion process. Increased pore penetration efficiency by cells in early G1 phase also coincided with their lower nuclear volume and higher cell deformability, compared with the later cell cycle stages. Furthermore, artificial decondensation of chromatin induced an increase in cell and nuclear deformability and improved pore penetration efficiency of cells in G1. Together, these results underline that along the cell cycle cells have different abilities to dynamically remodel their actin cytoskeleton and induce nuclear shape changes, which determines their pore penetration efficiency. Thus, our results support a mechanism in which cell proliferation and pore penetration are functionally linked to favor the interstitial dissemination of metastatic cells.

Load Rate and Temperature Dependent Mechanical Properties of the Cortical Neuron and Its Pericellular Layer Measured by Atomic Force Microscopy

When studying the mechanical properties of cells by an indentation technique, it is important to take into account the nontrivial pericellular interface (or pericellular "brush") which includes a pericellular coating and corrugation of the pericellular membrane (microvilli and microridges). Here we use atomic force microscopy (AFM) to study the mechanics of cortical neurons taking into account the presence of the above pericellular brush surrounding cell soma. We perform a systematic study of the mechanical properties of both the brush layer and the underlying neuron soma and demonstrate that the brush layer is likely responsible for the low elastic modulus (<1 kPa) typically reported for cortical neurons. When the contribution of the pericellular brush is excluded, the average elastic modulus of the cortical neuron soma is found to be 3-4 times larger than previously reported values measured under similar physiological conditions. We also demonstrate that the underlying soma behaves as a nonviscous elastic material over the indentation rates studied (1-10 μm/s). As a result, it seems that the brush layer is responsible for the previously reported viscoelastic response measured for the neuronal cell body as a whole, within these indentation rates. Due to of the similarities between the macroscopic brain mechanics and the effective modulus of the pericellular brush, we speculate that the pericellular brush layer might play an important role in defining the macroscopic mechanical properties of the brain.

Bridging structure and mechanics of three-dimensional porous hydrogel with X-ray ultramicroscopy and atomic force microscopy

By combining phase contrast X-ray ultramicroscopy and nanoindentation with atomic force microscopy, the mechanics of individual hydrogel pores as well as their collective performance as a scaffold can be modelled and simulated.

Topography design concept of a tissue engineering scaffold for controlling cell function and fate through actin cytoskeletal modulation

The physiological role of the actin cytoskeleton is well known: it provides mechanical support and endogenous force generation for formation of a cell shape and for migration. Furthermore, a growing number of studies have demonstrated another significant role of the actin cytoskeleton: it offers dynamic epigenetic memory for guiding cell fate, in particular, proliferation and differentiation. Because instantaneous imbalance in the mechanical homeostasis is adjusted through actin remodeling, a synthetic extracellular matrix (ECM) niche as a source of topographical and mechanical cues is expected to be effective at modulation of the actin cytoskeleton. In this context, the synthetic ECM niche determines cell migration, proliferation, and differentiation, all of which have to be controlled in functional tissue engineering scaffolds to ensure proper regulation of tissue/organ formation, maintenance of tissue integrity and repair, and regeneration. Here, with an emphasis on the epigenetic role of the actin cytoskeletal system, we propose a design concept of micro/nanotopography of a tissue engineering scaffold for control of cell migration, proliferation, and differentiation in a stable and well-defined manner, both in vitro and in vivo.

Three-dimensional fibroblast morphology on compliant substrates of controlled negative curvature

Traditionally, cell biological investigations have mostly employed cells growing on flat, two-dimensional, hard substrates, which are of questionable utility in mimicking microenvironments in vivo. We engineered a novel scaffold to achieve cell culture in the third dimension (3D), where fibroblasts lose the strong dorsal-ventral asymmetry in the distribution of cytoskeletal and adhesion components that is induced by growth on flat substrates. The design principle of our new 3D substrate was inspired by recent advances in engineering cellular microenvironments in which rigidity and the patterning of adhesion ligands were tuned on two-dimensional substrates; the engineered substrates enable independent control over biochemical and mechanical factors to elucidate how mechanical cues affect cellular behaviours. The 3D substrates consisted of polyacrylamide scaffolds of highly ordered, uniform pores coated with extracellular matrix proteins. We characterized important parameters for fabrication and the mechanical properties of polyacrylamide scaffolds. We then grew individual fibroblasts in the identical pores of the polyacrylamide scaffolds, examining cellular morphological, actin cytoskeletal, and adhesion properties. We found that fibroblasts sense the local rigidity of the scaffold, and exhibit a 3D distribution of actin cytoskeleton and adhesions that became more pronounced as the pore size was reduced. In small pores, we observed that elongated adhesions can exist without attachment to any solid support. Taken together, our results show that the use of negatively curved surfaces is a simple method to induce cell adhesions in 3D, opening up new degrees of freedom to explore cellular behaviours.

