Defined Electrical Stimulation Emphasizing Excitability for the Development and Testing of Engineered Skeletal Muscle

Steering Muscle-Based Bio-Syncretic Robot Through Bionic Optimized Biped Mechanical Design

Bio-syncretic robots consisting of artificial structures and living muscle cells have attracted much attention owing to their potential advantages, such as high drive efficiency, miniaturization, and compatibility. Motion controllability, as an important factor related to the main performance of bio-syncretic robots, has been explored in numerous studies. However, most of the existing bio-syncretic robots still face challenges related to the further development of steerable kinematic dexterity. In this study, a bionic optimized biped fully soft bio-syncretic robot actuated by two muscle tissues and steered with a direction-controllable electric field generated by external circularly distributed multiple electrodes has been developed. The developed bio-syncretic robot could realize wirelessly steerable motion and effective transportation of microparticle cargo on artificial polystyrene and biological pork tripe surfaces. This study may provide an effective strategy for the development of bio-syncretic robots and other related studies, such as nonliving soft robot design and muscle tissue engineering.

Biohybrid Soft Robots Powered by Myocyte: Current Progress and Future Perspectives

Myocyte-driven robots, a type of biological actuator that combines myocytes with abiotic systems, have gained significant attention due to their high energy efficiency, sensitivity, biocompatibility, and self-healing capabilities. These robots have a unique advantage in simulating the structure and function of human tissues and organs. This review covers the research progress in this field, detailing the benefits of myocyte-driven robots over traditional methods, the materials used in their fabrication (including myocytes and extracellular materials), and their properties and manufacturing techniques. Additionally, the review explores various control methods, robot structures, and motion types. Lastly, the potential applications and key challenges faced by myocyte-driven robots are discussed and summarized.

Contractile force assessment methods for in vitro skeletal muscle tissues

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

Biobased Elastomer Nanofibers Guide Light-Controlled Human-iPSC-Derived Skeletal Myofibers

Generating skeletal muscle tissue that mimics the cellular alignment, maturation, and function of native skeletal muscle is an ongoing challenge in disease modeling and regenerative therapies. Skeletal muscle cultures require extracellular guidance and mechanical support to stabilize contractile myofibers. Existing microfabrication-based solutions are limited by complex fabrication steps, low throughput, and challenges in measuring dynamic contractile function. Here, the synthesis and characterization of a new biobased nanohybrid elastomer, which is electrospun into aligned nanofiber sheets to mimic the skeletal muscle extracellular matrix, is presented. The polymer exhibits remarkable hyperelasticity well-matched to that of native skeletal muscle (≈11-50 kPa), with ultimate strain ≈1000%, and elastic modulus ≈25 kPa. Uniaxially aligned nanofibers guide myoblast alignment, enhance sarcomere formation, and promote a ≈32% increase in myotube fusion and ≈50% increase in myofiber maturation. The elastomer nanofibers stabilize optogenetically controlled human induced pluripotent stem cell derived skeletal myofibers. When activated by blue light, the myofiber-nanofiber hybrid constructs maintain a significantly higher (>200%) contraction velocity and specific force (>280%) compared to conventional culture methods. The engineered myofibers exhibit a power density of ≈35 W m-3 . This system is a promising new skeletal muscle tissue model for applications in muscular disease modeling, drug discovery, and muscle regeneration.

High-throughput, real-time monitoring of engineered skeletal muscle function using magnetic sensing

Engineered muscle tissues represent powerful tools for examining tissue level contractile properties of skeletal muscle. However, limitations in the throughput associated with standard analysis methods limit their utility for longitudinal study, high throughput drug screens, and disease modeling. Here we present a method for integrating 3D engineered skeletal muscles with a magnetic sensing system to facilitate non-invasive, longitudinal analysis of developing contraction kinetics. Using this platform, we show that engineered skeletal muscle tissues derived from both induced pluripotent stem cell and primary sources undergo improvements in contractile output over time in culture. We demonstrate how magnetic sensing of contractility can be employed for simultaneous assessment of multiple tissues subjected to different doses of known skeletal muscle inotropes as well as the stratification of healthy versus diseased functional profiles in normal and dystrophic muscle cells. Based on these data, this combined culture system and magnet-based contractility platform greatly broadens the potential for 3D engineered skeletal muscle tissues to impact the translation of novel therapies from the lab to the clinic.

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.

Mechanical loading of bioengineered skeletal muscle in vitro recapitulates gene expression signatures of resistance exercise in vivo

Understanding the role of mechanical loading and exercise in skeletal muscle (SkM) is paramount for delineating the molecular mechanisms that govern changes in muscle mass. However, it is unknown whether loading of bioengineered SkM in vitro adequately recapitulates the molecular responses observed after resistance exercise (RE) in vivo. To address this, the transcriptional and epigenetic (DNA methylation) responses were compared after mechanical loading in bioengineered SkM in vitro and after RE in vivo. Specifically, genes known to be upregulated/hypomethylated after RE in humans were analyzed. Ninety-three percent of these genes demonstrated similar changes in gene expression post-loading in the bioengineered muscle when compared to acute RE in humans. Furthermore, similar differences in gene expression were observed between loaded bioengineered SkM and after programmed RT in rat SkM tissue. Hypomethylation occurred for only one of the genes analysed (GRIK2) post-loading in bioengineered SkM. To further validate these findings, DNA methylation and mRNA expression of known hypomethylated and upregulated genes post-acute RE in humans were also analyzed at 0.5, 3, and 24 h post-loading in bioengineered muscle. The largest changes in gene expression occurred at 3 h, whereby 82% and 91% of genes responded similarly when compared to human and rodent SkM respectively. DNA methylation of only a small proportion of genes analyzed (TRAF1, MSN, and CTTN) significantly increased post-loading in bioengineered SkM alone. Overall, mechanical loading of bioengineered SkM in vitro recapitulates the gene expression profile of human and rodent SkM after RE in vivo. Although some genes demonstrated differential DNA methylation post-loading in bioengineered SkM, such changes across the majority of genes analyzed did not closely mimic the epigenetic response to acute-RE in humans.

