Scalable alignment of three-dimensional cellular constructs in a microfluidic chip

Hydrodynamic shear stress' impact on mammalian cell properties and its applications in 3D bioprinting

As an effective cell assembly method, three-dimensional bioprinting has been widely used in building organ models and tissue repair over the past decade. However, different shear stresses induced throughout the entire printing process can cause complex impacts on cell integrity, including reducing cell viability, provoking morphological changes and altering cellular functionalities. The potential effects that may occur and the conditions under which these effects manifest are not clearly understood. Here, we review systematically how different mammalian cells respond under shear stress. We enumerate available experimental apparatus, and we categorise properties that can be affected under disparate stress patterns. We also summarise cell damaging mathematical models as a predicting reference for the design of bioprinting systems. We concluded that it is essential to quantify specific cell resistance to shear stress for the optimisation of bioprinting systems. Besides, as substantial positive impacts, including inducing cell alignment and promoting cell motility, can be generated by shear stress, we suggest that we find the proper range of shear stress and actively utilise its positive influences in the development of future systems.

Integration of Extracellular Matrices into Organ-on-Chip Systems

The extracellular matrix (ECM) is a complex, dynamic network present within all tissues and organs that not only acts as a mechanical support and anchorage point but can also direct fundamental cell behavior, function, and characteristics. Although the importance of the ECM is well established, the integration of well-controlled ECMs into Organ-on-Chip (OoC) platforms remains challenging and the methods to modulate and assess ECM properties on OoCs remain underdeveloped. In this review, current state-of-the-art design and assessment of in vitro ECM environments is discussed with a focus on their integration into OoCs. Among other things, synthetic and natural hydrogels, as well as polydimethylsiloxane (PDMS) used as substrates, coatings, or cell culture membranes are reviewed in terms of their ability to mimic the native ECM and their accessibility for characterization. The intricate interplay among materials, OoC architecture, and ECM characterization is critically discussed as it significantly complicates the design of ECM-related studies, comparability between works, and reproducibility that can be achieved across research laboratories. Improving the biomimetic nature of OoCs by integrating properly considered ECMs would contribute to their further adoption as replacements for animal models, and precisely tailored ECM properties would promote the use of OoCs in mechanobiology.

Supplementation of GelMA with Minimally Processed Tissue Promotes the Formation of Densely Packed Skeletal-Muscle-Like Tissues

We present a simple and cost-effective strategy for developing gelatin methacryloyl (GelMA) hydrogels supplemented with minimally processed tissue (MPT) to fabricate densely packed skeletal-muscle-like tissues. MPT powder was prepared from skeletal muscle by freeze-drying, grinding, and sieving. Cell-culture experiments showed that the incorporation of 0.5-2.0% (w/v) MPT into GelMA hydrogels enhances the proliferation of murine myoblasts (C2C12 cells) compared to proliferation in pristine GelMA hydrogels and GelMA supplemented with decellularized skeletal-muscle tissues (DCTs). MPT-supplemented constructs also preserved their three-dimensional (3D) integrity for 28 days. By contrast, analogous pristine GelMA constructs only maintained their structure for 14 days or less. C2C12 cells embedded in MPT-supplemented constructs exhibited a higher degree of cell alignment and reached a significantly higher density than cells loaded in pristine GelMA constructs. Our results suggest that the addition of MPT incorporates a rich source of biochemical and topological cues, such as growth factors, glycosaminoglycans (GAGs), and structurally preserved proteins (e.g., collagen). In addition, GelMA supplemented with MPT showed suitable rheological properties for use as bioinks for extrusion bioprinting. We envision that this simple and cost-effective strategy of hydrogel supplementation will evolve into an exciting spectrum of applications for tissue engineers, primarily in the biofabrication of relevant microtissues for in vitro models and cultured meat and ultimately for the biofabrication of transplant materials using autologous MPT.

Organotypic cultures as aging associated disease models

Aging remains a primary risk factor for a host of diseases, including leading causes of death. Aging and associated diseases are inherently multifactorial, with numerous contributing factors and phenotypes at the molecular, cellular, tissue, and organismal scales. Despite the complexity of aging phenomena, models currently used in aging research possess limitations. Frequently used in vivo models often have important physiological differences, age at different rates, or are genetically engineered to match late disease phenotypes rather than early causes. Conversely, routinely used in vitro models lack the complex tissue-scale and systemic cues that are disrupted in aging. To fill in gaps between in vivo and traditional in vitro models, researchers have increasingly been turning to organotypic models, which provide increased physiological relevance with the accessibility and control of in vitro context. While powerful tools, the development of these models is a field of its own, and many aging researchers may be unaware of recent progress in organotypic models, or hesitant to include these models in their own work. In this review, we describe recent progress in tissue engineering applied to organotypic models, highlighting examples explicitly linked to aging and associated disease, as well as examples of models that are relevant to aging. We specifically highlight progress made in skin, gut, and skeletal muscle, and describe how recently demonstrated models have been used for aging studies or similar phenotypes. Throughout, this review emphasizes the accessibility of these models and aims to provide a resource for researchers seeking to leverage these powerful tools.