An overview of inverted colloidal crystal systems for tissue engineering

Scaffolding is at the heart of tissue engineering but the number of techniques available for turning biomaterials into scaffolds displaying the features required for a tissue engineering application is somewhat limited. Inverted colloidal crystals (ICCs) are inverse replicas of an ordered array of monodisperse colloidal particles, which organize themselves in packed long-range crystals. The literature on ICC systems has grown enormously in the past 20 years, driven by the need to find organized macroporous structures. Although replicating the structure of packed colloidal crystals (CCs) into solid structures has produced a wide range of advanced materials (e.g., photonic crystals, catalysts, and membranes) only in recent years have ICCs been evaluated as devices for medical/pharmaceutical and tissue engineering applications. The geometry, size, pore density, and interconnectivity are features of the scaffold that strongly affect the cell environment with consequences on cell adhesion, proliferation, and differentiation. ICC scaffolds are highly geometrically ordered structures with increased porosity and connectivity, which enhances oxygen and nutrient diffusion, providing optimum cellular development. In comparison to other types of scaffolds, ICCs have three major unique features: the isotropic three-dimensional environment, comprising highly uniform and size-controllable pores, and the presence of windows connecting adjacent pores. Thus far, this is the only technique that guarantees these features with a long-range order, between a few nanometers and thousands of micrometers. In this review, we present the current development status of ICC scaffolds for tissue engineering applications.

The use of porous scaffold as a tumor model

Background. Human cancer is a three-dimensional (3D) structure consisting of neighboring cells, extracellular matrix, and blood vessels. It is therefore critical to mimic the cancer cells and their surrounding environment during in vitro study. Our aim was to establish a 3D cancer model using a synthetic composite scaffold. Methods. High-density low-volume seeding was used to promote attachment of a non-small-cell lung cancer cell line (NCI-H460) to scaffolds. Growth patterns in 3D culture were compared with those of monolayers. Immunohistochemistry was conducted to compare the expression of Ki67, CD44, and carbonic anhydrase IX. Results. NCI-H460 readily attached to the scaffold without surface pretreatment at a rate of 35% from a load of 1.5 × 10⁶ cells. Most cells grew vertically to form clumps along the surface of the scaffold, and cell morphology resembled tissue origin; 2D cultures exhibited characteristics of adherent epithelial cancer cell lines. Expression patterns of Ki67, CD44, and CA IX varied markedly between 3D and monolayer cultures. Conclusions. The behavior of cancer cells in our 3D model is similar to tumor growth in vivo. This model will provide the basis for future study using 3D cancer culture.

Mechanics in neuronal development and repair

Biological cells are well known to respond to a multitude of chemical signals. In the nervous system, chemical signaling has been shown to be crucially involved in development, normal functioning, and disorders of neurons and glial cells. However, there are an increasing number of studies showing that these cells also respond to mechanical cues. Here, we summarize current knowledge about the mechanical properties of nervous tissue and its building blocks, review recent progress in methodology and understanding of cellular mechanosensitivity in the nervous system, and provide an outlook on the implications of neuromechanics for future developments in biomedical engineering to aid overcoming some of the most devastating and currently incurable CNS pathologies such as spinal cord injuries and multiple sclerosis.

