Rapid formation of functional muscle in vitro using fibrin gels

Degradable conductive self-healing hydrogels based on dextran-graft-tetraaniline and N-carboxyethyl chitosan as injectable carriers for myoblast cell therapy and muscle regeneration

Injectable conductive hydrogels have great potential as tissue engineering scaffolds and delivery vehicles for electrical signal sensitive cell therapy. In this work, we present the synthesis of a series of injectable electroactive degradable hydrogels with rapid self-healing ability and their potential application as cell delivery vehicles for skeletal muscle regeneration. Self-healable conductive injectable hydrogels based on dextran-graft-aniline tetramer-graft-4-formylbenzoic acid and N-carboxyethyl chitosan were synthesized at physiological conditions. The dynamic Schiff base bonds between the formylbenzoic acid and amine group from N-carboxyethyl chitosan endowed the hydrogels with rapid self-healing ability, which was verified by rheological test. Equilibrated swelling ratio, morphology, mechanical strength, electrochemistry and conductivity of the injectable hydrogels were fully investigated. The self-healable conductive hydrogels showed an in vivo injectability and a linear-like degradation behavior. Two different kinds of cells (C2C12 myoblasts and human umbilical vein endothelial cells (HUVEC)) were encapsulated in the hydrogels by self-healing effect. The L929 fibroblast cell culture results indicated the biocompatibility of the hydrogels. Moreover, the C2C12 myoblast cells were released from the conductive hydrogels with a linear-like profile. The in vivo skeletal muscle regeneration was also studied in a volumetric muscle loss injury model. All these data indicated that these biodegradable self-healing conductive hydrogels are potential candidates as cell delivery vehicles and scaffolds for skeletal muscle repair. STATEMENT OF SIGNIFICANCE: Injectable hydrogels with self-healing and electrical conductivity properties are excellent candidates as tissue-engineered scaffolds for myoblast cell therapy and skeletal muscle regeneration. The self-healing property of these hydrogels can prolong their lifespan. However, most of the reported conductive hydrogels are not degradable or do not have the self-healing ability. Herein, we synthesized antibacterial conductive self-healing hydrogels as a cell delivery carrier for cardiac cell therapy based on chitosan-grafted-tetraaniline hydrogels synthesized in our previous work. However, an acid solution was used to dissolve the polymers in that study, which may induce toxicity to cells. In this work, we synthesized a series of injectable electroactive biodegradable hydrogels with rapid self-healing ability composed of N-carboxyethyl chitosan (CECS) and dextran-graft-aniline oligomers, and these hydrogel precusor can dissolve in PBS solution of pH 7.4; we further demonstrated their potential application as cell delivery vehicles for skeletal muscle regeneration.

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.

A Novel Microplate 3D Bioprinting Platform for the Engineering of Muscle and Tendon Tissues

Two-dimensional (2D) cell cultures do not reflect the in vivo situation, and thus it is important to develop predictive three-dimensional (3D) in vitro models with enhanced reliability and robustness for drug screening applications. Treatments against muscle-related diseases are becoming more prominent due to the growth of the aging population worldwide. In this study, we describe a novel drug screening platform with automated production of 3D musculoskeletal-tendon-like tissues. With 3D bioprinting, alternating layers of photo-polymerized gelatin-methacryloyl-based bioink and cell suspension tissue models were produced in a dumbbell shape onto novel postholder cell culture inserts in 24-well plates. Monocultures of human primary skeletal muscle cells and rat tenocytes were printed around and between the posts. The cells showed high viability in culture and good tissue differentiation, based on marker gene and protein expressions. Different printing patterns of bioink and cells were explored and calcium signaling with Fluo4-loaded cells while electrically stimulated was shown. Finally, controlled co-printing of tenocytes and myoblasts around and between the posts, respectively, was demonstrated followed by co-culture and co-differentiation. This screening platform combining 3D bioprinting with a novel microplate represents a promising tool to address musculoskeletal diseases.

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.

Engineering of Human Skeletal Muscle With an Autologous Deposited Extracellular Matrix

Adult skeletal muscle progenitor cells can be embedded in an extracellular matrix (ECM) and tissue-engineered to form bio-artificial muscles (BAMs), composed of aligned post-mitotic myofibers. The ECM proteins which have been used most commonly are collagen type I and fibrin. Fibrin allows for in vitro vasculogenesis, however, high concentrations of fibrinolysis inhibitors are needed to inhibit degradation of the ECM and subsequent loss of BAM tissue structure. For in vivo implantation, fibrinolysis inhibition may prove difficult or even harmful to the host. Therefore, we adapted in vitro culture conditions to enhance the deposition of de novo synthesized collagen type I gradually replacing the degrading fibrin ECM. The in vitro viscoelastic properties of the fibrin BAMs and deposition of collagen were characterized. BAMs engineered with the addition of proline, hydroxyproline, and ascorbic acid in the tissue culture medium had a twofold increase in Young’s Modulus, a 2.5-fold decrease in maximum strain, and a 1.6-fold increase in collagen deposition. Lowering the fibrin content of the BAMs also increased Young’s Modulus, decreased maximum strain, and increased collagen deposition. Tissue engineering of BAMs with autologous ECM may allow for prolonged in vivo survival.

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.

