Aligned nanofibers of decellularized muscle ECM support myogenic activity in primary satellite cells in vitro

Suspension electrospinning of decellularized extracellular matrix: A new method to preserve bioactivity

Decellularized extracellular matrices (dECM) have strong regenerative potential as tissue engineering scaffolds; however, current clinical options for dECM scaffolds are limited to freeze-drying its native form into sheets. Electrospinning is a versatile scaffold fabrication technique that allows control of macro- and microarchitecture. It remains challenging to electrospin dECM, which has led researchers to either blend it with synthetic materials or use enzymatic digestion to fully solubilize the dECM. Both strategies reduce the innate bioactivity of dECM and limit its regenerative potential. Herein, we developed a new suspension electrospinning method to fabricate a pure dECM fibrous mesh that retains its innate bioactivity. Systematic investigation of suspension parameters was used to identify critical rheological properties required to instill "spinnability," including homogenization, concentration, and particle size. Homogenization enhanced particle interaction to impart the requisite elastic behavior to withstand electrostatic drawing without breaking. A direct correlation between concentration and viscosity was observed that altered fiber morphology; whereas, particle size had minimal impact on suspension properties and fiber morphology. The versatility of this new method was demonstrated by electrospinning dECM with three common decellularization techniques (Abraham, Badylak, Luo) and tissue sources (intestinal submucosa, heart, skin). Bioactivity retention after electrospinning was confirmed using cell proliferation, angiogenesis, and macrophage polarization assays. Collectively, these findings provide a framework for researchers to electrospin dECM for diverse tissue engineering applications.

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.

In vitro development of a muscle-tendon junction construct using decellularised extracellular matrix: Effect of cyclic tensile loading

The muscle tendon junction (MTJ) plays a crucial role in transmitting the force generated by muscles to the tendon and then to the bone. Injuries such as tears and strains frequently happen at the MTJ, where the regenerative process is limited due to poor vascularization and the complex structure of the tissue. Current solutions for a complete tear at the MTJ have not been successful and therefore, the development of a tissue-engineered MTJ may provide a more effective treatment. In this study, decellularised extracellular matrix (DECM) derived from sheep MTJ was used to provide a scaffold for the MTJ with the relevant mechanical properties and differentiation cues such as the relase of growth factors. Human mesenchymal stem cells (MSCs) were seeded on DECM and 10 % cyclic strain was applied using a bioreactor. MSCs cultured on DECM showed significantly higher gene and protein expression of MTJ markers such as collagen 22, paxillin and talin, than MSCs in 2D culture. Although collagen 22 protein expression was higher in the cells with strain than without strain, reduced gene expression of other MTJ markers was observed when the strain was applied. DECM combined with 10 % strain enhanced myogenic differentiation, while tenogenic differentiation was reduced when compared to static cultures of MSCs on DECM. For the first time, these results showed that DECM derived from the MTJ can induce MTJ marker gene and protein expression by MSCs, however, the effect of strain on the MTJ development in DECM culture needs further investigation.

Decellularized Bovine Skeletal Muscle Scaffolds: Structural Characterization and Preliminary Cytocompatibility Evaluation

Skeletal muscle degeneration is responsible for major mobility complications, and this muscle type has little regenerative capacity. Several biomaterials have been proposed to induce muscle regeneration and function restoration. Decellularized scaffolds present biological properties that allow efficient cell culture, providing a suitable microenvironment for artificial construct development and being an alternative for in vitro muscle culture. For translational purposes, biomaterials derived from large animals are an interesting and unexplored source for muscle scaffold production. Therefore, this study aimed to produce and characterize bovine muscle scaffolds to be applied to muscle cell 3D cultures. Bovine muscle fragments were immersed in decellularizing solutions for 7 days. Decellularization efficiency, structure, composition, and three-dimensionality were evaluated. Bovine fetal myoblasts were cultured on the scaffolds for 10 days to attest cytocompatibility. Decellularization was confirmed by DAPI staining and DNA quantification. Histological and immunohistochemical analysis attested to the preservation of main ECM components. SEM analysis demonstrated that the 3D structure was maintained. In addition, after 10 days, fetal myoblasts were able to adhere and proliferate on the scaffolds, attesting to their cytocompatibility. These data, even preliminary, infer that generated bovine muscular scaffolds were well structured, with preserved composition and allowed cell culture. This study demonstrated that biomaterials derived from bovine muscle could be used in tissue engineering.