Engineered meatballs via scalable skeletal muscle cell expansion and modular micro-tissue assembly using porous gelatin micro-carriers

The emerging field of cultured meat faces several technical hurdles, including the scale-up production of quality muscle and adipose progenitor cells, and the differentiation and bioengineering of these cellular materials into large, meat-like tissue. Here, we present edible, 3D porous gelatin micro-carriers (PoGelat-MCs), as efficient cell expansion scaffolds, as well as modular tissue-engineering building blocks for lab-grown meat. PoGelat-MC culture in spinner flasks, not only facilitated the scalable expansion of porcine skeletal muscle satellite cells and murine myoblasts, but also triggered their spontaneous myogenesis, in the absence of myogenic reagents. Using 3D-printed mold and transglutaminase, we bio-assembled pork muscle micro-tissues into centimeter-scale meatballs, which exhibited similar mechanical property and higher protein content compared to conventional ground pork meatballs. PoGelat-MCs also supported the expansion and differentiation of 3T3L1 murine pre-adipocytes into mature adipose micro-tissues, which could be used as modular assembly unit for engineered fat-containing meat products. Together, our results highlight PoGelat-MCs, in combination with dynamic bioreactors, as a scalable culture system to produce large quantity of highly-viable muscle and fat micro-tissues, which could be further bio-assembled into ground meat analogues.

Polystyrene Pocket Lithography: Sculpting Plastic with Light

Tissue-culture-ware polystyrene is the gold standard for in vitro cell culture. While microengineering techniques can create advanced cell microenvironments in polystyrene, they require specialized equipment and reagents, which hinder their accessibility for most biological researchers. An economical and easily accessible method is developed and validated for fabricating microstructures directly in polystyrene with sizes approaching subcellular dimensions while requiring minimal processing time. The process involves deep ultraviolet irradiation through a shadow mask or ink pattern using inexpensive, handheld devices followed by selective chemical development with common reagents to generate micropatterns with depths/heights between 5 and 10 µm, which can be used to guide cell behavior. The remarkable straightforwardness of the process enables this class of microengineering techniques to be broadly accessible to diverse research communities.

Contractile force assessment methods for in vitro skeletal muscle tissues

Over the last few years, there has been growing interest in measuring the contractile force (CF) of engineered muscle tissues to evaluate their functionality. However, there are still no standards available for selecting the most suitable experimental platform, measuring system, culture protocol, or stimulation patterns. Consequently, the high variability of published data hinders any comparison between different studies. We have identified that cantilever deflection, post deflection, and force transducers are the most commonly used configurations for CF assessment in 2D and 3D models. Additionally, we have discussed the most relevant emerging technologies that would greatly complement CF evaluation with intracellular and localized analysis. This review provides a comprehensive analysis of the most significant advances in CF evaluation and its critical parameters. In order to compare contractile performance across experimental platforms, we have used the specific force (sF, kN/m2), CF normalized to the calculated cross-sectional area (CSA). However, this parameter presents a high variability throughout the different studies, which indicates the need to identify additional parameters and complementary analysis suitable for proper comparison. We propose that future contractility studies in skeletal muscle constructs report detailed information about construct size, contractile area, maturity level, sarcomere length, and, ideally, the tetanus-to-twitch ratio. These studies will hopefully shed light on the relative impact of these variables on muscle force performance of engineered muscle constructs. Prospective advances in muscle tissue engineering, particularly in muscle disease models, will require a joint effort to develop standardized methodologies for assessing CF of engineered muscle tissues.

Micropatterned conductive elastomer patch based on poly(glycerol sebacate)-graphene for cardiac tissue repair

Preparing a micropatterned elastomer film with characteristics that can simulate the mechanical properties, anisotropy, and electroactivity of natural myocardial tissues is crucial in cardiac tissue engineering after myocardial infarction (MI). Therefore, in this study, we developed several elastomeric films with a surface micropattern based on poly (glycerol sebacate) (PGS) and graphene (Gr). These films have sufficient mechanical strength (0.6 ± 0.1-3.2 ± 0.08 MPa) to withstand heartbeats, and the micropatterned structure also satisfies the natural myocardium anisotropy in the transverse and vertical. Moreover, Gr makes these films conductive (up to 5.80 × 10-7 S/m), which is necessary for the conduction of electrical signals between cardiomyocytes and the cardiac tissue. Furthermore, they have good cytocompatibility and can promote cell proliferation in H9c2 rat cardiomyocyte cell lines. In vivo test results indicate that these films have good biocompatibility. Notably, a film with 1 wt% Gr content (PGS-Gr1) significantly affects the recovery of myocardial function in rats after MI. This film effectively decreased the infarct size and degree of myocardial fibrosis and reduced collagen deposition. Echocardiographic evaluation showed that after treatment with this film, the left ventricular internal dimension in systole and left ventricular internal dimension in diastole of rats exhibited a significant downward trend, whereas the fractional shortening and ejection fraction were significantly increased compared with the control group. These data indicate that this electroactive micropatterned anisotropic elastomer film can be applied in cardiac tissue engineering.

