Naturally derived and synthetic scaffolds for skeletal muscle reconstruction

Multitasking muscle: engineering iPSC-derived myogenic progenitors to do more

The generation of myogenic progenitors from iPSCs (iMPs) with therapeutic potential for in vivo tissue regeneration has long been a goal in the skeletal muscle community. Today, protocols enable the production of potent, albeit immature, iMPs that resemble Pax7+ adult muscle stem cells. While muscular dystrophies are often the primary therapeutic target for these cells, an underexplored application is their use in treating traumatic muscle injuries. Notably absent from recent reviews on iMPs is the concept of engineering these cells to perform functions post-transplantation that non-transgenic cells cannot. Here, we highlight protocols to enhance the generation, purification, and maturation of iMPs, and introduce the idea of engineering these cells to perform functions beyond their normal capacities, envisioning novel therapeutic applications.

Engineering Nanoclusters of Cell Adhesive Ligands on Biomaterial Surfaces: Superior Cell Proliferation and Myotube Formation for Skeletal Muscle Tissue Regeneration

Engineering biointerfaces with nanoscale clustering of integrin-binding cell adhesive peptides is critical for promoting receptor redistribution into signaling complexes. Skeletal muscle cells are exquisitely sensitive to integrin-mediated signaling, yet biomaterials supporting myogenesis through control of the density and nanodistribution of ligands have not been developed. Here, materials are developed with tailorable cell adhesive ligands distribution at the interface by independently controlling their global and local density to enhance myogenesis, by promoting myoblast growth and myotube formation. To this end, RGD-functionalized low-fouling polymer surfaces with global ligand densities (G) from 0-7 µg peptide/mg polymer and average local ligand densities (L) from 1-6.3 ligands/cluster, are generated and characterized. Cell studies demonstrate improvements in cell adhesion, spreading, growth, and myotube formation up to a density of 7 µg peptide/mg polymer with 4 ligands/cluster. Optimizing ligand density and distribution also promotes early myofiber maturation, identified by increased MF20 marker protein expression and sarcomere-forming myotubes. At higher ligand densities, these cell properties are decreased, indicating that ligand multivalency is a critical parameter for tailoring cell-material interactions, to a certain threshold. The findings provide new insights for designing next-generation biomaterials and hold promise for improved engineering of skeletal muscle.

Neurotization of decellularized muscle graft increases de novo type I slow muscle fiber formation and large fiber size frequency

Volumetric muscle loss (VML) injuries are the result of extreme trauma from battlefield injuries, tumor ablations, and other physical traumas such as car crash injuries. The abrupt loss of muscle restricts the tissue's remaining regenerative capacity, leading to loss of satellite cells, peripheral nerve connections, and aberrant fibrosis. Prior research from our lab demonstrated that decellularized muscle matrix (DMM) supported regeneration of de novo fibers within the graft. The goal of this study was to determine whether DMM treated with a peripheral nerve using neurotization surgeries would enhance muscle regeneration and innervation. Forty-eight male Sprague Dawley rats were randomized and received a 1.5×1 cm defect treated with no treatment empty defect (ED), DMM, or autograft with a direct peroneal (antagonist) neurotization or tibial via end to side graft (agonist) neurotization. DMM grafts treated with neurotization utilizing either peroneal or tibial nerve axons increased fast twitch fibers within the grafted area compared to untreated DMM or ED. Additionally, the frequency distribution of myofiber size shifted toward a healthier morphology in the tibial nerve axon neurotized DMM compared to the uninjured medial head. Lastly, Nanostring gene results showed DMM treated with a neurotization shifted expression towards a more regenerative phenotype with some myogenic markers returning to sham levels. These data indicate that injured muscle treated with DMM and neurotization becomes pro-regenerative and can contribute to the functionalization of DMM. STATEMENT OF SIGNIFICANCE: Extremity soft tissue trauma like volumetric muscle loss (VML) can result in permanent loss of skeletal muscle mass, denervation, and ischemia posing a significant clinical challenge. VML injuries disrupt normal tissue architecture in addition to intramuscular axons which are critical elements in muscle function and regeneration. The overall objective of this study was to enhance axon growth into a VML injury treated with decellularized muscle matrix (DMM) using neurotization. DMM is an acellular biomaterial capable of regenerating skeletal muscle; however, without bona fide neuromuscular connections, functional gains are small. This study demonstrates that introducing motor axons into an acellular regenerative material using neurotization enhanced muscle regeneration and promoted slow twitch fiber formation.

Tissue Engineered 3D Constructs for Volumetric Muscle Loss

Severe injuries to skeletal muscles, including cases of volumetric muscle loss (VML), are linked to substantial tissue damage, resulting in functional impairment and lasting disability. While skeletal muscle can regenerate following minor damage, extensive tissue loss in VML disrupts the natural regenerative capacity of the affected muscle tissue. Existing clinical approaches for VML, such as soft-tissue reconstruction and advanced bracing methods, need to be revised to restore tissue function and are associated with limitations in tissue availability and donor-site complications. Advancements in tissue engineering (TE), particularly in scaffold design and the delivery of cells and growth factors, show promising potential for regenerating damaged skeletal muscle tissue and restoring function. This article provides a brief overview of the pathophysiology of VML and critiques the shortcomings of current treatments. The subsequent section focuses on the criteria for designing TE scaffolds, offering insights into various natural and synthetic biomaterials and cell types for effectively regenerating skeletal muscle. We also review multiple TE strategies involving both acellular and cellular scaffolds to encourage the development and maturation of muscle tissue and facilitate integration, vascularization, and innervation. Finally, the article explores technical challenges hindering successful translation into clinical applications.

Bioactive Glass and Silica Particles for Skeletal and Cardiac Muscle Tissue Regeneration

When skeletal and cardiac tissues are damaged, surgical approaches are not always successful and tissue regeneration approaches are investigated. Reports in the literature indicate that silica nanoparticles and bioactive glasses (BGs), including silicate bioactive glasses (e.g., 45S5 BG), phosphate glass fibers, boron-doped mesoporous BGs, borosilicate glasses, and aluminoborates, are promising for repairing skeletal muscle tissue. Silica nanoparticles and BGs have been combined with polymers to obtain aligned nanofibers and to maintain controlled delivery of nanoparticles for skeletal muscle repair. The literature indicates that cardiac muscle regeneration can be also triggered by the ionic products of BGs. This was observed to be due to the release of vascular endothelial growth factor and other growth factors from cardiomyocytes, which regulate endothelial cells to form capillary structures (angiogenesis). Specific studies, including both in vitro and in vivo approaches, are reviewed in this article. The analysis of the literature indicates that although the research field is still very limited, BGs are showing great promise for muscle tissue engineering and further research in the field should be carried out to expand our basic knowledge on the application of BGs in muscle (skeletal and cardiac) tissue regeneration. Impact statement This review highlights the potential of silica particles and bioactive glasses (BGs) for skeletal and cardiac tissue regeneration. These biomaterials create scaffolds triggering muscle cell differentiation. Ionic products from BGs stimulate growth factors, supporting angiogenesis in cardiac tissue repair. Further research is required to expand our know-how on silica particles and BGs in muscle tissue engineering.

