Automated fabrication of photopatterned gelatin hydrogels for organ-on-chips applications

Developing a soft micropatterned substrate to enhance maturation of human induced pluripotent stem cell-derived cardiomyocytes

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSCCMs) offer numerous advantages as a biological model, yet their inherent immaturity compared to adult cardiomyocytes poses significant limitations. This study addresses hiPSCCM immaturity by introducing a physiologically relevant micropatterned substrate for long-term culture and maturation. An innovative microfabrication methodology combining laser etching and casting creates a micropatterned polydimethylsiloxane (PDMS) substrate with varying stiffness, from 2 to 50 kPa, mimicking healthy and fibrotic cardiac tissue. Platinum electrodes were integrated into the cell culture chamber enable pacing of cells at various frequencies. Subsequently, cells were transferred to the incubator for time-course analysis, ensuring contamination-free conditions. Cell contractility, cytosolic Ca2+ transient, sarcomere orientation, and nucleus aspect ratio were analyzed in a 2D hiPSCCM monolayer up to 90 days post-replating in relation to substrate micropattern dimensions. Culturing hiPSCCMs for three weeks on a micropatterned PDMS substrate (2.5-5 µm deep, 20 µm center-to-center spacing of grooves, 2-5 kPa stiffness) emerges as optimal for cardiomyocyte alignment, contractility, and cytosolic Ca2+ transient. The study provides insights into substrate stiffness effects on hiPSCCM contractility and Ca2+ transient at immature and mature states. Maximum contractility and fastest Ca2+transient kinetics occur in mature hiPSCCMs cultured for two to four weeks, with the optimum at three weeks, on a soft micropatterned PDMS substrate. MS proteomic analysis further revealed that hiPSCCMs cultured on soft micropatterned substrates exhibit advanced maturation, marked by significant upregulation of key structural, electrophysiological, and metabolic proteins. This new substrate offers a promising platform for disease modeling and therapeutic interventions. STATEMENT OF SIGNIFICANCE: Human induced pluripotent stem cell derived cardiomyocytes (hiPSCCMs) have been transformative to disease-in-a-dish modeling, drug discovery and testing, and autologous regeneration for human hearts and their role will continue to expand dramatically. However, one of the major limitations of hiPSCCMs is that without intervention, the cells are immature and represent those in the fetal heart. We developed protocols for the fabrication of the PDMS matrices that includes variations in its stiffness and micropatterning. Growing our hiPSCCMs on matrices of comparable stiffness to a healthy heart (5 kPa) and grooves of 20 μm, generate heart cells typical of the healthy adult human heart.

Organoids-On-a-Chip for Personalized Precision Medicine

The development of personalized precision medicine has become a pivotal focus in modern healthcare. Organoids-on-a-Chip (OoCs), a groundbreaking fusion of organoid culture and microfluidic chip technology, has emerged as a promising approach to advancing patient-specific treatment strategies. In this review, the diverse applications of OoCs are explored, particularly their pivotal role in personalized precision medicine, and their potential as a cutting-edge technology is highlighted. By utilizing patient-derived organoids, OoCs offer a pathway to optimize treatments, create precise disease models, investigate disease mechanisms, conduct drug screenings, and individualize therapeutic strategies. The emphasis is on the significance of this technological fusion in revolutionizing healthcare and improving patient outcomes. Furthermore, the transformative potential of personalized precision medicine, future prospects, and ongoing advancements in the field, with a focus on genomic medicine, multi-omics integration, and ethical frameworks are discussed. The convergence of these innovations can empower patients, redefine treatment approaches, and shape the future of healthcare.

Evolution of Hybrid Hydrogels: Next-Generation Biomaterials for Drug Delivery and Tissue Engineering

Hydrogels, being hydrophilic polymer networks capable of absorbing and retaining aqueous fluids, hold significant promise in biomedical applications owing to their high water content, permeability, and structural similarity to the extracellular matrix. Recent chemical advancements have bolstered their versatility, facilitating the integration of the molecules guiding cellular activities and enabling their controlled activation under time constraints. However, conventional synthetic hydrogels suffer from inherent weaknesses such as heterogeneity and network imperfections, which adversely affect their mechanical properties, diffusion rates, and biological activity. In response to these challenges, hybrid hydrogels have emerged, aiming to enhance their strength, drug release efficiency, and therapeutic effectiveness. These hybrid hydrogels, featuring improved formulations, are tailored for controlled drug release and tissue regeneration across both soft and hard tissues. The scientific community has increasingly recognized the versatile characteristics of hybrid hydrogels, particularly in the biomedical sector. This comprehensive review delves into recent advancements in hybrid hydrogel systems, covering the diverse types, modification strategies, and the integration of nano/microstructures. The discussion includes innovative fabrication techniques such as click reactions, 3D printing, and photopatterning alongside the elucidation of the release mechanisms of bioactive molecules. By addressing challenges, the review underscores diverse biomedical applications and envisages a promising future for hybrid hydrogels across various domains in the biomedical field.

Automated fabrication of a scalable heart-on-a-chip device by 3D printing of thermoplastic elastomer nanocomposite and hot embossing

The successful translation of organ-on-a-chip devices requires the development of an automated workflow for device fabrication, which is challenged by the need for precise deposition of multiple classes of materials in micro-meter scaled configurations. Many current heart-on-a-chip devices are produced manually, requiring the expertise and dexterity of skilled operators. Here, we devised an automated and scalable fabrication method to engineer a Biowire II multiwell platform to generate human iPSC-derived cardiac tissues. This high-throughput heart-on-a-chip platform incorporated fluorescent nanocomposite microwires as force sensors, produced from quantum dots and thermoplastic elastomer, and 3D printed on top of a polystyrene tissue culture base patterned by hot embossing. An array of built-in carbon electrodes was embedded in a single step into the base, flanking the microwells on both sides. The facile and rapid 3D printing approach efficiently and seamlessly scaled up the Biowire II system from an 8-well chip to a 24-well and a 96-well format, resulting in an increase of platform fabrication efficiency by 17,5000-69,000% per well. The device's compatibility with long-term electrical stimulation in each well facilitated the targeted generation of mature human iPSC-derived cardiac tissues, evident through a positive force-frequency relationship, post-rest potentiation, and well-aligned sarcomeric apparatus. This system's ease of use and its capacity to gauge drug responses in matured cardiac tissue make it a powerful and reliable platform for rapid preclinical drug screening and development.