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.

Decellularized extracellular matrix biomaterials for regenerative therapies: Advances, challenges and clinical prospects

Tissue engineering and regenerative medicine have shown potential in the repair and regeneration of tissues and organs via the use of engineered biomaterials and scaffolds. However, current constructs face limitations in replicating the intricate native microenvironment and achieving optimal regenerative capacity and functional recovery. To address these challenges, the utilization of decellularized tissues and cell-derived extracellular matrix (ECM) has emerged as a promising approach. These biocompatible and bioactive biomaterials can be engineered into porous scaffolds and grafts that mimic the structural and compositional aspects of the native tissue or organ microenvironment, both in vitro and in vivo. Bioactive dECM materials provide a unique tissue-specific microenvironment that can regulate and guide cellular processes, thereby enhancing regenerative therapies. In this review, we explore the emerging frontiers of decellularized tissue-derived and cell-derived biomaterials and bio-inks in the field of tissue engineering and regenerative medicine. We discuss the need for further improvements in decellularization methods and techniques to retain structural, biological, and physicochemical characteristics of the dECM products in a way to mimic native tissues and organs. This article underscores the potential of dECM biomaterials to stimulate in situ tissue repair through chemotactic effects for the development of growth factor and cell-free tissue engineering strategies. The article also identifies the challenges and opportunities in developing sterilization and preservation methods applicable for decellularized biomaterials and grafts and their translation into clinical products.

Exploring polysaccharide and protein-enriched decellularized matrix scaffolds for tendon and ligament repair: A review

Tissue engineering (TE) has become a primary research topic for the treatment of diseased or damaged tendon/ligament (T/L) tissue. T/L injuries pose a severe clinical burden worldwide, necessitating the development of effective strategies for T/L repair and tissue regeneration. TE has emerged as a promising strategy for restoring T/L function using decellularized extracellular matrix (dECM)-based scaffolds. dECM scaffolds have gained significant prominence because of their native structure, relatively high bioactivity, low immunogenicity, and ability to function as scaffolds for cell attachment, proliferation, and differentiation, which are difficult to imitate using synthetic materials. Here, we review the recent advances and possible future prospects for the advancement of dECM scaffolds for T/L tissue regeneration. We focus on crucial scaffold properties and functions, as well as various engineering strategies employed for biomaterial design in T/L regeneration. dECM provides both the physical and mechanical microenvironments required by cells to survive and proliferate. Various decellularization methods and sources of allogeneic and xenogeneic dECM in T/L repair and regeneration are critically discussed. Additionally, dECM hydrogels, bio-inks in 3D bioprinting, and nanofibers are briefly explored. Understanding the opportunities and challenges associated with dECM-based scaffold development is crucial for advancing T/L repairs in tissue engineering and regenerative medicine.

Decellularized extracellular matrix-based composite scaffolds for tissue engineering and regenerative medicine

Despite the considerable advancements in fabricating polymeric-based scaffolds for tissue engineering, the clinical transformation of these scaffolds remained a big challenge because of the difficulty of simulating native organs/tissues' microenvironment. As a kind of natural tissue-derived biomaterials, decellularized extracellular matrix (dECM)-based scaffolds have gained attention due to their unique biomimetic properties, providing a specific microenvironment suitable for promoting cell proliferation, migration, attachment and regulating differentiation. The medical applications of dECM-based scaffolds have addressed critical challenges, including poor mechanical strength and insufficient stability. For promoting the reconstruction of damaged tissues or organs, different types of dECM-based composite platforms have been designed to mimic tissue microenvironment, including by integrating with natural polymer or/and syntenic polymer or adding bioactive factors. In this review, we summarized the research progress of dECM-based composite scaffolds in regenerative medicine, highlighting the critical challenges and future perspectives related to the medical application of these composite materials.