An Electrical Stimulation Culture System for Daily Maintenance-Free Muscle Tissue Production

Low-labor production of tissue-engineered muscles (TEMs) is one of the key technologies to realize the practical use of muscle-actuated devices. This study developed and then demonstrated the daily maintenance-free culture system equipped with both electrical stimulation and medium replacement functions. To avoid ethical issues, immortal myoblast cells C2C12 were used. The system consisting of gel culture molds, a medium replacement unit, and an electrical stimulation unit could produce 12 TEMs at one time. The contractile forces of the TEMs were measured with a newly developed microforce measurement system. Even the TEMs cultured without electrical stimulation generated forces of almost 2 mN and were shortened by 10% in tetanic contractions. Regarding the contractile forces, electrical stimulation by a single pulse at 1 Hz was most effective, and the contractile forces in tetanus were over 2.5 mN. On the other hand, continuous pulses decreased the contractile forces of TEMs. HE-stained cross-sections showed that myoblast cells proliferated and fused into myotubes mainly in the peripheral regions, and fewer cells existed in the internal region. This must be due to insufficient supplies of oxygen and nutrients inside the TEMs. By increasing the supplies, one TEM might be able to generate a force up to around 10 mN. The tetanic forces of the TEMs produced by the system were strong enough to actuate microstructures like previously reported crawling robots. This daily maintenance-free culture system which could stably produce TEMs strong enough to be utilized for microrobots should contribute to the advancement of biohybrid devices.

Tissue-Engineered Skeletal Muscle Models to Study Muscle Function, Plasticity, and Disease

Skeletal muscle possesses remarkable plasticity that permits functional adaptations to a wide range of signals such as motor input, exercise, and disease. Small animal models have been pivotal in elucidating the molecular mechanisms regulating skeletal muscle adaptation and plasticity. However, these small animal models fail to accurately model human muscle disease resulting in poor clinical success of therapies. Here, we review the potential of in vitro three-dimensional tissue-engineered skeletal muscle models to study muscle function, plasticity, and disease. First, we discuss the generation and function of in vitro skeletal muscle models. We then discuss the genetic, neural, and hormonal factors regulating skeletal muscle fiber-type in vivo and the ability of current in vitro models to study muscle fiber-type regulation. We also evaluate the potential of these systems to be utilized in a patient-specific manner to accurately model and gain novel insights into diseases such as Duchenne muscular dystrophy (DMD) and volumetric muscle loss. We conclude with a discussion on future developments required for tissue-engineered skeletal muscle models to become more mature, biomimetic, and widely utilized for studying muscle physiology, disease, and clinical use.

Mechanical loading of tissue engineered skeletal muscle prevents dexamethasone induced myotube atrophy

Skeletal muscle atrophy as a consequence of acute and chronic illness, immobilisation, muscular dystrophies and aging, leads to severe muscle weakness, inactivity and increased mortality. Mechanical loading is thought to be the primary driver for skeletal muscle hypertrophy, however the extent to which mechanical loading can offset muscle catabolism has not been thoroughly explored. In vitro 3D-models of skeletal muscle provide a controllable, high throughput environment and mitigating many of the ethical and methodological constraints present during in vivo experimentation. This work aimed to determine if mechanical loading would offset dexamethasone (DEX) induced skeletal muscle atrophy, in muscle engineered using the C2C12 murine cell line. Mechanical loading successfully offset myotube atrophy and functional degeneration associated with DEX regardless of whether the loading occurred before or after 24 h of DEX treatment. Furthermore, mechanical load prevented increases in MuRF-1 and MAFbx mRNA expression, critical regulators of muscle atrophy. Overall, we demonstrate the application of tissue engineered muscle to study skeletal muscle health and disease, offering great potential for future use to better understand treatment modalities for skeletal muscle atrophy.

Extracellular Matrix-Derived Hydrogels as Biomaterial for Different Skeletal Muscle Tissue Replacements

Recently, skeletal muscle represents a complex and challenging tissue to be generated in vitro for tissue engineering purposes. Several attempts have been pursued to develop hydrogels with different formulations resembling in vitro the characteristics of skeletal muscle tissue in vivo. This review article describes how different types of cell-laden hydrogels recapitulate the multiple interactions occurring between extracellular matrix (ECM) and muscle cells. A special attention is focused on the biochemical cues that affect myocytes morphology, adhesion, proliferation, and phenotype maintenance, underlining the importance of topographical cues exerted on the hydrogels to guide cellular orientation and facilitate myogenic differentiation and maturation. Moreover, we highlight the crucial role of 3D printing and bioreactors as useful platforms to finely control spatial deposition of cells into ECM based hydrogels and provide the skeletal muscle native-like tissue microenvironment, respectively.

Glucose Uptake and Insulin Response in Tissue-engineered Human Skeletal Muscle

BACKGROUND

Tissue-engineered muscles ("myobundles") offer a promising platform for developing a human in vitro model of healthy and diseased muscle for drug development and testing. Compared to traditional monolayer cultures, myobundles better model the three-dimensional structure of native skeletal muscle and are amenable to diverse functional measures to monitor the muscle health and drug response. Characterizing the metabolic function of human myobundles is of particular interest to enable their utilization in mechanistic studies of human metabolic diseases, identification of related drug targets, and systematic studies of drug safety and efficacy.

METHODS

To this end, we studied glucose uptake and insulin responsiveness in human tissue-engineered skeletal muscle myobundles in the basal state and in response to drug treatments.

