Advanced maturation by electrical stimulation: Differences in response between C2C12 and primary muscle progenitor cells

Biomaterials-Based Technologies in Skeletal Muscle Tissue Engineering

For many clinically prevalent severe injuries, the inherent regenerative capacity of skeletal muscle remains inadequate. Skeletal muscle tissue engineering (SMTE) seeks to meet this clinical demand. With continuous progress in biomedicine and related technologies including micro/nanotechnology and 3D printing, numerous studies have uncovered various intrinsic mechanisms regulating skeletal muscle regeneration and developed tailored biomaterial systems based on these understandings. Here, the skeletal muscle structure and regeneration process are discussed and the diverse biomaterial systems derived from various technologies are explored in detail. Biomaterials serve not merely as local niches for cell growth, but also as scaffolds endowed with structural or physicochemical properties that provide tissue regenerative cues such as topographical, electrical, and mechanical signals. They can also act as delivery systems for stem cells and bioactive molecules that have been shown as key participants in endogenous repair cascades. To achieve bench-to-bedside translation, the typical effect enabled by biomaterial systems and the potential underlying molecular mechanisms are also summarized. Insights into the roles of biomaterials in SMTE from cellular and molecular perspectives are provided. Finally, perspectives on the advancement of SMTE are provided, for which gene therapy, exosomes, and hybrid biomaterials may hold promise to make important contributions.

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.

Self-organization of vascularized skeletal muscle from bovine embryonic stem cells

Cultured beef holds promising potential as an alternative to traditional meat options. While adult stem cells are commonly used as the cell source for cultured beef, their proliferation and differentiation capacities are limited. To produce cultured beef steaks, current manufacturing plans often require the separate preparation of multiple cell types and intricate engineering for assembling them into structured tissues. In this study, we propose and report the co-induction of skeletal muscle, neuronal, and endothelial cells from bovine embryonic stem cells (ESCs) and the self-organization of tissue structures in 2- and 3-dimensional cultures. Bovine myocytes were induced in a stepwise manner through the induction of presomitic mesoderm (PSM) from bovine ESCs. Muscle fibers with sarcomeres appeared within 15 days, displaying calcium oscillations responsive to inputs from co-induced bovine spinal neurons. Bovine endothelial cells were also co-induced via PSM, forming uniform vessel networks inside tissues. Our serum-free, rapid co-induction protocols represent a milestone toward self-organizing beef steaks with integrated vasculature and innervation.

An ex vivo animal model to study the effect of transverse mechanical loading on skeletal muscle

In many populations like wheelchair and prosthetic users, the soft tissue is subject to excessive or repetitive loading, making it prone to Deep Tissue Injury (DTI). To study the skeletal muscle response to physical stress, numerous in vitro and in vivo models exist. Yet, accuracy, variability, and ethical considerations pose significant trade-offs. Here, we present an ex vivo approach to address these limitations and offer additional quantitative information on cellular damage. In this study, skeletal muscle tissue from Sprague Dawley rats was isolated and transversely loaded. Histological analysis and fluorescence staining demonstrated that the setup was suitable to keep the tissue alive throughout the experimental procedure. Mechanically induced cell damage was readily distinguishable through morphological changes and uptake of a membrane impermeable dye. Our comparably simple experimental setup can be adapted to different loading conditions and tissues to assess the cell response to mechanical loading in future studies.

Development of a multifunctional bioreactor to evaluate the promotion effects of cyclic stretching and electrical stimulation on muscle differentiation

A multifunctional bioreactor was fabricated in this study to investigate the facilitation efficiency of electrical and mechanical stimulations on myogenic differentiation. This bioreactor consisted of a highly stretchable conductive membrane prepared by depositing polypyrrole (PPy) on a flexible polydimethylsiloxane (PDMS) film. The tensile deformation of the PPy/PDMS membrane can be tuned by adjusting the channel depth. In addition, PPy/PDMS maintained its electrical conductivity under continuous cyclic stretching in the strain range of 6.5%-13% for 24 h. This device can be used to individually or simultaneously perform cyclic stretching and electrical stimulation. The results of single stimulation showed that either cyclic stretching or electrical stimulation upregulated myogenic gene expression and promoted myotube formation, where electrical stimulation improved better than cyclic stretching. However, only cyclic stretching can align C2C12 cells perpendicular to the stretching direction, and electrical stimulation did not affect cell morphology. Myosin heavy chain (MHC) immunostaining demonstrated that oriented cells under cyclic stretching resulted in parallel myotubes. The combination of these two stimuli exhibited synergetic effects on both myogenic gene regulation and myotube formation, and the incorporated electrical field did not affect the orientation effect of the cyclic stretching. These results suggested that these two treatments likely influenced cells through different pathways. Overall, the simultaneous application of cyclic stretching and electrical stimulation preserved both stimuli's advantages, so myo-differentiation can be highly improved to obtain abundant parallel myotubes, suggesting that our developed multifunctional bioreactor should benefit muscle tissue engineering applications.

Pulsed Electromagnetic Fields Induce Skeletal Muscle Cell Repair by Sustaining the Expression of Proteins Involved in the Response to Cellular Damage and Oxidative Stress

Pulsed electromagnetic fields (PEMF) are employed as a non-invasive medicinal therapy, especially in the orthopedic field to stimulate bone regeneration. However, the effect of PEMF on skeletal muscle cells (SkMC) has been understudied. Here, we studied the potentiality of 1.5 mT PEMF to stimulate early regeneration of human SkMC. We showed that human SkMC stimulated with 1.5 mT PEMF for four hours repeated for two days can stimulate cell proliferation without inducing cell apoptosis or significant impairment of the metabolic activity. Interestingly, when we simulated physical damage of the muscle tissue by a scratch, we found that the same PEMF treatment can speed up the regenerative process, inducing a more complete cell migration to close the scratch and wound healing. Moreover, we investigated the molecular pattern induced by PEMF among 26 stress-related cell proteins. We found that the expression of 10 proteins increased after two consecutive days of PEMF stimulation for 4 h, and most of them were involved in response processes to oxidative stress. Among these proteins, we found that heat shock protein 70 (HSP70), which can promote muscle recovery, inhibits apoptosis and decreases inflammation in skeletal muscle, together with thioredoxin, paraoxonase, and superoxide dismutase (SOD2), which can also promote skeletal muscle regeneration following injury. Altogether, these data support the possibility of using PEMF to increase SkMC regeneration and, for the first time, suggest a possible molecular mechanism, which consists of sustaining the expression of antioxidant enzymes to control the important inflammatory and oxidative process occurring following muscle damage.