Aligned Collagen Sponges with Tunable Pore Size for Skeletal Muscle Tissue Regeneration

Volumetric muscle loss (VML) is a traumatic injury where at least 20% of the mass of a skeletal muscle has been destroyed and functionality is lost. The standard treatment for VML, autologous tissue transfer, is limited as approximately 1 in 10 grafts fail because of necrosis or infection. Tissue engineering strategies seek to develop scaffolds that can regenerate injured muscles and restore functionality. Many of these scaffolds, however, are limited in their ability to restore muscle functionality because of an inability to promote the alignment of regenerating myofibers. For aligned myofibers to form on a scaffold, myoblasts infiltrate the scaffold and receive topographical cues to direct targeted myofiber growth. We seek to determine the optimal pore size for myoblast infiltration and differentiation. We developed a method of tuning the pore size within collagen scaffolds while inducing longitudinal alignment of these pores. Significantly different pore sizes were generated by adjusting the freezing rate of the scaffolds. Scaffolds frozen at -20 °C contained the largest pores. These scaffolds promoted the greatest level of cell infiltration and orientation in the direction of pore alignment. Further research will be conducted to induce higher levels of myofiber formation, to ultimately create an off-the-shelf treatment for VML injuries.

The Use of Collagen Methacrylate in Actuating Polyethylene Glycol Diacrylate-Acrylic Acid Scaffolds for Muscle Regeneration

After muscle loss or injury, skeletal muscle tissue has the ability to regenerate and return its function. However, large volume defects in skeletal muscle tissue pose a challenge to regenerate due to the absence of regenerative elements such as biophysical and biochemical cues, making the development of new treatments necessary. One potential solution is to utilize electroactive polymers that can change size or shape in response to an external electric field. Poly(ethylene glycol) diacrylate (PEGDA) is one such polymer, which holds great potential as a scaffold for muscle tissue regeneration due to its mechanical properties. In addition, the versatile chemistry of this polymer allows for the conjugation of new functional groups to enhance its electroactive properties and biocompatibility. Herein, we have developed an electroactive copolymer of PEGDA and acrylic acid (AA) in combination with collagen methacrylate (CMA) to promote cell adhesion and proliferation. The electroactive properties of the CMA + PEGDA:AA constructs were investigated through actuation studies. Furthermore, the biological properties of the hydrogel were investigated in a 14-day in vitro study to evaluate myosin light chain (MLC) expression and metabolic activity of C2C12 mouse myoblast cells. The addition of CMA improved some aspects of material bioactivity, such as MLC expression in C2C12 mouse myoblast cells. However, the incorporation of CMA in the PEGDA:AA hydrogels reduced the sample movement when placed under an electric field, possibly due to steric hindrance from the CMA. Further research is needed to optimize the use of CMA in combination with PEGDA:AA as a potential scaffold for skeletal muscle tissue engineering.

Electrospun Poly(L-lactide-co-ε-caprolactone) Scaffold Potentiates C2C12 Myoblast Bioactivity and Acts as a Stimulus for Cell Commitment in Skeletal Muscle Myogenesis

Tissue engineering combines a scaffold, cells and regulatory signals, reproducing a biomimetic extracellular matrix capable of supporting cell attachment and proliferation. We examined the role of an electrospun scaffold made of a biocompatible polymer during the myogenesis of skeletal muscle (SKM) as an alternative approach to tissue regeneration. The engineered nanostructure was obtained by electrospinning poly(L-lactide-co-ε-caprolactone) (PLCL) in the form of a 3D porous nanofibrous scaffold further coated with collagen. C2C12 were cultured on the PLCL scaffold, and cell morphology and differentiation pathways were thoroughly investigated. The functionalized PLCL scaffold recreated the SKM nanostructure and performed its biological functions, guiding myoblast morphogenesis and promoting cell differentiation until tissue formation. The scaffold enabled cell-cell interactions through the development of cellular adhesions that were fundamental during myoblast fusion and myotube formation. Expression of myogenic regulatory markers and muscle-specific proteins at different stages of myogenesis suggested that the PLCL scaffold enhanced myoblast differentiation within a shorter time frame. The functionalized PLCL scaffold impacts myoblast bioactivity and acts as a stimulus for cell commitment, surpassing traditional 2D cell culture techniques. We developed a screening model for tissue development and a device for tissue restoration.