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.

The Wound Environment Agent-based Model (WEABM): a digital twin platform for characterization and complex therapeutic discovery for volumetric muscle loss

Volumetric Muscle Loss (VML) injuries are characterized by significant loss of muscle mass, usually due to trauma or surgical resection, often with a residual open wound in clinical settings and subsequent loss of limb function due to the replacement of the lost muscle mass with non-functional scar. Being able to regrow functional muscle in VML injuries is a complex control problem that needs to override robust, evolutionarily conserved healing processes aimed at rapidly closing the defect in lieu of restoration of function. We propose that discovering and implementing this complex control can be accomplished by the development of a Medical Digital Twin of VML. Digital Twins (DTs) are the subject of a recent report from the National Academies of Science, Engineering and Medicine (NASEM), which provides guidance as to the definition, capabilities and research challenges associated with the development and implementation of DTs. Specifically, DTs are defined as dynamic computational models that can be personalized to an individual real world "twin" and are connected to that twin via an ongoing data link. DTs can be used to provide control on the real-world twin that is, by the ongoing data connection, adaptive. We have developed an anatomic scale cell-level agent-based model of VML termed the Wound Environment Agent Based Model (WEABM) that can serve as the computational specification for a DT of VML. Simulations of the WEABM provided fundamental insights into the biology of VML, and we used the WEABM in our previously developed pipeline for simulation-based Deep Reinforcement Learning (DRL) to train an artificial intelligence (AI) to implement a robust generalizable control policy aimed at increasing the healing of VML with functional muscle. The insights into VML obtained include: 1) a competition between fibrosis and myogenesis due to spatial constraints on available edges of intact myofibrils to initiate the myoblast differentiation process, 2) the need to biologically "close" the wound from atmospheric/environmental exposure, which represents an ongoing inflammatory stimulus that promotes fibrosis and 3) that selective, multimodal and adaptive local mediator-level control can shift the trajectory of healing away from a highly evolutionarily beneficial imperative to close the wound via fibrosis. Control discovery with the WEABM identified the following design principles: 1) multimodal adaptive tissue-level mediator control to mitigate pro-inflammation as well as the pro-fibrotic aspects of compensatory anti-inflammation, 2) tissue-level mediator manipulation to promote myogenesis, 3) the use of an engineered extracellular matrix (ECM) to functionally close the wound and 4) the administration of an anti-fibrotic agent focused on the collagen-producing function of fibroblasts and myofibroblasts. The WEABM-trained DRL AI integrates these control modalities and provides design specifications for a potential device that can implement the required wound sensing and intervention delivery capabilities needed. The proposed cyber-physical system integrates the control AI with a physical sense-and-actuate device that meets the tenets of DTs put forth in the NASEM report and can serve as an example schema for the future development of Medical DTs.

Therapeutic applications of biological macromolecules and scaffolds for skeletal muscle regeneration: A review

Skeletal muscle (SM) mass and strength maintenance are important requirements for human well-being. SM regeneration to repair minor injuries depends upon the myogenic activities of muscle satellite (stem) cells. However, losses of regenerative properties following volumetric muscle loss or severe trauma or due to congenital muscular abnormalities are not self-restorable, and thus, these conditions have major healthcare implications and pose clinical challenges. In this context, tissue engineering based on different types of biomaterials and scaffolds provides an encouraging means of structural and functional SM reconstruction. In particular, biomimetic (able to transmit biological signals) and several porous scaffolds are rapidly evolving. Several biological macromolecules/biomaterials (collagen, gelatin, alginate, chitosan, and fibrin etc.) are being widely used for SM regeneration. However, available alternatives for SM regeneration must be redesigned to make them more user-friendly and economically feasible with longer shelf lives. This review aimed to explore the biological aspects of SM regeneration and the roles played by several biological macromolecules and scaffolds in SM regeneration in cases of volumetric muscle loss.

A biomimetic approach to modulating the sustained release of fibroblast growth factor 2 from fibrin microthread scaffolds

Aim

The pleiotropic effect of fibroblast growth factor 2 (FGF2) on promoting myogenesis, angiogenesis, and innervation makes it an ideal growth factor for treating volumetric muscle loss (VML) injuries. While an initial delivery of FGF2 has demonstrated enhanced regenerative potential, the sustained delivery of FGF2 from scaffolds with robust structural properties as well as biophysical and biochemical signaling cues has yet to be explored for treating VML. The goal of this study is to develop an instructive fibrin microthread scaffold with intrinsic topographic alignment cues as well as regenerative signaling cues and a physiologically relevant, sustained release of FGF2 to direct myogenesis and ultimately enhance functional muscle regeneration.

Methods

Heparin was passively adsorbed or carbodiimide-conjugated to microthreads, creating a biomimetic binding strategy, mimicking FGF2 sequestration in the extracellular matrix (ECM). It was also evaluated whether FGF2 incorporated into fibrin microthreads would yield sustained release. It was hypothesized that heparin-conjugated and co-incorporated (co-inc) fibrin microthreads would facilitate sustained release of FGF2 from the scaffold and enhance in vitro myoblast proliferation and outgrowth.

Results

Toluidine blue staining and Fourier transform infrared spectroscopy confirmed that carbodiimide-conjugated heparin bound to fibrin microthreads in a dose-dependent manner. Release kinetics revealed that heparin-conjugated fibrin microthreads exhibited sustained release of FGF2 over a period of one week. An in vitro assay demonstrated that FGF2 released from microthreads remained bioactive, stimulating myoblast proliferation over four days. Finally, a cellular outgrowth assay suggests that FGF2 promotes increased outgrowth onto microthreads.

Conclusions

It was anticipated that the combined effects of fibrin microthread structural properties, topographic alignment cues, and FGF2 release profiles will facilitate the fabrication of a biomimetic scaffold that enhances the regeneration of functional muscle tissue for the treatment of VML injuries.

Novel muscle-derived extracellular matrix hydrogel promotes angiogenesis and neurogenesis in volumetric muscle loss

Volumetric muscle loss (VML) represents a clinical challenge due to the limited regenerative capacity of skeletal muscle. Most often, it results in scar tissue formation and loss of function, which cannot be prevented by current therapies. Decellularized extracellular matrix (DEM) has emerged as a native biomaterial for the enhancement of tissue repair. Here, we report the generation and characterization of hydrogels derived from DEM prepared from WT or thrombospondin (TSP)-2 null muscle tissue. TSP2-null hydrogels, when compared to WT, displayed altered architecture, protein composition, and biomechanical properties and allowed enhanced invasion of C2C12 myocytes and chord formation by endothelial cells. They also displayed enhanced cell invasion, innervation, and angiogenesis following subcutaneous implantation. To evaluate their regenerative capacity, WT or TSP2 null hydrogels were used to treat VML injury to tibialis anterior muscles and the latter induced greater recruitment of repair cells, innervation, and blood vessel formation and reduced inflammation. Taken together, these observations indicate that TSP2-null hydrogels enhance angiogenesis and promote muscle repair in a VML model.