Porous biomaterial scaffolds for skeletal muscle tissue engineering

Volumetric muscle loss is a traumatic injury which overwhelms the innate repair mechanisms of skeletal muscle and results in significant loss of muscle functionality. Tissue engineering seeks to regenerate these injuries through implantation of biomaterial scaffolds to encourage endogenous tissue formation and to restore mechanical function. Many types of scaffolds are currently being researched for this purpose. Scaffolds are typically made from either natural, synthetic, or conductive polymers, or any combination therein. A major criterion for the use of scaffolds for skeletal muscle is their porosity, which is essential for myoblast infiltration and myofiber ingrowth. In this review, we summarize the various methods of fabricating porous biomaterial scaffolds for skeletal muscle regeneration, as well as the various types of materials used to make these scaffolds. We provide guidelines for the fabrication of scaffolds based on functional requirements of skeletal muscle tissue, and discuss the general state of the field for skeletal 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.

Effects of Protein Hydrolysate from Silkworm (Bombyx mori) pupae on the C2C12 Myogenic Differentiation

This study investigated the effects and active compounds of silkworm pupae, an edible insect, on C2C12 muscle differentiation. The protein of silkworm pupae was extracted using sonication after defatting with hexane. Subsequently, the extract was rehydrated using Alcalase to obtain a protein hydrolysate. The silkworm pupae protein hydrolysate effectively promoted C2C12 myogenic differentiation without cytotoxicity. Subsequently, the hydrolysate was fractionated into four subfractions using preparative high-performance liquid chromatography (Prep-HPLC). Subfraction 1 was the most effective in promoting C2C12 myogenic differentiation and significantly upregulated the expression of myoblast transcription factors, 1.5-fold of myoblast determination protein 1 (MyoD), 2-fold of myogenin, and 3-fold of myosin heavy chain (MyHC). Liquid chromatography-tandem mass spectrometry (LC-MS/MS) and multivariate statistical analysis were used to identify the active peptides in silkworm pupae responsible for the observed effects; then, dipeptides and essential amino acids, such as isoleucine (Ile), valine (Val), and methionine (Met), were identified. In addition, Val, Ile, and two dipeptides underwent quantification to determine the potential bioactive peptides that enhanced C2C12 myogenic differentiation. This study suggests that the peptides from silkworm pupae could be used as a nutraceutical to enhance muscle growth.

Unlocking the potential of stem cells: Their crucial role in the production of cultivated meat

Cellular agriculture is an emerging research field of agribiotechnology that aims to produce agricultural products using stem cells, without sacrificing animals or cultivating crops. Cultivated meat, as a representative cellular product of cellular agriculture, is being actively researched due to global food insecurity, environmental, and ethical concerns. This review focuses on the application of stem cells, which are the seeds of cellular agriculture, for the production of cultivated meat, with emphasis on deriving and culturing muscle and adipose stem cells for imitating fresh meat. Establishing standards and safety regulations for culturing stem cells is crucial for the market entry of cultured muscle tissue-based biomaterials. Understanding stem cells is a prerequisite for creating reliable cultivated meat and other cellular agricultural biomaterials. The techniques and regulations from the cultivated meat industry could pave the way for new cellular agriculture industries in the future.

Culturing characteristics of Hanwoo myosatellite cells and C2C12 cells incubated at 37°C and 39°C for cultured meat

To improve culture efficiency of Hanwoo myosatellite cells, these cells were cultured at different temperatures. Hanwoo myosatellite cells were compared with C2C12 cells to observe proliferation and differentiation at culture temperatures of 37°C and 39°C and determine the possibility of using them as cultured meat. Immunofluorescence staining using Pax7 and Hoechst, both cells cultured at 37°C proliferated better than cultured at 39°C (p < 0.05). When differentiated cells were stained with myosin and Hoechst, there was no significant difference in myotube thickness and Fusion index (p > 0.05). In Western blotting analysis, Hanwoo myosatellite cells were no significant difference in the expression of myosin between cells differentiated at the two temperatures (p > 0.05). C2C12 cells were no significant difference in the expression of myosin between cells differentiated at the two temperatures (p > 0.05). In reverse transcription and quantitative polymerase chain reaction (RT-qPCR) analysis, Hanwoo myosatellite cells cultured at 39°C had significantly (p < 0.05) higher expression levels of MyHC, MYF6, and MB than those cultured at 37°C. C2C12 cells cultured at 39°C showed significantly (p < 0.05) higher expression levels of MYOG and MB than those cultured at 37°C. To increase culture efficiency of Hanwoo myosatellite cells, proliferating at 37°C and differentiating at 39°C are appropriate. Since results of temperature differences of Hanwoo myosatellite cells were similar to those of C2C12 cells, they could be used as a reference for producing cultured meat using Hanwoo satellite cells.

Electrically Triggered Quercetin Release from Polycaprolactone/Bismuth Ferrite Microfibrous Scaffold for Skeletal Muscle Tissue

Skeletal muscle tissue engineering presents a promising avenue to address the limitations pertaining to the regenerative potential of stem cells in case of injury or damage. The objective of this research was to evaluate the effects of utilizing novel microfibrous scaffolds, containing the compound quercetin (Q), on skeletal muscle regeneration. Morphological test results showed us that the combination of bismuth ferrite (BFO), polycaprolactone (PCL), and Q were bonded and well-ordered with each other, and a uniform microfibrous structure was obtained. Antimicrobial susceptibility testing of PCL/BFO/Q was conducted, and microbial reduction was found to be over 90% in the highest concentration of Q-loaded microfibrous scaffolds with the most inhibitory effect on S. aureus strains. Further, biocompatibility was investigated by performing MTT testing, fluorescence testing, and SEM imaging on mesenchymal stem cells (MSCs) to determine whether they could act as suitable microfibrous scaffolds for skeletal muscle tissue engineering. Incremental changes in the concentration of Q led to increased strength and strain, allowing muscles to withstand stretching during the healing process. In addition, electrically conductive microfibrous scaffolds enhanced the drug release capability by revealing that Q can be released significantly more quickly by applying the appropriate electric field, compared with conventional drug-release techniques. These findings suggest a possible use for PCL/BFO/Q microfibrous scaffolds in skeletal muscle regeneration by demonstrating that the combined action of both guidance biomaterials was more successful than Q itself acting alone.

nNOS-derived NO modulates force production and iNO-derived NO the excitability in C2C12-derived 3D tissue engineering skeletal muscle via different NO signaling pathways