RESULTS

In the human skeletal muscle myobundle system, insulin stimulates a 50% increase in 2-deoxyglucose (2-DG) uptake with a compiled EC₅₀ of 0.27 ± 0.03 nM. Treatment of myobundles with 400 µM metformin increased basal 2-DG uptake 1.7-fold and caused a significant drop in twitch and tetanus contractile force along with decreased fatigue resistance. Treatment with the histone deacetylase inhibitor 4-phenylbutyrate (4-PBA) increased the magnitude of insulin response from a 1.2-fold increase in glucose uptake in the untreated state to a 1.4-fold increase after 4-PBA treatment. 4-PBA treated myobundles also exhibited increased fatigue resistance and increased twitch half-relaxation time.

CONCLUSION

Although tissue-engineered human myobundles exhibit a modest increase in glucose uptake in response to insulin, they recapitulate key features of in vivo insulin sensitivity and exhibit relevant drug-mediated perturbations in contractile function and glucose metabolism.

Engineered skeletal muscles for disease modeling and drug discovery

Skeletal muscle is the largest organ of human body with several important roles in everyday movement and metabolic homeostasis. The limited ability of small animal models of muscle disease to accurately predict drug efficacy and toxicity in humans has prompted the development in vitro models of human skeletal muscle that fatefully recapitulate cell and tissue level functions and drug responses. We first review methods for development of three-dimensional engineered muscle tissues and organ-on-a-chip microphysiological systems and discuss their potential utility in drug discovery research and development of new regenerative therapies. Furthermore, we describe strategies to increase the functional maturation of engineered muscle, and motivate the importance of incorporating multiple tissue types on the same chip to model organ cross-talk and generate more predictive drug development platforms. Finally, we review the ability of available in vitro systems to model diseases such as type II diabetes, Duchenne muscular dystrophy, Pompe disease, and dysferlinopathy.

Mechanical loading stimulates hypertrophy in tissue-engineered skeletal muscle: Molecular and phenotypic responses

Mechanical loading of skeletal muscle results in molecular and phenotypic adaptations typified by enhanced muscle size. Studies on humans are limited by the need for repeated sampling, and studies on animals have methodological and ethical limitations. In this investigation, three‐dimensional skeletal muscle was tissue‐engineered utilizing the murine cell line C2C12, which bears resemblance to native tissue and benefits from the advantages of conventional in vitro experiments. The work aimed to determine if mechanical loading induced an anabolic hypertrophic response, akin to that described in vivo after mechanical loading in the form of resistance exercise. Specifically, we temporally investigated candidate gene expression and Akt‐mechanistic target of rapamycin 1 signalling along with myotube growth and tissue function. Mechanical loading (construct length increase of 15%) significantly increased insulin‐like growth factor‐1 and MMP‐2 messenger RNA expression 21 hr after overload, and the levels of the atrophic gene MAFbx were significantly downregulated 45 hr after mechanical overload. In addition, p70S6 kinase and 4EBP‐1 phosphorylation were upregulated immediately after mechanical overload. Maximal contractile force was augmented 45 hr after load with a 265% increase in force, alongside significant hypertrophy of the myotubes within the engineered muscle. Overall, mechanical loading of tissue‐engineered skeletal muscle induced hypertrophy and improved force production.

Oxygen consumption in human, tissue-engineered myobundles during basal and electrical stimulation conditions

During three-dimensional culture of skeletal muscle in vitro, electrical stimulation provides an important cue to enhance skeletal muscle mimicry of the in vivo structure and function. However, increased respiration can cause oxygen transport limitations in these avascular three-dimensional constructs, leading to a hypoxic, necrotic core, or nonuniform cell distributions in larger constructs. To enhance oxygen transport with convection, oxygen concentrations were measured using an optical sensor at the inlet and outlet of an 80 μl fluid volume microphysiological system (MPS) flow chamber containing three-dimensional human skeletal muscle myobundles. Finite element model simulations of convection around myobundles and oxygen metabolism by the myobundles in the 80 μl MPS flow chamber agreed well with the oxygen consumption rate (OCR) at different flow rates, suggesting that under basal conditions, mass transfer limitations were negligible for flow rates above 1.5 μl s^-1. To accommodate electrodes for electrical stimulation, a modified 450 μl chamber was constructed. Electrical stimulation for 30 min increased the measured rate of oxygen consumption by the myobundles to slightly over 2 times the basal OCR. Model simulations indicate that mass transfer limitations were significant during electrical stimulation and, in the absence of mass transfer limitations, electrical stimulation induced about a 20-fold increase in the maximum rate of oxygen consumption. The results indicate that simulated exercise conditions increase respiration of skeletal muscle and mass transfer limitations reduce the measured levels of oxygen uptake, which may affect previous studies that model exercise with engineered muscle.

Electrical stimulation increases hypertrophy and metabolic flux in tissue-engineered human skeletal muscle

In vitro models of contractile human skeletal muscle hold promise for use in disease modeling and drug development, but exhibit immature properties compared to native adult muscle. To address this limitation, 3D tissue-engineered human muscles (myobundles) were electrically stimulated using intermittent stimulation regimes at 1 Hz and 10 Hz. Dystrophin in myotubes exhibited mature membrane localization suggesting a relatively advanced starting developmental maturation. One-week stimulation significantly increased myobundle size, sarcomeric protein abundance, calcium transient amplitude (∼2-fold), and tetanic force (∼3-fold) resulting in the highest specific force generation (19.3mN/mm2) reported for engineered human muscles to date. Compared to 1 Hz electrical stimulation, the 10 Hz stimulation protocol resulted in greater myotube hypertrophy and upregulated mTORC1 and ERK1/2 activity. Electrically stimulated myobundles also showed a decrease in fatigue resistance compared to control myobundles without changes in glycolytic or mitochondrial protein levels. Greater glucose consumption and decreased abundance of acetylcarnitine in stimulated myobundles indicated increased glycolytic and fatty acid metabolic flux. Moreover, electrical stimulation of myobundles resulted in a metabolic shift towards longer-chain fatty acid oxidation as evident from increased abundances of medium- and long-chain acylcarnitines. Taken together, our study provides an advanced in vitro model of human skeletal muscle with improved structure, function, maturation, and metabolic flux.