Tailoring biomaterials for biomimetic organs-on-chips

Organs-on-chips are microengineered microfluidic living cell culture devices with continuously perfused chambers penetrating to cells. By mimicking the biological features of the multicellular constructions, interactions among organs, vascular perfusion, physicochemical microenvironments, and so on, these devices are imparted with some key pathophysiological function levels of living organs that are difficult to be achieved in conventional 2D or 3D culture systems. In this technology, biomaterials are extremely important because they affect the microstructures and functionalities of the organ cells and the development of the organs-on-chip functions. Thus, herein, we provide an overview on the advances of biomaterials for the construction of organs-on-chips. After introducing the general components, structures, and fabrication techniques of the biomaterials, we focus on the studies of the functions and applications of these biomaterials in the organs-on-chips systems. Applications of the biomaterial-based organs-on-chips as alternative animal models for pharmaceutical, chemical, and environmental tests are described and highlighted. The prospects for exciting future directions and the challenges of biomaterials for realizing the further functionalization of organs-on-chips are also presented.

Tunable Light-Actuated Interpenetrating Networks of Silk Fibroin and Gelatin for Tissue Engineering and Flexible Biodevices

Soft materials with tunable properties are valuable for applications such as tissue engineering, electronic skins, and human-machine interfaces. Materials that are nature-derived offer additional advantages such as biocompatibility, biodegradability, low-cost sourcing, and sustainability. However, these materials often have contrasting properties that limit their use. For example, silk fibroin (SF) has high mechanical strength but lacks processability and cell-adhesive domains. Gelatin, derived from collagen, has excellent biological properties, but is fragile and lacks stability. To overcome these limitations, composites of gelatin and SF have been explored. However, mechanically robust self-supported matrices and electrochemically active or micropatterned substrates were not demonstrated. In this study, we present a composite of photopolymerizable SF and photogelatin, termed photofibrogel (PFG). By incorporating photoreactive properties in both SF and gelatin, control over material properties can be achieved. The PFG composite can be easily and rapidly formed into free-standing, high-resolution architectures with tunable properties. By optimizing the ratio of SF to gelatin, properties such as swelling, mechanical behavior, enzymatic degradation, and patternability are tailored. The PFG composite allows for macroscale and microscale patterning without significant swelling, enabling the fabrication of structures using photolithography and laser cutting techniques. PFG can be patterned with electrically conductive materials, making it suitable for cell guidance and stimulation. The versatility, mechanical robustness, bioactivity, and electrochemical properties of PFG are shown for skeletal muscle tissue engineering using C2C12 cells as a model. Overall, such composite biomaterials with tunable properties have broad potential in flexible bioelectronics, wound healing, regenerative medicine, and food systems.

An Azomethine-H-Based Fluorogenic Sensor for Formic Acid

Formic acid (FA) is an important C1-containing feedstock that serves as a masked source of dihydrogen gas (H2). To encourage the adoption of cleaner (noncarbonaceous) energy sources, FA detection and sensing is thus of considerable interest. Here, we examine the use of a commercially available dye, azomethine-H (Az-H), for FA sensing. Solution studies confirm that FA quenches both the absorbance and the luminescence properties of Az-H. FA was additionally found to attenuate a known Az-H (E)-to-(Z) conformational change, suggesting an Az-H/FA interaction, possibly through hydrogen bonding; this phenomenon was probed using 1H NMR spectroscopy. Moving toward a solid-state sensor, the Az-H probe was incorporated into a gelatin-based matrix. On exposure to FA, the luminescence of this system was found to increase in a FA-dependent manner, attributed to the formation of stable hydrogen-bonded structures, facilitating a (Z)-to-(E) isomerization via imine protonation, allowing for production of the more luminescent (E)-isomer. This fluorogenic signal was used as a FA sensor with an estimated detection limit of ca. 0.4 ppb FA vapor. This work constitutes an important step toward a highly sensitive FA sensor in both the solution and solid state, opening new space for the detection of organic acids in differing chemical environments.

Dynamic Stimulations with Bioengineered Extracellular Matrix-Mimicking Hydrogels for Mechano Cell Reprogramming and Therapy

Cells interact with their surrounding environment through a combination of static and dynamic mechanical signals that vary over stimulus types, intensity, space, and time. Compared to static mechanical signals such as stiffness, porosity, and topography, the current understanding on the effects of dynamic mechanical stimulations on cells remains limited, attributing to a lack of access to devices, the complexity of experimental set-up, and data interpretation. Yet, in the pursuit of emerging translational applications (e.g., cell manufacturing for clinical treatment), it is crucial to understand how cells respond to a variety of dynamic forces that are omnipresent in vivo so that they can be exploited to enhance manufacturing and therapeutic outcomes. With a rising appreciation of the extracellular matrix (ECM) as a key regulator of biofunctions, researchers have bioengineered a suite of ECM-mimicking hydrogels, which can be fine-tuned with spatiotemporal mechanical cues to model complex static and dynamic mechanical profiles. This review first discusses how mechanical stimuli may impact different cellular components and the various mechanobiology pathways involved. Then, how hydrogels can be designed to incorporate static and dynamic mechanical parameters to influence cell behaviors are described. The Scopus database is also used to analyze the relative strength in evidence, ranging from strong to weak, based on number of published literatures, associated citations, and treatment significance. Additionally, the impacts of static and dynamic mechanical stimulations on clinically relevant cell types including mesenchymal stem cells, fibroblasts, and immune cells, are evaluated. The aim is to draw attention to the paucity of studies on the effects of dynamic mechanical stimuli on cells, as well as to highlight the potential of using a cocktail of various types and intensities of mechanical stimulations to influence cell fates (similar to the concept of biochemical cocktail to direct cell fate). It is envisioned that this progress report will inspire more exciting translational development of mechanoresponsive hydrogels for biomedical applications.

Thinking in 3 dimensions: philosophies of the microenvironment in organoids and organs-on-chip

Organoids and organs-on-a-chip are currently the two major families of 3D advanced organotypic in vitro culture systems, aimed at reconstituting miniaturized models of physiological and pathological states of human organs. Both share the tenets of the so-called "three-dimensional thinking", a Systems Physiology approach focused on recapitulating the dynamic interactions between cells and their microenvironment. We first review the arguments underlying the "paradigm shift" toward three-dimensional thinking in the in vitro culture community. Then, through a historically informed account of the technical affordances and the epistemic commitments of these two approaches, we highlight how they embody two distinct experimental cultures. We finally argue that the current systematic effort for their integration requires not only innovative "synergistic" engineering solutions, but also conceptual integration between different perspectives on biological causality.