Nitric oxide (NO) is a bioactive gas produced by one of the three NO synthases: neuronal NOS (nNOS), inducible (iNOS), and endothelial NOS (eNOS). NO has a relevant modulatory role in muscle contraction; this takes place through two major signaling pathways: (i) activation of soluble guanylate cyclase and, thus, protein kinase G or (ii) nitrosylation of sulfur groups of cysteine. Although it has been suggested that nNOS-derived NO is the responsible isoform in muscle contraction, the roles of eNOS and iNOS and their signaling pathways have not yet been clarified. To elucidate the action of each pathway, we optimized the generation of myooids, an engineered skeletal muscle tissue based on the C2C12 cell line. In comparison with diaphragm strips from wild-type mice, 180 myooids were analyzed, which expressed all relevant excitation–contraction coupling proteins and both nNOS and iNOS isoforms. Along with the biochemical results, myooids treated with NO donor (SNAP) and unspecific NOS blocker (L-NAME) revealed a comparable NO modulatory effect on force production as was observed in the diaphragm strips. Under the effects of pharmacological tools, we analyzed the myooids in response to electrical stimulation of two possible signaling pathways and NO sources. The nNOS-derived NO exerted its negative effect on force production via the sGG-PKG pathway, while iNOS-derived NO increased the excitability in response to sub-threshold electrical stimulation. These results strengthen the hypotheses of previous reports on the mechanism of action of NO during force production, showed a novel function of iNOS-derived NO, and establish the myooid as a novel and robust alternative model for pathophysiological skeletal muscle research.

Development of a simple and versatile in vitro method for production, stimulation, and analysis of bioengineered muscle

In recent years, 3D in vitro modeling of human skeletal muscle has emerged as a subject of increasing interest, due to its applicability in basic studies or screening platforms. These models strive to recapitulate key features of muscle architecture and function, such as cell alignment, maturation, and contractility in response to different stimuli. To this end, it is required to culture cells in biomimetic hydrogels suspended between two anchors. Currently available protocols are often complex to produce, have a high rate of breakage, or are not adapted to imaging and stimulation. Therefore, we sought to develop a simplified and reliable protocol, which still enabled versatility in the study of muscle function. In our method, we have used human immortalized myoblasts cultured in a hydrogel composed of MatrigelTM and fibrinogen, to create muscle strips suspended between two VELCROTM anchors. The resulting muscle constructs show a differentiated phenotype and contractile activity in response to electrical, chemical and optical stimulation. This activity is analyzed by two alternative methods, namely contraction analysis and calcium analysis with Fluo-4 AM. In all, our protocol provides an optimized version of previously published methods, enabling individual imaging of muscle bundles and straightforward analysis of muscle response with standard image analysis software. This system provides a start-to-finish guide on how to produce, validate, stimulate, and analyze bioengineered muscle. This ensures that the system can be quickly established by researchers with varying degrees of expertise, while maintaining reliability and similarity to native muscle.

Microphysiological system for studying contractile differences in young, active, and old, sedentary adult derived skeletal muscle cells

Microphysiological systems (MPS), also referred to as tissue chips, incorporating 3D skeletal myobundles are a novel approach for physiological and pharmacological studies to uncover new medical treatments for sarcopenia. We characterize a MPS in which engineered skeletal muscle myobundles derived from donor-specific satellite cells that model aged phenotypes are encapsulated in a perfused tissue chip platform containing platinum electrodes. Our myobundles were derived from CD56+ myogenic cells obtained via percutaneous biopsy of the vastus lateralis from adults phenotyped by age and physical activity. Following 17 days differentiation including 5 days of a 3 V, 2 Hz electrical stimulation regime, the myobundles exhibited fused myotube alignment and upregulation of myogenic, myofiber assembly, signaling and contractile genes as demonstrated by gene array profiling and localization of key components of the sarcomere. Our results demonstrate that myobundles derived from the young, active (YA) group showed high intensity immunofluorescent staining of α-actinin proteins and responded to electrical stimuli with a ~1 μm displacement magnitude compared with non-stimulated myobundles. Myobundles derived from older sedentary group (OS) did not display a synchronous contraction response. Hypertrophic potential is increased in YA-derived myobundles in response to stimulation as shown by upregulation of insulin growth factor (IGF-1), α-actinin (ACTN3, ACTA1) and fast twitch troponin protein (TNNI2) compared with OS-derived myobundles. Our MPS mimics disease states of muscle decline and thus provides an aged system and experimental platform to investigate electrical stimulation mimicking exercise regimes and may be adapted to long duration studies of compound efficacy and toxicity for therapeutic evaluation against sarcopenia.

Enhancing Myoblast Fusion and Myotube Diameter in Human 3D Skeletal Muscle Constructs by Electromagnetic Stimulation

Skeletal muscle tissue engineering (SMTE) aims at the in vitro generation of 3D skeletal muscle engineered constructs which mimic the native muscle structure and function. Although native skeletal muscle is a highly dynamic tissue, most research approaches still focus on static cell culture methods, while research on stimulation protocols indicates a positive effect, especially on myogenesis. A more mature muscle construct may be needed especially for the potential applications for regenerative medicine purposes, disease or drug disposition models. Most efforts towards dynamic cell or tissue culture methods have been geared towards mechanical or electrical stimulation or a combination of those. In the context of dynamic methods, pulsed electromagnetic field (PEMF) stimulation has been extensively used in bone tissue engineering, but the impact of PEMF on skeletal muscle development is poorly explored. Here, we evaluated the effects of PEMF stimulation on human skeletal muscle cells both in 2D and 3D experiments. First, PEMF was applied on 2D cultures of human myoblasts during differentiation. In 2D, enhanced myogenesis was observed, as evidenced by an increased myotube diameter and fusion index. Second, 2D results were translated towards 3D bioartificial muscles (BAMs). BAMs were subjected to PEMF for varying exposure times, where a 2-h daily stimulation was found to be effective in enhancing 3D myotube formation. Third, applying this protocol for the entire 16-days culture period was compared to a stimulation starting at day 8, once the myotubes were formed. The latter was found to result in significantly higher myotube diameter, fusion index, and increased myosin heavy chain 1 expression. This work shows the potential of electromagnetic stimulation for enhancing myotube formation both in 2D and 3D, warranting its further consideration in dynamic culturing techniques.

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.