Exercising Bioengineered Skeletal Muscle In Vitro: Biopsy to Bioreactor

The bioengineering of skeletal muscle tissue in-vitro has enabled researchers to more closely mimic the in-vivo skeletal muscle niche. The three-dimensional (3-D) structure of the tissue engineered systems employed to date enable the generation of highly aligned and differentiated myofibers within a representative biological matrix. The use of electrical stimulation to model concentric contraction, via innervation of the myofibers, and the use of mechanical loading to model passive lengthening or stretch has begun to provide a manipulable environment to investigate the cellular and molecular responses following exercise mimicking stimuli in-vitro. Currently available bioreactor systems allow either electrical stimulation or mechanical loading to be utilized at any given time. In the present manuscript, we describe in detail the methodological procedures to create 3-D bioengineered skeletal muscle using both cell lines and/or primary human muscle derived cells from a tissue biopsy, through to modeling exercising stimuli using a bioreactor that can provide both electrical stimulation and mechanical loading simultaneously within the same in-vitro system.

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.

Long-Term High-Density Extracellular Recordings Enable Studies of Muscle Cell Physiology

Skeletal (voluntary) muscle is the most abundant tissue in the body, thus making it an important biomedical research subject. Studies of neuromuscular transmission, including disorders of ion channels or receptors in autoimmune or genetic neuromuscular disorders, require high-spatial-resolution measurement techniques and an ability to acquire repeated recordings over time in order to track pharmacological interventions. Preclinical techniques for studying diseases of neuromuscular transmission can be enhanced by physiologic ex vivo models of tissue-tissue and cell-cell interactions. Here, we present a method, which allows tracking the development of primary skeletal muscle cells from myoblasts into mature contracting myotubes over more than 2 months. In contrast to most previous studies, the myotubes did not detach from the surface but instead formed functional networks between the myotubes, whose electrical signals were observed over the entire culturing period. Primary cultures of mouse myoblasts differentiated into contracting myotubes on a chip that contained an array of 26,400 platinum electrodes at a density of 3,265 electrodes per mm². Our ability to track extracellular action potentials at subcellular resolution enabled study of skeletal muscle development and kinetics, modes of spiking and spatio-temporal relationships between muscles. The developed system in turn enables creation of a novel electrophysiological platform for establishing ex vivo disease models.

The Importance of Biophysical and Biochemical Stimuli in Dynamic Skeletal Muscle Models

Classical approaches to engineer skeletal muscle tissue based on current regenerative and surgical procedures still do not meet the desired outcome for patient applications. Besides the evident need to create functional skeletal muscle tissue for the repair of volumetric muscle defects, there is also growing demand for platforms to study muscle-related diseases, such as muscular dystrophies or sarcopenia. Currently, numerous studies exist that have employed a variety of biomaterials, cell types and strategies for maturation of skeletal muscle tissue in 2D and 3D environments. However, researchers are just at the beginning of understanding the impact of different culture settings and their biochemical (growth factors and chemical changes) and biophysical cues (mechanical properties) on myogenesis. With this review we intend to emphasize the need for new in vitro skeletal muscle (disease) models to better recapitulate important structural and functional aspects of muscle development. We highlight the importance of choosing appropriate system components, e.g., cell and biomaterial type, structural and mechanical matrix properties or culture format, and how understanding their interplay will enable researchers to create optimized platforms to investigate myogenesis in healthy and diseased tissue. Thus, we aim to deliver guidelines for experimental designs to allow estimation of the potential influence of the selected skeletal muscle tissue engineering setup on the myogenic outcome prior to their implementation. Moreover, we offer a workflow to facilitate identifying and selecting different analytical tools to demonstrate the successful creation of functional skeletal muscle tissue. Ultimately, a refinement of existing strategies will lead to further progression in understanding important aspects of muscle diseases, muscle aging and muscle regeneration to improve quality of life of patients and enable the establishment of new treatment options.

In Vitro Tissue-Engineered Skeletal Muscle Models for Studying Muscle Physiology and Disease

Healthy skeletal muscle possesses the extraordinary ability to regenerate in response to small-scale injuries; however, this self-repair capacity becomes overwhelmed with aging, genetic myopathies, and large muscle loss. The failure of small animal models to accurately replicate human muscle disease, injury and to predict clinically-relevant drug responses has driven the development of high fidelity in vitro skeletal muscle models. Herein, the progress made and challenges ahead in engineering biomimetic human skeletal muscle tissues that can recapitulate muscle development, genetic diseases, regeneration, and drug response is discussed. Bioengineering approaches used to improve engineered muscle structure and function as well as the functionality of satellite cells to allow modeling muscle regeneration in vitro are also highlighted. Next, a historical overview on the generation of skeletal muscle cells and tissues from human pluripotent stem cells, and a discussion on the potential of these approaches to model and treat genetic diseases such as Duchenne muscular dystrophy, is provided. Finally, the need to integrate multiorgan microphysiological systems to generate improved drug discovery technologies with the potential to complement or supersede current preclinical animal models of muscle disease is described.

