Bioinspired Three-Dimensional Human Neuromuscular Junction Development in Suspended Hydrogel Arrays

Recent trends in 3D bioprinting technology for skeletal muscle regeneration

Skeletal muscle is a pro-regenerative tissue, that utilizes a tissue-resident stem cell system to effect repair upon injury. Despite the demonstrated efficiency of this system in restoring muscle mass after many acute injuries, in conditions of severe trauma such as those evident in volumetric muscle loss (VML) (>20 % by mass), this self-repair capability is unable to restore tissue architecture, requiring interventions which currently are largely surgical. As a possible alternative, the generation of artificial muscle using tissue engineering approaches may also be of importance in the treatment of VML and muscle diseases such as dystrophies. Three-dimensional (3D) bioprinting has been identified as a promising technique for regeneration of the complex architecture of skeletal muscle. This review discusses existing treatment strategies following muscle damage, recent progress in bioprinting techniques, the bioinks used for muscle regeneration, the immunogenicity of scaffold materials, and in vitro and in vivo maturation techniques for 3D bio-printed muscle constructs. The pros and cons of these bioink formulations are also highlighted. Finally, we present the current limitations and challenges in the field and critical factors to consider for bioprinting approaches to become more translationa and to produce clinically relevant engineered muscle. STATEMENT OF SIGNIFICANCE: This review discusses the physiopathology of muscle injuries and existing clinical treatment strategies for muscle damage, the types of bioprinting techniques that have been applied to bioprinting of muscle, and the bioinks commonly used for muscle regeneration. The pros and cons of these bioinks are highlighted. We present a discussion of existing gaps in the literature and critical factors to consider for the translation of bioprinting approaches and to produce clinically relevant engineered muscle. Finally, we provide insights into what we believe will be the next steps required before the realization of the application of tissue-engineered muscle in humans. We believe this manuscript is an insightful, timely, and instructive review that will guide future muscle bioprinting research from a fundamental construct creation approach, down a translational pathway to achieve the desired impact in the clinic.

Unlocking the Complexity of Neuromuscular Diseases: Insights from Human Pluripotent Stem Cell-Derived Neuromuscular Junctions

Over the past 20 years, the use of pluripotent stem cells to mimic the complexities of the human neuromuscular junction has received much attention. Deciphering the key mechanisms underlying the establishment and maturation of this complex synapse has been driven by the dual goals of addressing developmental questions and gaining insight into neuromuscular disorders. This review aims to summarise the evolution and sophistication of in vitro neuromuscular junction models developed from the first differentiation of human embryonic stem cells into motor neurons to recent neuromuscular organoids. We also discuss the potential offered by these models to decipher different neuromuscular diseases characterised by defects in the presynaptic compartment, the neuromuscular junction, and the postsynaptic compartment. Finally, we discuss the emerging field that considers the use of these techniques in drug screening assay and the challenges they will face in the future.

Engineering Cell-ECM-Material Interactions for Musculoskeletal Regeneration

The extracellular microenvironment regulates many of the mechanical and biochemical cues that direct musculoskeletal development and are involved in musculoskeletal disease. The extracellular matrix (ECM) is a main component of this microenvironment. Tissue engineered approaches towards regenerating muscle, cartilage, tendon, and bone target the ECM because it supplies critical signals for regenerating musculoskeletal tissues. Engineered ECM-material scaffolds that mimic key mechanical and biochemical components of the ECM are of particular interest in musculoskeletal tissue engineering. Such materials are biocompatible, can be fabricated to have desirable mechanical and biochemical properties, and can be further chemically or genetically modified to support cell differentiation or halt degenerative disease progression. In this review, we survey how engineered approaches using natural and ECM-derived materials and scaffold systems can harness the unique characteristics of the ECM to support musculoskeletal tissue regeneration, with a focus on skeletal muscle, cartilage, tendon, and bone. We summarize the strengths of current approaches and look towards a future of materials and culture systems with engineered and highly tailored cell-ECM-material interactions to drive musculoskeletal tissue restoration. The works highlighted in this review strongly support the continued exploration of ECM and other engineered materials as tools to control cell fate and make large-scale musculoskeletal regeneration a reality.