Unraveling Muscle Impairment Associated With COVID-19 and the Role of 3D Culture in Its Investigation

COVID-19, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has been considered a public health emergency, extensively investigated by researchers. Accordingly, the respiratory tract has been the main research focus, with some other studies outlining the effects on the neurological, cardiovascular, and renal systems. However, concerning SARS-CoV-2 outcomes on skeletal muscle, scientific evidence is still not sufficiently strong to trace, treat and prevent possible muscle impairment due to the COVID-19. Simultaneously, there has been a considerable amount of studies reporting skeletal muscle damage in the context of COVID-19. Among the detrimental musculoskeletal conditions associated with the viral infection, the most commonly described are sarcopenia, cachexia, myalgia, myositis, rhabdomyolysis, atrophy, peripheral neuropathy, and Guillain-Barré Syndrome. Of note, the risk of developing sarcopenia during or after COVID-19 is relatively high, which poses special importance to the condition amid the SARS-CoV-2 infection. The yet uncovered mechanisms by which musculoskeletal injury takes place in COVID-19 and the lack of published methods tailored to study the correlation between COVID-19 and skeletal muscle hinder the ability of healthcare professionals to provide SARS-CoV-2 infected patients with an adequate treatment plan. The present review aims to minimize this burden by both thoroughly exploring the interaction between COVID-19 and the musculoskeletal system and examining the cutting-edge 3D cell culture techniques capable of revolutionizing the study of muscle dynamics.

Biomaterial-based physical regulation of macrophage behaviour

Macrophages play a critical role in regulating immune reactions induced by implanted biomaterials. They are highly plastic and in response to diverse stimuli in the microenvironment can exhibit a spectrum of phenotypes and functions. In addition to biochemical signals, the physical properties of biomaterials are becoming increasingly appreciated for their significant impact on macrophage behaviour, and the underlying mechanisms deserve more in-depth investigations. This review first summarises the effects of key physical cues - including stiffness, topography, physical confinement and applied force - on macrophage behaviour. Then, it reviews the current knowledge of cellular sensing and transduction of physical cues into intracellular signals. Finally, it discusses the major challenges in understanding mechanical regulation that could provide insights for biomaterial design.

Effects of topological constraints on the alignment and maturation of multinucleated myotubes

Microfluidic-based technologies enable the development of cell culture systems that provide tailored microenvironmental inputs to mammalian cells. Primary myoblasts can be induced to differentiate into multinucleated skeletal muscle cells, myotubes, which are a relevant model system for investigating skeletal muscle metabolism and physiology in vitro. However, it remains challenging to differentiate primary myoblasts into mature myotubes in microfluidics devices. Here we investigated the effects of integrating continuous (solid) and intermittent (dashed) walls in microfluidic channels as topological constraints in devices designed to promote the alignment and maturation of primary myoblast-derived myotubes. The topological constraints caused alignment of the differentiated myotubes, mimicking the native anisotropic organization of skeletal muscle cells. Interestingly, dashed walls facilitated the maturation of skeletal muscle cells, as measured by quantifying myotube cell area and the number of nuclei per myotube. Together, our results suggest that integrating dashed walls as topographic constraints in microfluidic devices supports the alignment and maturation of primary myoblast-derived myotubes.

Milliscale Substrate Curvature Promotes Myoblast Self-Organization and Differentiation

Biological tissues comprise complex structural environments known to influence cell behavior via multiple interdependent sensing and transduction mechanisms. Yet, and despite the predominantly nonplanar geometry of these environments, the impact of tissue-size (milliscale) curvature on cell behavior is largely overlooked or underestimated. This study explores how concave, hemicylinder-shaped surfaces 3-50 mm in diameter affect the migration, proliferation, orientation, and differentiation of C2C12 myoblasts. Notably, these milliscale cues significantly affect cell responses compared with planar substrates, with myoblasts grown on surfaces 7.5-15 mm in diameter showing prevalent migration and alignment parallel to the curvature axis. Moreover, surfaces within this curvature range promote myoblast differentiation and the formation of denser, more compact tissues comprising highly oriented multinucleated myotubes. Based on the similarity of effects, it is further proposed that myoblast susceptibility to substrate curvature depends on mechanotransduction signaling. This model thus supports the notion that cellular responses to substrate curvature and compliance share the same molecular pathways and that control of cell behavior can be achieved via modulation of either individual parameter or in combination. This correlation is relevant for elucidating how muscle tissue forms and heals, as well as for designing better biomaterials and more appropriate cell-surface interfaces.

Toward Vasculature in Skeletal Muscle-on-a-Chip through Thermo-Responsive Sacrificial Templates

Developing new approaches for vascularizing synthetic tissue systems will have a tremendous impact in diverse areas. One area where this is particularly important is developing new skeletal muscle tissue systems, which could be utilized in physiological model studies and tissue regeneration. To develop vascularized approaches a microfluidic on-chip design for creating channels in polymer systems can be pursued. Current microfluidic tissue engineering methods include soft lithography, rapid prototyping, and cell printing; however, these have limitations such as having their scaffolding being inorganic, less desirable planar vasculature geometry, low fabrication efficiency, and limited resolution. Here we successfully developed a circular microfluidic channel embedded in a 3D extracellular matrix scaffolding with 3D myogenesis. We used a thermo-responsive polymer approach with micromilling-molding and designed a mixture of polyester wax and paraffin wax to fabricate the sacrificial template for microfluidic channel generation in the scaffolding. These findings will impact a number of fields including biomaterials, biomimetic structures, and personalized medicine in the future.