Translational Nanomedicines Across Human Reproductive Organs Modeling on Microfluidic Chips: State-of-the-Art and Future Prospects

Forecasting the consequence of nanoparticles (NPs) and therapeutically significant molecules before materializing for human clinical trials is a mainstay for drug delivery and screening processes. One of the noteworthy obstacles that has prevented the clinical translation of NP-based drug delivery systems and novel drugs is the lack of effective preclinical platforms. As a revolutionary technology, the organ-on-a-chip (OOC), a coalition of microfluidics and tissue engineering, has surfaced as an alternative to orthodox screening platforms. OOC technology recapitulates the structural and physiological features of human organs along with intercommunications between tissues on a chip. The current review discusses the concept of microfluidics and confers cutting-edge fabrication processes for chip designing. We also outlined the advantages of microfluidics in analyzing NPs in terms of characterization, transport, and degradation in biological systems. The review further elaborates the scope and research on translational nanomedicines in human reproductive organs (testis, placenta, uterus, and menstrual cycle) by taking the advantages offered by microfluidics and shedding light on their potential future implications. Finally, we accentuate the existing challenges for clinical translation and scale-up dynamics for microfluidics chips and emphasize its future perspectives.

Biomaterial-assisted biotherapy: A brief review of biomaterials used in drug delivery, vaccine development, gene therapy, and stem cell therapy

Biotherapy has recently become a hotspot research topic with encouraging prospects in various fields due to a wide range of treatments applications, as demonstrated in preclinical and clinical studies. However, the broad applications of biotherapy have been limited by critical challenges, including the lack of safe and efficient delivery systems and serious side effects. Due to the unique potentials of biomaterials, such as good biocompatibility and bioactive properties, biomaterial-assisted biotherapy has been demonstrated to be an attractive strategy. The biomaterial-based delivery systems possess sufficient packaging capacity and versatile functions, enabling a sustained and localized release of drugs at the target sites. Furthermore, the biomaterials can provide a niche with specific extracellular conditions for the proliferation, differentiation, attachment, and migration of stem cells, leading to tissue regeneration. In this review, the state-of-the-art studies on the applications of biomaterials in biotherapy, including drug delivery, vaccine development, gene therapy, and stem cell therapy, have been summarized. The challenges and an outlook of biomaterial-assisted biotherapies have also been discussed.

Technological and structural aspects of scaffold manufacturing for cultured meat: recent advances, challenges, and opportunities

In vitro cultured meat is an emerging area of research focus with an innovative approach through tissue engineering (i.e., cellular engineering) to meet the global food demand. The manufacturing of lab-cultivated meat is an innovative business that alleviates life-threatening environmental issues concerning public health and animal well-being on the global platform. There has been a noteworthy advancement in cultivating artificial meat, but still, there are numerous challenges that impede the swift headway of lab-grown meat production at a commercially large scale. In this review, we focus on the manufacturing of edible scaffolds for cultured meat production. In brief, first an introduction to cultivating artificial meat and its current scenario in the market is provided. Further, a discussion on the understanding of composition, cellular, and molecular communications in muscle tissue is presented, which are vital to scaling up the production of lab-grown meat. In continuation, the major components (e.g., cells, biomaterial scaffolds, and their manufacturing technologies, media, and potential bioreactors) for cultured meat production are conferred followed by a comprehensive discussion on the most recent advances in lab-cultured meat. Finally, existing challenges and opportunities including future research perspectives for scaling-up cultured meat production are discussed with conclusive interpretations.

Characterization and Optimization of Real-Time Photoresponsive Gelatin for Direct Laser Writing

There is an abundance of plastic materials used in the widest range of applications, such as packaging, machine parts, biomedical devices and components, etc. However, most materials used today are non-decomposable in the environment, producing a huge burden on ecosystems. The search for better, safer alternatives is still on. Here we present a detailed analysis of a simple, cheap, non-toxic, even edible, eco-friendly material, which can be easily manufactured, laser patterned and used for the fabrication of complex structures. The base substance is gelatin which is made photoresponsive by adding plasticizers and sensitizers. The resulting films were analyzed with respect to their optical, thermal and mechanical properties, which can be modified by a slight variation of chemical composition. The material is optimized for rapid laser-manufacturing of elastic microstructures (lenses, gratings, cantilevers, etc.) without any waste or residues. Overall, the material properties were tailored to increase photothermal responsivity, improve the surface quality and achieve material homogeneity, transparency and long-term stability (as verified using electron microscopy, infrared spectroscopy and differential scanning calorimetry).

Applications of Polymers for Organ-on-Chip Technology in Urology

Organ-on-chips (OOCs) are microfluidic devices used for creating physiological organ biomimetic systems. OOC technology brings numerous advantages in the current landscape of preclinical models, capable of recapitulating the multicellular assemblage, tissue-tissue interaction, and replicating numerous human pathologies. Moreover, in cancer research, OOCs emulate the 3D hierarchical complexity of in vivo tumors and mimic the tumor microenvironment, being a practical cost-efficient solution for tumor-growth investigation and anticancer drug screening. OOCs are compact and easy-to-use microphysiological functional units that recapitulate the native function and the mechanical strain that the cells experience in the human bodies, allowing the development of a wide range of applications such as disease modeling or even the development of diagnostic devices. In this context, the current work aims to review the scientific literature in the field of microfluidic devices designed for urology applications in terms of OOC fabrication (principles of manufacture and materials used), development of kidney-on-chip models for drug-toxicity screening and kidney tumors modeling, bladder-on-chip models for urinary tract infections and bladder cancer modeling and prostate-on-chip models for prostate cancer modeling.

Advances in microfabrication technologies in tissue engineering and regenerative medicine

BACKGROUND

Tissue engineering provides various strategies to fabricate an appropriate microenvironment to support the repair and regeneration of lost or damaged tissues. In this matter, several technologies have been implemented to construct close-to-native three-dimensional structures at numerous physiological scales, which are essential to confer the functional characteristics of living tissues.