Recent trends in bioartificial muscle engineering and their applications in cultured meat, biorobotic systems and biohybrid implants

Recent advances in tissue engineering and biofabrication technology have yielded a plethora of biological tissues. Among these, engineering of bioartificial muscle stands out for its exceptional versatility and its wide range of applications. From the food industry to the technology sector and medicine, the development of this tissue has the potential to affect many different industries at once. However, to date, the biofabrication of cultured meat, biorobotic systems, and bioartificial muscle implants are still considered in isolation by individual peer groups. To establish common ground and share advances, this review outlines application-specific requirements for muscle tissue generation and provides a comprehensive overview of commonly used biofabrication strategies and current application trends. By solving the individual challenges and merging various expertise, synergetic leaps of innovation that inspire each other can be expected in all three industries in the future.

Optimization of Application-Driven Development of In Vitro Neuromuscular Junction Models

Neuromuscular junctions (NMJs) are specialized synapses responsible for signal transduction between motor neurons (MNs) and skeletal muscle tissue. Malfunction at this site can result from developmental disorders, toxic environmental exposures, and neurodegenerative diseases leading to severe neurological dysfunction. Exploring these conditions in human or animal subjects is restricted by ethical concerns and confounding environmental factors. Therefore, in vitro NMJ models provide exciting opportunities for advancements in tissue engineering. In the last two decades, multiple NMJ prototypes and platforms have been reported, and each model system design is strongly tied to a specific application: exploring developmental physiology, disease modeling, or high-throughput screening. Directing the differentiation of stem cells into mature MNs and/or skeletal muscle for NMJ modeling has provided critical cues to recapitulate early-stage development. Patient-derived inducible pluripotent stem cells provide a personalized approach to investigating NMJ disease, especially when disease etiology cannot be resolved down to a specific gene mutation. Having reproducible NMJ culture replicates is useful for high-throughput screening to evaluate drug toxicity and determine the impact of environmental threat exposures. Cutting-edge bioengineering techniques have propelled this field forward with innovative microfabrication and design approaches allowing both two-dimensional and three-dimensional NMJ culture models. Many of these NMJ systems require further validation for broader application by regulatory agencies, pharmaceutical companies, and the general research community. In this summary, we present a comprehensive review on the current state-of-art research in NMJ models and discuss their ability to provide valuable insight into cell and tissue interactions. Impact statement In vitro neuromuscular junction (NMJ) models reveal the specialized mechanisms of communication between neurons and muscle tissue. This site can be disrupted by developmental disorders, toxic environmental exposures, or neurodegenerative diseases, which often lead to fatal outcomes and is therefore of critical importance to the medical community. Many bioengineering approaches for in vitro NMJ modeling have been designed to mimic development and disease; other approaches include in vitro NMJ models for high-throughput toxicology screening, providing a platform to limit or replace animal testing. This review describes various NMJ applications and the bioengineering advancements allowing for human NMJ characteristics to be more accurately recapitulated. While the extensive range of NMJ device structures has hindered standardization attempts, there is still a need to harmonize these devices for broader application and to continue advancing the field of NMJ modeling.

Autologous iPSC-Derived Human Neuromuscular Junction to Model the Pathophysiology of Hereditary Spastic Paraplegia

Hereditary spastic paraplegia (HSP) is a heterogeneous group of genetic neurodegenerative disorders, characterized by progressive lower limb spasticity and weakness resulting from retrograde axonal degeneration of motor neurons (MNs). Here, we generated in vitro human neuromuscular junctions (NMJs) from five HSP patient-specific induced pluripotent stem cell (hiPSC) lines, by means of microfluidic strategy, to model disease-relevant neuropathologic processes. The strength of our NMJ model lies in the generation of lower MNs and myotubes from autologous hiPSC origin, maintaining the genetic background of the HSP patient donors in both cell types and in the cellular organization due to the microfluidic devices. Three patients characterized by a mutation in the SPG3a gene, encoding the ATLASTIN GTPase 1 protein, and two patients with a mutation in the SPG4 gene, encoding the SPASTIN protein, were included in this study. Differentiation of the HSP-derived lines gave rise to lower MNs that could recapitulate pathological hallmarks, such as axonal swellings with accumulation of Acetyl-α-TUBULIN and reduction of SPASTIN levels. Furthermore, NMJs from HSP-derived lines were lower in number and in contact point complexity, denoting an impaired NMJ profile, also confirmed by some alterations in genes encoding for proteins associated with microtubules and responsible for axonal transport. Considering the complexity of HSP, these patient-derived neuronal and skeletal muscle cell co-cultures offer unique tools to study the pathologic mechanisms and explore novel treatment options for rescuing axonal defects and diverse cellular processes, including membrane trafficking, intracellular motility and protein degradation in HSP.