An Assessment of Myotube Morphology, Matrix Deformation, and Myogenic mRNA Expression in Custom-Built and Commercially Available Engineered Muscle Chamber Configurations

There are several three-dimensional (3D) skeletal muscle (SkM) tissue engineered models reported in the literature. 3D SkM tissue engineering (TE) aims to recapitulate the structure and function of native (in vivo) tissue, within an in vitro environment. This requires the differentiation of myoblasts into aligned multinucleated myotubes surrounded by a biologically representative extracellular matrix (ECM). In the present work, a new commercially available 3D SkM TE culture chamber manufactured from polyether ether ketone (PEEK) that facilitates suitable development of these myotubes is presented. To assess the outcomes of the myotubes within these constructs, morphological, gene expression, and ECM remodeling parameters were compared against a previously published custom-built model. No significant differences were observed in the morphological and gene expression measures between the newly introduced and the established construct configuration, suggesting biological reproducibility irrespective of manufacturing process. However, TE SkM fabricated using the commercially available PEEK chambers displayed reduced variability in both construct attachment and matrix deformation, likely due to increased reproducibility within the manufacturing process. The mechanical differences between systems may also have contributed to such differences, however, investigation of these variables was beyond the scope of the investigation. Though more expensive than the custom-built models, these PEEK chambers are also suitable for multiple use after autoclaving. As such this would support its use over the previously published handmade culture chamber system, particularly when seeking to develop higher-throughput systems or when experimental cost is not a factor.

Stem Cells for Skeletal Muscle Tissue Engineering

Volumetric muscle loss (VML) is a debilitating condition wherein muscle loss overwhelms the body's normal physiological repair mechanism. VML is particularly common among military service members who have sustained war injuries. Because of the high social and medical cost associated with VML and suboptimal current surgical treatments, there is great interest in developing better VML therapies. Skeletal muscle tissue engineering (SMTE) is a promising alternative to traditional VML surgical treatments that use autogenic tissue grafts, and rather uses isolated stem cells with myogenic potential to generate de novo skeletal muscle tissues to treat VML. Satellite cells are the native precursors to skeletal muscle tissue, and are thus the most commonly studied starting source for SMTE. However, satellite cells are difficult to isolate and purify, and it is presently unknown whether they would be a practical source in clinical SMTE applications. Alternative myogenic stem cells, including adipose-derived stem cells, bone marrow-derived mesenchymal stem cells, perivascular stem cells, umbilical cord mesenchymal stem cells, induced pluripotent stem cells, and embryonic stem cells, each have myogenic potential and have been identified as possible starting sources for SMTE, although they have yet to be studied in detail for this purpose. These alternative stem cell varieties offer unique advantages and disadvantages that are worth exploring further to advance the SMTE field toward highly functional, safe, and practical VML treatments. The following review summarizes the current state of satellite cell-based SMTE, details the properties and practical advantages of alternative myogenic stem cells, and offers guidance to tissue engineers on how alternative myogenic stem cells can be incorporated into SMTE research.

Hydrogel biomaterials and their therapeutic potential for muscle injuries and muscular dystrophies

Muscular diseases such as muscular dystrophies and muscle injuries constitute a large group of ailments that manifest as muscle weakness, atrophy or fibrosis. Although cell therapy is a promising treatment option, the delivery and retention of cells in the muscle is difficult and prevents sustained regeneration needed for adequate functional improvements. Various types of biomaterials with different physical and chemical properties have been developed to improve the delivery of cells and/or growth factors for treating muscle injuries. Hydrogels are a family of materials with distinct advantages for use as cell delivery systems in muscle injuries and ailments, including their mild processing conditions, their similarities to natural tissue extracellular matrix, and their ability to be delivered with less invasive approaches. Moreover, hydrogels can be made to completely degrade in the body, leaving behind their biological payload in a process that can enhance the therapeutic process. For these reasons, hydrogels have shown great potential as cell delivery matrices. This paper reviews a few of the hydrogel systems currently being applied together with cell therapy and/or growth factor delivery to promote the therapeutic repair of muscle injuries and muscle wasting diseases such as muscular dystrophies.

Organ-On-A-Chip Platforms: A Convergence of Advanced Materials, Cells, and Microscale Technologies

Significant advances in biomaterials, stem cell biology, and microscale technologies have enabled the fabrication of biologically relevant tissues and organs. Such tissues and organs, referred to as organ-on-a-chip (OOC) platforms, have emerged as a powerful tool in tissue analysis and disease modeling for biological and pharmacological applications. A variety of biomaterials are used in tissue fabrication providing multiple biological, structural, and mechanical cues in the regulation of cell behavior and tissue morphogenesis. Cells derived from humans enable the fabrication of personalized OOC platforms. Microscale technologies are specifically helpful in providing physiological microenvironments for tissues and organs. In this review, biomaterials, cells, and microscale technologies are described as essential components to construct OOC platforms. The latest developments in OOC platforms (e.g., liver, skeletal muscle, cardiac, cancer, lung, skin, bone, and brain) are then discussed as functional tools in simulating human physiology and metabolism. Future perspectives and major challenges in the development of OOC platforms toward accelerating clinical studies of drug discovery are finally highlighted.

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

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

Three-dimensional tissue-engineered skeletal muscle for laryngeal reconstruction

OBJECTIVE

There is an unmet need for tissue-engineered three-dimensional (3D) muscle constructs for laryngeal reconstruction. Functional engineered muscle could be used to repair postoncologic or traumatic defects or to medialize the vocal fold in cases of paresis/paralysis. Autologous, organized, engineered muscle that has adequate bulk integrates into host tissue and restores function currently does not exist.

METHODS

Primary skeletal muscle progenitor cells (MPCs) were isolated from F344 rats. Three-dimensional muscle constructs were created by encapsulating MPCs via flow alignment in a customized collagen formulation and cultured under passive tension. Muscle-specific immunohistochemistry and confocal microscopy were used to evaluate muscle tissue differentiation. After 2 weeks of culture, muscle constructs were implanted into surgically created defects in the rat larynx. Postmortem function testing and histology was performed at 1 and 3 months.