3D printed oxidized alginate-gelatin bioink provides guidance for C2C12 muscle precursor cell orientation and differentiation via shear stress during bioprinting

Biofabrication can be a tool to three-dimensionally (3D) print muscle cells embedded inside hydrogel biomaterials, ultimately aiming to mimic the complexity of the native muscle tissue and to create in-vitro muscle analogues for advanced repair therapies and drug testing. However, to 3D print muscle analogues of high cell alignment and synchronous contraction, the effect of biofabrication process parameters on myoblast growth has to be understood. A suitable biomaterial matrix is required to provide 3D printability as well as matrix degradation to create space for cell proliferation, matrix remodelling capacity, and cell differentiation. We demonstrate that by the proper selection of nozzle size and extrusion pressure, the shear stress during extrusion-bioprinting of mouse myoblast cells (C2C12) can achieve cell orientation when using oxidized alginate-gelatin (ADA-GEL) hydrogel bionk. The cells grow in the direction of printing, migrate to the hydrogel surface over time, and differentiate into ordered myotube segments in areas of high cell density. Together, our results show that ADA-GEL hydrogel can be a simple and cost-efficient biodegradable bioink that allows the successful 3D bioprinting and cultivation of C2C12 cells in-vitro to study muscle engineering.

Biohybrid robotics with living cell actuation

This review comprehensively discusses recent advances in the basic components, controlling methods and especially in the applications of biohybrid robots.

4D anisotropic skeletal muscle tissue constructs fabricated by staircase effect strategy

Like the morphology of native tissue fiber arrangement (such as skeletal muscle), unidirectional anisotropic scaffolds are highly desired as a means to guide cell behavior in anisotropic tissue engineering. In contrast, contour-like staircases exhibit directional topographical cues and are judged as an inevitable defect of fused deposition modeling (FDM). In this study, we will translate this staircase defect into an effective bioengineering strategy by integrating FDM with surface coating technique (FCT) to investigate the effect of topographical cues on regulating behaviors of human mesenchymal stem cells (hMSCs) toward skeletal muscle tissues. This integrated approach serves to fabricate shape-specific, multiple dimensional, anisotropic scaffolds using different biomaterials. 2D anisotropic scaffolds, first demonstrated with different polycaprolactone concentrations herein, efficiently direct hMSC alignment, especially when the scaffold is immobilized on a support ring. By surface coating the polymer solution inside FDM-printed sacrificial structures, 3D anisotropic scaffolds with thin wall features are developed and used to regulate seeded hMSCs through a self-established rotating bioreactor. Using layer-by-layer coating, along with a shape memory polymer, smart constructs exhibiting shape fix and recovery processes are prepared, bringing this study into the realm of 4D printing. Immunofluorescence staining and real-time quantitative polymerase chain reaction analysis confirm that the topographical cues created via FCT significantly enhance the expression of myogenic genes, including myoblast differentiation protein-1, desmin, and myosin heavy chain-2. We conclude that there are broad application potentials for this FCT strategy in tissue engineering as many tissues and organs, including skeletal muscle, possess highly organized and anisotropic extracellular matrix components.

Cell membrane engineering with synthetic materials: Applications in cell spheroids, cellular glues and microtissue formation

Biologically inspired materials with tunable bio- and physicochemical properties provide an essential framework to actively control and support cellular behavior. Cell membrane remodeling approaches benefit from the advances in polymer science and bioconjugation methods, which allow for the installation of un-/natural molecules and particles on the cells' surface. Synthetically remodeled cells have superior properties and are under intense investigation in various therapeutic scenarios as cell delivery systems, bio-sensing platforms, injectable biomaterials and bioinks for 3D bioprinting applications. In this review article, recent advances in the field of cell surface remodeling via bio-chemical means and the potential biomedical applications of these emerging cell hybrids are discussed. STATEMENT OF SIGNIFICANCE: Recent advances in bioconjugation methods, controlled/living polymerizations, microfabrication techniques and 3D printing technologies have enabled researchers to probe specific cellular functions and cues for therapeutic and research purposes through the formation of cell spheroids and polymer-cell chimeras. This review article highlights recent non-genetic cell membrane engineering strategies towards the fabrication of cellular ensembles and microtissues with interest in 3D in vitro modeling, cell therapeutics and tissue engineering. From a wider perspective, these approaches may provide a roadmap for future advances in cell therapies which will expedite the clinical use of cells, thereby improving the quality and accessibility of disease treatments.

In vitro micro-physiological model of the inflamed human adipose tissue for immune-metabolic analysis in type II diabetes

Chronic inflammation mediated by the interaction of immune cells and adipocytes is a key underlying factor in obesity-associated type 2 diabetes mellitus (T2DM). Therefore, methods to investigate adipocyte-immune cells interaction and their immuno-metabolic status in obese/T2DM subjects not only serve as an early indicator of disease development but also provide an insight into disease mechanism. A microfluidic-based in vitro model of the human adipose that is interfaced with a co-culture of immune cell has been developed for in vitro immune-metabolic analysis. This miniaturized system integrates a biologically active in vitro cellular system within a perfusion-based microfluidic device for mimicking the major processes that characterize the interaction of adipose tissue with immune cells. A viable immune competent model of the adipocytes/PBMCs co-culture has been demonstrated and characterized. Our testing results showed that the inflammatory cytokine profile obtained from the on-chip culture agrees with those from static transwell based co-culture with more intense responses observed in the chip-based system. The microfluidic chip also allows time-resolved measurement of cytokines that provide reliable data and detailed mechanisms of inflammation. In addition, glucose uptake by the adipocytes from the chip-based cultures showed correlated insulin responsivity/resistivity to the expression of the cytokine profile in different dynamic culture conditions. Testing of the known diabetic drug, metformin, and neutraceutical compound, omega-3, on-chip show agreeable results as compared to the previously reported data. This organotypic culture system offers a physiologically relevant model that exhibits a key characteristic of type 2 diabetic adipose tissues and can be used to study the T2DM mechanisms and diabetic drug screening.