Biomaterials in Tendon and Skeletal Muscle Tissue Engineering: Current Trends and Challenges

Tissue engineering is a promising approach to repair tendon and muscle when natural healing fails. Biohybrid constructs obtained after cells’ seeding and culture in dedicated scaffolds have indeed been considered as relevant tools for mimicking native tissue, leading to a better integration in vivo. They can also be employed to perform advanced in vitro studies to model the cell differentiation or regeneration processes. In this review, we report and analyze the different solutions proposed in literature, for the reconstruction of tendon, muscle, and the myotendinous junction. They classically rely on the three pillars of tissue engineering, i.e., cells, biomaterials and environment (both chemical and physical stimuli). We have chosen to present biomimetic or bioinspired strategies based on understanding of the native tissue structure/functions/properties of the tissue of interest. For each tissue, we sorted the relevant publications according to an increasing degree of complexity in the materials’ shape or manufacture. We present their biological and mechanical performances, observed in vitro and in vivo when available. Although there is no consensus for a gold standard technique to reconstruct these musculo-skeletal tissues, the reader can find different ways to progress in the field and to understand the recent history in the choice of materials, from collagen to polymer-based matrices.

Electrical pulse stimulation of cultured skeletal muscle cells as a model for in vitro exercise - possibilities and limitations

The beneficial health-related effects of exercise are well recognized, and numerous studies have investigated underlying mechanism using various in vivo and in vitro models. Although electrical pulse stimulation (EPS) for the induction of muscle contraction has been used for quite some time, its application on cultured skeletal muscle cells of animal or human origin as a model of in vitro exercise is a more recent development. In this review, we compare in vivo exercise and in vitro EPS with regard to effects on signalling, expression level and metabolism. We provide a comprehensive overview of different EPS protocols and their applications, discuss technical aspects of this model including critical controls and the importance of a proper maintenance procedure and finally discuss the limitations of the EPS model.

Leucine elicits myotube hypertrophy and enhances maximal contractile force in tissue engineered skeletal muscle in vitro

The amino acid leucine is thought to be important for skeletal muscle growth by virtue of its ability to acutely activate mTORC1 and enhance muscle protein synthesis, yet little data exist regarding its impact on skeletal muscle size and its ability to produce force. We utilized a tissue engineering approach in order to test whether supplementing culture medium with leucine could enhance mTORC1 signaling, myotube growth, and muscle function. Phosphorylation of the mTORC1 target proteins 4EBP-1 and rpS6 and myotube hypertrophy appeared to occur in a dose dependent manner, with 5 and 20 mM of leucine inducing similar effects, which were greater than those seen with 1 mM. Maximal contractile force was also elevated with leucine supplementation; however, although this did not appear to be enhanced with increasing leucine doses, this effect was completely ablated by co-incubation with the mTOR inhibitor rapamycin, showing that the augmented force production in the presence of leucine was mTOR sensitive. Finally, by using electrical stimulation to induce chronic (24 hr) contraction of engineered skeletal muscle constructs, we were able to show that the effects of leucine and muscle contraction are additive, since the two stimuli had cumulative effects on maximal contractile force production. These results extend our current knowledge of the efficacy of leucine as an anabolic nutritional aid showing for the first time that leucine supplementation may augment skeletal muscle functional capacity, and furthermore validates the use of engineered skeletal muscle for highly-controlled investigations into nutritional regulation of muscle physiology.

Factors That Affect Tissue-Engineered Skeletal Muscle Function and Physiology

Tissue-engineered skeletal muscle has the promise to be a tool for studying physiology, screening muscle-active drugs, and clinical replacement of damaged muscle. To maximize the potential benefits of engineered muscle, it is important to understand the factors required for tissue formation and how these affect muscle function. In this review, we evaluate how biomaterials, cell source, media components, and bioreactor interventions impact muscle function and phenotype.

Creating Interactions between Tissue-Engineered Skeletal Muscle and the Peripheral Nervous System

Effective models of mammalian tissues must allow and encourage physiologically (mimetic) correct interactions between co-cultured cell types in order to produce culture microenvironments as similar as possible to those that would normally occur in vivo. In the case of skeletal muscle, the development of such a culture model, integrating multiple relevant cell types within a biomimetic scaffold, would be of significant benefit for investigations into the development, functional performance, and pathophysiology of skeletal muscle tissue. Although some work has been published regarding the behaviour of in vitro muscle models co-cultured with organotypic slices of CNS tissue or with stem cell-derived neurospheres, little investigation has so far been made regarding the potential to maintain isolated motor neurons within a 3D biomimetic skeletal muscle culture platform. Here, we review the current state of the art for engineering neuromuscular contacts in vitro and provide original data detailing the development of a 3D collagen-based model for the co-culture of primary muscle cells and motor neurons. The devised culture system promotes increased myoblast differentiation, forming arrays of parallel, aligned myotubes on which areas of nerve-muscle contact can be detected by immunostaining for pre- and post-synaptic proteins. Quantitative RT-PCR results indicate that motor neuron presence has a positive effect on myotube maturation, suggesting neural incorporation influences muscle development and maturation in vitro. The importance of this work is discussed in relation to other published neuromuscular co-culture platforms along with possible future directions for the field.

Striated muscle function, regeneration, and repair

As the only striated muscle tissues in the body, skeletal and cardiac muscle share numerous structural and functional characteristics, while exhibiting vastly different size and regenerative potential. Healthy skeletal muscle harbors a robust regenerative response that becomes inadequate after large muscle loss or in degenerative pathologies and aging. In contrast, the mammalian heart loses its regenerative capacity shortly after birth, leaving it susceptible to permanent damage by acute injury or chronic disease. In this review, we compare and contrast the physiology and regenerative potential of native skeletal and cardiac muscles, mechanisms underlying striated muscle dysfunction, and bioengineering strategies to treat muscle disorders. We focus on different sources for cellular therapy, biomaterials to augment the endogenous regenerative response, and progress in engineering and application of mature striated muscle tissues in vitro and in vivo. Finally, we discuss the challenges and perspectives in translating muscle bioengineering strategies to clinical practice.