Three-dimensional in vitro models of neuromuscular tissue

Skeletal muscle is a dynamic tissue in which homeostasis and function are guaranteed by a very defined three-dimensional organization of myofibers in respect to other non-muscular components, including the extracellular matrix and the nervous network. In particular, communication between myofibers and the nervous system is essential for the overall correct development and function of the skeletal muscle. A wide range of chronic, acute and genetic-based human pathologies that lead to the alteration of muscle function are associated with modified preservation of the fine interaction between motor neurons and myofibers at the neuromuscular junction. Recent advancements in the development of in vitro models for human skeletal muscle have shown that three-dimensionality and integration of multiple cell types are both key parameters required to unveil pathophysiological relevant phenotypes. Here, we describe recent achievement reached in skeletal muscle modeling which used biomaterials for the generation of three-dimensional constructs of myotubes integrated with motor neurons.

Bioinks and Bioprinting Strategies for Skeletal Muscle Tissue Engineering

Skeletal muscles play important roles in critical body functions and their injury or disease can lead to limitation of mobility and loss of independence. Current treatments result in variable functional recovery, while reconstructive surgery, as the gold-standard approach, is limited due to donor shortage, donor-site morbidity, and limited functional recovery. Skeletal muscle tissue engineering (SMTE) has generated enthusiasm as an alternative solution for treatment of injured tissue and serves as a functional disease model. Recently, bioprinting has emerged as a promising tool for recapitulating the complex and highly organized architecture of skeletal muscles at clinically relevant sizes. Here, skeletal muscle physiology, muscle regeneration following injury, and current treatments following muscle loss are discussed, and then bioprinting strategies implemented for SMTE are critically reviewed. Subsequently, recent advancements that have led to improvement of bioprinting strategies to construct large muscle structures, boost myogenesis in vitro and in vivo, and enhance tissue integration are discussed. Bioinks for muscle bioprinting, as an essential part of any bioprinting strategy, are discussed, and their benefits, limitations, and areas to be improved are highlighted. Finally, the directions the field should expand to make bioprinting strategies more translational and overcome the clinical unmet needs are discussed.

Hydrogel-Based Fiber Biofabrication Techniques for Skeletal Muscle Tissue Engineering

The functional capabilities of skeletal muscle are strongly correlated with its well-arranged microstructure, consisting of parallelly aligned myotubes. In case of extensive muscle loss, the endogenous regenerative capacity is hindered by scar tissue formation, which compromises the native muscle structure, ultimately leading to severe functional impairment. To address such an issue, skeletal muscle tissue engineering (SMTE) attempts to fabricate in vitro bioartificial muscle tissue constructs to assist and accelerate the regeneration process. Due to its dynamic nature, SMTE strategies must employ suitable biomaterials (combined with muscle progenitors) and proper 3D architectures. In light of this, 3D fiber-based strategies are gaining increasing interest for the generation of hydrogel microfibers as advanced skeletal muscle constructs. Indeed, hydrogels possess exceptional biomimetic properties, while the fiber-shaped morphology allows for the creation of geometrical cues to guarantee proper myoblast alignment. In this review, we summarize commonly used hydrogels in SMTE and their main properties, and we discuss the first efforts to engineer hydrogels to guide myoblast anisotropic orientation. Then, we focus on presenting the main hydrogel fiber-based techniques for SMTE, including molding, electrospinning, 3D bioprinting, extrusion, and microfluidic spinning. Furthermore, we describe the effect of external stimulation (i.e., mechanical and electrical) on such constructs and the application of hydrogel fiber-based methods on recapitulating complex skeletal muscle tissue interfaces. Finally, we discuss the future developments in the application of hydrogel microfibers for SMTE.

Current Progress in the Creation, Characterization, and Application of Human Stem Cell-derived in Vitro Neuromuscular Junction Models

Human pluripotent stem cells (PSCs) such as embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) are of great value for studying developmental processes, disease modeling, and drug testing. One area in which the use of human PSCs has become of great interest in recent years is for in vitro models of the neuromuscular junction (NMJ). The NMJ is a synapse at which a motor neuron releases acetylcholine to bind to skeletal muscle and stimulate contraction. Degeneration of the NMJ and subsequent loss of muscle function is a common feature of many neuromuscular diseases such as myasthenia gravis, spinal muscular atrophy, and amyotrophic lateral sclerosis. In order to develop new therapies for patients with neuromuscular diseases, it is essential to understand mechanisms taking place at the NMJ. However, we have limited ability to study the NMJ in living human patients, and animal models are limited by physiological relevance. Therefore, an in vitro model of the NMJ consisting of human cells is of great value. The use of stem cells for in vitro NMJ models is still in progress and requires further optimization in order to yield reliable, reproducible results. The objective of this review is (1) to outline the current progress towards fully PSC-derived in vitro co-culture models of the human NMJ and (2) to discuss future directions and challenges that must be overcome in order to create reproducible fully PSC-derived models that can be used for developmental studies, disease modeling, and drug testing.