The Effect of Microfluidic Geometry on Myoblast Migration

In vitro systems comprised of wells interconnected by microchannels have emerged as a platform for the study of cell migration or multicellular models. In the present study, we systematically evaluated the effect of microchannel width on spontaneous myoblast migration across these microchannels-from the proximal to the distal chamber. Myoblast migration was examined in microfluidic devices with varying microchannel widths of 1.5⁻20 µm, and in chips with uniform microchannel widths over time spans that are relevant for myoblast-to-myofiber differentiation in vitro. We found that the likelihood of spontaneous myoblast migration was microchannel width dependent and that a width of 3 µm was necessary to limit spontaneous migration below 5% of cells in the seeded well after 48 h. These results inform the future design of Polydimethylsiloxane (PDMS) microchannel-based co-culture platforms as well as future in vitro studies of myoblast migration.

Stem-cell based organ-on-a-chip models for diabetes research

Diabetes mellitus (DM) ranks among the severest global health concerns of the 21st century. It encompasses a group of chronic disorders characterized by a dysregulated glucose metabolism, which arises as a consequence of progressive autoimmune destruction of pancreatic beta-cells (type 1 DM), or as a result of beta-cell dysfunction combined with systemic insulin resistance (type 2 DM). Human cohort studies have provided evidence of genetic and environmental contributions to DM; yet, these studies are mostly restricted to investigating statistical correlations between DM and certain risk factors. Mechanistic studies, on the other hand, aimed at re-creating the clinical picture of human DM in animal models. A translation to human biology is, however, often inadequate owing to significant differences between animal and human physiology, including the species-specific glucose regulation. Thus, there is an urgent need for the development of advanced human in vitro models with the potential to identify novel treatment options for DM. This review provides an overview of the technological advances in research on DM-relevant stem cells and their integration into microphysiological environments as provided by the organ-on-a-chip technology.

Ex Vivo Tumor-on-a-Chip Platforms to Study Intercellular Interactions within the Tumor Microenvironment

The emergence of immunotherapies and recent FDA approval of several of them makes them a promising therapeutic strategy for cancer. While these advancements underscore the potential of engaging the immune system to target tumors, this approach has so far been efficient only for certain cancers. Extending immunotherapy as a widely acceptable treatment for various cancers requires a deeper understanding of the interactions of tumor cells within the tumor microenvironment (TME). The immune cells are a key component of the TME, which also includes other stromal cells, soluble factors, and extracellular matrix-based cues. While in vivo studies function as a gold standard, tissue-engineered microphysiological tumor models can offer patient-specific insights into cancer-immune interactions. These platforms, which recapitulate cellular and non-cellular components of the TME, enable a systematic understanding of the contribution of each component toward disease progression in isolation and in concert. Microfluidic-based microphysiological platforms recreating these environments, also known as "tumor-on-a-chip," are increasingly being utilized to study the effect of various elements of TME on tumor development. Herein are reviewed advancements in tumor-on-a-chip technology that are developed and used to understand the interaction of tumor cells with other surrounding cells, including immune cells, in the TME.

Uniaxially crumpled graphene as a platform for guided myotube formation

Graphene, owing to its inherent chemical inertness, biocompatibility, and mechanical flexibility, has great potential in guiding cell behaviors such as adhesion and differentiation. However, due to the two-dimensional (2D) nature of graphene, the microfabrication of graphene into micro/nanoscale patterns has been widely adopted for guiding cellular assembly. In this study, we report crumpled graphene, i.e., monolithically defined graphene with a nanoscale wavy surface texture, as a tissue engineering platform that can efficiently promote aligned C2C12 mouse myoblast cell differentiation. We imparted out-of-plane, nanoscale crumpled morphologies to flat graphene via compressive strain-induced deformation. When C2C12 mouse myoblast cells were seeded on the uniaxially crumpled graphene, not only were the alignment and elongation promoted at a single-cell level but also the differentiation and maturation of myotubes were enhanced compared to that on flat graphene. These results demonstrate the utility of the crumpled graphene platform for tissue engineering and regenerative medicine for skeletal muscle tissues.