Using Vertebrate Stem and Progenitor Cells for Cellular Agriculture, State-of-the-Art, Challenges, and Future Perspectives

Global food systems are under significant pressure to provide enough food, particularly protein-rich foods whose demand is on the rise in times of crisis and inflation, as presently existing due to post-COVID-19 pandemic effects and ongoing conflict in Ukraine and resulting in looming food insecurity, according to FAO. Cultivated meat (CM) and cultivated seafood (CS) are protein-rich alternatives for traditional meat and fish that are obtained via cellular agriculture (CA) i.e., tissue engineering for food applications. Stem and progenitor cells are the building blocks and starting point for any CA bioprocess. This review presents CA-relevant vertebrate cell types and procedures needed for their myogenic and adipogenic differentiation since muscle and fat tissue are the primary target tissues for CM/CS production. The review also describes existing challenges, such as a need for immortalized cell lines, or physical and biochemical parameters needed for enhanced meat/fat culture efficiency and ways to address them.

Scaffolding Biomaterials for 3D Cultivated Meat: Prospects and Challenges

Cultivating meat from stem cells rather than by raising animals is a promising solution to concerns about the negative externalities of meat production. For cultivated meat to fully mimic conventional meat's organoleptic and nutritional properties, innovations in scaffolding technology are required. Many scaffolding technologies are already developed for use in biomedical tissue engineering. However, cultivated meat production comes with a unique set of constraints related to the scale and cost of production as well as the necessary attributes of the final product, such as texture and food safety. This review discusses the properties of vertebrate skeletal muscle that will need to be replicated in a successful product and the current state of scaffolding innovation within the cultivated meat industry, highlighting promising scaffold materials and techniques that can be applied to cultivated meat development. Recommendations are provided for future research into scaffolds capable of supporting the growth of high-quality meat while minimizing production costs. Although the development of appropriate scaffolds for cultivated meat is challenging, it is also tractable and provides novel opportunities to customize meat properties.

Available In Vitro Models for Human Satellite Cells from Skeletal Muscle

Skeletal muscle accounts for almost 40% of the total adult human body mass. This tissue is essential for structural and mechanical functions such as posture, locomotion, and breathing, and it is endowed with an extraordinary ability to adapt to physiological changes associated with growth and physical exercise, as well as tissue damage. Moreover, skeletal muscle is the most age-sensitive tissue in mammals. Due to aging, but also to several diseases, muscle wasting occurs with a loss of muscle mass and functionality, resulting from disuse atrophy and defective muscle regeneration, associated with dysfunction of satellite cells, which are the cells responsible for maintaining and repairing adult muscle. The most established cell lines commonly used to study muscle homeostasis come from rodents, but there is a need to study skeletal muscle using human models, which, due to ethical implications, consist primarily of in vitro culture, which is the only alternative way to vertebrate model organisms. This review will survey in vitro 2D/3D models of human satellite cells to assess skeletal muscle biology for pre-clinical investigations and future directions.

Contractile Activity of Myotubes Derived from Human Induced Pluripotent Stem Cells: A Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a genetic disorder that results from deficiency of the dystrophin protein. In recent years, DMD pathological models have been created using induced pluripotent stem (iPS) cells derived from DMD patients. In addition, gene therapy using CRISPR-Cas9 technology to repair the dystrophin gene has been proposed as a new treatment method for DMD. However, it is not known whether the contractile function of myotubes derived from gene-repaired iPS cells can be restored. We therefore investigated the maturation of myotubes in electrical pulse stimulation culture and examined the effect of gene repair by observing the contractile behaviour of myotubes. The contraction activity of myotubes derived from dystrophin-gene repaired iPS cells was improved by electrical pulse stimulation culture. The iPS cell method used in this study for evaluating muscle contractile activity is a useful technique for analysing the mechanism of hereditary muscular disease pathogenesis and for evaluating the efficacy of new drugs and gene therapy.

3D anisotropic conductive fibers electrically stimulated myogenesis

Recapitulation of in vivo environments that drive muscle cells to organize into a physiologically relevant 3D architecture remains a major challenge for muscle tissue engineering. To recreate electrophysiology of muscle tissues, electroactive biomaterials have been used to stimulate muscle cells with exogenous electrical fields. In particular, the use of electroactive biomaterials with an anisotropic micro-/nanostructure that closely mimic the native skeletal-muscle extracellular matrix (ECM) is desirable for skeletal muscle tissue engineering. Herein, we present a hierarchically organized, anisotropic, and conductive Polycaprolactone/gold (PCL/Au) scaffold for guiding myoblasts alignment and promoting the elongation and maturation of myotubes under electrical stimulation. Culturing with H9c2 myoblasts cells indicated that the nanotopographic cues was crucial for nuclei alignment, while the presence of microscale grooves effectively enhanced both the formation and elongation of myotubes. The anisotropic structure also leads to anisotropic conductivity. Under electrical stimulation, the elongation and maturation of myotubes were significantly enhanced along the anisotropic scaffold. Specifically, compared to the unstimulated group (0 V), the myotube area percentage increased by 1.4, 1.9 and 2.4 times in the 1 V, 2 V, 3 V groups, respectively. In addition, the myotube average length in the 1 V group increased by 1.3 times compared to that of the unstimulated group, and significantly increased by 1.8 and 2.0 times in the 2 V, 3 V groups, respectively. Impressively, the longest myotubes reached more than 4 mm in both 2 V and 3 V groups. Overall, our conductive, anisotropic 3D nano/microfibrous scaffolds with the application of electrical stimulation provides a desirable platform for skeletal muscle tissue engineering.

A muscle fatigue-like contractile decline was recapitulated using skeletal myotubes from Duchenne muscular dystrophy patient-derived iPSCs

Duchenne muscular dystrophy (DMD) is a muscle degenerating disease caused by dystrophin deficiency, for which therapeutic options are limited. To facilitate drug development, it is desirable to develop in vitro disease models that enable the evaluation of DMD declines in contractile performance. Here, we show MYOD1-induced differentiation of hiPSCs into functional skeletal myotubes in vitro with collagen gel and electrical field stimulation (EFS). Long-term EFS training (0.5 Hz, 20 V, 2 ms, continuous for 2 weeks) mimicking muscle overuse recapitulates declines in contractile performance in dystrophic myotubes. A screening of clinically relevant drugs using this model detects three compounds that ameliorate this decline. Furthermore, we validate the feasibility of adapting the model to a 96-well culture system using optogenetic technology for large-scale screening. Our results support a disease model using patient-derived iPSCs that allows for the recapitulation of the contractile pathogenesis of DMD and a screening strategy for drug development.