Organotypic and Microphysiological Human Tissue Models for Drug Discovery and Development-Current State-of-the-Art and Future Perspectives

The number of successful drug development projects has been stagnant for decades despite major breakthroughs in chemistry, molecular biology, and genetics. Unreliable target identification and poor translatability of preclinical models have been identified as major causes of failure. To improve predictions of clinical efficacy and safety, interest has shifted to three-dimensional culture methods in which human cells can retain many physiologically and functionally relevant phenotypes for extended periods of time. Here, we review the state of the art of available organotypic culture techniques and critically review emerging models of human tissues with key importance for pharmacokinetics, pharmacodynamics, and toxicity. In addition, developments in bioprinting and microfluidic multiorgan cultures to emulate systemic drug disposition are summarized. We close by highlighting important trends regarding the fabrication of organotypic culture platforms and the choice of platform material to limit drug absorption and polymer leaching while supporting the phenotypic maintenance of cultured cells and allowing for scalable device fabrication. We conclude that organotypic and microphysiological human tissue models constitute promising systems to promote drug discovery and development by facilitating drug target identification and improving the preclinical evaluation of drug toxicity and pharmacokinetics. There is, however, a critical need for further validation, benchmarking, and consolidation efforts ideally conducted in intersectoral multicenter settings to accelerate acceptance of these novel models as reliable tools for translational pharmacology and toxicology. SIGNIFICANCE STATEMENT: Organotypic and microphysiological culture of human cells has emerged as a promising tool for preclinical drug discovery and development that might be able to narrow the translation gap. This review discusses recent technological and methodological advancements and the use of these systems for hit discovery and the evaluation of toxicity, clearance, and absorption of lead compounds.

Human muscle production in vitro from pluripotent stem cells: Basic and clinical applications

Human pluripotent stem cells (PSCs), which have the capacity to self-renew and differentiate into multiple cell types, offer tremendous therapeutic potential and invaluable flexibility as research tools. Recently, remarkable progress has been made in directing myogenic differentiation of human PSCs. The differentiation strategies, which were inspired by our knowledge of myogenesis in vivo, have provided an important platform for the study of human muscle development and modeling of muscular diseases, as well as a promising source of cells for cell therapy to treat muscular dystrophies. In this review, we summarize the current state of skeletal muscle generation from human PSCs, including transgene-based and transgene-free differentiation protocols, and 3D muscle tissue production through bioengineering approaches. We also highlight their basic and clinical applications, which facilitate the study of human muscle biology and deliver new hope for muscular disease treatment.

Recent Trends in Biofabrication Technologies for Studying Skeletal Muscle Tissue-Related Diseases

In native skeletal muscle, densely packed myofibers exist in close contact with surrounding motor neurons and blood vessels, which are embedded in the fibrous connective tissue. In comparison to conventional two-dimensional (2D) cultures, the three-dimensional (3D) engineered skeletal muscle models allow structural and mechanical resemblance with native skeletal muscle tissue by providing geometric confinement and physiological matrix stiffness to the cells. In addition, various external stimuli applied to these models enhance muscle maturation along with cell-cell and cell-extracellular matrix interaction. Therefore, 3D in vitro muscle models can adequately recapitulate the pathophysiologic events occurring in tissue-tissue interfaces inside the native skeletal muscle such as neuromuscular junction. Moreover, 3D muscle models can induce pathological phenotype of human muscle dystrophies such as Duchenne muscular dystrophy by incorporating patient-derived induced pluripotent stem cells and human primary cells. In this review, we discuss the current biofabrication technologies for modeling various skeletal muscle tissue-related diseases (i.e., muscle diseases) including muscular dystrophies and inflammatory muscle diseases. In particular, these approaches would enable the discovery of novel phenotypic markers and the mechanism study of human muscle diseases with genetic mutations.