Thermally-triggered fabrication of cell sheets for tissue engineering and regenerative medicine

Cell transplantation is a promising approach for promoting tissue regeneration in the treatment of damaged tissues or organs. Although cells have conventionally been delivered by direct injection to damaged tissues, cell injection has limited efficiency to deliver therapeutic cells to the target sites. Progress in tissue engineering has moved scaffold-based cell/tissue delivery into the mainstream of tissue regeneration. A variety of scaffolds can be fabricated from natural or synthetic polymers to provide the appropriate culture conditions for cell growth and achieve in-vitro tissue formation. Tissue engineering has now become the primary approach for cell-based therapies. However, there are still serious limitations, particularly for engineering of cell-dense tissues. "Cell sheet engineering" is a scaffold-free tissue technology that holds even greater promise in the field of tissue engineering and regenerative medicine. Thermoresponsive poly(N-isopropylacrylamide)-grafted surfaces allow the fabrication of a tissue-like cell monolayer, a "cell sheet", and efficiently delivers this cell-dense tissue to damaged sites without the use of scaffolds. At present, this unique approach has been applied to human clinical studies in regenerative medicine. Furthermore, this thermally triggered cell manipulation system allows us to produce various types of 3D tissue models not only for regenerative medicine but also for tissue modeling, which can be used for drug discovery. Here, new cell sheet-based technologies are described including vascularization for scaled-up 3D tissue constructs, induced pluripotent stem (iPS) cell technology for human cell sheet fabrication and microfabrication for arranging tissue microstructures, all of which are expected to produce more complex tissues based on cell sheet tissue engineering.

Hydrodynamics in Cell Studies

Hydrodynamic phenomena are ubiquitous in living organisms and can be used to manipulate cells or emulate physiological microenvironments experienced in vivo. Hydrodynamic effects influence multiple cellular properties and processes, including cell morphology, intracellular processes, cell–cell signaling cascades and reaction kinetics, and play an important role at the single-cell, multicellular, and organ level. Selected hydrodynamic effects can also be leveraged to control mechanical stresses, analyte transport, as well as local temperature within cellular microenvironments. With a better understanding of fluid mechanics at the micrometer-length scale and the advent of microfluidic technologies, a new generation of experimental tools that provide control over cellular microenvironments and emulate physiological conditions with exquisite accuracy is now emerging. Accordingly, we believe that it is timely to assess the concepts underlying hydrodynamic control of cellular microenvironments and their applications and provide some perspective on the future of such tools in in vitro cell-culture models. Generally, we describe the interplay between living cells, hydrodynamic stressors, and fluid flow-induced effects imposed on the cells. This interplay results in a broad range of chemical, biological, and physical phenomena in and around cells. More specifically, we describe and formulate the underlying physics of hydrodynamic phenomena affecting both adhered and suspended cells. Moreover, we provide an overview of representative studies that leverage hydrodynamic effects in the context of single-cell studies within microfluidic systems.

Skeletal muscle-on-a-chip: an in vitro model to evaluate tissue formation and injury

Engineered skeletal muscle tissues can be used for in vitro studies that require physiologically relevant models of native tissues. Herein, we describe the development of a three-dimensional (3D) skeletal muscle tissue that recapitulates the architectural and structural complexities of muscle within a microfluidic device. Using a 3D photo-patterning approach, we spatially confined a cell-laden gelatin network around two bio-inert hydrogel pillars, which induce uniaxial alignment of the cells and serve as anchoring sites for the encapsulated cells and muscle tissues as they form and mature. We have characterized the tissue morphology and strain profile during differentiation of the cells and skeletal muscle tissue formation by using a combination of fluorescence microscopy and computational tools. The time-dependent strain profile suggests the existence of individual cells within the gelatin matrix, which differentiated to form a multinucleated skeletal muscle tissue bundle as a function of culture time. We have also developed a method to calculate the passive tension generated by the engineered muscle tissue bundles suspended between two pillars. Finally, as a proof-of-concept we have examined the applicability of the skeletal muscle-on-chip system as a screening platform and in vitro muscle injury model. We studied the dose-dependent effect of cardiotoxin on the engineered muscle tissue architecture and its subsequent effect on the passive tension. This simple yet effective tool can be appealing for studies that necessitate the analysis of skeletal muscle structure and function, including preclinical drug discovery and development.

Biomimicry, Biofabrication, and Biohybrid Systems: The Emergence and Evolution of Biological Design

The discipline of biological design has a relatively short history, but has undergone very rapid expansion and development over that time. This Progress Report outlines the evolution of this field from biomimicry to biofabrication to biohybrid systems' design, showcasing how each subfield incorporates bioinspired dynamic adaptation into engineered systems. Ethical implications of biological design are discussed, with an emphasis on establishing responsible practices for engineering non-natural or hypernatural functional behaviors in biohybrid systems. This report concludes with recommendations for implementing biological design into educational curricula, ensuring effective and responsible practices for the next generation of engineers and scientists.

A pump-free microfluidic 3D perfusion platform for the efficient differentiation of human hepatocyte-like cells

The practical application of microfluidic liver models for in vitro drug testing is partly hampered by their reliance on human primary hepatocytes, which are limited in number and have batch-to-batch variation. Human stem cell-derived hepatocytes offer an attractive alternative cell source, although their 3D differentiation and maturation in a microfluidic platform have not yet been demonstrated. We develop a pump-free microfluidic 3D perfusion platform to achieve long-term and efficient differentiation of human liver progenitor cells into hepatocyte-like cells (HLCs). The device contains a micropillar array to immobilize cells three-dimensionally in a central cell culture compartment flanked by two side perfusion channels. Constant pump-free medium perfusion is accomplished by controlling the differential heights of horizontally orientated inlet and outlet media reservoirs. Computational fluid dynamic simulation is used to estimate the hydrostatic pressure heads required to achieve different perfusion flow rates, which are experimentally validated by micro-particle image velocimetry, as well as viability and functional assessments in a primary rat hepatocyte model. We perform on-chip differentiation of HepaRG, a human bipotent progenitor cell, and discover that 3D microperfusion greatly enhances the hepatocyte differentiation efficiency over static 2D and 3D cultures. However, HepaRG progenitor cells are highly sensitive to the time-point at which microperfusion is applied. Isolated HepaRG cells that are primed as static 3D spheroids before being subjected to microperfusion yield a significantly higher proportion of HLCs (92%) than direct microperfusion of isolated HepaRG cells (62%). This platform potentially offers a simple and efficient means to develop highly functional microfluidic liver models incorporating human stem cell-derived HLCs. Biotechnol. Bioeng. 2017;114: 2360-2370. © 2017 Wiley Periodicals, Inc.