RESULTS

Immunohistochemistry with confocal microscopy demonstrated well-differentiated myotubes, which were well aligned and distributed throughout the engineered construct in vitro. There was evidence of restoration of normal laryngeal function at 1 month postoperative, as indicated by safe swallow (no aspiration events), weight gain, and excellent animal survival. Postmortem specimens demonstrated functional muscle contraction on ex vivo testing, and histology confirmed integration into host tissue.

CONCLUSION

This is the first study to demonstrate that functional, 3D tissue-engineered skeletal muscle can be developed from primary MPCs and standardized oligomeric collagen. Collectively, these findings may have tremendous clinical implications for autologous laryngeal muscle repair and reconstruction.

LEVEL OF EVIDENCE

NA. Laryngoscope, 128:603-609, 2018.

3D porous polyurethanes featured by different mechanical properties: Characterization and interaction with skeletal muscle cells

The fabrication of biomaterials for interaction with muscle cells has attracted significant interest in the last decades. However, 3D porous scaffolds featured by a relatively low stiffness (almost matching the natural muscle one) and highly stable in response to cyclic loadings are not available at present, in this context. This work describes 3D polyurethane-based porous scaffolds featured by different mechanical properties. Biomaterial stiffness was finely tuned by varying the cross-linking degree of the starting foam. Compression tests revealed, for the softest material formulation, stiffness values close to the ones possessed by natural skeletal muscles. The materials were also characterized in terms of local nanoindenting, rheometric properties and long-term stability through cyclic compressions, in a strain range reflecting the contraction extent of natural muscles. Preliminary in vitro tests revealed a preferential adhesion of C2C12 skeletal muscle cells over the softer, rougher and more porous structures. All the material formulations showed low cytotoxicity.

Laminin-111 enriched fibrin hydrogels for skeletal muscle regeneration

Laminin (LM)-111 supplementation has improved muscle regeneration in several models of disease and injury. This study investigated a novel hydrogel composed of fibrinogen and LM-111. Increasing LM-111 concentration (50-450 μg/mL) in fibrin hydrogels resulted in highly fibrous scaffolds with progressively thinner interlaced fibers. Rheological testing showed that all hydrogels had viscoelastic behavior and the Young's modulus ranged from 2-6KPa. C₂C₁₂ myobalsts showed a significant increase in VEGF production and decrease in IL-6 production on LM-111 enriched fibrin hydrogels as compared to pure fibrin hydrogels on day 4. Western blotting results showed a significant increase in MyoD and desmin protein quantity but a significant decrease in myogenin protein quantity in myoblasts cultured on the LM-111 (450 μg/mL) enriched fibrin hydrogel. Combined application of electromechanical stimulation significantly enhanced the production of VEGF and IGF-1 from myoblast seeded fibrin-LM-111 hydrogels. Taken together, these observations offer an important first step toward optimizing a tissue engineered constructs for skeletal muscle regeneration.

Injectable scaffold materials differ in their cell instructive effects on primary human myoblasts

Scaffolds are materials used for delivery of cells for regeneration of tissues. They support three-dimensional organization and improve cell survival. For the repair of small skeletal muscles, injections of small volumes of cells are attractive, and injectable scaffolds for delivery of cells offer a minimally invasive technique. In this study, we examined in vitro the cell instructive effects of three types of injectable scaffolds, fibrin, alginate, and poly(lactic-co-glycolic acid)-based microparticles on primary human myoblasts. The myoblast morphology and progression in the myogenic program differed, depending on the type of scaffold material. In alginate gel, the cells obtained a round morphology, they ceased to proliferate, and entered quiescence. In the fibrin gels, differentiation was promoted, and myotubes were observed within a few days in culture, while poly(lactic-co-glycolic acid)-based microparticles supported prolonged proliferation. Myoblasts released from the alginate and fibrin gels were studied, and cells released from these scaffolds had retained the ability to proliferate and differentiate. Thus, the study shows that human myogenic cells combined with injectable scaffold materials are guided into different states depending on the choice of scaffold. This opens for in vivo experiments, including testing of the significance of the cell state on regeneration potential of primary human myoblasts.

Three-Dimensional Culture Model of Skeletal Muscle Tissue with Atrophy Induced by Dexamethasone

Drug screening systems for muscle atrophy based on the contractile force of cultured skeletal muscle tissues are required for the development of preventive or therapeutic drugs for atrophy. This study aims to develop a muscle atrophy model by inducing atrophy in normal muscle tissues constructed on microdevices capable of measuring the contractile force and to verify if this model is suitable for drug screening using the contractile force as an index. Tissue engineered skeletal muscles containing striated myotubes were prepared on the microdevices for the study. The addition of 100 µM dexamethasone (Dex), which is used as a muscle atrophy inducer, for 24 h reduced the contractile force significantly. An increase in the expression of Atrogin-1 and MuRF-1 in the tissues treated with Dex was established. A decrease in the number of striated myotubes was also observed in the tissues treated with Dex. Treatment with 8 ng/mL Insulin-like Growth Factor (IGF-I) for 24 h significantly increased the contractile force of the Dex-induced atrophic tissues. The same treatment, though, had no impact on the force of the normal tissues. Thus, it is envisaged that the atrophic skeletal muscle tissues induced by Dex can be used for drug screening against atrophy.