METHODS

In this article, we review a variety of microfabrication technologies that are currently utilized for several tissue engineering applications, such as soft lithography, microneedles, templated and self-assembly of microstructures, microfluidics, fiber spinning, and bioprinting.

RESULTS

These technologies have considerably helped us to precisely manipulate cells or cellular constructs for the fabrication of biomimetic tissues and organs. Although currently available tissues still lack some crucial functionalities, including vascular networks, innervation, and lymphatic system, microfabrication strategies are being proposed to overcome these issues. Moreover, the microfabrication techniques that have progressed to the preclinical stage are also discussed.

CONCLUSIONS

This article aims to highlight the advantages and drawbacks of each technique and areas of further research for a more comprehensive and evolving understanding of microfabrication techniques in terms of tissue engineering and regenerative medicine applications.

Monitoring the maturation of the sarcomere network: a super-resolution microscopy-based approach

The in vitro generation of human cardiomyocytes derived from induced pluripotent stem cells (iPSC) is of great importance for cardiac disease modeling, drug-testing applications and for regenerative medicine. Despite the development of various cultivation strategies, a sufficiently high degree of maturation is still a decisive limiting factor for the successful application of these cardiac cells. The maturation process includes, among others, the proper formation of sarcomere structures, mediating the contraction of cardiomyocytes. To precisely monitor the maturation of the contractile machinery, we have established an imaging-based strategy that allows quantitative evaluation of important parameters, defining the quality of the sarcomere network. iPSC-derived cardiomyocytes were subjected to different culture conditions to improve sarcomere formation, including prolonged cultivation time and micro patterned surfaces. Fluorescent images of α-actinin were acquired using super-resolution microscopy. Subsequently, we determined cell morphology, sarcomere density, filament alignment, z-Disc thickness and sarcomere length of iPSC-derived cardiomyocytes. Cells from adult and neonatal heart tissue served as control. Our image analysis revealed a profound effect on sarcomere content and filament orientation when iPSC-derived cardiomyocytes were cultured on structured, line-shaped surfaces. Similarly, prolonged cultivation time had a beneficial effect on the structural maturation, leading to a more adult-like phenotype. Automatic evaluation of the sarcomere filaments by machine learning validated our data. Moreover, we successfully transferred this approach to skeletal muscle cells, showing an improved sarcomere formation cells over different differentiation periods. Overall, our image-based workflow can be used as a straight-forward tool to quantitatively estimate the structural maturation of contractile cells. As such, it can support the establishment of novel differentiation protocols to enhance sarcomere formation and maturity.

Bioengineering approaches to treat the failing heart: from cell biology to 3D printing

Successfully engineering a functional, human, myocardial pump would represent a therapeutic alternative for the millions of patients with end-stage heart disease and provide an alternative to animal-based preclinical models. Although the field of cardiac tissue engineering has made tremendous advances, major challenges remain, which, if properly resolved, might allow the clinical implementation of engineered, functional, complex 3D structures in the future. In this Review, we provide an overview of state-of-the-art studies, challenges that have not yet been overcome and perspectives on cardiac tissue engineering. We begin with the most clinically relevant cell sources used in this field and discuss the use of topological, biophysical and metabolic stimuli to obtain mature phenotypes of cardiomyocytes, particularly in relation to organized cytoskeletal and contractile intracellular structures. We then move from the cellular level to engineering planar cardiac patches and discuss the need for proper vascularization and the main strategies for obtaining it. Finally, we provide an overview of several different approaches for the engineering of volumetric organs and organ parts - from whole-heart decellularization and recellularization to advanced 3D printing technologies.

Insights to Heart Development and Cardiac Disease Models Using Pluripotent Stem Cell Derived 3D Organoids

Medical research in the recent years has achieved significant progress due to the increasing prominence of organoid technology. Various developed tissue organoids bridge the limitations of conventional 2D cell culture and animal models by recapitulating in vivo cellular complexity. Current 3D cardiac organoid cultures have shown their utility in modelling key developmental hallmarks of heart organogenesis, but the complexity of the organ demands a more versatile model that can investigate more fundamental parameters, such as structure, organization and compartmentalization of a functioning heart. This review will cover the prominence of cardiac organoids in recent research, unpack current in vitro 3D models of the developing heart and look into the prospect of developing physiologically appropriate cardiac organoids with translational applicability. In addition, we discuss some of the limitations of existing cardiac organoid models in modelling embryonic development of the heart and manifestation of cardiac diseases.

Local Renin-Angiotensin System Signaling Mediates Cellular Function of Aortic Valves

The renin-angiotensin system (RAS) is activated in aortic valve disease, yet little is understood about how it affects the acute functional response of valve interstitial cells (VICs). Herein, we developed a gelatin-based valve thin film (vTF) platform to investigate whether the contractile response of VICs can be regulated via RAS mediators and inhibitors. First, the impact of culture medium (quiescent, activated, and osteogenic medium) on VIC phenotype and function was assessed. Contractility of VICs was measured upon treatment with angiotensin I (Ang I), angiotensin II (Ang II), angiotensin-converting enzyme (ACE) inhibitor, and Angiotensin II type 1 receptor (AT1R) inhibitor. Anisotropic cell alignment on gelatin vTF was achieved independent of culture conditions. Cells cultured in activated and osteogenic conditions were found to be more elongated than in quiescent medium. Increased α-SMA expression was observed in activated medium and no RUNX2 expression were observed in cells. VIC contractile stress increased with increasing concentrations (from 10-10 to 10-6 M) of Ang I and Ang II. Moreover, cell contraction was significantly reduced in all ACE and AT1R inhibitor-treated groups. Together, these findings suggest that local RAS is active in VICs, and our vTF may provide a powerful platform for valve drug screening and development.

Revisiting tissue tensegrity: Biomaterial-based approaches to measure forces across length scales

Cell-generated forces play a foundational role in tissue dynamics and homeostasis and are critically important in several biological processes, including cell migration, wound healing, morphogenesis, and cancer metastasis. Quantifying such forces in vivo is technically challenging and requires novel strategies that capture mechanical information across molecular, cellular, and tissue length scales, while allowing these studies to be performed in physiologically realistic biological models. Advanced biomaterials can be designed to non-destructively measure these stresses in vitro, and here, we review mechanical characterizations and force-sensing biomaterial-based technologies to provide insight into the mechanical nature of tissue processes. We specifically and uniquely focus on the use of these techniques to identify characteristics of cell and tissue "tensegrity:" the hierarchical and modular interplay between tension and compression that provide biological tissues with remarkable mechanical properties and behaviors. Based on these observed patterns, we highlight and discuss the emerging role of tensegrity at multiple length scales in tissue dynamics from homeostasis, to morphogenesis, to pathological dysfunction.