On The Origin of Shear Stress Induced Myogenesis Using PMMA Based Lab-on-Chip

One of the central themes in cell and tissue engineering is to develop an understanding as to how biophysical cues can influence cell functionality changes. The flow induced shear stress is regarded as one such biophysical cue to influence physiological changes in shear-sensitive tissues, in vivo. The origin of such phenomena is, however, poorly understood. While addressing such an issue, the present work demonstrates the intriguing synergistic effect of shear stress and spatial constraints in inducing aligned growth and differentiation of myoblast cells to myotubes. In a planned set of in vitro experiments, the regulation of laminar flow regime within a narrow window was obtained in a PMMA-based Lab-on-Chip (LOC) device, wherein the murine muscle cells (C2C12), chosen for their phenotypical differentiation stages, were cultured under graded shear conditions. The two factors of shear stress and spatial allowance were decoupled by another two sets of experiments. This aspect has been conclusively established using a PMMA device having a fixed width microchannel with varying shear and an identical amount of shear with different width of channels. On the basis of the extensive analysis of biochemical assays (WST-1, picogreen) together with gene expression using qRT-PCR and cell morphological changes (fluorescence/confocal microscopy), extensive differentiation of the myoblasts into myotubes is found to be dependent on both shear stress and spatial allocation with a maximum at an optimal shear of ca. 16 mPa. Quantitatively, the mRNA expression of myogenic biomarkers, i.e., myogenin, MyoD, and neogenin, exhibited 10- to 50-fold changes at ca. 16 mPa shear flow, compared to that under static conditions. Also, myotube aspect ratio and myotube density are modulated with shear stress and are in commensurate with gene expression changes. The flow cytometry analysis further confirmed that the cell cycle arrest at the G1/G0 phase triggers the onset of myogenesis. Taken together, the present study unambiguously establishes qualitative and quantitative biophysical basis for the origin of myogenesis toward the critical shear stress of murine myoblasts in a microfludic device, in vitro.

Simplified Murine 3D Neuronal Cultures for Investigating Neuronal Activity and Neurodegeneration

The ability to model brain tissue in three-dimensions offers new potential for elucidating functional cellular interactions and corruption of such functions during pathogenesis. Many protocols now exist for growing neurones in three-dimensions and these vary in complexity and cost. Herein, we describe a straight-forward method for generating three-dimensional, terminally differentiated central nervous system cultures from adult murine neural stem cells. The protocol requires no specialist equipment, is not labour intensive or expensive and produces mature cultures within 10 days that can survive beyond a month. Populations of functional glutamatergic neurones could be identified within cultures. Additionally, the three dimensional neuronal cultures can be used to investigate tissue changes during the development of neurodegenerative disease where demonstration of hallmark features, such as plaque generation, has not previously been possible using two-dimensional cultures of neuronal cells. Using a prion model of acquired neurodegenerative disease, biochemical changes indicative of prion pathology were induced within 2-3 weeks in the three dimensional cultures. Our findings show that tissue differentiated in this simplified three dimensional culture model is physiologically competent to model central nervous system cellular behaviour as well as manifest the functional failures and pathological changes associated with neurodegenerative disease.

Rapid Formation of Cell Aggregates and Spheroids Induced by a "Smart" Boronic Acid Copolymer

Cell surface engineering has emerged as a powerful approach to forming cell aggregates/spheroids and cell-biomaterial ensembles with significant uses in tissue engineering and cell therapeutics. Herein, we demonstrate that cell membrane remodeling with a thermoresponsive boronic acid copolymer induces the rapid formation of spheroids using either cancer or cardiac cell lines under conventional cell culture conditions at minute concentrations. It is shown that the formation of well-defined spheroids is accelerated by at least 24 h compared to non-polymer-treated controls, and, more importantly, the polymer allows for fine control of the aggregation kinetics owing to its stimulus response to temperature and glucose content. On the basis of its simplicity and effectiveness to promote cellular aggregation, this platform holds promise in three-dimensional tissue/tumor modeling and tissue engineering applications.