Natural polymeric hydrogel evaluation for skeletal muscle tissue engineering

Although skeletal muscle has a remarkable ability to repair/regenerate after most types of injuries, there is limited regeneration after volumetric muscle loss (VML). A number of scaffold materials have been used in the development of grafts to treat VML, however, there is still a need to better understand the most appropriate material with regards to its ability to maintain mechanical integrity while also supporting myogenesis. Five commonly used natural polymeric materials (Collagen I, Agarose, Alginate, Fibrin, and Collagen Chitosan) used in skeletal muscle tissue engineering grafts were evaluated for their mechanical properties and myogenic capacity. Rheological properties, water absorption rates, degradation stability, tensile characteristics, and the ability to support in vitro myogenesis were compared in all five materials. Collagen, Collagen Chitosan, and Fibrin demonstrated high elasticity and 100% stretch without failure, Agarose was the most brittle (20% max stretch), and Alginate demonstrated poor handleabilty. While Collagen was supportive of myogenesis, overall, Fibrin demonstrated the highest myogenic potential as indicated by the earliest and highest increases in myogenin and myosin heavy chain mRNA in satellite cells along with the most extensive myotube development as evaluated with immunohistochemistry. The findings herein support the notion that under the conditions used in this study, Fibrin is the most suitable scaffold for the development of scaffolds for skeletal muscle tissue engineering. Future studies are required to determine whether the differences in mechanical properties and myogenic potential observed in vitro in the current study translate to better skeletal muscle development in a VML injury model. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 672-679, 2018.

Anisotropic Materials for Skeletal-Muscle-Tissue Engineering

Repair of damaged skeletal-muscle tissue is limited by the regenerative capacity of the native tissue. Current clinical approaches are not optimal for the treatment of large volumetric skeletal-muscle loss. As an alternative, tissue engineering represents a promising approach for the functional restoration of damaged muscle tissue. A typical tissue-engineering process involves the design and fabrication of a scaffold that closely mimics the native skeletal-muscle extracellular matrix (ECM), allowing organization of cells into a physiologically relevant 3D architecture. In particular, anisotropic materials that mimic the morphology of the native skeletal-muscle ECM, can be fabricated using various biocompatible materials to guide cell alignment, elongation, proliferation, and differentiation into myotubes. Here, an overview of fundamental concepts associated with muscle-tissue engineering and the current status of muscle-tissue-engineering approaches is provided. Recent advances in the development of anisotropic scaffolds with micro- or nanoscale features are reviewed, and how scaffold topographical, mechanical, and biochemical cues correlate to observed cellular function and phenotype development is examined. Finally, some recent developments in both the design and utility of anisotropic materials in skeletal-muscle-tissue engineering are highlighted, along with their potential impact on future research and clinical applications.

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.

Growth Factors for Skeletal Muscle Tissue Engineering

Tissue-engineered skeletal muscle holds promise as a source of graft tissue for repair of volumetric muscle loss and as a model system for pharmaceutical testing. To reach this potential, engineered tissues must advance past the neonatal phenotype that characterizes the current state of the art. In this review, we describe native skeletal muscle development and identify important growth factors controlling this process. By comparing in vivo myogenesis to in vitro satellite cell cultures and tissue engineering approaches, several key similarities and differences that may potentially advance tissue-engineered skeletal muscle were identified. In particular, hepatocyte and fibroblast growth factors used to accelerate satellite cell activation and proliferation, followed by addition of insulin-like growth factor as a potent inducer of differentiation, are proven methods for increased myogenesis in engineered muscle. Additionally, we review our recent novel application of dexamethasone (DEX), a glucocorticoid that stimulates myoblast differentiation, in skeletal muscle tissue engineering. Using our established skeletal muscle unit (SMU) fabrication protocol, timing- and dose-dependent effects of DEX were measured. The supplemented SMUs demonstrated advanced sarcomeric structure and significantly increased myotube diameter and myotube fusion compared to untreated controls. Most significantly, these SMUs exhibited a fivefold rise in force production. Thus, we concluded that DEX may serve to improve myogenesis, advance muscle structure, and increase force production in engineered skeletal muscle.

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.

Synergizing Engineering and Biology to Treat and Model Skeletal Muscle Injury and Disease

Although skeletal muscle is one of the most regenerative organs in our body, various genetic defects, alterations in extrinsic signaling, or substantial tissue damage can impair muscle function and the capacity for self-repair. The diversity and complexity of muscle disorders have attracted much interest from both cell biologists and, more recently, bioengineers, leading to concentrated efforts to better understand muscle pathology and develop more efficient therapies. This review describes the biological underpinnings of muscle development, repair, and disease, and discusses recent bioengineering efforts to design and control myomimetic environments, both to study muscle biology and function and to aid in the development of new drug, cell, and gene therapies for muscle disorders. The synergy between engineering-aided biological discovery and biology-inspired engineering solutions will be the path forward for translating laboratory results into clinical practice.

The Effect of Sterilization Methods on the Structural and Chemical Properties of Fibrin Microthread Scaffolds

A challenge for the design of scaffolds in tissue engineering is to determine a terminal sterilization method that will retain the structural and biochemical properties of the materials. Since commonly used heat and ionizing energy-based sterilization methods have been shown to alter the material properties of protein-based scaffolds, the effects of ethanol and ethylene oxide (EtO) sterilization on the cellular compatibility and the structural, chemical, and mechanical properties of uncrosslinked, UV crosslinked, or 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) crosslinked fibrin microthreads in neutral (EDCn) or acidic (EDCa) buffers are evaluated. EtO sterilization significantly reduces the tensile strength of uncrosslinked microthreads. Surface chemistry analyses show that EtO sterilization induces alkylation of EDCa microthreads leading to a significant reduction in myoblast attachment. The material properties of EDCn microthreads do not appear to be affected by the sterilization method. These results significantly enhance the understanding of how sterilization or crosslinking techniques affect the material properties of protein scaffolds.