Promoting Long-Term Cultivation of Motor Neurons for 3D Neuromuscular Junction Formation of 3D In Vitro Using Central-Nervous-Tissue-Derived Bioink

3D cell printing technology is in the spotlight for producing 3D tissue or organ constructs useful for various medical applications. In printing of neuromuscular tissue, a bioink satisfying all the requirements is a challenging issue. Gel integrity and motor neuron activity are two major characters because a harmonious combination of extracellular materials essential to motor neuron activity consists of disadvantages in mechanical properties. Here, a method for fabrication of 3D neuromuscular tissue is presented using a porcine central nervous system tissue decellularized extracellular matrix (CNSdECM) bioink. CNSdECM retains CNS tissue-specific extracellular molecules, provides rheological properties crucial for extrusion-based 3D cell printing, and reveals positive effects on the growth and maturity of axons of motor neurons compared with Matrigel. It also allows long-term cultivation of human-induced-pluripotent-stem-cell-derived lower motor neurons and sufficiently supports their cellular behavior to carry motor signals to muscle fibers. CNSdECM bioink holds great promise for producing a tissue-engineered motor system using 3D cell printing.

Replace and repair: Biomimetic bioprinting for effective muscle engineering

The debilitating effects of muscle damage, either through ischemic injury or volumetric muscle loss (VML), can have significant impacts on patients, and yet there are few effective treatments. This challenge arises when function is degraded due to significant amounts of skeletal muscle loss, beyond the regenerative ability of endogenous repair mechanisms. Currently available surgical interventions for VML are quite invasive and cannot typically restore function adequately. In response to this, many new bioengineering studies implicate 3D bioprinting as a viable option. Bioprinting for VML repair includes three distinct phases: printing and seeding, growth and maturation, and implantation and application. Although this 3D bioprinting technology has existed for several decades, the advent of more advanced and novel printing techniques has brought us closer to clinical applications. Recent studies have overcome previous limitations in diffusion distance with novel microchannel construct architectures and improved myotubule alignment with highly biomimetic nanostructures. These structures may also enhance angiogenic and nervous ingrowth post-implantation, though further research to improve these parameters has been limited. Inclusion of neural cells has also shown to improve myoblast maturation and development of neuromuscular junctions, bringing us one step closer to functional, implantable skeletal muscle constructs. Given the current state of skeletal muscle 3D bioprinting, the most pressing future avenues of research include furthering our understanding of the physical and biochemical mechanisms of myotube development and expanding our control over macroscopic and microscopic construct structures. Further to this, current investigation needs to be expanded from immunocompromised rodent and murine myoblast models to more clinically applicable human cell lines as we move closer to viable therapeutic implementation.

3D electron-beam writing at sub-15 nm resolution using spider silk as a resist

Electron beam lithography (EBL) is renowned to provide fabrication resolution in the deep nanometer scale. One major limitation of current EBL techniques is their incapability of arbitrary 3d nanofabrication. Resolution, structure integrity and functionalization are among the most important factors. Here we report all-aqueous-based, high-fidelity manufacturing of functional, arbitrary 3d nanostructures at a resolution of sub-15 nm using our developed voltage-regulated 3d EBL. Creating arbitrary 3d structures of high resolution and high strength at nanoscale is enabled by genetically engineering recombinant spider silk proteins as the resist. The ability to quantitatively define structural transitions with energetic electrons at different depths within the 3d protein matrix enables polymorphic spider silk proteins to be shaped approaching the molecular level. Furthermore, genetic or mesoscopic modification of spider silk proteins provides the opportunity to embed and stabilize physiochemical and/or biological functions within as-fabricated 3d nanostructures. Our approach empowers the rapid and flexible fabrication of heterogeneously functionalized and hierarchically structured 3d nanocomponents and nanodevices, offering opportunities in biomimetics, therapeutic devices and nanoscale robotics.

Nanoclay Reinforced Biomaterials for Mending Musculoskeletal Tissue Disorders

Nanoclay-reinforced biomaterials have sparked a new avenue in advanced healthcare materials that can potentially revolutionize treatment of musculoskeletal defects. Native tissues display many important chemical, mechanical, biological, and physical properties that engineered biomaterials need to mimic for optimal tissue integration and regeneration. However, it is time-consuming and difficult to endow such combinatorial properties on materials via feasible and nontoxic procedures. Fortunately, a number of nanomaterials such as graphene, carbon nanotubes, MXenes, and nanoclays already display a plethora of material properties that can be transferred to biomaterials through a simple incorporation procedure. In this direction, the members of the nanoclay family are easy to functionalize chemically, they can significantly reinforce the mechanical performance of biomaterials, and can provide bioactive properties by ionic dissolution products to upregulate cartilage and bone tissue formation. For this reason, nanoclays can become a key component for future orthopedic biomaterials. In this review, we specifically focus on the rapidly decreasing gap between clinic and laboratory by highlighting their application in a number of promising in vivo studies.