Integrating biomaterials and food biopolymers for cultured meat production

Cultured meat has recently achieved mainstream prominence due to the emergence of societal and industrial interest. In contrast to animal-based production of traditional meat, the cultured meat approach entails laboratory cultivation of engineered muscle tissue. However, bioengineers have hitherto engineered tissues to fulfil biomedical endpoints, and have had limited experience in engineering muscle tissue for its post-mortem traits, which broadly govern consumer definitions of meat quality. Furthermore, existing tissue engineering approaches face fundamental challenges in technical feasibility and industrial scalability for cultured meat production. This review discusses how animal-based meat production variables influence meat properties at both the molecular and functional level, and whether current cultured meat approaches recapitulate these properties. In addition, this review considers how conventional meat producers employ exogenous biopolymer-based meat ingredients and processing techniques to mimic desirable meat properties in meat products. Finally, current biomaterial strategies for engineering muscle and adipose tissue are surveyed in the context of emerging constraints that pertain to cultured meat production, such as edibility, sustainability and scalability, and potential areas for integrating biomaterials and food biopolymer approaches to address these constraints are discussed. STATEMENT OF SIGNIFICANCE: Laboratory-grown or cultured meat has gained increasing interest from industry and the public, but currently faces significant impediment to market feasibility. This is due to fundamental knowledge gaps in producing realistic meat tissues via conventional tissue engineering approaches, as well as translational challenges in scaling up these approaches in an efficient, sustainable and high-volume manner. By defining the molecular basis for desirable meat quality attributes, such as taste and texture, and introducing the fundamental roles of food biopolymers in mimicking these properties in conventional meat products, this review aims to bridge the historically disparate fields of meat science and biomaterials engineering in order to inspire potentially synergistic strategies that address some of these challenges.

Formation of contractile 3D bovine muscle tissue for construction of millimetre-thick cultured steak

Owing to the increase in the global demand of meat, cultured meat technology is being developed to circumvent a shortage of meat in the future. However, methods for construction of millimetre-thick bovine muscle tissues with highly aligned myotubes have not yet been established. Here, we propose a culture method for constructing 3D-cultured bovine muscle tissue containing myotubes aligned along its long-axial direction, which contracted in response to electrical stimulation. First, we optimised the composition of biomaterials used in the construction and the electrical stimulation applied to the tissue during culture. Subsequently, we fabricated millimetre-thick bovine muscle tissues containing highly aligned myotubes by accumulating bovine myoblast-laden hydrogel modules. The microbial content of the bovine muscle tissue cultured for 14 days was below the detection limit, indicating that the muscle tissues were sterile, unlike commercial meat. Therefore, the proposed construction method for bovine muscle tissues will be useful for the production of clean cultured steak meat simulating real meat.

Muscle stem cell isolation and in vitro culture for meat production: A methodological review

Cultured muscle tissue-based protein products, also known as cultured meat, are produced through in vitro myogenesis involving muscle stem cell culture and differentiation, and mature muscle cell processing for flavor and texture. This review focuses on the in vitro myogenesis for cultured meat production. The muscle stem cell-based in vitro muscle tissue production consists of a sequential process: (1) muscle sampling for stem cell collection, (2) muscle tissue dissociation and muscle stem cell isolation, (3) primary cell culture, (4) upscaled cell culture, (5) muscle differentiation and maturation, and (6) muscle tissue harvest. Although muscle stem cell research is a well-established field, the majority of these steps remain to be underoptimized to enable the in vitro creation of edible muscle-derived meat products. The profound understanding of the process would help not only cultured meat production but also business sectors that have been seeking new biomaterials for the food industry. In this review, we discuss comprehensively and in detail each step of cutting-edge methods for cultured meat production. This would be meaningful for both academia and industry to prepare for the new era of cellular agriculture.

Bioactive polymeric materials and electrical stimulation strategies for musculoskeletal tissue repair and regeneration

Electrical stimulation (ES) is predominantly used as a physical therapy modality to promote tissue healing and functional recovery. Research efforts in both laboratory and clinical settings have shown the beneficial effects of this technique for the repair and regeneration of damaged tissues, which include muscle, bone, skin, nerve, tendons, and ligaments. The collective findings of these studies suggest ES enhances cell proliferation, extracellular matrix (ECM) production, secretion of several cytokines, and vasculature development leading to better tissue regeneration in multiple tissues. However, there is still a gap in the clinical relevance for ES to better repair tissue interfaces, as ES applied clinically is ineffective on deeper tissue. The use of a conducting material can transmit the stimulation applied from skin electrodes to the desired tissue and lead to an increased function on the repair of that tissue. Ionically conductive (IC) polymeric scaffolds in conjunction with ES may provide solutions to utilize this approach effectively. Injectable IC formulations and their scaffolds may provide solutions for applying ES into difficult to reach tissue types to enable tissue repair and regeneration. A better understanding of ES-mediated cell differentiation and associated molecular mechanisms including the immune response will allow standardization of procedures applicable for the next generation of regenerative medicine. ES, along with the use of IC scaffolds is more than sufficient for use as a treatment option for single tissue healing and may fulfill a role in interfacing multiple tissue types during the repair process.

New volumetric CNT-doped gelatin-cellulose scaffolds for skeletal muscle tissue engineering

Currently, the fabrication of scaffolds for engineered skeletal muscle tissues is unable to reach the millimeter size. The main drawbacks are the poor nutrient diffusion, lack of an internal structure to align the precursor cells, and poor mechanical and electric properties. Herein, we present a combination of gelatin-carboxymethyl cellulose materials polymerised by a cryogelation process that allowed us to reach scaffold fabrication up to millimeter size and solve the main problems related to the large size muscle tissue constructs. (1) By incorporating carbon nanotubes (CNT), we can improve the electrical properties of the scaffold, thereby enhancing tissue maturation when applying an electric pulse stimulus (EPS). (2) We have fabricated an anisotropic internal three-dimensional microarchitecture with good pore distribution and highly aligned morphology to enhance the cell alignment, cell fusion and myotube formation. With this set up, we were able to generate a fully functional skeletal muscle tissue using a combination of EPS and our doped-biocomposite scaffold and obtain a mature tissue on the millimeter scale. We also characterized the pore distribution, swelling, stiffness and conductivity of the scaffold. Moreover, we proved that the cells were viable and could fuse in three-dimensional (3D) functional myotubes throughout the scaffold. In conclusion, we fabricated a biocompatible and customizable scaffold for 3D cell culture suitable for a wide range of applications such as organ-on-a-chip, drug screening, transplantation and disease modelling.