Design, evaluation, and application of engineered skeletal muscle

For over two decades, research groups have been developing methods to engineer three-dimensional skeletal muscle tissues. These tissues hold great promise for use in disease modeling and pre-clinical drug development, and have potential to serve as therapeutic grafts for functional muscle repair. Recent advances in the field have resulted in the engineering of regenerative muscle constructs capable of survival, vascularization, and functional maturation in vivo as well as the first-time creation of functional human engineered muscles for screening of therapeutics in vitro. In this review, we will discuss the methodologies that have progressed work in the muscle tissue engineering field to its current state. The emphasis will be placed on the existing procedures to generate myogenic cell sources and form highly functional muscle tissues in vitro, techniques to monitor and evaluate muscle maturation and performance in vitro and in vivo, and surgical strategies to both create diseased environments and ensure implant survival and rapid integration into the host. Finally, we will suggest the most promising methodologies that will enable continued progress in the field.

Fabrication of biomimetic vascular scaffolds for 3D tissue constructs using vascular corrosion casts

Vascularization is among the most pressing technical challenges facing tissue engineering of 3D organs. While small engineered constructs can rely solely on vascular infiltration and diffusion from host tissues following implantation, larger avascular constructs do not survive long enough for vessel ingrowth to occur. To address this challenge, strategies for pre-vascularization of engineered constructs have been developed. Various biofabrication techniques have been utilized for pre-vascularization, but limitations exist with respect to the size and complexity of the resulting vessels. To this end, we developed a simple and novel fabrication method to create biomimetic microvascular scaffolds using vascular corrosion casting as a template for pre-vascularization of engineered tissue constructs. Gross and electron microscopic analysis demonstrates that polycaprolactone (PCL)-derived kidney vascular corrosion casts are able to capture the architecture of normal renal tissue and can serve as a sacrificial template for the creation of a collagen-based vascular scaffold. Histological analysis demonstrates that the collagen vascular scaffolds are biomimetic in structure and can be perfused, endothelialized, and embedded in hydrogel tissue constructs. Our scaffold creation method is simple, cost effective, and provides a biomimetic, tissue-specific option for pre-vascularization that is broadly applicable in tissue engineering.

STATEMENT OF SIGNIFICANCE

Tissues in the body are vascularized to provide nutrients to the cells within the tissues and carry away waste, but creating tissue engineered constructs with functional vascular networks has been challenging. Current biofabrication techniques can incorporate blood vessel-like structures with straight or simple branching patterns into tissue constructs. Unfortunately, these techniques are expensive, complicated and create simplified versions of the complex vessel structures seen in native tissue. Our technique uses novel vascular corrosion casts of normal tissue as templates to create vascular scaffolds that are a copy of normal vessels. These vascular scaffolds can be easily incorporated into 3D tissue constructs. Our process is simple, inexpensive and inherently tissue-specific, making it widely applicable in the field of tissue engineering.

3D tissue-engineered bone marrow as a novel model to study pathophysiology and drug resistance in multiple myeloma

PURPOSE

Multiple myeloma (MM) is the second most prevalent hematological malignancy and it remains incurable despite the introduction of several novel drugs. The discrepancy between preclinical and clinical outcomes can be attributed to the failure of classic two-dimensional (2D) culture models to accurately recapitulate the complex biology of MM and drug responses observed in patients.

EXPERIMENTAL DESIGN

We developed 3D tissue engineered bone marrow (3DTEBM) cultures derived from the BM supernatant of MM patients to incorporate different BM components including MM cells, stromal cells, and endothelial cells. Distribution and growth were analyzed by confocal imaging, and cell proliferation of cell lines and primary MM cells was tested by flow cytometry. Oxygen and drug gradients were evaluated by immunohistochemistry and flow cytometry, and drug resistance was studied by flow cytometry.

RESULTS

3DTEBM cultures allowed proliferation of MM cells, recapitulated their interaction with the microenvironment, recreated 3D aspects observed in the bone marrow niche (such as oxygen and drug gradients), and induced drug resistance in MM cells more than 2D or commercial 3D tissue culture systems.

CONCLUSIONS

3DTEBM cultures not only provide a better model for investigating the pathophysiology of MM, but also serve as a tool for drug development and screening in MM. In the future, we will use the 3DTEBM cultures for developing personalized therapeutic strategies for individual MM patients.

Biocompatible Hydrogels for Microarray Cell Printing and Encapsulation

Conventional drug screening processes are a time-consuming and expensive endeavor, but highly rewarding when they are successful. To identify promising lead compounds, millions of compounds are traditionally screened against therapeutic targets on human cells grown on the surface of 96-wells. These two-dimensional (2D) cell monolayers are physiologically irrelevant, thus, often providing false-positive or false-negative results, when compared to cells grown in three-dimensional (3D) structures such as hydrogel droplets. However, 3D cell culture systems are not easily amenable to high-throughput screening (HTS), thus inherently low throughput, and requiring relatively large volume for cell-based assays. In addition, it is difficult to control cellular microenvironments and hard to obtain reliable cell images due to focus position and transparency issues. To overcome these problems, miniaturized 3D cell cultures in hydrogels were developed via cell printing techniques where cell spots in hydrogels can be arrayed on the surface of glass slides or plastic chips by microarray spotters and cultured in growth media to form cells encapsulated 3D droplets for various cell-based assays. These approaches can dramatically reduce assay volume, provide accurate control over cellular microenvironments, and allow us to obtain clear 3D cell images for high-content imaging (HCI). In this review, several hydrogels that are compatible to microarray printing robots are discussed for miniaturized 3D cell cultures.