A Comprehensive Review of Muscle-Tendon Junction: Structure, Function, Injury and Repair

The muscle-tendon junction (MTJ) is a highly specific tissue interface where the muscle's fascia intersects with the extracellular matrix of the tendon. The MTJ functions as the particular structure facilitating the transmission of force from contractive muscle fibers to the skeletal system, enabling movement. Considering that the MTJ is continuously exposed to constant mechanical forces during physical activity, it is susceptible to injuries. Ruptures at the MTJ often accompany damage to both tendon and muscle tissues. In this review, we attempt to provide a precise definition of the MTJ, describe its subtle structure in detail, and introduce therapeutic approaches related to MTJ tissue engineering. We hope that our detailed illustration of the MTJ and summary of the representative research achievements will help researchers gain a deeper understanding of the MTJ and inspire fresh insights and breakthroughs for future research.

A comprehensive review on 3D tissue models: Biofabrication technologies and preclinical applications

The limitations of traditional two-dimensional (2D) cultures and animal testing, when it comes to precisely foreseeing the toxicity and clinical effectiveness of potential drug candidates, have resulted in a notable increase in the rate of failure during the process of drug discovery and development. Three-dimensional (3D) in-vitro models have arisen as substitute platforms with the capacity to accurately depict in-vivo conditions and increasing the predictivity of clinical effects and toxicity of drug candidates. It has been found that 3D models can accurately represent complex tissue structure of human body and can be used for a wide range of disease modeling purposes. Recently, substantial progress in biomedicine, materials and engineering have been made to fabricate various 3D in-vitro models, which have been exhibited better disease progression predictivity and drug effects than convention models, suggesting a promising direction in pharmaceutics. This comprehensive review highlights the recent developments in 3D in-vitro tissue models for preclinical applications including drug screening and disease modeling targeting multiple organs and tissues, like liver, bone, gastrointestinal tract, kidney, heart, brain, and cartilage. We discuss current strategies for fabricating 3D models for specific organs with their strengths and pitfalls. We expand future considerations for establishing a physiologically-relevant microenvironment for growing 3D models and also provide readers with a perspective on intellectual property, industry, and regulatory landscape.

Extracellular matrix: the critical contributor to skeletal muscle regeneration-a comprehensive review

The regenerative ability of skeletal muscle (SM) in response to damage, injury, or disease is a highly intricate process that involves the coordinated activities of multiple cell types and biomolecular factors. Of these, extracellular matrix (ECM) is considered a fundamental component of SM regenerative ability. This review briefly discusses SM myogenesis and regeneration, the roles played by muscle satellite cells (MSCs), other cells, and ECM components, and the effects of their dysregulations on these processes. In addition, we review the various types of ECM scaffolds and biomaterials used for SM regeneration, their applications, recent advances in ECM scaffold research, and their impacts on tissue engineering and SM regeneration, especially in the context of severe muscle injury, which frequently results in substantial muscle loss and impaired regenerative capacity. This review was undertaken to provide a comprehensive overview of SM myogenesis and regeneration, the stem cells used for muscle regeneration, the significance of ECM in SM regeneration, and to enhance understanding of the essential role of the ECM scaffold during SM regeneration.

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.

Bioprocessing Considerations towards the Manufacturing of Therapeutic Skeletal and Smooth Muscle Cells

Tissue engineering approaches within the muscle context represent a promising emerging field to address the current therapeutic challenges related with multiple pathological conditions affecting the muscle compartments, either skeletal muscle or smooth muscle, responsible for involuntary and voluntary contraction, respectively. In this review, several features and parameters involved in the bioprocessing of muscle cells are addressed. The cell isolation process is depicted, depending on the type of tissue (smooth or skeletal muscle), followed by the description of the challenges involving the use of adult donor tissue and the strategies to overcome the hurdles of reaching relevant cell numbers towards a clinical application. Specifically, the use of stem/progenitor cells is highlighted as a source for smooth and skeletal muscle cells towards the development of a cellular product able to maintain the target cell's identity and functionality. Moreover, taking into account the need for a robust and cost-effective bioprocess for cell manufacturing, the combination of muscle cells with biomaterials and the need for scale-up envisioning clinical applications are also approached.

Upscaled Skeletal Muscle Engineered Tissue with In Vivo Vascularization and Innervation Potential

Engineering functional tissues of clinically relevant size (in mm-scale) in vitro is still a challenge in tissue engineering due to low oxygen diffusion and lack of vascularization. To address these limitations, a perfusion bioreactor was used to generate contractile engineered muscles of a 3 mm-thickness and a 8 mm-diameter. This study aimed to upscale the process to 50 mm in diameter by combining murine skeletal myoblasts (SkMbs) with human adipose-derived stromal vascular fraction (SVF) cells, providing high neuro-vascular potential in vivo. SkMbs were cultured on a type-I-collagen scaffold with (co-culture) or without (monoculture) SVF. Large-scale muscle-like tissue showed an increase in the maturation index over time (49.18 ± 1.63% and 76.63 ± 1.22%, at 9 and 11 days, respectively) and a similar force of contraction in mono- (43.4 ± 2.28 µN) or co-cultured (47.6 ± 4.7 µN) tissues. Four weeks after implantation in subcutaneous pockets of nude rats, the vessel length density within the constructs was significantly higher in SVF co-cultured tissues (5.03 ± 0.29 mm/mm²) compared to monocultured tissues (3.68 ± 0.32 mm/mm²) (p < 0.005). Although no mature neuromuscular junctions were present, nerve-like structures were predominantly observed in the engineered tissues co-cultured with SVF cells. This study demonstrates that SVF cells can support both in vivo vascularization and innervation of contractile muscle-like tissues, making significant progress towards clinical translation.

Nanocarbon-Enforced Anisotropic MusCAMLR for Rapid Rescue of Mechanically Damaged Skeletal Muscles

Mechanical damages to skeletal muscles could be detrimental to the active work hours and lifestyle of athletes, mountaineers, and security personnel. In this regard, the slowness of conventional treatment strategies and drug-associated side effects greatly demand the design and development of novel biomaterials, which can rescue such mechanically damaged skeletal muscles. To accomplish this demand, we have developed a musculoresponsive polymer-carbon composite for assisting myotubular regeneration (MusCAMLR). The MusCAMLR is enforced to attain anisotropic muscle-like characteristics while incorporating a smartly passivated nanoscale carbon material in the PNIPAM gel under physiological conditions as a stimulus, which is not achieved by the pristine nanocarbon system. The MusCAMLR establishes a specific mechanical interaction with muscle cells, supports myotube regeneration, maintains excellent mechanical similarity with the myotube, and restores the structural integrity and biochemical parameters of mechanically damaged muscles in a delayed onset muscle soreness (DOMS) rat model within a short period of 72 h. Concisely, this study discloses the potential of smartly passivated nanocarbon in generating an advanced biomaterial system, MusCAMLR, from a regularly used polymeric hydrogel system. This engineered polymer-carbon composite reveals its possible potential to be used as a nondrug therapeutic alternative for rescuing mechanically damaged muscles and probably can be extended for therapy of various other diseases including muscular dystrophy.