Creating stem cell-derived neuromuscular junctions in vitro

Recent development of novel therapies has improved mobility and quality of life for people suffering from inheritable neuromuscular disorders. Despite this progress, the majority of neuromuscular disorders are still incurable, in part due to a lack of predictive models of neuromuscular junction (NMJ) breakdown. Improvement of predictive models of a human NMJ would be transformative in terms of expanding our understanding of the mechanisms that underpin development, maintenance, and disease, and as a testbed with which to evaluate novel therapeutics. Induced pluripotent stem cells (iPSCs) are emerging as a clinically relevant and non‐invasive cell source to create human NMJs to study synaptic development and maturation, as well as disease modeling and drug discovery. This review will highlight the recent advances and remaining challenges to generating an NMJ capable of eliciting contraction of stem cell‐derived skeletal muscle in vitro. We explore the advantages and shortcomings of traditional NMJ culturing platforms, as well as the pioneering technologies and novel, biomimetic culturing systems currently in use to guide development and maturation of the neuromuscular synapse and extracellular microenvironment. Then, we will explore how this NMJ‐in‐a‐dish can be used to study normal assembly and function of the efferent portion of the neuromuscular arc, and how neuromuscular disease‐causing mutations disrupt structure, signaling, and function.

Advances in 3D neuronal microphysiological systems: towards a functional nervous system on a chip

Microphysiological systems (MPS) designed to study the complexities of the peripheral and central nervous systems have made marked improvements over the years and have allowed researchers to assess in two and three dimensions the functional interconnectivity of neuronal tissues. The recent generation of brain organoids has further propelled the field into the nascent recapitulation of structural, functional, and effective connectivities which are found within the native human nervous system. Herein, we will review advances in culture methodologies, focused especially on those of human tissues, which seek to bridge the gap from 2D cultures to hierarchical and defined 3D MPS with the end goal of developing a robust nervous system-on-a-chip platform. These advances have far-reaching implications within basic science, pharmaceutical development, and translational medicine disciplines.

Freeform 3D printing of soft matters: recent advances in technology for biomedical engineering

In the last decade, an emerging three-dimensional (3D) printing technique named freeform 3D printing has revolutionized the biomedical engineering field by allowing soft matters with or without cells to be printed and solidified with high precision regardless of their poor self-supportability. The key to this freeform 3D printing technology is the supporting matrices that hold the printed soft ink materials during omnidirectional writing and solidification. This approach not only overcomes structural design restrictions of conventional layer-by-layer printing but also helps to realize 3D printing of low-viscosity or slow-curing materials. This article focuses on the recent developments in freeform 3D printing of soft matters such as hydrogels, cells, and silicone elastomers, for biomedical engineering. Herein, we classify the reported freeform 3D printing systems into positive, negative, and functional based on the fabrication process, and discuss the rheological requirements of the supporting matrix in accordance with the rheological behavior of counterpart inks, aiming to guide development and evaluation of new freeform printing systems. We also provide a brief overview of various material systems used as supporting matrices for freeform 3D printing systems and explore the potential applications of freeform 3D printing systems in different areas of biomedical engineering.

Bioprinting Neural Systems to Model Central Nervous System Diseases

To date, pharmaceutical progresses in central nervous system (CNS) diseases are clearly hampered by the lack of suitable disease models. Indeed, animal models do not faithfully represent human neurodegenerative processes and human in vitro 2D cell culture systems cannot recapitulate the in vivo complexity of neural systems. The search for valuable models of neurodegenerative diseases has recently been revived by the addition of 3D culture that allows to re-create the in vivo microenvironment including the interactions among different neural cell types and the surrounding extracellular matrix (ECM) components. In this review, the new challenges in the field of CNS diseases in vitro 3D modeling are discussed, focusing on the implementation of bioprinting approaches enabling positional control on the generation of the 3D microenvironments. The focus is specifically on the choice of the optimal materials to simulate the ECM brain compartment and the biofabrication technologies needed to shape the cellular components within a microenvironment that significantly represents brain biochemical and biophysical parameters.