Modulation of cell function by electric field: a high-resolution analysis

Regulation of cell function by a non-thermal, physiological-level electromagnetic field has potential for vascular tissue healing therapies and advancing hybrid bioelectronic technology. We have recently demonstrated that a physiological electric field (EF) applied wirelessly can regulate intracellular signalling and cell function in a frequency-dependent manner. However, the mechanism for such regulation is not well understood. Here, we present a systematic numerical study of a cell-field interaction following cell exposure to the external EF. We use a realistic experimental environment that also recapitulates the absence of a direct electric contact between the field-sourcing electrodes and the cells or the culture medium. We identify characteristic regimes and present their classification with respect to frequency, location, and the electrical properties of the model components. The results show a striking difference in the frequency dependence of EF penetration and cell response between cells suspended in an electrolyte and cells attached to a substrate. The EF structure in the cell is strongly inhomogeneous and is sensitive to the physical properties of the cell and its environment. These findings provide insight into the mechanisms for frequency-dependent cell responses to EF that regulate cell function, which may have important implications for EF-based therapies and biotechnology development.

Graphite Oxide to Graphene. Biomaterials to Bionics

The advent of implantable biomaterials has revolutionized medical treatment, allowing the development of the fields of tissue engineering and medical bionic devices (e.g., cochlea implants to restore hearing, vagus nerve stimulators to control Parkinson's disease, and cardiac pace makers). Similarly, future materials developments are likely to continue to drive development in treatment of disease and disability, or even enhancing human potential. The material requirements for implantable devices are stringent. In all cases they must be nontoxic and provide appropriate mechanical integrity for the application at hand. In the case of scaffolds for tissue regeneration, biodegradability in an appropriate time frame may be required, and for medical bionics electronic conductivity is essential. The emergence of graphene and graphene-family composites has resulted in materials and structures highly relevant to the expansion of the biomaterials inventory available for implantable medical devices. The rich chemistries available are able to ensure properties uncovered in the nanodomain are conveyed into the world of macroscopic devices. Here, the inherent properties of graphene, along with how graphene or structures containing it interface with living cells and the effect of electrical stimulation on nerves and cells, are reviewed.

Role of contraction duration in inducing fast-to-slow contractile and metabolic protein and functional changes in engineered muscle

The role of factors such as frequency, contraction duration and active time in the adaptation to chronic low-frequency electrical stimulation (CLFS) is widely disputed. In this study we explore the ability of contraction duration (0.6, 6, 60, and 600 sec) to induce a fast-to-slow shift in engineered muscle while using a stimulation frequency of 10 Hz and keeping active time constant at 60%. We found that all contraction durations induced similar slowing of time-to-peak tension. Despite similar increases in total myosin heavy (MHC) levels with stimulation, increasing contraction duration resulted in progressive decreases in total fast myosin. With contraction durations of 60 and 600 sec, MHC IIx levels decreased and MHC IIa levels increased. All contraction durations resulted in fast-to-slow shifts in TnT and TnC but increased both fast and slow TnI levels. Half-relaxation slowed to a greater extent with contraction durations of 60 and 600 sec despite similar changes in the calcium sequestering proteins calsequestrin and parvalbumin and the calcium uptake protein SERCA. All CLFS groups resulted in greater fatigue resistance than control. Similar increases in GLUT4, mitochondrial enzymes (SDH and ATPsynthase), the fatty acid transporter CPT-1, and the metabolic regulators PGC-1α and MEF2 were found with all contraction durations. However, the mitochondrial enzymes cytochrome C and citrate synthase were increased to greater levels with contraction durations of 60 and 600 sec. These results demonstrate that contraction duration plays a pivotal role in dictating the level of CLFS-induced contractile and metabolic adaptations in tissue-engineered skeletal muscle.

Neuromuscular Junction Formation in Tissue-Engineered Skeletal Muscle Augments Contractile Function and Improves Cytoskeletal Organization

Neuromuscular and neurodegenerative diseases are conditions that affect both motor neurons and the underlying skeletal muscle tissue. At present, the majority of neuromuscular research utilizes animal models and there is a growing need to develop novel methodologies that can be used to help understand and develop treatments for these diseases. Skeletal muscle tissue-engineered constructs exhibit many of the characteristics of the native tissue such as accurate fascicular structure and generation of active contractions. However, to date, there has been little consideration toward the integration of engineered skeletal muscle with motor neurons with the aim of neuromuscular junction (NMJ) formation, which would provide a model to investigate neuromuscular diseases and basic biology. In the present work we isolated primary embryonic motor neurons and neonatal myoblasts from Sprague-Dawley rats, and cocultured the two cell types in three-dimensional tissue-engineered fibrin hydrogels with the aim of NMJ formation. Immunohistochemistry revealed myotube formation in a fascicular arrangement and neurite outgrowth from motor neuron cell bodies toward the aligned myotubes. Furthermore, colocalization of pre- and postsynaptic proteins and chemical inhibition of spontaneous myotube twitch indicated the presence of NMJs in the innervated constructs. When electrical field stimulation was employed to evoke isometric contractions, maximal twitch and tetanic force were higher in the constructs cocultured with motor neurons, which may, in part, be explained by improved myotube cytoskeletal organization in these constructs. The fabrication of such constructs may be useful tools for investigating neuromuscular pharmaceuticals and improving the understanding of neuromuscular pathologies.