Mechanical environment modulates biological properties of oligodendrocyte progenitor cells

Myelination and its regenerative counterpart remyelination represent one of the most complex cell-cell interactions in the central nervous system (CNS). The biochemical regulation of axon myelination via the proliferation, migration, and differentiation of oligodendrocyte progenitor cells (OPCs) has been characterized extensively. However, most biochemical analysis has been conducted in vitro on OPCs adhered to substrata of stiffness that is orders of magnitude greater than that of the in vivo CNS environment. Little is known of how variation in mechanical properties over the physiological range affects OPC biology. Here, we show that OPCs are mechanosensitive. Cell survival, proliferation, migration, and differentiation capacity in vitro depend on the mechanical stiffness of polymer hydrogel substrata. Most of these properties are optimal at the intermediate values of CNS tissue stiffness. Moreover, many of these properties measured for cells on gels of optimal stiffness differed significantly from those measured on glass or polystyrene. The dependence of OPC differentiation on the mechanical properties of the extracellular environment provides motivation to revisit results obtained on nonphysiological, rigid surfaces. We also find that OPCs stiffen upon differentiation, but that they do not change their compliance in response to substratum stiffness, which is similar to embryonic stem cells, but different from adult stem cells. These results form the basis for further investigations into the mechanobiology of cell function in the CNS and may specifically shed new light on the failure of remyelination in chronic demyelinating diseases such as multiple sclerosis.

Cancer cell migration within 3D layer-by-layer microfabricated photocrosslinked PEG scaffolds with tunable stiffness

Our current understanding of 3-dimensional (3D) cell migration is primarily based on results from fibrous scaffolds with randomly organized internal architecture. Manipulations that change the stiffness of these 3D scaffolds often alter other matrix parameters that can modulate cell motility independently or synergistically, making observations less predictive of how cells behave when migrating in 3D. In order to decouple microstructural influences and stiffness effects, we have designed and fabricated 3D polyethylene glycol (PEG) scaffolds that permit orthogonal tuning of both elastic moduli and microstructure. Scaffolds with log-pile architectures were used to compare the 3D migration properties of normal breast epithelial cells (HMLE) and Twist-transformed cells (HMLET). Our results indicate that the nature of cell migration is significantly impacted by the ability of cells to migrate in the third dimension. 2D ECM-coated PEG substrates revealed no statistically significant difference in cell migration between HMLE and HMLET cells among substrates of different stiffness. However, when cells were allowed to move along the third dimension, substantial differences were observed for cell displacement, velocity and path straightness parameters. Furthermore, these differences were sensitive to both substrate stiffness and the presence of the Twist oncogene. Importantly, these 3D modes of migration provide insight into the potential for oncogene-transformed cells to migrate within and colonize tissues of varying stiffness.

Single molecule experimentation in biological physics: exploring the living component of soft condensed matter one molecule at a time

The soft matter of biological systems consists of mesoscopic length scale building blocks, composed of a variety of different types of biological molecules. Most single biological molecules are so small that 1 billion would fit on the full-stop at the end of this sentence, but collectively they carry out the vital activities in living cells whose length scale is at least three orders of magnitude greater. Typically, the number of molecules involved in any given cellular process at any one time is relatively small, and so real physiological events may often be dominated by stochastics and fluctuation behaviour at levels comparable to thermal noise, and are generally heterogeneous in nature. This challenging combination of heterogeneity and stochasticity is best investigated experimentally at the level of single molecules, as opposed to more conventional bulk ensemble-average techniques. In recent years, the use of such molecular experimental approaches has become significantly more widespread in research laboratories around the world. In this review we discuss recent experimental approaches in biological physics which can be applied to investigate the living component of soft condensed matter to a precision of a single molecule.

Bio-inspired materials for parsing matrix physicochemical control of cell migration: a review

Cell motility is ubiquitous in both normal and pathophysiological processes. It is a complex biophysical response elicited via the integration of diverse extracellular physicochemical cues. The extracellular matrix directs cell motility via gradients in morphogens (a.k.a. chemotaxis), adhesive proteins (haptotaxis), and stiffness (durotaxis). Three-dimensional geometrical and proteolytic cues also constitute key regulators of motility. Therefore, cells process a variety of physicochemical signals simultaneously, while making informed decisions about migration via intracellular processing. Over the last few decades, bioengineers have created and refined natural and synthetic in vitro platforms in an attempt to isolate these extracellular cues and tease out how cells are able to translate this complex array of dynamic biochemical and biophysical features into functional motility. Here, we review how biomaterials have played a key role in the development of these types of model systems, and how recent advances in engineered materials have significantly contributed to our current understanding of the mechanisms of cell migration.