Hydrogel-Colloid Composite Bioinks for Targeted Tissue-Printing

The development of extrusion-based bioprinting for tissue engineering is conditioned by the design of bioinks displaying adequate printability, shape stability, and postprinting bioactivity. In this context, simple bioink formulations, made of cells supported by a polymer matrix, often lack the necessary versatility. To address this issue, intense research work has been focused on introducing colloidal particles into the ink formulation. By creating weak cross-links between polymer chains, added particles modify the rheology and mechanical behavior of bioinks to improve their printability and structural integrity. Additionally, nano- and microscopic particles display composition- and structure-specific properties that can affect the cellular behavior and enhance the formation of tissue within the printed material. This Review offers a comprehensive picture of the role of colloids in bioprinting from a physicochemical and biological perspective. As such, it provides guidance on devising adaptable bioinks for the fabrication of biomimetic tissues.

Microphysiological Systems: Design, Fabrication, and Applications

Microphysiological systems, including organoids, 3-D printed tissue constructs and organ-on-a-chips (organ chips), are physiologically relevant in vitro models and have experienced explosive growth in the past decades. Different from conventional, tissue culture plastic-based in vitro models or animal models, microphysiological systems recapitulate key microenvironmental characteristics of human organs and mimic their primary functions. The advent of microphysiological systems is attributed to evolving biomaterials, micro-/nanotechnologies and stem cell biology, which enable the precise control over the matrix properties and the interactions between cells, tissues and organs in physiological conditions. As such, microphysiological systems have been developed to model a broad spectrum of organs from microvasculature, eye, to lung and many others to understand human organ development and disease pathology and facilitate drug discovery. Multiorgans-on-a-chip systems have also been developed by integrating multiple associated organ chips in a single platform, which allows to study and employ the organ function in a systematic approach. Here we first discuss the design principles of microphysiological systems with a focus on the anatomy and physiology of organs, and then review the commonly used fabrication techniques and biomaterials for microphysiological systems. Subsequently, we discuss the recent development of microphysiological systems, and provide our perspectives on advancing microphysiological systems for preclinical investigation and drug discovery of human disease.

Effects of Graphene Oxide Nanofilm and Chicken Embryo Muscle Extract on Muscle Progenitor Cell Differentiation and Contraction

Finding an effective muscle regeneration technique is a priority for regenerative medicine. It is known that the key factors determining tissue formation include cells, capable of proliferating and/or differentiating, a niche (surface) allowing their colonization and growth factors. The interaction between these factors, especially between the surface of the artificial niche and growth factors, is not entirely clear. Moreover, it seems that the use of a complex of complementary growth factors instead of a few strictly defined ones could increase the effectiveness of tissue maturation, including muscle tissue. In this study, we evaluated whether graphene oxide (GO) nanofilm, chicken embryo muscle extract (CEME), and GO combined with CEME would affect the differentiation and functional maturation of muscle precursor cells, as well as the ability to spontaneously contract a pseudo-tissue muscle. CEME was extracted on day 18 of embryogenesis. Muscle cells obtained from an 8-day-old chicken embryo limb bud were treated with GO and CEME. Cell morphology and differentiation were observed using different microscopy methods. Cytotoxicity and viability of cells were measured by lactate dehydrogenase and Vybrant Cell Proliferation assays. Gene expression of myogenic regulatory genes was measured by Real-Time PCR. Our results demonstrate that CEME, independent of the culture surface, was the main factor influencing the intense differentiation of muscle progenitor cells. The present results, for the first time, clearly demonstrated that the cultured tissue-like structure was capable of inducing contractions without externally applied impulses. It has been indicated that a small amount of CEME in media (about 1%) allows the culture of pseudo-tissue muscle capable of spontaneous contraction. The study showed that the graphene oxide may be used as a niche for differentiating muscle cells, but the decisive influence on the maturation of muscle tissue, especially muscle contractions, depends on the complexity of the applied growth factors.

Microcarriers for Upscaling Cultured Meat Production

Due to the considerable environmental impact and the controversial animal welfare associated with industrial meat production, combined with the ever-increasing global population and demand for meat products, sustainable production alternatives are indispensable. In 2013, the world's first laboratory grown hamburger made from cultured muscle cells was developed. However, coming at a price of $300.000, and being produced manually, substantial effort is still required to reach sustainable large-scale production. One of the main challenges is scalability. Microcarriers (MCs), offering a large surface/volume ratio, are the most promising candidates for upscaling muscle cell culture. However, although many MCs have been developed for cell lines and stem cells typically used in the medical field, none have been specifically developed for muscle stem cells and meat production. This paper aims to discuss the MCs' design criteria for skeletal muscle cell proliferation and subsequently for meat production based on three scenarios: (1) MCs are serving only as a temporary substrate for cell attachment and proliferation and therefore they need to be separated from the cells at some stage of the bioprocess, (2) MCs serve as a temporary substrate for cell proliferation but are degraded or dissolved during the bioprocess, and (3) MCs are embedded in the final product and therefore need to be edible. The particularities of each of these three bioprocesses will be discussed from the perspective of MCs as well as the feasibility of a one-step bioprocess. Each scenario presents advantages and drawbacks, which are discussed in detail, nevertheless the third scenario appears to be the most promising one for a production process. Indeed, using an edible material can limit or completely eliminate dissociation/degradation/separation steps and even promote organoleptic qualities when embedded in the final product. Edible microcarriers could also be used as a temporary substrate similarly to scenarios 1 and 2, which would limit the risk of non-edible residues.

Tissue Engineering for Musculoskeletal Regeneration and Disease Modeling

Musculoskeletal injuries and associated conditions are the leading cause of physical disability worldwide. The concept of tissue engineering has opened up novel approaches to repair musculoskeletal defects in a fast and/or efficient manner. Biomaterials, cells, and signaling molecules constitute the tissue engineering triad. In the past 40 years, significant progress has been made in developing and optimizing all three components, but only a very limited number of technologies have been successfully translated into clinical applications. A major limiting factor of this barrier to translation is the insufficiency of two-dimensional cell cultures and traditional animal models in informing the safety and efficacy of in-human applications. In recent years, microphysiological systems, often referred to as organ or tissue chips, generated according to tissue engineering principles, have been proposed as the next-generation drug testing models. This chapter aims to first review the current tissue engineering-based approaches that are being applied to fabricate and develop the individual critical elements involved in musculoskeletal organ/tissue chips. We next highlight the general strategy of generating musculoskeletal tissue chips and their potential in future regenerative medicine research. Exemplary microphysiological systems mimicking musculoskeletal tissues are described. With sufficient physiological accuracy and relevance, the human cell-derived, three-dimensional, multi-tissue systems have been used to model a number of orthopedic disorders and to test new treatments. We anticipate that the novel emerging tissue chip technology will continually reshape and improve our understanding of human musculoskeletal pathophysiology, ultimately accelerating the development of advanced pharmaceutics and regenerative therapies.