The effect of serum origin on tissue engineered skeletal muscle function

Skeletal muscle phenotype is regulated by a complex interaction between genetic, hormonal, and electrical inputs. However, because of the interrelatedness of these factors in vivo it is difficult to determine the importance of one over the other. Over the last 5 years, we have engineered skeletal muscles in the European Union (EU) and the United States (US) using the same clone of C2C12 cells. Strikingly, the dynamics of contraction of the muscles was dramatically different. Therefore, in this study we sought to determine whether the hormonal milieu (source of fetal bovine serum (FBS)) could alter engineered muscle phenotype. In muscles engineered in serum of US origin time-to-peak tension (2.2-fold), half relaxation (2.6-fold), and fatigue resistance (improved 25%) all showed indications of a shift towards a slower phenotype. Even though there was a dramatic shift in the rate of contraction, myosin heavy chain expression was the same. The contraction speed was instead related to a shift in calcium release/sensitivity proteins (DHPR = 3.1-fold lower, slow CSQ = 3.4-fold higher, and slow TnT = 2.4-fold higher) and calcium uptake proteins (slow SERCA = 1.7-fold higher and parvalbumin = 41-fold lower). These shifts in calcium dynamics were accompanied by a partial shift in metabolic enzymes, but could not be explained by purported regulators of muscle phenotype. These data suggest that hormonal differences in serum of USDA and EU origin cause a shift in calcium handling resulting in a dramatic change in engineered muscle function.

Engineered in vitro disease models

The ultimate goal of most biomedical research is to gain greater insight into mechanisms of human disease or to develop new and improved therapies or diagnostics. Although great advances have been made in terms of developing disease models in animals, such as transgenic mice, many of these models fail to faithfully recapitulate the human condition. In addition, it is difficult to identify critical cellular and molecular contributors to disease or to vary them independently in whole-animal models. This challenge has attracted the interest of engineers, who have begun to collaborate with biologists to leverage recent advances in tissue engineering and microfabrication to develop novel in vitro models of disease. As these models are synthetic systems, specific molecular factors and individual cell types, including parenchymal cells, vascular cells, and immune cells, can be varied independently while simultaneously measuring system-level responses in real time. In this article, we provide some examples of these efforts, including engineered models of diseases of the heart, lung, intestine, liver, kidney, cartilage, skin and vascular, endocrine, musculoskeletal, and nervous systems, as well as models of infectious diseases and cancer. We also describe how engineered in vitro models can be combined with human inducible pluripotent stem cells to enable new insights into a broad variety of disease mechanisms, as well as provide a test bed for screening new therapies.

Mechanistic investigation of adult myotube response to exercise and drug treatment in vitro using a multiplexed functional assay system

The ability to accurately measure skeletal muscle functional performance at the single-cell level would be advantageous for exercise physiology studies and disease modeling applications. To that end, this study characterizes the functional response of individual skeletal muscle myotubes derived from adult rodent tissue to creatine treatment and chronic exercise. The observed improvements to functional performance in response to these treatments appear to correlate with alterations in hypertrophic and mitochondrial biogenesis pathways, supporting previously published in vivo and in vitro data, which highlights the role of these pathways in augmenting skeletal muscle output. The developed system represents a multiplexed functional in vitro assay capable of long-term assessment of contractile cellular outputs in real-time that is compatible with concomitant molecular biology analysis. Adoption of this system in drug toxicity and efficacy studies would improve understanding of compound activity on physical cellular outputs and provide more streamlined and predictive data for future preclinical analyses.

Utilization of microscale silicon cantilevers to assess cellular contractile function in vitro

The development of more predictive and biologically relevant in vitro assays is predicated on the advancement of versatile cell culture systems which facilitate the functional assessment of the seeded cells. To that end, microscale cantilever technology offers a platform with which to measure the contractile functionality of a range of cell types, including skeletal, cardiac, and smooth muscle cells, through assessment of contraction induced substrate bending. Application of multiplexed cantilever arrays provides the means to develop moderate to high-throughput protocols for assessing drug efficacy and toxicity, disease phenotype and progression, as well as neuromuscular and other cell-cell interactions. This manuscript provides the details for fabricating reliable cantilever arrays for this purpose, and the methods required to successfully culture cells on these surfaces. Further description is provided on the steps necessary to perform functional analysis of contractile cell types maintained on such arrays using a novel laser and photo-detector system. The representative data provided highlights the precision and reproducible nature of the analysis of contractile function possible using this system, as well as the wide range of studies to which such technology can be applied. Successful widespread adoption of this system could provide investigators with the means to perform rapid, low cost functional studies in vitro, leading to more accurate predictions of tissue performance, disease development and response to novel therapeutic treatment.

Induction of functional tissue-engineered skeletal muscle constructs by defined electrical stimulation

Electrical impulses are necessary for proper in vivo skeletal muscle development. To fabricate functional skeletal muscle tissues in vitro, recapitulation of the in vivo niche, including physical stimuli, is crucial. Here, we report a technique to engineer skeletal muscle tissues in vitro by electrical pulse stimulation (EPS). Electrically excitable tissue-engineered skeletal muscle constructs were stimulated with continuous electrical pulses of 0.3 V/mm amplitude, 4 ms width, and 1 Hz frequency, resulting in a 4.5-fold increase in force at day 14. In myogenic differentiation culture, the percentage of peak twitch force (%Pt) was determined as the load on the tissue constructs during the artificial exercise induced by continuous EPS. We optimized the stimulation protocol, wherein the tissues were first subjected to 24.5%Pt, which was increased to 50-60%Pt as the tissues developed. This technique may be a useful approach to fabricate tissue-engineered functional skeletal muscle constructs.

Use of flow, electrical, and mechanical stimulation to promote engineering of striated muscles

The field of tissue engineering involves design of high-fidelity tissue substitutes for predictive experimental assays in vitro and cell-based regenerative therapies in vivo. Design of striated muscle tissues, such as cardiac and skeletal muscle, has been particularly challenging due to a high metabolic demand and complex cellular organization and electromechanical function of the native tissues. Successful engineering of highly functional striated muscles may thus require creation of biomimetic culture conditions involving medium perfusion, electrical and mechanical stimulation. When optimized, these external cues are expected to synergistically and dynamically activate important intracellular signaling pathways leading to accelerated muscle growth and development. This review will discuss the use of different types of tissue culture bioreactors aimed at providing conditions for enhanced structural and functional maturation of engineered striated muscles.