Volumetric Muscle Loss: A Bibliometric Analysis of a Decade of Progress

The consequences of military conflict, accidents, and diseases have led to the definition-and subsequent study-of the pathological condition now known as volumetric muscle loss (VML). VML is a significant injury to skeletal muscle tissue on a scale that is endogenously irrecoverable and leads to chronic functional deficits and long-term disability. Currently, there lacks a definitive approach to meaningfully restore the tissue and function lost by those afflicted, ushering a need for scientific activities and associated funding to both facilitate a deeper understanding of the pathobiology of VML as well as to develop and assess clinically relevant therapeutics and treatment strategies. Thereby, evaluation of the VML field is crucial to gauging the return on resource expenditures and to understand the evolution of the field to guide future directions. This article presents a bibliometric analysis of publicly available data to explore the growth of the VML field since its genesis and to highlight its prosperity through its expanding literature, its development and evaluation of promising treatment strategies, rising financial investments, and innovation. Altogether, the bibliometric analysis reveals the field of VML as an emergent research focus that is productive and translational. Impact statement Analyses of a research topic are fundamental toward evaluating the returns on investment and appreciating the evolution of the research toward novel directions. This study aims to highlight the growing field of volumetric muscle loss (VML), defined as a significant injury to skeletal muscle tissue that leads to functional impairment and is irrecoverable through inherent regenerative mechanisms. The analysis of bibliometric and publicly available data provides evidence that the field of VML has an expanding research interest and investment, with biomaterials at the forefront of study.

Chitosan and Pectin Hydrogels for Tissue Engineering and In Vitro Modeling

Hydrogels are fascinating biomaterials that can act as a support for cells, i.e., a scaffold, in which they can organize themselves spatially in a similar way to what occurs in vivo. Hydrogel use is therefore essential for the development of 3D systems and allows to recreate the cellular microenvironment in physiological and pathological conditions. This makes them ideal candidates for biological tissue analogues for application in the field of both tissue engineering and 3D in vitro models, as they have the ability to closely mimic the extracellular matrix (ECM) of a specific organ or tissue. Polysaccharide-based hydrogels, because of their remarkable biocompatibility related to their polymeric constituents, have the ability to interact beneficially with the cellular components. Although the growing interest in the use of polysaccharide-based hydrogels in the biomedical field is evidenced by a conspicuous number of reviews on the topic, none of them have focused on the combined use of two important polysaccharides, chitosan and pectin. Therefore, the present review will discuss the biomedical applications of polysaccharide-based hydrogels containing the two aforementioned natural polymers, chitosan and pectin, in the fields of tissue engineering and 3D in vitro modeling.

Recent advances in bioengineered scaffold for in vitro meat production

In vitro meat production via stem cell technology and tissue engineering provides hypothetically elevated resource efficiency which involves the differentiation of muscle cells from pluripotent stem cells. By applying the tissue engineering technique, muscle cells are cultivated and grown onto a scaffold, resulting in the development of muscle tissue. The studies related to in vitro meat production are advancing with a seamless pace, and scientists are trying to develop various approaches to mimic the natural meat. The formulation and fabrication of biodegradable and cost-effective edible scaffold is the key to the successful development of downstream culture and meat production. Non-mammalian biopolymers such as gelatin and alginate or plant-derived proteins namely soy protein and decellularized leaves have been suggested as potential scaffold materials for in vitro meat production. Thus, this article is aimed to furnish recent updates on bioengineered scaffolds, covering their formulation, fabrication, features, and the mode of utilization.

Recent advances in engineering hydrogels for niche biomimicking and hematopoietic stem cell culturing

Hematopoietic stem cells (HSCs) provide a life-long supply of haemopoietic cells and are indispensable for clinical transplantation in the treatment of malignant hematological diseases. Clinical applications require vast quantities of HSCs with maintained stemness characteristics. Meeting this demand poses often insurmountable challenges for traditional culture methods. Creating a supportive artificial microenvironment for the culture of HSCs, which allows the expansion of the cells while maintaining their stemness, is becoming a new solution for the provision of these rare multipotent HSCs. Hydrogels with good biocompatibility, excellent hydrophilicity, tunable biochemical and biophysical properties have been applied in mimicking the hematopoietic niche for the efficient expansion of HSCs. This review focuses on recent progress in the use of hydrogels in this specialized application. Advanced biomimetic strategies use for the creation of an artificial haemopoietic niche are discussed, advances in combined use of hydrogel matrices and microfluidics, including the emerging organ-on-a-chip technology, are summarized. We also provide a brief description of novel stimulus-responsive hydrogels that are used to establish an intelligent dynamic cell microenvironment. Finally, current challenges and future perspectives of engineering hydrogels for HSC biomedicine are explored.

Advances in electrospinning and 3D bioprinting strategies to enhance functional regeneration of skeletal muscle tissue

Skeletal muscles are essential for body movement, and the loss of motor function due to volumetric muscle loss (VML) limits the mobility of patients. Current therapeutic approaches are insufficient to offer complete functional recovery of muscle damages. Tissue engineering provides viable ways to fabricate scaffolds to regenerate damaged tissues. Hence, tissue engineering options are explored to address existing challenges in the treatment options for muscle regeneration. Electrospinning is a widely employed fabrication technique to make muscle mimetic nanofibrous scaffolds for tissue regeneration. 3D bioprinting has also been utilized to fabricate muscle-like tissues in recent times. This review discusses the anatomy of skeletal muscle, defects, the healing process, and various treatment options for VML. Further, the advanced strategies in electrospinning of natural and synthetic polymers are discussed, along with the recent developments in the fabrication of hybrid scaffolds. Current approaches in 3D bioprinting of skeletal muscle tissues are outlined with special emphasis on the combination of electrospinning and 3D bioprinting towards the development of fully functional muscle constructs. Finally, the current challenges and future perspectives of these convergence techniques are discussed.

Electrospun hydroxyapatite loaded L-polylactic acid aligned nanofibrous membrane patch for rotator cuff repair

Rotator cuff repair remains a challenge clinically due to the high retear rate after surgical intervention. There is a significant need to develop functional biomaterials facilitating tendon-to-bone integration. In this study, hydroxyapatite (HA) incorporated polylactic acid (PLLA) aligned nanofibrous membranes were fabricated by electrospinning as a low-cost sustainable rotator cuff patch. The morphology, physical, mechanical and in vitro cell assays of the nanofibrous membranes were characterized. The results showed that the nanofibrous membrane maintained a rough surface and weakened hydrophobicity. It has excellent cytocompatibility, and the cells were oriented along the direction of fiber arrangement. What's more, the PLLA-HA nanofibrous membrane could increase the alkaline phosphatase (ALP) expression in rat bone marrow mesenchymal stem cells (BMSCs), indicating that the electrospinning PLLA-HA nanofibrous membrane can better induce the bone formation of rat BMSCs cells. When the mass ratio of PLLA to HA exceeds 3: 1, with the increase of the HA content, the patch showed rising induction ability. The results suggested that electrospinning PLLA-HA nanofibrous membranes are an ideal patch for promoting tendon-bone healing and reducing the secondary tear rate. Furthermore, the use of biodegradable polymers and low-cost preparation methods presented the possibility for commercial production of these nanofibrous membranes.