Regenerative Medicine for the Treatment of Ischemic Heart Disease; Status and Future Perspectives

Cardiovascular disease is now the leading cause of adult death in the world. According to new estimates from the World Health Organization, myocardial infarction (MI) is responsible for four out of every five deaths due to cardiovascular disease. Conventional treatments of MI are taking aspirin and nitroglycerin as intermediate treatments and injecting antithrombotic agents within the first 3 h after MI. Coronary artery bypass grafting and percutaneous coronary intervention are the most common long term treatments. Since none of these interventions will fully regenerate the infarcted myocardium, there is value in pursuing more innovative therapeutic approaches. Regenerative medicine is an innovative interdisciplinary method for rebuilding, replacing, or repairing the missed part of different organs in the body, as similar as possible to the primary structure. In recent years, regenerative medicine has been widely utilized as a treatment for ischemic heart disease (one of the most fatal factors around the world) to repair the lost part of the heart by using stem cells. Here, the development of mesenchymal stem cells causes a breakthrough in the treatment of different cardiovascular diseases. They are easily obtainable from different sources, and expanded and enriched easily, with no need for immunosuppressing agents before transplantation, and fewer possibilities of genetic abnormality accompany them through multiple passages. The production of new cardiomyocytes can result from the transplantation of different types of stem cells. Accordingly, due to its remarkable benefits, stem cell therapy has received attention in recent years as it provides a drug-free and surgical treatment for patients and encourages a more safe and feasible cardiac repair. Although different clinical trials have reported on the promising benefits of stem cell therapy, there is still uncertainty about its mechanism of action. It is important to conduct different preclinical and clinical studies to explore the exact mechanism of action of the cells. After reviewing the pathophysiology of MI, this study addresses the role of tissue regeneration using various materials, including different types of stem cells. It proves some appropriate data about the importance of ethical problems, which leads to future perspectives on this scientific method.

Assessment of the effects of four crosslinking agents on gelatin hydrogel for myocardial tissue engineering applications

Cardiomyocyte (CM) transplantation is a promising option for regenerating infarcted myocardium. However, poor cell survival and residence rates reduce the efficacy of cell transplantation. Gelatin (GA) hydrogel as a frequently-used cell carrier is a possible approach to increase the survival rate of CMs. In this study, microbial transglutaminase (mTG) and chemical crosslinkers glutaraldehyde, genipin, and 1-ethyl-3-(3-dimethyl aminopropyl)-carbodiimide were employed to prepare GA hydrogels. The mechanical properties and degradation characteristics of these hydrogels were then evaluated. Neonatal rat CMs (NRCMs) were isolated and inoculated on the surface of these hydrogels or encapsulated in mTG-hydrogels. Cellular growth morphology and beating behavior were observed. Cellular viability and immunofluorescence were analyzed. Intracellular Ca²⁺transient and membrane potential propagation were detected using fluorescence dyes (Fluo-3 and di-4-ANEPPS, respectively). Results showed that the chemical crosslinkers exhibited high cytotoxicity and resulted in high rates of cell death. By contrast, mTG-hydrogels showed excellent cell compatibility. The CMs cultured in mTG-hydrogels for a week expressed CM maturation markers. The NRCMs begun independently beating on the third day of culture, and their beating synchronized after a week of culture. Furthermore, intracellular Ca²⁺transient events with periodicity were observed. In conclusion, the novel mTG-crosslinked GA hydrogel synthesized herein has good biocompatibility, and it supports CM adhesion, growth, and maturation.

Biomimetic microsystems for cardiovascular studies

Traditional tissue culture platforms have been around for several decades and have enabled key findings in the cardiovascular field. However, these platforms failed to recreate the mechanical and dynamic features found within the body. Organs-on-chips (OOCs) are cellularized microfluidic-based devices that can mimic the basic structure, function, and responses of organs. These systems have been successfully utilized in disease, development, and drug studies. OOCs are designed to recapitulate the mechanical, electrical, chemical, and structural features of the in vivo microenvironment. Here, we review cardiovascular-themed OOC studies, design considerations, and techniques used to generate these cellularized devices. Furthermore, we will highlight the advantages of OOC models over traditional cell culture vessels, discuss implementation challenges, and provide perspectives on the state of the field.

Design and Development of Hybrid Hydrogels for Biomedical Applications: Recent Trends in Anticancer Drug Delivery and Tissue Engineering

The applications of hydrogels in biomedical field has been since multiple decades. Discoveries in biology and chemistry render this platform endowed with much engineering potentials and growing continuously. Novel approaches in constructing these materials have led to the production of complex hybrid hydrogels systems that can incorporate both natural and synthetic polymers and other functional moieties for mediated cell response, tunable release kinetic profiles, thus they are used and research for diverse biomedical applications. Recent advancement in this field has established promising techniques for the development of biorelevant materials for construction of hybrid hydrogels with potential applications in the delivery of cancer therapeutics, drug discovery, and re-generative medicines. In this review, recent trends in advanced hybrid hydrogels systems incorporating nano/microstructures, their synthesis, and their potential applications in tissue engineering and anticancer drug delivery has been discussed. Examples of some new approaches including click reactions implementation, 3D printing, and photopatterning for the development of these materials has been briefly discussed. In addition, the application of biomolecules and motifs for desired outcomes, and tailoring of their transport and kinetic behavior for achieving desired outcomes in hybrid nanogels has also been reviewed.

Multiorgan-on-a-Chip: A Systemic Approach To Model and Decipher Inter-Organ Communication

Multiorgan-on-a-chip (multi-OoC) platforms have great potential to redefine the way in which human health research is conducted. After briefly reviewing the need for comprehensive multiorgan models with a systemic dimension, we highlight scenarios in which multiorgan models are advantageous. We next overview existing multi-OoC platforms, including integrated body-on-a-chip devices and modular approaches involving interconnected organ-specific modules. We highlight how multi-OoC models can provide unique information that is not accessible using single-OoC models. Finally, we discuss remaining challenges for the realization of multi-OoC platforms and their worldwide adoption. We anticipate that multi-OoC technology will metamorphose research in biology and medicine by providing holistic and personalized models for understanding and treating multisystem diseases.