Biomimetic scaffolds for regeneration of volumetric muscle loss in skeletal muscle injuries

Skeletal muscle injuries typically result from traumatic incidents such as combat injuries where soft-tissue extremity injuries are present in one of four cases. Further, about 4.5 million reconstructive surgical procedures are performed annually as a result of car accidents, cancer ablation, or cosmetic procedures. These combat- and trauma-induced skeletal muscle injuries are characterized by volumetric muscle loss (VML), which significantly reduces the functionality of the injured muscle. While skeletal muscle has an innate repair mechanism, it is unable to compensate for VML injuries because large amounts of tissue including connective tissue and basement membrane are removed or destroyed. This results in a significant need to develop off-the-shelf biomimetic scaffolds to direct skeletal muscle regeneration. Here, the structure and organization of native skeletal muscle tissue is described in order to reveal clear design parameters that are necessary for scaffolds to mimic in order to successfully regenerate muscular tissue. We review the literature with respect to the materials and methodologies used to develop scaffolds for skeletal muscle tissue regeneration as well as the limitations of these materials. We further discuss the variety of cell sources and different injury models to provide some context for the multiple approaches used to evaluate these scaffold materials. Recent findings are highlighted to address the state of the field and directions are outlined for future strategies, both in scaffold design and in the use of different injury models to evaluate these materials, for regenerating functional skeletal muscle.

STATEMENT OF SIGNIFICANCE

Volumetric muscle loss (VML) injuries result from traumatic incidents such as those presented from combat missions, where soft-tissue extremity injuries are represented in one of four cases. These injuries remove or destroy large amounts of skeletal muscle including the basement membrane and connective tissue, removing the structural, mechanical, and biochemical cues that usually direct its repair. This results in a significant need to develop off-the-shelf biomimetic scaffolds to direct skeletal muscle regeneration. In this review, we examine current strategies for the development of scaffold materials designed for skeletal muscle regeneration, highlighting advances and limitations associated with these methodologies. Finally, we identify future approaches to enhance skeletal muscle regeneration.

Activated Schwann Cell-Like Cells on Aligned Fibrin-Poly(Lactic-Co-Glycolic Acid) Structures: A Novel Construct for Application in Peripheral Nerve Regeneration

Tissue engineering approaches in nerve regeneration search for ways to support gold standard therapy (autologous nerve grafts) and to improve results by bridging nerve defects with different kinds of conduits. In this study, we describe electrospinning of aligned fibrin-poly(lactic-co-glycolic acid) (PLGA) fibers in an attempt to create a biomimicking tissue-like material seeded with Schwann cell-like cells (SCLs) in vitro for potential use as an in vivo scaffold. Rat adipose-derived stem cells (rASCs) were differentiated into SCLs and evaluated with flow cytometry concerning their differentiation and activation status [S100b, P75, myelin-associated glycoprotein (MAG), and protein 0 (P0)]. After receiving the proliferation stimulus forskolin, SCLs expressed S100b and P75; comparable to native, activated Schwann cells, while cultured without forskolin, cells switched to a promyelinating phenotype and expressed S100b, MAG, and P0. Human fibrinogen and thrombin, blended with PLGA, were electrospun and the alignment and homogeneity of the fibers were proven by scanning electron microscopy. Electrospun scaffolds were seeded with SCLs and the formation of Büngner-like structures in SCLs was evaluated with phalloidin/propidium iodide staining. Carrier fibrin gels containing rASCs acted as a self-shaping matrix to form a tubular structure. In this study, we could show that rASCs can be differentiated into activated, proliferating SCLs and that these cells react to minimal changes in stimulus, switching to a promyelinating phenotype. Aligned electrospun fibrin-PLGA fibers promoted the formation of Büngner-like structures in SCLs, which also rolled the fibrin-PLGA matrix into a tubular scaffold. These in vitro findings favor further in vivo testing.

Three dimensional multi-cellular muscle-like tissue engineering in perfusion-based bioreactors

Conventional tissue engineering strategies often rely on the use of a single progenitor cell source to engineer in vitro biological models; however, multi-cellular environments can better resemble the complexity of native tissues. Previous described co-culture models used skeletal myoblasts, as parenchymal cell source, and mesenchymal or endothelial cells, as stromal component. Here, we propose instead the use of adipose tissue-derived stromal vascular fraction cells, which include both mesenchymal and endothelial cells, to better resemble the native stroma. Percentage of serum supplementation is one of the crucial parameters to steer skeletal myoblasts toward either proliferation (20%) or differentiation (5%) in two-dimensional culture conditions. On the contrary, three-dimensional (3D) skeletal myoblast culture often simply adopts the serum content used in monolayer, without taking into account the new cell environment. When considering 3D cultures of mm-thick engineered tissues, homogeneous and sufficient oxygen supply is paramount to avoid formation of necrotic cores. Perfusion-based bioreactor culture can significantly improve the oxygen access to the cells, enhancing the viability and the contractility of the engineered tissues. In this study, we first investigated the influence of different serum supplementations on the skeletal myoblast ability to proliferate and differentiate during 3D perfusion-based culture. We tested percentages of serum promoting monolayer skeletal myoblast-proliferation (20%) and differentiation (5%) and suitable for stromal cell culture (10%) with a view to identify the most suitable condition for the subsequent co-culture. The 10% serum medium composition resulted in the highest number of mature myotubes and construct functionality. Co-culture with stromal vascular fraction cells at 10% serum also supported the skeletal myoblast differentiation and maturation, hence providing a functional engineered 3D muscle model that resembles the native multi-cellular environment.

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.

Mechanical cues in orofacial tissue engineering and regenerative medicine

Cleft lip and palate patients suffer from functional, aesthetical, and psychosocial problems due to suboptimal regeneration of skin, mucosa, and skeletal muscle after restorative cleft surgery. The field of tissue engineering and regenerative medicine (TE/RM) aims to restore the normal physiology of tissues and organs in conditions such as birth defects or after injury. A crucial factor in cell differentiation, tissue formation, and tissue function is mechanical strain. Regardless of this, mechanical cues are not yet widely used in TE/RM. The effects of mechanical stimulation on cells are not straight-forward in vitro as cellular responses may differ with cell type and loading regime, complicating the translation to a therapeutic protocol. We here give an overview of the different types of mechanical strain that act on cells and tissues and discuss the effects on muscle, and skin and mucosa. We conclude that presently, sufficient knowledge is lacking to reproducibly implement external mechanical loading in TE/RM approaches. Mechanical cues can be applied in TE/RM by fine-tuning the stiffness and architecture of the constructs to guide the differentiation of the seeded cells or the invading surrounding cells. This may already improve the treatment of orofacial clefts and other disorders affecting soft tissues.