Biohybrid Actuators for Soft Robotics: Challenges in Scaling Up

Living systems have evolved to survive in a wide range of environments and safely interact with other objects and organisms. Thus, living systems have been the source of inspiration for many researchers looking to apply their mechanics and unique characteristics in engineering robotics. Moving beyond bioinspiration, biohybrid actuators, with compliance and self-healing capabilities enabled by living cells or tissue interfaced with artificial structures, have drawn great interest as ways to address challenges in soft robotics, and in particular have seen success in small-scale robotic actuation. However, macro-scale biohybrid actuators beyond the centimeter scale currently face many practical obstacles. In this perspective, we discuss the challenges in scaling up biohybrid actuators and the path to realize large-scale biohybrid soft robotics.

Neural cell integration into 3D bioprinted skeletal muscle constructs accelerates restoration of muscle function

A bioengineered skeletal muscle construct that mimics structural and functional characteristics of native skeletal muscle is a promising therapeutic option to treat extensive muscle defect injuries. We previously showed that bioprinted human skeletal muscle constructs were able to form multi-layered bundles with aligned myofibers. In this study, we investigate the effects of neural cell integration into the bioprinted skeletal muscle construct to accelerate functional muscle regeneration in vivo. Neural input into this bioprinted skeletal muscle construct shows the improvement of myofiber formation, long-term survival, and neuromuscular junction formation in vitro. More importantly, the bioprinted constructs with neural cell integration facilitate rapid innervation and mature into organized muscle tissue that restores normal muscle weight and function in a rodent model of muscle defect injury. These results suggest that the 3D bioprinted human neural-skeletal muscle constructs can be rapidly integrated with the host neural network, resulting in accelerated muscle function restoration.

3D bioprinting of skeletal muscle using primary human muscle progenitor cells results in correct muscle architecture, but functional restoration in rodent models is limited. Here the authors include human neural stem cells into bioprinted skeletal muscle and observe improved architecture and function in vivo.

Enzymatically crosslinked gelatin-laminin hydrogels for applications in neuromuscular tissue engineering

We report a water-soluble and non-toxic method to incorporate additional extracellular matrix proteins into gelatin hydrogels, while obviating the use of chemical crosslinkers such as glutaraldehyde. Gelatin hydrogels were fabricated using a range of gelatin concentrations (4%-10%) that corresponded to elastic moduli of approximately 1 kPa-25 kPa, respectively, a substrate stiffness relevant for multiple cell types. Microbial transglutaminase was then used to enzymatically crosslink a layer of laminin on top of gelatin hydrogels, resulting in 2-component gelatin-laminin hydrogels. Human induced pluripotent stem cell derived spinal spheroids readily adhered and rapidly extended axons on GEL-LN hydrogels. Axons displayed a more mature morphology and superior electrophysiological properties on GEL-LN hydrogels compared to the controls. Schwann cells on GEL-LN hydrogels adhered and proliferated normally, displayed a healthy morphology, and maintained the expression of Schwann cell specific markers. Lastly, skeletal muscle cells on GEL-LN hydrogels achieved long-term culture for up to 28 days without delamination, while expressing higher levels of terminal genes including myosin heavy chain, MyoD, MuSK, and M-cadherin suggesting enhanced maturation potential and myotube formation compared to the controls. Future studies will employ the superior culture outcomes of this hybrid substrate for engineering functional neuromuscular junctions and related organ on a chip applications.

Vascularized and Innervated Skeletal Muscle Tissue Engineering

Volumetric muscle loss (VML) is a devastating loss of muscle tissue that overwhelms the native regenerative properties of skeletal muscle and results in lifelong functional deficits. There are currently no treatments for VML that fully recover the lost muscle tissue and function. Tissue engineering presents a promising solution for VML treatment and significant research has been performed using tissue engineered muscle constructs in preclinical models of VML with a broad range of defect locations and sizes, tissue engineered construct characteristics, and outcome measures. Due to the complex vascular and neural anatomy within skeletal muscle, regeneration of functional vasculature and nerves is vital for muscle recovery following VML injuries. This review aims to summarize the current state of the field of skeletal muscle tissue engineering using 3D constructs for VML treatment with a focus on studies that have promoted vascular and neural regeneration within the muscle tissue post-VML.

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.