Glow-in-the-Dark Patterned PET Nonwoven Using Air-Atmospheric Plasma Treatment and Vitamin B2-Derivative (FMN)

Flavin mononucleotide (FMN) derived from Vitamin B2, a bio-based fluorescent water-soluble molecule with visible yellow-green fluorescence, has been used in the scope of producing photoluminescent and glow-in-the-dark patterned polyester (PET) nonwoven panels. Since the FMN molecule cannot diffuse inside the PET fiber, screen printing, coating, and padding methods were used in an attempt to immobilize FMN molecules at the PET fiber surface of a nonwoven, using various biopolymers such as gelatin and sodium alginate as well as a water-based commercial polyacrylate. In parallel, air atmospheric plasma activation of PET nonwoven was carried for improved spreading and adhesion of FMN bearing biopolymer/polymer mixture. Effectively, the plasma treatment yielded a more hydrophilic PET nonwoven, reduction in wettability, and surface roughness of the plasma treated fiber with reduced water contact angle and increased capillary uptake were observed. The standard techniques of morphological properties were explored by a scanning electron microscope (SEM) and atomic force microscopy (AFM). Films combining each biopolymer and FMN were formed on PS (polystyrene) Petri-dishes. However, only the gelatin and polyacrylate allowed the yellow-green fluorescence of FMN molecule to be maintained on the film and PET fabric (seen under ultraviolet (UV) light). No yellow-green fluorescence of FMN was observed with sodium alginate. Thus, when the plasma-activated PET was coated with the gelatin mixture or polyacrylate bearing FMN, the intense photoluminescent yellow-green glowing polyester nonwoven panel was obtained in the presence of UV light (370 nm). Screen printing of FMN using a gelatin mixture was possible. The biopolymer exhibited appropriate viscosity and rheological behavior, thus creating a glow-in-the-dark pattern on the polyester nonwoven, with the possibility of one expression in daylight and another in darkness (in presence of UV light). A bio-based natural product such as FMN is potentially an interesting photoluminescent molecule with which textile surface pattern designers may create light-emitting textiles and interesting aesthetic expressions.

Advanced Fabrication Techniques of Microengineered Physiological Systems

The field of organs-on-chips (OOCs) has experienced tremendous growth over the last decade. However, the current main limiting factor for further growth lies in the fabrication techniques utilized to reproducibly create multiscale and multifunctional devices. Conventional methods of photolithography and etching remain less useful to complex geometric conditions with high precision needed to manufacture the devices, while laser-induced methods have become an alternative for higher precision engineering yet remain costly. Meanwhile, soft lithography has become the foundation upon which OOCs are fabricated and newer methods including 3D printing and injection molding show great promise to innovate the way OOCs are fabricated. This review is focused on the advantages and disadvantages associated with the commonly used fabrication techniques applied to these microengineered physiological systems (MPS) and the obstacles that remain in the way of further innovation in the field.

A Facile Fabrication of Biodegradable and Biocompatible Cross-Linked Gelatin as Screen Printing Substrates

This study focuses on preparation and valuation of the biodegradable, native, and modified gelatin film as screen-printing substrates. Modified gelatin film was prepared by crosslinking with various crosslinking agents and the electrode array was designed by screen-printing. It was observed that the swelling ratio of C-2, crosslinked with glutaraldehyde and EDC/NHS (1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide) was found to be lower (3.98%) than that of C-1 (crosslinked with only glutaraldehyde) (8.77%) and C-0 (without crosslinking) (28.15%). The obtained results indicate that the swelling ratios of both C-1 and C-2 were found to be lower than that of C-0 (control one without crosslinking). The Young's modulus for C-1 and C-2 was found to be 8.55 ± 0.57 and 23.72 ± 2.04 kPa, respectively. Hence, it was conveyed that the mechanical strength of C-2 was found to be two times higher than that of C-l, suggesting that the mechanical strength was enhanced upon dual crosslinking in this study also. The adhesion study indicates that silver ink adhesion on the gelation surface is better than that of carbon ink. In addition, the electrical response of C-2 with a screen-printed electrode (SPE) was found to be the same as the commercial polycarbonate (PC) substrate. The result of MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) assay suggested that the silver SPE on C-2 was non-cytotoxic toward L929 fibroblast cells proliferation. The results indicated that C-2 gelatin is a promising material to act as a screen-printing substrate with excellent biodegradable and biocompatible properties.

Extracellular matrix-based biomaterials for cardiac regeneration and repair

The spectrum of ischemic heart diseases, encompassing acute myocardial infarction to heart failure, represents the leading cause of death worldwide. Although extensive progress in cardiovascular diagnoses and therapy has been made, the prevalence of the disease continues to increase. Cardiac regeneration has a promising perspective for the therapy of heart failure. Recently, extracellular matrix (ECM) has been shown to play an important role in cardiac regeneration and repair after cardiac injury. There is also evidence that the ECM could be directly used as a drug to promote cardiomyocyte proliferation and cardiac regeneration. Increasing evidence supports that applying ECM biomaterials to maintain heart function recovery is an important approach to apply the concept of cardiac regenerative medicine to clinical practice in the future. Here, we will introduce the essential role of cardiac ECM in cardiac regeneration and summarize the approaches of delivering ECM biomaterials to promote cardiac repair in this review.

Ferritin Nanocage Conjugated Hybrid Hydrogel for Tissue Engineering and Drug Delivery Applications

Hydrogels have recently been attractive in various drug delivery and tissue engineering applications because of their structural similarities to the natural extracellular matrix. Despite enormous advances in the application of hydrogels, poor mechanical properties and lack of control for the release of drugs and biomolecules act as major barriers for widespread clinical applications. To overcome these challenges, we developed both physically and covalently conjugated nanocage-laden hydrogels between the surface of the nanocage and a gelatin methacryloyl (GelMA) hydrogel matrix. Ferritin and its empty-core equivalent apoferritin were used as nanocages that could be easily incorporated into a GelMA hydrogel via physical bonding. To fabricate covalently conjugated nanocage-laden GelMA hydrogels, ferritin and apoferritin were chemically modified to present the methacryloyl groups, ferritin methacryloyl (FerMA) and apoferritin methacryloyl (ApoMA), respectively. The covalently conjugated FerMA- and ApoMA-GelMA hydrogels offered a better ability to tune mechanical properties compared with those prepared by direct dispersion of ferritin and apoferritin into GelMA hydrogels with physical bonding, without affecting their porosity or cell growth. Furthermore, the ability of the nanocage to release small chemical compounds was confirmed by performing a cumulative release test on fluorescein isothiocyanate (FITC) encapsulated apoferritin and ApoMA incorporated GelMA hydrogels by pH stimulus. Thus, the nanocage incorporated hydrogels have emerged as excellent materials for drug delivery and tissue engineering applications.