Biodegradable and biocompatible synthetic saccharide-Peptide hydrogels for three-dimensional stem cell culture

Saccharide-peptide hydrogels have been developed in our laboratory as new synthetic extracellular matrices for regenerative medicine applications. In this work, we have expanded on our previously reported system and applied copolymerization of cysteine (Cys) and vinyl sulfone (VS)-functionalized saccharide-peptide polymers via Michael-type addition for encapsulation and 3D culture of cells. Specifically, our aims were to (1) develop a novel hydrogel platform, which could be applied for encapsulating and culturing mesenchymal stem cells (MSCs) in a 3D environment, (2) characterize the tunable properties of the hydrogel, specifically, degradation, mechanical, and gel network properties, and (3) determine the biocompatibility of the saccharide-peptide hydrogel material with MSCs. Hydrogel mechanical properties were tunable by varying the VS:Cys ratio (= 0.5, 1, or 2) as well as the pH (6, 7, or 8) of the cross-linking components. Stiffer gels were formed at VS:Cys = 1 and pH 6 or 7. Gels formed at pH 8 or with excess Cys (VS:Cys = 0.5) or VS (VS:Cys = 2) were significantly softer. Cross-linking pH and VS:Cys ratio also had an effect on the degradation behavior of the VS:Cys gels, with higher cross-linking pH resulting in an accelerated loss of mass. On the basis of environmental scanning electron microscopy (ESEM) analysis and fluorescence microscopy, all hydrogels appeared to exhibit porous gel networks. MSCs cultured in monolayer and exposed to soluble Cys or VS copolymers (0.1-5 mg/mL) did not exhibit measurable cytotoxicity. In addition, MSCs were cultured in 3D for up to 14 days in vitro without deleterious effects on cell viability. In summary, we have established and characterized a tunable 3D saccharide-peptide hybrid copolymer hydrogel platform for culturing MSCs. Future studies will focus on utilizing the hydrogel system for controlling the differentiation of MSCs.

Marrow-derived stem cell motility in 3D synthetic scaffold is governed by geometry along with adhesivity and stiffness

Design of 3D scaffolds that can facilitate proper survival, proliferation, and differentiation of progenitor cells is a challenge for clinical applications involving large connective tissue defects. Cell migration within such scaffolds is a critical process governing tissue integration. Here, we examine effects of scaffold pore diameter, in concert with matrix stiffness and adhesivity, as independently tunable parameters that govern marrow-derived stem cell motility. We adopted an "inverse opal" processing technique to create synthetic scaffolds by crosslinking poly(ethylene glycol) at different densities (controlling matrix elastic moduli or stiffness) and small doses of a heterobifunctional monomer (controlling matrix adhesivity) around templating beads of different radii. As pore diameter was varied from 7 to 17 µm (i.e., from significantly smaller than the spherical cell diameter to approximately cell diameter), it displayed a profound effect on migration of these stem cells-including the degree to which motility was sensitive to changes in matrix stiffness and adhesivity. Surprisingly, the highest probability for substantive cell movement through pores was observed for an intermediate pore diameter, rather than the largest pore diameter, which exceeded cell diameter. The relationships between migration speed, displacement, and total path length were found to depend strongly on pore diameter. We attribute this dependence to convolution of pore diameter and void chamber diameter, yielding different geometric environments experienced by the cells within.

Critical review: cellular mechanobiology and amoeboid migration

Cell motility is important for tissue homeostasis and plays a central role in various pathologies, notably inflammation and cancer. Research into the critical processes involved in cell migration has so far mostly focused on cell adhesion and proteolytic degradation of the extracellular matrix. However, pharmacological interference with these processes only partially blocks cell motility in vivo. In this review we summarize the arising evidence that the mechanical properties of the cell body have a major role to play in cell motility--especially in a low-adhesion, amoeboid-like migration mode in three-dimensional tissue structures. We summarize the processes determining cell mechanics and discuss relevant measurement technologies including their applications in medical cell biology.