A Novel Conductive and Micropatterned PEG-Based Hydrogel Enabling the Topographical and Electrical Stimulation of Myoblasts

In this study, we designed a cell-adhesive poly(ethylene glycol) (PEG)-based hydrogel that simultaneously provides topographical and electrical stimuli to C2C12 myoblasts. Specifically, PEG hydrogels with microgroove structures of 3 μm ridges and 3 μm grooves were prepared by micromolding; in situ polymerization of poly(3,4-ethylenedioxythiophene) (PEDOT) was then performed within the micropatterned PEG hydrogels to create a microgrooved conductive hydrogel (CH/P). The CH/P had clear replica patterns of the silicone mold and a conductivity of 2.49 × 10^-3 S/cm, with greater than 85% water content. In addition, the CH exhibited Young's modulus (45.84 ± 7.12 kPa) similar to that of a muscle tissue. The surface of the CH/P was further modified via covalent bonding with cell-adhesive peptides to facilitate cell adhesion without affecting conductivity. An in vitro cell assay revealed that the CH/P was cytocompatible and enhanced the cell alignment and elongation of C2C12 myoblasts. The microgrooves and conductivity of the CH/P had the greatest positive effect on the myogenesis of C2C12 myoblasts compared to the other PEG hydrogel samples without conductivity or/and microgrooves, even in the absence of electrical stimulation. Electrical stimulation studies indicated that the combination of topographical and electrical cues maximized the differentiation of C2C12 myoblasts into myotubes, confirming the synergetic effect of incorporating microgroove surface features and a conductive PEDOT component into hydrogels.

Vascularization of tissue-engineered skeletal muscle constructs

Skeletal muscle tissue can be created in vitro by tissue engineering approaches, based on differentiation of muscle stem cells. Several approaches exist and generally result in three dimensional constructs composed of multinucleated myofibers to which we refer as myooids. Engineering methods date back to 3 decades ago and meanwhile a wide range of cell types and scaffold types have been evaluated. Nevertheless, in most approaches, myooids remain very small to allow for diffusion-mediated nutrient supply and waste product removal, typically less than 1 mm thick. One of the shortcomings of current in vitro skeletal muscle organoid development is the lack of a functional vascular structure, thus limiting the size of myooids. This is a challenge which is nowadays applicable to almost all organoid systems. Several approaches to obtain a vascular structure within myooids have been proposed. The purpose of this review is to give a concise overview of these approaches.

Effects of vitamin D on primary human skeletal muscle cell proliferation, differentiation, protein synthesis and bioenergetics

The active form of Vitamin D (1,25(OH)2D), has been suggested to have a regulatory role in skeletal muscle function and metabolism, however, the effects and mechanisms of vitamin D (VitD) action in this tissue remain to be fully established. In this study, we have used primary human skeletal muscle myoblast (HSMM) cells that display typical characteristics of human skeletal muscle function and protein levels, to investigate the effects of the active form of VitD on proliferation, differentiation, protein synthesis and bioenergetics. Myoblast cells were treated with 100 nM of VitD for 24 h, 48 h, 72 h and five days (cells were differentiated into myotubes) and then analyses were performed. We report that VitD inhibits myoblast proliferation and enhances differentiation by altering the expression of myogenic regulatory factors. In addition, we found that protein synthesis signaling improved in myotubes after VitD treatment in the presence of insulin. We also report an increase in oxygen consumption rate after 24 h of treatment in myoblasts and after 5 days of treatment in myotubes after VitD exposure. VitD significantly impacted HSMM myogenesis, as well as protein synthesis in the presence of insulin.

Bioprinting of a Cell-Laden Conductive Hydrogel Composite

Bioprinting has gained significant attention for creating biomimetic tissue constructs with potential to be used in biomedical applications such as drug screening or regenerative medicine. Ideally, biomaterials used for three-dimensional (3D) bioprinting should match the mechanical, hydrostatic, bioelectric, and physicochemical properties of the native tissues. However, many materials with these tissue-like properties are not compatible with printing techniques without modifying their compositions. In addition, integration of cell-laden biomaterials with bioprinting methodologies that preserve their physicochemical properties remains a challenge. In this work, a biocompatible conductive hydrogel composed of gelatin methacryloyl (GelMA) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was synthesized and bioprinted to form complex, 3D cell-laden structures. The biofabricated conductive hydrogels were formed by an initial cross-linking step of the PEDOT:PSS with bivalent calcium ions and a secondary photopolymerization step with visible light to cross-link the GelMA component. These modifications enabled tuning the mechanical properties of the hydrogels, with Young's moduli ranging from ∼40-150 kPa, as well as tunable conductivity by varying the concentration of PEDOT:PSS. In addition, the hydrogels degraded in vivo with no substantial inflammatory responses as demonstrated by haematoxylin and eosin (H&E) and immunofluorescent staining of subcutaneously implanted samples in Wistar rats. The parameters for forming a slurry of microgel particles to support 3D bioprinting of the engineered cell-laden hydrogel were optimized to form constructs with improved resolution. High cytocompatibility and cell spreading were demonstrated in both wet-spinning and 3D bioprinting of cell-laden hydrogels with the new conductive hydrogel-based bioink and printing methodology. The synergy of an advanced fabrication method and conductive hydrogel presented here is promising for engineering complex conductive and cell-laden structures.

Electrical stimulation promotes the angiogenic potential of adipose-derived stem cells

Autologous fat transfer (AFT) is limited by post-operative volume loss due to ischemia-induced cell death in the fat graft. Previous studies have demonstrated that electrical stimulation (ES) promotes angiogenesis in a variety of tissues and cell types. In this study we investigated the effects of ES on the angiogenic potential of adipose-derived stem cells (ASC), important progenitor cells in fat grafts with proven angiogenic potential. Cultured human ASC were electrically stimulated for 72 hours after which the medium of stimulated (ES) and non-stimulated (control) ASC was analysed for angiogenesis-related proteins by protein array and ELISA. The functional effect of ES on angiogenesis was then assessed in vitro and in vivo. Nine angiogenesis-related proteins were detected in the medium of electrically (non-)stimulated ASC and were quantified by ELISA. The pro-angiogenic proteins VEGF and MCP-1 were significantly increased following ES compared to controls, while the anti-angiogenic factor Serpin E1/PAI-1 was significantly decreased. Despite increased levels of anti-angiogenic TSP-1 and TIMP-1, medium of ES-treated ASC significantly increased vessel density, total vessel network length and branching points in chorio-allantoic membrane assays. In conclusion, our proof-of-concept study showed that ES increased the angiogenic potential of ASC both in vitro and in vivo.