Engineering anisotropic cardiac monolayers on microelectrode arrays for non-invasive analyses of electrophysiological properties

Engineering cardiac tissues with physiological architectural and mechanical properties on microelectrode arrays enables long term culture and non-invasive collection of electrophysiological readouts.

3D Printed Hydrogel Multiassay Platforms for Robust Generation of Engineered Contractile Tissues

We present a method for reproducible manufacture of multiassay platforms with tunable mechanical properties for muscle tissue strip analysis. The platforms result from stereolithographic 3D printing of low protein-binding poly(ethylene glycol) diacrylate (PEGDA) hydrogels. Contractile microtissues have previously been engineered by immobilizing suspended cells in a confined hydrogel matrix with embedded anchoring cantilevers to facilitate muscle tissue strip formation. The 3D shape and mechanical properties of the confinement and the embedded cantilevers are critical for the tissue robustness. High-resolution 3D printing of PEGDA hydrogels offers full design freedom to engineer cantilever stiffness, while minimizing unwanted cell attachment. We demonstrate the applicability by generating suspended muscle tissue strips from C2C12 mouse myoblasts in a compliant fibrin-based hydrogel matrix. The full design freedom allows for new platform geometries that reduce local stress in the matrix and tissue, thus, reducing the risk of tissue fracture.

Advances in Hydrogels in Organoids and Organs-on-a-Chip

Significant advances in materials, microscale technology, and stem cell biology have enabled the construction of 3D tissues and organs, which will ultimately lead to more effective diagnostics and therapy. Organoids and organs-on-a-chip (OOC), evolved from developmental biology and bioengineering principles, have emerged as major technological breakthrough and distinct model systems to revolutionize biomedical research and drug discovery by recapitulating the key structural and functional complexity of human organs in vitro. There is growing interest in the development of functional biomaterials, especially hydrogels, for utilization in these promising systems to build more physiologically relevant 3D tissues with defined properties. The remarkable properties of defined hydrogels as proper extracellular matrix that can instruct cellular behaviors are presented. The recent trend where functional hydrogels are integrated into organoids and OOC systems for the construction of 3D tissue models is highlighted. Future opportunities and perspectives in the development of advanced hydrogels toward accelerating organoids and OOC research in biomedical applications are also discussed.

Matrix-guided control of mitochondrial function in cardiac myocytes

In ventricular myocardium, extracellular matrix (ECM) remodeling is a hallmark of physiological and pathological growth, coincident with metabolic rewiring of cardiac myocytes. However, the direct impact of the biochemical and mechanical properties of the ECM on the metabolic function of cardiac myocytes is mostly unknown. Furthermore, understanding the impact of distinct biomaterials on cardiac myocyte metabolism is critical for engineering physiologically-relevant models of healthy and diseased myocardium. For these reasons, we systematically measured morphological and metabolic responses of neonatal rat ventricular myocytes cultured on fibronectin- or gelatin-coated polydimethylsiloxane (PDMS) of three elastic moduli and gelatin hydrogels with four elastic moduli. On all substrates, total protein content, cell morphology, and the ratio of mitochondrial DNA to nuclear DNA were preserved. Cytotoxicity was low on all substrates, although slightly higher on PDMS compared to gelatin hydrogels. We also quantified oxygen consumption rates and extracellular acidification rates using a Seahorse extracellular flux analyzer. Our data indicate that several metrics associated with baseline glycolysis and baseline and maximum mitochondrial function are enhanced when cardiac myocytes are cultured on gelatin hydrogels compared to all PDMS substrates, irrespective of substrate rigidity. These results yield new insights into how mechanical and biochemical cues provided by the ECM impact mitochondrial function in cardiac myocytes. STATEMENT OF SIGNIFICANCE: Cardiac development and disease are associated with remodeling of the extracellular matrix coincident with metabolic rewiring of cardiac myocytes. However, little is known about the direct impact of the biochemical and mechanical properties of the extracellular matrix on the metabolic function of cardiac myocytes. In this study, oxygen consumption rates were measured in neonatal rat ventricular myocytes maintained on several commonly-used biomaterial substrates to reveal new relationships between the extracellular matrix and cardiac myocyte metabolism. Several mitochondrial parameters were enhanced on gelatin hydrogels compared to synthetic PDMS substrates. These data are important for comprehensively understanding matrix-regulation of cardiac myocyte physiology. Additionally, these data should be considered when selecting scaffolds for engineering in vitro cardiac tissue models.

Hybrid hydrogels for biomedical applications

The use of hydrogels in biomedical applications dates back multiple decades, and the engineering potential of these materials continues to grow with discoveries in chemistry and biology. The approaches have led to increasing complex hydrogels that incorporate both synthetic and natural polymers and functional domains for tunable release kinetics, mediated cell response, and ultimately use in clinical and research applications in biomedical practice. This review focuses on recent advances in hybrid hydrogels that incorporate nano/microstructures, their synthesis, and applications in biomedical research. Examples discussed include the implementation of click reactions, photopatterning, and 3D printing for the facile production of these hybrid hydrogels, the use of biological molecules and motifs to promote a desired cellular outcome, and the tailoring of kinetic and transport behavior through hybrid-hydrogel engineering to achieve desired biomedical outcomes. Recent progress in the field has established promising approaches for the development of biologically relevant hybrid hydrogel materials with potential applications in drug discovery, drug/gene delivery, and regenerative medicine.