Correlation of embryonic skeletal muscle myotube physical characteristics with contractile force generation on an atomic force microscope-based bio-microelectromechanical systems device

Rigorous analysis of muscle function in in vitro systems is needed for both acute and chronic biomedical applications. Forces generated by skeletal myotubes on bio-microelectromechanical cantilevers were calculated using a modified version of Stoney's thin-film equation and finite element analysis (FEA), then analyzed for regression to physical parameters. The Stoney's equation results closely matched the more intensive FEA and the force correlated to cross-sectional area (CSA). Normalizing force to measured CSA significantly improved the statistical sensitivity and now allows for close comparison of in vitro data to in vivo measurements for applications in exercise physiology, robotics, and modeling neuromuscular diseases.

Microfabricated mammalian organ systems and their integration into models of whole animals and humans

While in vitro cell based systems have been an invaluable tool in biology, they often suffer from a lack of physiological relevance. The discrepancy between the in vitro and in vivo systems has been a bottleneck in drug development process and biological sciences. The recent progress in microtechnology has enabled manipulation of cellular environment at a physiologically relevant length scale, which has led to the development of novel in vitro organ systems, often termed 'organ-on-a-chip' systems. By mimicking the cellular environment of in vivo tissues, various organ-on-a-chip systems have been reported to reproduce target organ functions better than conventional in vitro model systems. Ultimately, these organ-on-a-chip systems will converge into multi-organ 'body-on-a-chip' systems composed of functional tissues that reproduce the dynamics of the whole-body response. Such microscale in vitro systems will open up new possibilities in medical science and in the pharmaceutical industry.

Electrical stimulation as a biomimicry tool for regulating muscle cell behavior

There is a growing need to understand muscle cell behaviors and to engineer muscle tissues to replace defective tissues in the body. Despite a long history of the clinical use of electric fields for muscle tissues in vivo, electrical stimulation (ES) has recently gained significant attention as a powerful tool for regulating muscle cell behaviors in vitro. ES aims to mimic the electrical environment of electroactive muscle cells (e.g., cardiac or skeletal muscle cells) by helping to regulate cell-cell and cell-extracellular matrix (ECM) interactions. As a result, it can be used to enhance the alignment and differentiation of skeletal or cardiac muscle cells and to aid in engineering of functional muscle tissues. Additionally, ES can be used to control and monitor force generation and electrophysiological activity of muscle tissues for bio-actuation and drug-screening applications in a simple, high-throughput, and reproducible manner. In this review paper, we briefly describe the importance of ES in regulating muscle cell behaviors in vitro, as well as the major challenges and prospective potential associated with ES in the context of muscle tissue engineering.

Formation and optogenetic control of engineered 3D skeletal muscle bioactuators

Densely arrayed skeletal myotubes are activated individually and as a group using precise optical stimulation with high spatiotemporal resolution. Skeletal muscle myoblasts are genetically encoded to express a light-activated cation channel, Channelrhodopsin-2, which allows for spatiotemporal coordination of a multitude of skeletal myotubes that contract in response to pulsed blue light. Furthermore, ensembles of mature, functional 3D muscle microtissues have been formed from the optogenetically encoded myoblasts using a high-throughput device. The device, called "skeletal muscle on a chip", not only provides the myoblasts with controlled stress and constraints necessary for muscle alignment, fusion and maturation, but also facilitates the measurement of forces and characterization of the muscle tissue. We measured the specific static and dynamic stresses generated by the microtissues and characterized the morphology and alignment of the myotubes within the constructs. The device allows testing of the effect of a wide range of parameters (cell source, matrix composition, microtissue geometry, auxotonic load, growth factors and exercise) on the maturation, structure and function of the engineered muscle tissues in a combinatorial manner. Our studies integrate tools from optogenetics and microelectromechanical systems (MEMS) technology with skeletal muscle tissue engineering to open up opportunities to generate soft robots actuated by a multitude of spatiotemporally coordinated 3D skeletal muscle microtissues.

Interdigitated array of Pt electrodes for electrical stimulation and engineering of aligned muscle tissue

Engineered skeletal muscle tissues could be useful for applications in tissue engineering, drug screening, and bio-robotics. It is well-known that skeletal muscle cells are able to differentiate under electrical stimulation (ES), with an increase in myosin production, along with the formation of myofibers and contractile proteins. In this study, we describe the use of an interdigitated array of electrodes as a novel platform to electrically stimulate engineered muscle tissues. The resulting muscle myofibers were analyzed and quantified in terms of their myotube characteristics and gene expression. The engineered muscle tissues stimulated through the interdigitated array of electrodes demonstrated superior performance and maturation compared to the corresponding tissues stimulated through a conventional setup (i.e., through Pt wires in close proximity to the muscle tissue). In particular, the ES of muscle tissue (voltage 6 V, frequency 1 Hz and duration 10 ms for 1 day) through the interdigitated array of electrodes resulted in a higher degree of C2C12 myotube alignment (∼80%) as compared to ES using Pt wires (∼65%). In addition, higher amounts of C2C12 myotube coverage area, myotube length, muscle transcription factors and protein biomarkers were found for myotubes stimulated through the interdigitated array of electrodes compared to those stimulated using the Pt wires. Due to the wide array of potential applications of ES for two- and three-dimensional (2D and 3D) engineered tissues, the suggested platform could be employed for a variety of cell and tissue structures to more efficiently investigate their response to electrical fields.