Functional regeneration of tissue engineered skeletal muscle in vitro is dependent on the inclusion of basement membrane proteins

Skeletal muscle has a high regenerative capacity, injuries trigger a regenerative program which restores tissue function to a level indistinguishable to the pre-injury state. However, in some cases where significant trauma occurs, such as injuries seen in military populations, the regenerative process is overwhelmed and cannot restore full function. Limited clinical interventions exist which can be used to promote regeneration and prevent the formation of non-regenerative defects following severe skeletal muscle trauma. Robust and reproducible techniques for modelling complex tissue responses are essential to promote the discovery of effective clinical interventions. Tissue engineering has been highlighted as an alternative method, allowing the generation of three-dimensional in vivo like tissues without laboratory animals. Reducing the requirement for animal models promotes rapid screening of potential clinical interventions, as these models are more easily manipulated, genetically and pharmacologically, and reduce the associated cost and complexity, whilst increasing access to models for laboratories without animal facilities. In this study, an in vitro chemical injury using barium chloride is validated using the C2C12 myoblast cell line, and is shown to selectively remove multinucleated myotubes, whilst retaining a regenerative mononuclear cell population. Monolayer cultures showed limited regenerative capacity, with basement membrane supplementation or extended regenerative time incapable of improving the regenerative response. Conversely tissue engineered skeletal muscles, supplemented with basement membrane proteins, showed full functional regeneration, and a broader in vivo like inflammatory response. This work outlines a freely available and open access methodology to produce a cell line-based tissue engineered model of skeletal muscle regeneration.

Making Sense of Making Meat: Key Moments in the First 20 Years of Tissue Engineering Muscle to Make Food

Cultured/clean/cell-based meat (CM) now has a near two decade history of laboratory research, commencing with the early NASA-funded work at Touro College and the bioarts practice of the Tissue Culture and Art project. Across this period the field, or as it is now more commonly termed, the "space," has developed significantly while promoting different visions for what CM is and can do, and the best mechanisms for delivery. Here we both analyse and critically engage with this near-twenty year period as a productive provocation to those engaged with CM, or considering becoming so. This paper is not a history of the field, and does not offer a comprehensive timeline. Instead it identifies significant activities, transitions, and moments in which key meanings and practices have taken form or exerted influence. We do this through analyzing two related themes: the CM "institutional context" and the CM "interpretative package." The former, the institutional context, refers to events and infrastructures that have come into being to support and shape the CM field, including university activities, conferences, third sector groups, various potential funding mechanisms, and the establishment of a start-up sector. The latter, the interpretative package, refers to the constellation of factors that shape or assert how CM should be understood, including the various names used to describe it, accounts of what it will achieve, and most recently, the emergent regulatory discussions that frame its legal standing. Across the paper we argue it is productive to think of the CM community in terms of a first and second wave. The first wave was more university-based and broadly covers the period from the millennium until around the 2013 cultured burger event. The second wave saw the increasing prevalence of a start-up culture and the circuits of venture capital interest that support it. Through this analysis we seek to provoke further reflection upon how the CM community has come to be as it is, and how this could develop in the future.

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.

Skeletal Muscle Regenerative Engineering

Skeletal muscles have the intrinsic ability to regenerate after minor injury, but under certain circumstances such as severe trauma from accidents, chronic diseases or battlefield injuries the regeneration process is limited. Skeletal muscle regenerative engineering has emerged as a promising approach to address this clinical issue. The regenerative engineering approach involves the convergence of advanced materials science, stem cell science, physical forces, insights from developmental biology, and clinical translation. This article reviews recent studies showing the potential of the convergences of technologies involving biomaterials, stem cells and bioactive factors in concert with clinical translation, in promoting skeletal muscle regeneration. Several types of biomaterials such as electrospun nanofibers, hydrogels, patterned scaffolds, decellularized tissues, and conductive matrices are being investigated. Detailed discussions are given on how these biomaterials can interact with cells and modulate their behavior through physical, chemical and mechanical cues. In addition, the application of physical forces such as mechanical and electrical stimulation are reviewed as strategies that can further enhance muscle contractility and functionality. The review also discusses established animal models to evaluate regeneration in two clinically relevant muscle injuries; volumetric muscle loss (VML) and muscle atrophy upon rotator cuff injury. Regenerative engineering approaches using advanced biomaterials, cells, and physical forces, developmental cues along with insights from immunology, genetics and other aspects of clinical translation hold significant potential to develop promising strategies to support skeletal muscle regeneration.

Scalable 3D Printed Molds for Human Tissue Engineered Skeletal Muscle

Tissue engineered skeletal muscle allows investigation of the cellular and molecular mechanisms that regulate skeletal muscle pathology. The fabricated model must resemble characteristics of in vivo tissue and incorporate cost-effective and high content primary human tissue. Current models are limited by low throughput due to the complexities associated with recruiting tissue donors, donor specific variations, as well as cellular senescence associated with passaging. This research presents a method using fused deposition modeling (FDM) and laser sintering (LS) 3D printing to generate reproducible and scalable tissue engineered primary human muscle, possessing aligned mature myotubes reminiscent of in vivo tissue. Many existing models are bespoke causing variability when translated between laboratories. To this end, a scalable model has been developed (25–500 μL construct volumes) allowing fabrication of mature primary human skeletal muscle. This research provides a strategy to overcome limited biopsy cell numbers, enabling high throughput screening of functional human tissue.

Efficacy of Biophysical Energies on Healing of Diabetic Skin Wounds in Cell Studies and Animal Experimental Models: A Systematic Review

We have systematically assessed published cell studies and animal experimental reports on the efficacy of selected biophysical energies (BPEs) in the treatment of diabetic foot ulcers. These BPEs include electrical stimulation (ES), pulsed electromagnetic field (PEMF), extracorporeal shockwave (ECSW), photo energies and ultrasound (US). Databases searched included CINAHL, MEDLINE and PubMed from 1966 to 2018. Studies reviewed include animal and cell studies on treatment with BPEs compared with sham, control or other BPEs. Information regarding the objective measures of tissue healing and data was extracted. Eighty-two studies were eventually selected for the critical appraisal: five on PEMF, four each on ES and ECSW, sixty-six for photo energies, and three about US. Based on the percentage of original wound size affected by the BPEs, both PEMF and low-level laser therapy (LLL) demonstrated a significant clinical benefit compared to the control or sham treatment, whereas the effect of US did not reveal a significance. Our results indicate potential benefits of selected BPEs in diabetic wound management. However, due to the heterogeneity of the current clinical trials, comprehensive studies using well-designed trials are warranted to confirm the results.