Biomimetic design and fabrication of scaffolds integrating oriented micro-pores with branched channel networks for myocardial tissue engineering

The ability to fabricate three-dimensional (3D) thick vascularized myocardial tissue could enable scientific and technological advances in tissue engineering and drug screening, and may accelerate its application in myocardium repair. In this study, we developed a novel biomimetic scaffold integrating oriented micro-pores with branched channel networks to mimic the anisotropy and vasculature of native myocardium. The oriented micro-pores were fabricated using an 'Oriented Thermally Induced Phase Separation (OTIPS)' technique, and the channel network was produced by embedding and subsequently dissolving a 3D-printed carbohydrate template after crosslinking. Micro-holes were incorporated on the wall of channels, which greatly enhanced the permeability of channels. The effect of the sacrificial template on the formation of oriented micro- pores was assessed. The mechanical properties of the scaffold were tuned by varying the temperature gradient and chitosan/collagen ratio to match the specific stiffness of native heart tissue. The engineered cardiac tissue achieved synchronized beating with electrical stimulation. Calcium transient results suggested the formation of connection between cardiomyocytes within scaffold. All the results demonstrated that the reported scaffold has the potential to induce formation of a perfusable vascular network and to create thick vascularized cardiac tissue that may advance further clinical applications.

ESC Working Group on Cellular Biology of the Heart: position paper for Cardiovascular Research: tissue engineering strategies combined with cell therapies for cardiac repair in ischaemic heart disease and heart failure

Morbidity and mortality from ischaemic heart disease (IHD) and heart failure (HF) remain significant in Europe and are increasing worldwide. Patients with IHD or HF might benefit from novel therapeutic strategies, such as cell-based therapies. We recently discussed the therapeutic potential of cell-based therapies and provided recommendations on how to improve the therapeutic translation of these novel strategies for effective cardiac regeneration and repair. Despite major advances in optimizing these strategies with respect to cell source and delivery method, the clinical outcome of cell-based therapy remains unsatisfactory. Major obstacles are the low engraftment and survival rate of transplanted cells in the harmful microenvironment of the host tissue, and the paucity or even lack of endogenous cells with repair capacity. Therefore, new ways of delivering cells and their derivatives are required in order to empower cell-based cardiac repair and regeneration in patients with IHD or HF. Strategies using tissue engineering (TE) combine cells with matrix materials to enhance cell retention or cell delivery in the transplanted area, and have recently received much attention for this purpose. Here, we summarize knowledge on novel approaches emerging from the TE scenario. In particular, we will discuss how combinations of cell/bio-materials (e.g. hydrogels, cell sheets, prefabricated matrices, microspheres, and injectable matrices) combinations might enhance cell retention or cell delivery in the transplantation areas, thereby increase the success rate of cell therapies for IHD and HF. We will not focus on the use of classical engineering approaches, employing fully synthetic materials, because of their unsatisfactory material properties which render them not clinically applicable. The overall aim of this Position Paper from the ESC Working Group Cellular Biology of the Heart is to provide recommendations on how to proceed in research with these novel TE strategies combined with cell-based therapies to boost cardiac repair in the clinical settings of IHD and HF.

Emerging trends in multiscale modeling of vascular pathophysiology: Organ-on-a-chip and 3D printing

Most biomedical and pharmaceutical research of the human vascular system aims to unravel the complex mechanisms that drive disease progression from molecular to organ levels. The knowledge gained can then be used to innovate diagnostic and treatment strategies which can ultimately be determined precisely for patients. Despite major advancements, current modeling strategies are often limited at identifying, quantifying, and dissecting specific cellular and molecular targets that regulate human vascular diseases. Therefore, development of multiscale modeling approaches are needed that can advance our knowledge and facilitate the design of next-generation therapeutic approaches in vascular diseases. This article critically reviews animal models, static in vitro systems, and dynamic in vitro culture systems currently used to model vascular diseases. A leading emphasis on the potential of emerging approaches, specifically organ-on-a-chip and three-dimensional (3D) printing, to recapitulate the innate human vascular physiology and anatomy is described. The applications of these approaches and future outlook in designing and screening novel therapeutics are also presented.

Methods for producing microstructured hydrogels for targeted applications in biology

Hydrogels have been broadly studied for applications in clinically motivated fields such as tissue regeneration, drug delivery, and wound healing, as well as in a wide variety of consumer and industry uses. While the control of mechanical properties and network structures are important in all of these applications, for regenerative medicine applications in particular, matching the chemical, topographical and mechanical properties for the target use/tissue is critical. There have been multiple alternatives developed for fabricating materials with microstructures with goals of controlling the spatial location, phenotypic evolution, and signaling of cells. The commonly employed polymers such as poly(ethylene glycol) (PEG), polypeptides, and polysaccharides (as well as others) can be processed by various methods in order to control material heterogeneity and microscale structures. We review here the more commonly used polymers, chemistries, and methods for generating microstructures in biomaterials, highlighting the range of possible morphologies that can be produced, and the limitations of each method. With a focus in liquid-liquid phase separation, methods and chemistries well suited for stabilizing the interface and arresting the phase separation are covered. As the microstructures can affect cell behavior, examples of such effects are reviewed as well. STATEMENT OF SIGNIFICANCE: Heterogeneous hydrogels with enhanced matrix complexity have been studied for a variety of biomimetic materials. A range of materials based on poly(ethylene glycol), polypeptides, proteins, and/or polysaccharides, have been employed in the studies of materials that by virtue of their microstructure, can control the behaviors of cells. Methods including microfluidics, photolithography, gelation in the presence of porogens, and liquid-liquid phase separation, are presented as possible strategies for producing materials, and their relative advantages and disadvantages are discussed. We also describe in more detail the various processes involved in LLPS, and how they can be manipulated to alter the kinetics of phase separation and to yield different microstructured materials.

Development and application of human skeletal muscle microphysiological systems

A number of major disease states involve skeletal muscle, including type 2 diabetes, muscular dystrophy, sarcopenia and cachexia arising from cancer or heart disease. Animals do not accurately represent many of these disease states. Human skeletal muscle microphysiological systems derived from primary or induced pluripotent stem cells (hPSCs) can provide an in vitro model of genetic and chronic diseases and assess individual variations. Three-dimensional culture systems more accurately represent skeletal muscle function than do two-dimensional cultures. While muscle biopsies enable culture of primary muscle cells, hPSCs provide the opportunity to sample a wider population of donors. Recent advances to promote maturation of PSC-derived skeletal muscle provide an alternative to primary cells. While contractile function is often measured in three-dimensional cultures and several systems exist to characterize contraction of small numbers of muscle fibers, there is a need for functional measures of metabolism suited for microphysiological systems. Future research should address generation of well-differentiated hPSC-derived muscle cells, enabling muscle repair in vitro, and improved disease models.