Toward Building the Neuromuscular Junction: In Vitro Models To Study Synaptogenesis and Neurodegeneration

The neuromuscular junction (NMJ) is a unique, specialized chemical synapse that plays a crucial role in transmitting and amplifying information from spinal motor neurons to skeletal muscles. NMJ complexity ensures closely intertwined interactions between numerous synaptic vesicles, signaling molecules, ion channels, motor neurons, glia, and muscle fibers, making it difficult to dissect the underlying mechanisms and factors affecting neurodegeneration and muscle loss. Muscle fiber or motor neuron cell death followed by rapid axonal degeneration due to injury or disease has a debilitating effect on movement and behavior, which adversely affects the quality of life. It thus becomes imperative to study the synapse and intercellular signaling processes that regulate plasticity at the NMJ and elucidate mechanisms and pathways at the cellular level. Studies using in vitro 2D cell cultures have allowed us to gain a fundamental understanding of how the NMJ functions. However, they do not provide information on the intricate signaling networks that exist between NMJs and the biological environment. The advent of 3D cell cultures and microfluidic lab-on-a-chip technologies has opened whole new avenues to explore the NMJ. In this perspective, we look at the challenges involved in building a functional NMJ and the progress made in generating models for studying the NMJ, highlighting the current and future applications of these models.

Engineering complex muscle-tissue interfaces through microfabrication

Skeletal muscle is a tissue with a complex and hierarchical architecture that influences its functional properties. In order to exert its contractile function, muscle tissue is connected to neural, vascular and connective compartments, comprising finely structured interfaces which are orchestrated by multiple signalling pathways. Pathological conditions such as dystrophies and trauma, or physiological situations such as exercise and aging, modify the architectural organization of these structures, hence affecting muscle functionality. To overcome current limitations of in vivo and standard in vitro models, microfluidics and biofabrication techniques have been applied to better reproduce the microarchitecture and physicochemical environment of human skeletal muscle tissue. In the present review, we aim to critically discuss the role of those techniques, taken individually or in combination, in the generation of models that mimic the complex interfaces between muscle tissue and neural/vascular/tendon compartments. The exploitation of either microfluidics or biofabrication to model different muscle interfaces has led to the development of constructs with an improved spatial organization, thus presenting a better functionality as compared to standard models. However, the achievement of models replicating muscle-tissue interfaces with adequate architecture, presence of fundamental proteins and recapitulation of signalling pathways is still far from being achieved. Increased integration between microfluidics and biofabrication, providing the possibility to pattern cells in predetermined structures with higher resolution, will help to reproduce the hierarchical and heterogeneous structure of skeletal muscle interfaces. Such strategies will further improve the functionality of these techniques, providing a key contribution towards the study of skeletal muscle functions in physiology and pathology.

3D Bioprinting in Skeletal Muscle Tissue Engineering

Skeletal muscle tissue engineering (SMTE) aims at repairing defective skeletal muscles. Until now, numerous developments are made in SMTE; however, it is still challenging to recapitulate the complexity of muscles with current methods of fabrication. Here, after a brief description of the anatomy of skeletal muscle and a short state-of-the-art on developments made in SMTE with "conventional methods," the use of 3D bioprinting as a new tool for SMTE is in focus. The current bioprinting methods are discussed, and an overview of the bioink formulations and properties used in 3D bioprinting is provided. Finally, different advances made in SMTE by 3D bioprinting are highlighted, and future needs and a short perspective are provided.

Scaffolding kidney organoids on silk

End stage kidney disease affects hundreds of thousands of patients in the United States. The therapy of choice is kidney replacement, but availability of organs is limited, and alternative sources of tissue are needed. Generation of new kidney tissue in the laboratory has been made possible through pluripotent cell reprogramming and directed differentiation. In current procedures, aggregates of cells known as organoids are grown either submerged or at the air-liquid interface. These studies have demonstrated that kidney tissue can be generated from pluripotent stem cells, but they also identify limitations. The first is that perfusion of cell aggregates is limited, restricting the size to which they can be grown. The second is that aggregates lack the structural integrity required for convenient engraftment and suturing or adhesion to regions of kidney injury. In this study, we evaluated the capacity of silk to serve as a support for the growth and differentiation of kidney tissue from primary cells and from human induced pluripotent stem cells. We find that cells can differentiate to epithelia characteristic of the developing kidney on this material and that these structures are maintained following engraftment under the capsule of the adult kidney. Blood vessel investment can be promoted by the addition of vascular endothelial growth factor to the scaffold, but the proliferation of stromal cells within the graft presents a challenge, which will require some readjustment of cell growth and differentiation conditions. In summary, we find that silk can be used to support growth of stem cell derived kidney tissue.