Research progress of natural tissue-derived hydrogels for tissue repair and reconstruction

There are many different grafts to repair damaged tissue. Various types of biological scaffolds, including films, fibers, microspheres, and hydrogels, can be used for tissue repair. A hydrogel, which is composed a natural or synthetic polymer network with high water absorption capacity, can provide a microenvironment closely resembling the extracellular matrix (ECM) of natural tissues to stimulate cell adhesion, proliferation, and differentiation. It has been shown to have great application potential in the field of tissue repair and regeneration. Hydrogels derived from natural tissues retain a variety of proteins and growth factors in optimal proportions, which is beneficial for the regeneration of specific tissues. This article reviews the latest research advances in the field of hydrogels from a variety of natural tissue sources, including bone tissue, blood vessels, nerve tissue, adipose tissue, skin tissue, and muscle tissue, including preparation methods, advantages, and applications in tissue engineering and regenerative medicine. Finally, it summarizes and discusses the challenges faced by natural tissue-derived hydrogels used in tissue repair, as well as future research and application directions.

A myogenic niche with a proper mechanical stress environment improves abdominal wall muscle repair by modulating immunity and preventing fibrosis

Volumetric muscle loss (VML) healing is often complicated by fibrosis, which impairs muscle regeneration and function. Adjusting mechanical stress in the repair environment may modulate immunity and reduce fibrosis. In this study, we aimed to create a biomaterial with suitable tension conditions and bidirectional tissue-inducing abilities to prevent fibrosis thus promote muscle regeneration and induce aponeurosis-like structures to restore muscle force transmission. A protocol was developed to manufacture decellularized muscle aponeurosis (D-MA) patches with an intact extracellular matrix (ECM) and low cytotoxicity. D-MA optimized the mechanical stress distribution in muscle injury sites and decreased the number of proinflammatory macrophages and myofibroblasts, thereby attenuating muscle fibrosis. Muscle and aponeurosis ECM environments had different microstructures and mechanical properties, which specifically enhanced stem cell differentiation into muscle-like cells on muscle ECM and tenocyte-like cells on aponeurosis ECM in vitro. Four weeks after orthotopic implantation, the biphasic muscle-aponeurosis-like tissue was successfully regenerated by the D-MA scaffold. The regenerated muscle fibers in D-MA were more abundant than those in the fibrotic decellularized muscle (D-M) scaffold. D-MA can be used to repair abdominal defects, which significantly improves the repair outcomes. Our results suggest D-MA as a promising material for VML repair.

Natural Scaffolds Used for Liver Regeneration: A Narrative Update

Annually chronic liver diseases cause two million death worldwide. Although liver transplantation (LT) is still considered the best therapeutic option, the limited number of donated livers and lifelong side effects of LT has led researchers to seek alternative therapies. Tissue engineering (TE) as a promising method is considered for liver repair and regeneration. TE uses natural or synthetic scaffolds, functional somatic cells, multipotent stem cells, and growth factors to develop new organs. Biological scaffolds are notable in TE because of their capacity to mimic extracellular matrices, biodegradability, and biocompatibility. Moreover, natural scaffolds are classified based on their source and function in three separate groups. Hemostat-based scaffolds as the first group were reviewed for their application in coagulation in liver injury or surgery. Furthermore, recent studies showed improvement in the function of biological hydrogels in liver regeneration and vascularity. In addition, different applications of natural scaffolds were discussed and compared with synthetic scaffolds. Finally, we focused on the efforts to improve the performance of decellularized extracellular matrixes for liver implantation.

Engineering Hydrogels for the Development of Three-Dimensional In Vitro Models

The superiority of in vitro 3D cultures over conventional 2D cell cultures is well recognized by the scientific community for its relevance in mimicking the native tissue architecture and functionality. The recent paradigm shift in the field of tissue engineering toward the development of 3D in vitro models can be realized with its myriad of applications, including drug screening, developing alternative diagnostics, and regenerative medicine. Hydrogels are considered the most suitable biomaterial for developing an in vitro model owing to their similarity in features to the extracellular microenvironment of native tissue. In this review article, recent progress in the use of hydrogel-based biomaterial for the development of 3D in vitro biomimetic tissue models is highlighted. Discussions of hydrogel sources and the latest hybrid system with different combinations of biopolymers are also presented. The hydrogel crosslinking mechanism and design consideration are summarized, followed by different types of available hydrogel module systems along with recent microfabrication technologies. We also present the latest developments in engineering hydrogel-based 3D in vitro models targeting specific tissues. Finally, we discuss the challenges surrounding current in vitro platforms and 3D models in the light of future perspectives for an improved biomimetic in vitro organ system.

Future foods: Alternative proteins, food architecture, sustainable packaging, and precision nutrition

There are numerous challenges facing the modern food and agriculture industry that urgently need to be addressed, including feeding a growing global population, mitigating and adapting to climate change, decreasing pollution, waste, and biodiversity loss, and ensuring that people remain healthy. At the same time, foods should be safe, affordable, convenient, and delicious. The latest developments in science and technology are being deployed to address these issues. Some of the most important elements within this modern food design approach are encapsulated by the MATCHING model: Meat-reduced; Automation; Technology-driven; Consumer-centric; Healthy; Intelligent; Novel; and Globalization. In this review article, we focus on four key aspects that will be important for the creation of a new generation of healthier and more sustainable foods: emerging raw materials; structural design principles for creating innovative products; developments in eco-friendly packaging; and precision nutrition and customized production of foods. We also highlight some of the most important new developments in science and technology that are being used to create future foods, including food architecture, synthetic biology, nanoscience, and sensory perception.Supplemental data for this article is available online at https://doi.org/10.1080/10408398.2022.2033683.

Assembled Cell-Decorated Collagen (AC-DC) Fiber Bioprinted Implants with Musculoskeletal Tissue Properties Promote Functional Recovery in Volumetric Muscle Loss

Musculoskeletal tissue injuries, including volumetric muscle loss (VML), are commonplace and often lead to permanent disability and deformation. Addressing this healthcare need, an advanced biomanufacturing platform, assembled cell-decorated collagen (AC-DC) bioprinting, is invented to rapidly and reproducibly create living biomaterial implants, using clinically relevant cells and strong, microfluidic wet-extruded collagen microfibers. Quantitative analysis shows that the directionality and distribution of cells throughout AC-DC implants mimic native musculoskeletal tissue. AC-DC bioprinted implants further approximate or exceed the strength and stiffness of human musculoskeletal tissue and exceed collagen hydrogel tensile properties by orders of magnitude. In vivo, AC-DC implants are assessed in a critically sized muscle injury in the hindlimb, with limb torque generation potential measured over 12 weeks. Both acellular and cellular implants promote functional recovery compared to the unrepaired group, with AC-DC implants containing therapeutic muscle progenitor cells promoting the highest degree of recovery. Histological analysis and automated image processing of explanted muscle cross-sections reveal increased total muscle fiber count, median muscle fiber size, and increased cellularization for injuries repaired with cellularized implants. These studies introduce an advanced bioprinting method for generating musculoskeletal tissue analogs with near-native biological and biomechanical properties with the potential to repair myriad challenging musculoskeletal injuries.