Patterning of Fibroblast and Matrix Anisotropy within 3D Confinement is Driven by the Cytoskeleton

Effects of 3D confinement on cellular growth and matrix assembly are important in tissue engineering, developmental biology, and regenerative medicine. Polydimethylsiloxane wells with varying anisotropy are microfabicated using soft-lithography. Microcontact printing of bovine serum albumin is used to block cell adhesion to surfaces between wells. The orientations of fibroblast stress fibers, microtubules, and fibronectin fibrils are examined 1 day after cell seeding using laser scanning confocal microscopy, and anisotropy is quantified using a custom autocorrelation analysis. Actin, microtubules, and fibronectin exhibit higher anisotropy coefficients for cells grown in rectangular wells with aspect ratios of 1:4 and 1:8, as compared to those in wells with lower aspect ratios or in square wells. The effects of disabling individual cytoskeletal components on fibroblast responses to anisotropy are then tested by applying actin or microtubule polymerization inhibitors, Rho kinase inhibitor, or by siRNA-mediated knockdown of AXL or cofilin-1. Latrunculin A decreases cytoskeletal and matrix anisotropy, nocodazole ablates both, and Y27632 mutes cellular polarity while decreasing matrix anisotropy. AXL siRNA knockdown has little effect, as does siRNA knockdown of cofilin-1. These data identify several specific cytoskeletal strategies as targets for the manipulation of anisotropy in 3D tissue constructs.

Morphology, proliferation, alignment, and new collagen synthesis of mesenchymal stem cells on a microgrooved collagen membrane

The topographic cues of the extracellular matrix may have significant effects upon cellular behavior, such as adhesion, spreading, migration, proliferation, differentiation, and in particular, morphology and orientation. In this study, we examined the effects of microgrooved collagen membrane (MCM) on mesenchymal stem cell (MSC) behavior. The MCM (9 μm in periodicity, and 1-2 μm in depth) was fabricated on an untreated (nonpolar) and smooth polystyrene substrate, based on the absorption and self-assembly properties of collagen on the polystyrene substrate. Methyl thiazolyl tetrazolium assay revealed that cell proliferation on the MCM was enhanced compared with the smooth collagen membrane at day 2. Qualitative observation of MSC behavior using confocal laser scanning microscopy and scanning electron microscopy showed that MSCs grew with a highly elongated morphology and were aligned strictly along the direction of the microgrooves. Additionally, scanning electron microscopy revealed the oriented cells produced a collagenous matrix on the MCM that had a preferential orientation, whereas the collagenous matrix produced by randomly oriented MSCs on the smooth collagen membrane was disorganized. Future studies should investigate the fabrication of oriented topographical substrates, based on the natural biomaterial collagen, to guide cell alignment and oriented growth along definite directions. These substrates may help produce aligned collagenous matrices that could have good potential for the production of tissue substitutes.

Cell Sheet-Based Tissue Engineering for Organizing Anisotropic Tissue Constructs Produced Using Microfabricated Thermoresponsive Substrates

In some native tissues, appropriate microstructures, including orientation of the cell/extracellular matrix, provide specific mechanical and biological functions. For example, skeletal muscle is made of oriented myofibers that is responsible for the mechanical function. Native artery and myocardial tissues are organized three-dimensionally by stacking sheet-like tissues of aligned cells. Therefore, to construct any kind of complex tissue, the microstructures of cells such as myotubes, smooth muscle cells, and cardiomyocytes also need to be organized three-dimensionally just as in the native tissues of the body. Cell sheet-based tissue engineering allows the production of scaffold-free engineered tissues through a layer-by-layer construction technique. Recently, using microfabricated thermoresponsive substrates, aligned cells are being harvested as single continuous cell sheets. The cell sheets act as anisotropic tissue units to build three-dimensional tissue constructs with the appropriate anisotropy. This cell sheet-based technology is straightforward and has the potential to engineer a wide variety of complex tissues. In addition, due to the scaffold-free cell-dense environment, the physical and biological cell-cell interactions of these cell sheet constructs exhibit unique cell behaviors. These advantages will provide important clues to enable the production of well-organized tissues that closely mimic the structure and function of native tissues, required for the future of tissue engineering.

Skeletal muscle tissue engineering: methods to form skeletal myotubes and their applications

Skeletal muscle tissue engineering (SMTE) aims to repair or regenerate defective skeletal muscle tissue lost by traumatic injury, tumor ablation, or muscular disease. However, two decades after the introduction of SMTE, the engineering of functional skeletal muscle in the laboratory still remains a great challenge, and numerous techniques for growing functional muscle tissues are constantly being developed. This article reviews the recent findings regarding the methodology and various technical aspects of SMTE, including cell alignment and differentiation. We describe the structure and organization of muscle and discuss the methods for myoblast alignment cultured in vitro. To better understand muscle formation and to enhance the engineering of skeletal muscle, we also address the molecular basics of myogenesis and discuss different methods to induce myoblast differentiation into myotubes. We then provide an overview of different coculture systems involving skeletal muscle cells, and highlight major applications of engineered skeletal muscle tissues. Finally, potential challenges and future research directions for SMTE are outlined.

Accelerating drug discovery via organs-on-chips

Considerable advances have been made in the development of micro-physiological systems that seek to faithfully replicate the complexity and functionality of animal and human physiology in research laboratories. Sometimes referred to as "organs-on-chips", these systems provide key insights into physiological or pathological processes associated with health maintenance and disease control, and serve as powerful platforms for new drug development and toxicity screening. In this Focus article, we review the state-of-the-art designs and examples for developing multiple "organs-on-chips", and discuss the potential of this emerging technology to enhance our understanding of human physiology, and to transform and accelerate the drug discovery and preclinical testing process. This Focus article highlights some of the recent technological advances in this field, along with the challenges that must be addressed for these technologies to fully realize their potential.