Mechanical Characterization and Shape Optimization of Fascicle-Like 3D Skeletal Muscle Tissues Contracted with Electrical and Optical Stimuli

In this study, we present a quantitative approach to construct effective 3D muscle tissues through shape optimization and load impedance matching with electrical and optical stimulation. We have constructed long, thin, fascicle-like skeletal muscle tissue and optimized its form factor through mechanical characterization. A new apparatus was designed and built, which allowed us to measure force-displacement characteristics with diverse load stiffnesses. We have found that (1) there is an optimal form factor that maximizes the muscle stress, (2) the energy transmitted to the load can be maximized with matched load stiffness, and (3) optical stimulation using channelrhodopsin2 in the muscle tissue can generate a twitch force as large as its electrical counterpart for well-developed muscle tissue. Using our tissue construct method, we found that an optimal initial diameter of 500 μm outperformed tissues using 250 μm by more than 60% and tissues using 760 μm by 105%. Using optimal load stiffness, our tissues have generated 12 pJ of energy per twitch at a peak generated stress of 1.28 kPa. Additionally, the difference in optically stimulated twitch performance versus electrically stimulated is a function of how well the overall tissue performs, with average or better performing strips having less than 10% difference. The unique mechanical characterization method used is generalizable to diverse load conditions and will be used to match load impedance to muscle tissue impedance for a wide variety of applications.

Cell Therapy for Stress Urinary Incontinence

Urinary incontinence (UI) is the involuntary loss of urine and is a common condition in middle-aged and elderly women and men. Stress urinary incontinence (SUI) is caused by leakage of urine when coughing, sneezing, laughing, lifting, and exercise, even standing leads to increased intra-abdominal pressure. Other types of UI also exist such as urge incontinence (also called overactive bladder), which is a strong and unexpected sudden urge to urinate, mixed forms of UI that result in symptoms of both urge and stress incontinence, and functional incontinence caused by reduced mobility, cognitive impairment, or neuromuscular limitations that impair mobility or dexterity. However, for many SUI patients, there is significant loss of urethral sphincter muscle due to degeneration of tissue, the strain and trauma of pregnancy and childbirth, or injury acquired during surgery. Hence, for individuals with SUI, a cell-based therapeutic approach to regenerate the sphincter muscle offers the advantage of treating the cause rather than the symptoms. We discuss current clinically relevant cell therapy approaches for regeneration of the external urethral sphincter (striated muscle), internal urethral sphincter (smooth muscle), the neuromuscular synapse, and blood supply. The use of mesenchymal stromal/stem cells is a major step in the right direction, but they may not be enough for regeneration of all components of the urethral sphincter. Inclusion of other cell types or biomaterials may also be necessary to enhance integration and survival of the transplanted cells.

Engineering muscle tissue for the fetus: getting ready for a strong life

Congenital malformations frequently involve either skeletal, smooth or cardiac tissues. When large parts of those tissues are damaged, the repair of the malformations is challenged by the fact that so much autologous tissue is missing. Current treatments require the use of prostheses or other therapies and are associated with a significant morbidity and mortality. Nonetheless, affected children have generally good survival rates and mostly normal schooling. As such, new therapeutic modalities need to represent significant improvements with clear safety profiles. Regenerative medicine and tissue engineering technologies have the potential to dramatically improve the treatment of any disease or disorder involving a lack of viable tissue. With respect to congenital soft tissue anomalies, the development of, for example, implantable muscle constructs would provide not only the usual desired elasticity and contractile proprieties, but should also be able to grow with the fetus and/or in the postnatal life. Such an approach would eliminate the need for multiple surgeries. However, the more widespread clinical applications of regenerative medicine and tissue engineering technologies require identification of the optimal indications, as well as further elucidation of the precise mechanisms and best methods (cells, scaffolds/biomaterials) for achieving large functional tissue regeneration in those clinical indications. In short, despite some amazing scientific progress, significant safety and efficacy hurdles remain. However, the rapid preclinical advances in the field bode well for future applications. As such, translational researchers and clinicians alike need be informed and prepared to utilize these new techniques for the benefit of their patients, as soon as they are available. To this end, we review herein, the clinical need(s), potential applications, and the relevant preclinical studies that are currently guiding the field toward novel therapeutics.

Abnormalities of AMPK activation and glucose uptake in cultured skeletal muscle cells from individuals with chronic fatigue syndrome

BACKGROUND

Post exertional muscle fatigue is a key feature in Chronic Fatigue Syndrome (CFS). Abnormalities of skeletal muscle function have been identified in some but not all patients with CFS. To try to limit potential confounders that might contribute to this clinical heterogeneity, we developed a novel in vitro system that allows comparison of AMP kinase (AMPK) activation and metabolic responses to exercise in cultured skeletal muscle cells from CFS patients and control subjects.

METHODS

Skeletal muscle cell cultures were established from 10 subjects with CFS and 7 age-matched controls, subjected to electrical pulse stimulation (EPS) for up to 24h and examined for changes associated with exercise.