Engineering pro-angiogenic biomaterials via chemoselective extracellular vesicle immobilization

Nanoscale extracellular vesicles (EVs) represent a unique cellular derivative that reflect the therapeutic potential of mesenchymal stem cells (MSCs) toward tissue engineering and injury repair without the logistical and safety concerns of utilizing living cells. However, upon systemic administration in vivo,EVs undergo rapid clearance and typically lack controlled targeted delivery, thus reducing their effectiveness in therapeutic regenerative therapies. Here, we describe a strategy that enables long-term in vivo spatial EV retention by chemoselective immobilization of metabolically incoporated azido ligand-bearing EVs (azido-EVs) within a dibenzocyclooctyne-modified collagen hydrogel. MSC-derived azido-EVs exhibit comparable morphological and functional properties as their non-labeled EV counterparts and, when immobilized within collagen hydrogel implants via click chemistry, they elicited more robust host cell infiltration, angiogenic and immunoregulatory responses including vascular ingrowth and macrophage recruitment compared to ten times the higher dose required by non-immobilized EVs. We envision this technology will enable a wide range of applications to spatially promote vascularization and host integration relevant to tissue engineering and regenerative medicine applications.

Using Platelet-Rich Plasma Hydrogel to Deliver Mesenchymal Stem Cells into Three-Dimensional PLGA Scaffold for Cartilage Tissue Engineering

The synthetic biodegradable polyester-based rigid porous scaffolds and cell-laden hydrogels have been separately employed as therapeutic modality for cartilage repair. However, the synthetic rigid scaffolds alone may be limited due to the inherent lack of bioactivity for cartilage regeneration, while the hydrogels have insufficient mechanical properties that are not ideal for load-bearing cartilage applications. In the present study, a hybrid construct was designed to merge the advantage of 3D-printed rigid poly(lactic-co-glycolic acid) (PLGA) scaffolds with cell-laden platelet-rich plasma (PRP) hydrogels that can release growth factors to regulate the tissue healing process. PRP hydrogels potentially achieved the effective delivery of mesenchymal stem cells (MSCs) into PLGA scaffolds. This hybrid construct could obtain adequate mechanical properties and independently provide MSCs with appropriate clues for proliferation and differentiation. Real-time gene expression analysis showed that PRP stimulated both chondrogenic and osteogenic differentiation of MSC seeding into PLGA scaffolds. Finally, the hybrid constructs were implanted into rabbits to simultaneously regenerate both articular cartilage and subchondral bone within osteochondral defects. Our findings suggest that this unique hybrid system could be practically applied for osteochondral regeneration due to its capacity for cell transportation, growth factors release, and excellent mechanical strength, which would greatly contribute to the progress of cartilage tissue engineering.

Mobilizing Endogenous Repair Through Understanding Immune Reaction With Biomaterials

With few exceptions, humans are incapable of fully recovering from severe physical trauma. Due to these limitations, the field of regenerative medicine seeks to find clinically viable ways to repair permanently damaged tissue. There are two main approaches to regenerative medicine: promoting endogenous repair of the wound, or transplanting a material to replace the injured tissue. In recent years, these two methods have fused with the development of biomaterials that act as a scaffold and mobilize the body's natural healing capabilities. This process involves not only promoting stem cell behavior, but by also inducing activity of the immune system. Through understanding the immune interactions with biomaterials, we can understand how the immune system participates in regeneration and wound healing. In this review, we will focus on biomaterials that promote endogenous tissue repair, with discussion on their interactions with the immune system.

Scaffold biomaterials and nano-based therapeutic strategies for skeletal muscle regeneration

Skeletal muscle is integral to the functioning of the human body. Several pathological conditions, such as trauma (primary lesion) or genetic diseases such as Duchenne muscular dystrophy (DMD), can affect and impair its functions or exceed its regeneration capacity. Tissue engineering (TE) based on natural, synthetic and hybrid biomaterials provides a robust platform for developing scaffolds that promote skeletal muscle regeneration, strength recovery, vascularization and innervation. Recent 3D-cell printing technology and the use of nanocarriers for the release of drugs, peptides and antisense oligonucleotides support unique therapeutic alternatives. Here, the authors present recent advances in scaffold biomaterials and nano-based therapeutic strategies for skeletal muscle regeneration and perspectives for future endeavors.

Advanced Glycation End Products Are Retained in Decellularized Muscle Matrix Derived from Aged Skeletal Muscle

Decellularized tissues are biocompatible materials that engraft well, but the age of their source has not been explored for clinical translation. Advanced glycation end products (AGEs) are chemical cross-links that accrue on skeletal muscle collagen in old age, stiffening the matrix and increasing inflammation. Whether decellularized biomaterials derived from aged muscle would suffer from increased AGE collagen cross-links is unknown. We characterized gastrocnemii of 1-, 2-, and 20-month-old C57BL/6J mice before and after decellularization to determine age-dependent changes to collagen stiffness and AGE cross-linking. Total and soluble collagen was measured to assess if age-dependent increases in collagen and cross-linking persisted in decellularized muscle matrix (DMM). Stiffness of aged DMM was determined using atomic force microscopy. AGE levels and the effect of an AGE cross-link breaker, ALT-711, were tested in DMM samples. Our results show that age-dependent increases in collagen amount, cross-linking, and general stiffness were observed in DMM. Notably, we measured increased AGE-specific cross-links within old muscle, and observed that old DMM retained AGE cross-links using ALT-711 to reduce AGE levels. In conclusion, deleterious age-dependent modifications to collagen are present in DMM from old muscle, implying that age matters when sourcing skeletal muscle extracellular matrix as a biomaterial.

Immunomodulation and Biomaterials: Key Players to Repair Volumetric Muscle Loss

Volumetric muscle loss (VML) is defined as a condition in which a large volume of skeletal muscle is lost due to physical insult. VML often results in a heightened immune response, resulting in significant long-term functional impairment. Estimates indicate that ~250,000 fractures occur in the US alone that involve VML. Currently, there is no active treatment to fully recover or repair muscle loss in VML patients. The health economics burden due to VML is rapidly increasing around the world. Immunologists, developmental biologists, and muscle pathophysiologists are exploring both immune responses and biomaterials to meet this challenging situation. The inflammatory response in muscle injury involves a non-specific inflammatory response at the injured site that is coordination between the immune system, especially macrophages and muscle. The potential role of biomaterials in the regenerative process of skeletal muscle injury is currently an important topic. To this end, cell therapy holds great promise for the regeneration of damaged muscle following VML. However, the delivery of cells into the injured muscle site poses a major challenge as it might cause an adverse immune response or inflammation. To overcome this obstacle, in recent years various biomaterials with diverse physical and chemical nature have been developed and verified for the treatment of various muscle injuries. These biomaterials, with desired tunable physicochemical properties, can be used in combination with stem cells and growth factors to repair VML. In the current review, we focus on how various immune cells, in conjunction with biomaterials, can be used to promote muscle regeneration and, most importantly, suppress VML pathology.

Mini review: Biomaterials in repair and regeneration of nerve in a volumetric muscle loss

Volumetric muscle loss (VML) following a severe trauma or injury is beyond the intrinsic regenerative capacity of muscle tissues, and hence interventional therapy is required. Extensive muscle loss concomitant with damage to neuromuscular components overwhelms the muscles' remarkable regenerative capacity. The loss of nervous and vascular tissue leads to further damage and atrophy, so a combined treatment for neuromuscular junction (NMJ) along with the volumetric muscle regeneration is important. There have been immense advances in the field of tissue engineering for skeletal muscle tissue and peripheral nerve regeneration, but very few address the interdependence of the tissues and the need for combined therapies to repair and regenerate fully functional muscle tissue. This review addresses the problem and presents an overview of the biomaterials that have been studied for tissue engineering of neuromuscular tissues associated with skeletal muscles.

Synthesis of casein-γ-polyglutamic acid hydrogels by microbial transglutaminase-mediated gelation for controlled release of drugs

Casein-based hydrogels were reported as biodegradability, biocompatibility, and non-toxic materials that had potential in drug delivery. At present, we prepared two kinds of casein/γ-PGA hybrid hydrogels, 1/5 and 1/9, based on the ratio of γ-PGA to casein. The hydrogels were crosslinked by microbial transglutaminase (MTG), the physicochemical properties of the casein/γ-PGA hydrogels were investigated by scanning electron microscopy (SEM) observation, differential scanning calorimetry (DSC) analysis, texture analysis, swelling ratio test, and stability test. The hydrogels showed a well-interconnected sparse and porous structure. The 1/5 casein/γ-PGA hydrogel was much stable, hard, and cohesive than the 1/9 casein/γ-PGA hydrogel, and the 1/5 casein/γ-PGA hydrogel showed a higher swelling ratio and lower degradation rate. To investigate in vitro release behavior, we chose the hydrophilic vitamin B12 and hydrophobic aspirin as the model drugs incorporated into the casein/γ-PGA hydrogels. The 1/5 casein/γ-PGA hydrogel exhibited a good drug release behavior.

Manipulating Mesenchymal Stem Cell Differentiation on Nanopattern Constructed through Cell-Mediated Mineralization

The extracellular matrix microenvironment, including chemical constituents and topological structure, plays key role in regulating the cell behavior, such as adhesion, proliferation, differentiation, apoptosis, etc. Until now, to investigate the relationship between surface texture and cell response, various ordered patterns have been prepared on the surface of different matrixes, whereas almost all these strategies depend on advanced instruments or severe synthesis conditions. Herein, cell-mediated mineralization method has been applied to construct nanopattern on the surface of β-TCP scaffold. The formation process, morphology, and composition of the final pattern were characterized, and a possible mineralization mechanism has been proposed. Moreover, the cell behavior on the nanopattern has been investigated, and the results showed that the mouse bone marrow mesenchyme stem cells (mBMSCs) display good affinity with the nanopattern, which was manifested by the good proliferation and osteogenic differentiation status of cells. The synthetic strategy may shed light to construct advanced topological structures on other matrixes for bone repair.

Cell Sources for Cultivated Meat: Applications and Considerations throughout the Production Workflow

Cellular agriculture is an emerging scientific discipline that leverages the existing principles behind stem cell biology, tissue engineering, and animal sciences to create agricultural products from cells in vitro. Cultivated meat, also known as clean meat or cultured meat, is a prominent subfield of cellular agriculture that possesses promising potential to alleviate the negative externalities associated with conventional meat production by producing meat in vitro instead of from slaughter. A core consideration when producing cultivated meat is cell sourcing. Specifically, developing livestock cell sources that possess the necessary proliferative capacity and differentiation potential for cultivated meat production is a key technical component that must be optimized to enable scale-up for commercial production of cultivated meat. There are several possible approaches to develop cell sources for cultivated meat production, each possessing certain advantages and disadvantages. This review will discuss the current cell sources used for cultivated meat production and remaining challenges that need to be overcome to achieve scale-up of cultivated meat for commercial production. We will also discuss cell-focused considerations in other components of the cultivated meat production workflow, namely, culture medium composition, bioreactor expansion, and biomaterial tissue scaffolding.

Transdifferentiation of Human Fibroblasts into Skeletal Muscle Cells: Optimization and Assembly into Engineered Tissue Constructs through Biological Ligands

Simple Summary

Engineered human skeletal muscle tissue is a platform tool that can help scientists and physicians better understand human physiology, pharmacology, and disease modeling. Over the past few years this area of research has been actively being pursued by many labs worldwide. Significant challenges remain, including accessing an adequate cell source, and achieving proper physiological-like architecture of the engineered tissue. To address cell resourcing we aimed at further optimizing a process called transdifferentiation which involves the direct conversion of fibroblasts into skeletal muscle cells. The opportunity here is that fibroblasts are readily available and can be expanded sufficiently to meet the needs of a tissue engineering approach. Additionally, we aimed to demonstrate the applicability of transdifferentiation in assembling tissue engineered skeletal muscle. We implemented a screening process of protein ligands in an effort to refine transdifferentiation, and identified that most proteins resulted in a deficit in transdifferentiation efficiency, although one resulted in robust expansion of cultured cells. We were also successful in assembling engineered constructs consisting of transdifferentiated cells. Future directives involve demonstrating that the engineered tissues are capable of contractile and functional activity, and pursuit of optimizing factors such as electrical and chemical exposure, towards achieving physiological parameters observed in human muscle.

Abstract

The development of robust skeletal muscle models has been challenging due to the partial recapitulation of human physiology and architecture. Reliable and innovative 3D skeletal muscle models recently described offer an alternative that more accurately captures the in vivo environment but require an abundant cell source. Direct reprogramming or transdifferentiation has been considered as an alternative. Recent reports have provided evidence for significant improvements in the efficiency of derivation of human skeletal myotubes from human fibroblasts. Herein we aimed at improving the transdifferentiation process of human fibroblasts (tHFs), in addition to the differentiation of murine skeletal myoblasts (C2C12), and the differentiation of primary human skeletal myoblasts (HSkM). Differentiating or transdifferentiating cells were exposed to single or combinations of biological ligands, including Follistatin, GDF8, FGF2, GDF11, GDF15, hGH, TMSB4X, BMP4, BMP7, IL6, and TNF-α. These were selected for their critical roles in myogenesis and regeneration. C2C12 and tHFs displayed significant differentiation deficits when exposed to FGF2, BMP4, BMP7, and TNF-α, while proliferation was significantly enhanced by FGF2. When exposed to combinations of ligands, we observed consistent deficit differentiation when TNF-α was included. Finally, our direct reprogramming technique allowed for the assembly of elongated, cross-striated, and aligned tHFs within tissue-engineered 3D skeletal muscle constructs. In conclusion, we describe an efficient system to transdifferentiate human fibroblasts into myogenic cells and a platform for the generation of tissue-engineered constructs. Future directions will involve the evaluation of the functional characteristics of these engineered tissues.