RESULTS

In the basal state, CFS cultures showed increased myogenin expression but decreased IL6 secretion during differentiation compared with control cultures. Control cultures subjected to 16 h EPS showed a significant increase in both AMPK phosphorylation and glucose uptake compared with unstimulated cells. In contrast, CFS cultures showed no increase in AMPK phosphorylation or glucose uptake after 16 h EPS. However, glucose uptake remained responsive to insulin in the CFS cells pointing to an exercise-related defect. IL6 secretion in response to EPS was significantly reduced in CFS compared with control cultures at all time points measured.

CONCLUSION

EPS is an effective model for eliciting muscle contraction and the metabolic changes associated with exercise in cultured skeletal muscle cells. We found four main differences in cultured skeletal muscle cells from subjects with CFS; increased myogenin expression in the basal state, impaired activation of AMPK, impaired stimulation of glucose uptake and diminished release of IL6. The retention of these differences in cultured muscle cells from CFS subjects points to a genetic/epigenetic mechanism, and provides a system to identify novel therapeutic targets.

Bioengineered human myobundles mimic clinical responses of skeletal muscle to drugs

Existing in vitro models of human skeletal muscle cannot recapitulate the organization and function of native muscle, limiting their use in physiological and pharmacological studies. Here, we demonstrate engineering of electrically and chemically responsive, contractile human muscle tissues (‘myobundles’) using primary myogenic cells. These biomimetic constructs exhibit aligned architecture, multinucleated and striated myofibers, and a Pax7⁺ cell pool. They contract spontaneously and respond to electrical stimuli with twitch and tetanic contractions. Positive correlation between contractile force and GCaMP6-reported calcium responses enables non-invasive tracking of myobundle function and drug response. During culture, myobundles maintain functional acetylcholine receptors and structurally and functionally mature, evidenced by increased myofiber diameter and improved calcium handling and contractile strength. In response to diversely acting drugs, myobundles undergo dose-dependent hypertrophy or toxic myopathy similar to clinical outcomes. Human myobundles provide an enabling platform for predictive drug and toxicology screening and development of novel therapeutics for muscle-related disorders.

DOI:http://dx.doi.org/10.7554/eLife.04885.001

Scientists have developed realistic models of the human liver, lung, and heart that allow them to observe living tissue in the laboratory. These models have helped us to better understand how these organs work and what goes wrong in diseases that affect these organs. The models can also be used to test how new drugs may affect a particular organ without the risk of exposing patients to the drug.

Efforts to develop a realistic laboratory model of human muscle tissues that can contract like real muscles have not been as successful to date. This shortcoming has potentially hindered the development of drugs to treat numerous disorders that affect muscles and movement in humans—such as muscular dystrophies, which are diseases in which people progressively lose muscle strength.

Some important drugs, like cholesterol-lowering statins, have detrimental effects on muscle tissue; one statin was so harmful to muscles that it had to be withdrawn from the market. As such, it would be useful to have experimental models that would allow scientists to test whether potential drugs damage or treat muscle tissue.

Madden et al. have now bioengineered a three-dimensional laboratory model of living muscle tissue made of cells taken from biopsies of several different human patients. These tissues were grown into bundles of muscle fibers on special polymer frames in the laboratory. The bioengineered muscle bundles respond to electrical and chemical signals and contract just like normal muscle. They also exhibit the same structure and signaling as healthy muscle tissue in humans.

Madden et al. exposed the muscle tissue bundles to three drugs known to affect muscles to determine if the model could be used to test whether drugs have harmful effects. This revealed that the bundles had weaker contractions in response to statins and the malaria drug chloroquine, just like normal muscles do—and that this effect worsened if more of each drug was used. Madden et al. also found that a drug that strengthens muscle contractions at low doses and damages muscle at high doses in humans has similar effects in the model.

As well as this model being used to screen for harmful effects of drugs before clinical trials, the technique used to create the model could be used to grow muscle tissue from patients with muscle diseases. This would help researchers and doctors to better understand the patient's condition and potentially develop more efficient therapies. Also, the technique could be eventually developed to grow healthy muscle tissue to implant in patients who have been injured.

DOI:http://dx.doi.org/10.7554/eLife.04885.002

Bio-hybrid cell-based actuators for microsystems

As we move towards the miniaturization of devices to perform tasks at the nano and microscale, it has become increasingly important to develop new methods for actuation, sensing, and control. Over the past decade, bio-hybrid methods have been investigated as a promising new approach to overcome the challenges of scaling down robotic and other functional devices. These methods integrate biological cells with artificial components and therefore, can take advantage of the intrinsic actuation and sensing functionalities of biological cells. Here, the recent advancements in bio-hybrid actuation are reviewed, and the challenges associated with the design, fabrication, and control of bio-hybrid microsystems are discussed. As a case study, focus is put on the development of bacteria-driven microswimmers, which has been investigated as a targeted drug delivery carrier. Finally, a future outlook for the development of these systems is provided. The continued integration of biological and artificial components is envisioned to enable the performance of tasks at a smaller and smaller scale in the future, leading to the parallel and distributed operation of functional systems at the microscale.

Engineering muscle constructs for the creation of functional engineered musculoskeletal tissue

Volumetric muscle loss (VML) is a disabling condition in which current clinical procedures are suboptimal. The field of tissue engineering has many promising strategies for the creation of functional skeletal muscle in vitro. However, there are still two key limitations that prevent it from becoming a solution for treating VML. First, engineered muscle tissue must be biocompatible to facilitate muscle tissue regrowth without generating an immune response. Second, engineered muscle constructs must be scaled up to facilitate replacement of clinically relevant volumes of tissue (centimeters in diameter). There are currently no tissue engineering strategies to produce tissue constructs that are both biocompatible and large enough to facilitate clinical repair. However, recent advances in tissue engineering using synthetic scaffolds, native scaffolds, or scaffold-free approaches may lead to a solution for repair of VML injuries.