Current Strategies for the Regeneration of Skeletal Muscle Tissue

Traumatic injuries, tumor resections, and degenerative diseases can damage skeletal muscle and lead to functional impairment and severe disability. Skeletal muscle regeneration is a complex process that depends on various cell types, signaling molecules, architectural cues, and physicochemical properties to be successful. To promote muscle repair and regeneration, various strategies for skeletal muscle tissue engineering have been developed in the last decades. However, there is still a high demand for the development of new methods and materials that promote skeletal muscle repair and functional regeneration to bring approaches closer to therapies in the clinic that structurally and functionally repair muscle. The combination of stem cells, biomaterials, and biomolecules is used to induce skeletal muscle regeneration. In this review, we provide an overview of different cell types used to treat skeletal muscle injury, highlight current strategies in biomaterial-based approaches, the importance of topography for the successful creation of functional striated muscle fibers, and discuss novel methods for muscle regeneration and challenges for their future clinical implementation.

Promoting endogenous repair of skeletal muscle using regenerative biomaterials

Skeletal muscles normally have a remarkable ability to repair themselves; however, large muscle injuries and several myopathies diminish this ability leading to permanent loss of function. No clinical therapy yet exists that reliably restores muscle integrity and function following severe injury. Consequently, numerous tissue engineering techniques, both acellular and with cells, are being investigated to enhance muscle regeneration. Biomaterials are an essential part of these techniques as they can present physical and biochemical signals that augment the repair process. Successful tissue engineering strategies require regenerative biomaterials that either actively promote endogenous muscle repair or create an environment supportive of regeneration. This review will discuss several acellular biomaterial strategies for skeletal muscle regeneration with a focus on those under investigation in vivo. This includes materials that release bioactive molecules, biomimetic materials and immunomodulatory materials.

3D in vitro models of skeletal muscle: myopshere, myobundle and bioprinted muscle construct

Typical two-dimensional (2D) culture models of skeletal muscle-derived cells cannot fully recapitulate the organization and function of living muscle tissues, restricting their usefulness in in-depth physiological studies. The development of functional 3D culture models offers a major opportunity to mimic the living tissues and to model muscle diseases. In this respect, this new type of in vitro model significantly increases our understanding of the involvement of the different cell types present in the formation of skeletal muscle and their interactions, as well as the modalities of response of a pathological muscle to new therapies. This second point could lead to the identification of effective treatments. Here, we report the significant progresses that have been made the last years to engineer muscle tissue-like structures, providing useful tools to investigate the behavior of resident cells. Specifically, we interest in the development of myopshere- and myobundle-based systems as well as the bioprinting constructs. The electrical/mechanical stimulation protocols and the co-culture systems developed to improve tissue maturation process and functionalities are presented. The formation of these biomimetic engineered muscle tissues represents a new platform to study skeletal muscle function and spatial organization in large number of physiological and pathological contexts.

Promoting musculoskeletal system soft tissue regeneration by biomaterial-mediated modulation of macrophage polarization

Musculoskeletal disorders are common in clinical practice. Repairing critical-sized defects in musculoskeletal systems remains a challenge for researchers and surgeons, requiring the application of tissue engineering biomaterials. Successful application depends on the response of the host tissue to the biomaterial and specific healing process of each anatomical structure. The commonly-held view is that biomaterials should be biocompatible to minimize local host immune response. However, a growing number of studies have shown that active modulation of the immune cells, particularly macrophages, via biomaterials is an effective way to control immune response and promote tissue regeneration as well as biomaterial integration. Therefore, we critically review the role of macrophages in the repair of injured musculoskeletal system soft tissues, which have relatively poor regenerative capacities, as well as discuss further enhancement of target tissue regeneration via modulation of macrophage polarization by biomaterial-mediated immunomodulation (biomaterial properties and delivery systems). This active regulation approach rather than passive-evade strategy maximizes the potential of biomaterials to promote musculoskeletal system soft tissue regeneration and provides alternative therapeutic options for repairing critical-sized defects.

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Different phenotypes of macrophages play a crucial role in musculoskeletal system soft tissue regeneration.

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Biomaterials and biomaterial-based delivery systems can be utilized to modulate macrophage polarization.

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This review summarizes immunomodulatory biomaterials to spur musculoskeletal system soft tissue regeneration.

Preparation of Stretchable Nanofibrous Sheets with Sacrificial Coaxial Electrospinning for Treatment of Traumatic Muscle Injury

Traumatic muscle injury with massive loss of muscle volume requires intramuscular implantation of proper scaffolds for fast and successful recovery. Although many artificial scaffolds effectively accelerate formation and maturation of myotubes, limited studies are showing the therapeutic effect of artificial scaffolds in animal models with massive muscle injury. In this study, improved myotube differentiation is approved on stepwise stretched gelatin nanofibers and applied to damaged muscle recovery in an animal model. The gelatin nanofibers are fabricated by a two-step process composed of co-axial electrospinning of poly(ɛ-caprolactone) and gelatin and subsequent removal of the outer shells. When stepwise stretching is applied to the myoblasts on gelatin nanofibers for five days, enhanced myotube formation and polarized elongation are observed. Animal models with volumetric loss at quadriceps femoris muscles (>50%) are transplanted with the myotubes cultivated on thin and flexible gelatin nanofiber. Treated animals more efficiently recover exercising functions of the leg when myotubes and the gelatin nanofiber are co-implanted at the injury sites. This result suggests that mechanically stimulated myotubes on gelatin nanofiber is therapeutically feasible for the robust recovery of volumetric muscle loss.

Functional Skeletal Muscle Regeneration with Thermally Drawn Porous Fibers and Reprogrammed Muscle Progenitors for Volumetric Muscle Injury

Skeletal muscle has an inherent capacity for spontaneous regeneration. However, recovery after severe injuries such as volumetric muscle loss (VML) is limited. There is therefore a need to develop interventions to induce functional skeletal muscle restoration. One suggested approach includes tissue-engineered muscle constructs. Tissue-engineering treatments have so far been impeded by the lack of reliable cell sources and the challenges in engineering of suitable tissue scaffolds. To address these challenges, muscle extracellular matrix (MEM) and induced skeletal myogenic progenitor cells (iMPCs) are integrated within thermally drawn fiber based microchannel scaffolds. The microchannel fibers decorated with MEM enhance differentiation and maturation of iMPCs. Furthermore, engraftment of these bioengineered hybrid muscle constructs induce de novo muscle regeneration accompanied with microvessel and neuromuscular junction formation in a VML mouse model, ultimately leading to functional recovery of muscle activity.

The Future of Regenerative Medicine: Cell Therapy Using Pluripotent Stem Cells and Acellular Therapies Based on Extracellular Vesicles

The rapid progress in the field of stem cell research has laid strong foundations for their use in regenerative medicine applications of injured or diseased tissues. Growing evidences indicate that some observed therapeutic outcomes of stem cell-based therapy are due to paracrine effects rather than long-term engraftment and survival of transplanted cells. Given their ability to cross biological barriers and mediate intercellular information transfer of bioactive molecules, extracellular vesicles are being explored as potential cell-free therapeutic agents. In this review, we first discuss the state of the art of regenerative medicine and its current limitations and challenges, with particular attention on pluripotent stem cell-derived products to repair organs like the eye, heart, skeletal muscle and skin. We then focus on emerging beneficial roles of extracellular vesicles to alleviate these pathological conditions and address hurdles and operational issues of this acellular strategy. Finally, we discuss future directions and examine how careful integration of different approaches presented in this review could help to potentiate therapeutic results in preclinical models and their good manufacturing practice (GMP) implementation for future clinical trials.