Preparation and Use of Decellularized Extracellular Matrix for Tissue Engineering

The multidisciplinary fields of tissue engineering and regenerative medicine have the potential to revolutionize the practise of medicine through the abilities to repair, regenerate, or replace tissues and organs with functional engineered constructs. To this end, tissue engineering combines scaffolding materials with cells and biologically active molecules into constructs with the appropriate structures and properties for tissue/organ regeneration, where scaffolding materials and biomolecules are the keys to mimic the native extracellular matrix (ECM). For this, one emerging way is to decellularize the native ECM into the materials suitable for, directly or in combination with other materials, creating functional constructs. Over the past decade, decellularized ECM (or dECM) has greatly facilitated the advance of tissue engineering and regenerative medicine, while being challenged in many ways. This article reviews the recent development of dECM for tissue engineering and regenerative medicine, with a focus on the preparation of dECM along with its influence on cell culture, the modification of dECM for use as a scaffolding material, and the novel techniques and emerging trends in processing dECM into functional constructs. We highlight the success of dECM and constructs in the in vitro, in vivo, and clinical applications and further identify the key issues and challenges involved, along with a discussion of future research directions.

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.

Nanomedicine, a valuable tool for skeletal muscle disorders: Challenges, promises, and limitations

Muscular dystrophies are a group of rare genetic disorders characterized by progressive muscle weakness, which, in the most severe forms, leads to the patient's death due to cardiorespiratory problems. There is still no cure available for these diseases and significant effort is being placed into developing new strategies to either correct the genetic defect or to compensate muscle loss by stimulating skeletal muscle regeneration. However, the vast anatomical extension of the target tissue poses great challenges to these goals, highlighting the need for complementary strategies. Nanomedicine is an actively evolving field that merges nanotechnologies with biomedical and pharmaceutical sciences. It holds great potential in regenerative medicine, both in supporting tissue engineering and regeneration, and in optimizing drug and oligonucleotide delivery and gene therapy strategies. In this review, we will summarize the state‐of‐the‐art in the field of nanomedicine applied to skeletal muscle regeneration. We will discuss the recent work toward the development of nanopatterned scaffolds for tissue engineering, the efforts in the synthesis of organic and inorganic nanoparticles for gene therapy and drug delivery applications, as well as their use as immune modulators. Although nanomedicine holds great promise for muscle and other degenerative diseases, many challenges still need to be systematically addressed to assure a smooth transition from the bench to the bedside.

This article is categorized under:

Implantable Materials and Surgical Technologies > Nanotechnology in Tissue Repair and Replacement

Customized bioreactor enables the production of 3D diaphragmatic constructs influencing matrix remodeling and fibroblast overgrowth

The production of skeletal muscle constructs useful for replacing large defects in vivo, such as in congenital diaphragmatic hernia (CDH), is still considered a challenge. The standard application of prosthetic material presents major limitations, such as hernia recurrences in a remarkable number of CDH patients. With this work, we developed a tissue engineering approach based on decellularized diaphragmatic muscle and human cells for the in vitro generation of diaphragmatic-like tissues as a proof-of-concept of a new option for the surgical treatment of large diaphragm defects. A customized bioreactor for diaphragmatic muscle was designed to control mechanical stimulation and promote radial stretching during the construct engineering. In vitro tests demonstrated that both ECM remodeling and fibroblast overgrowth were positively influenced by the bioreactor culture. Mechanically stimulated constructs also increased tissue maturation, with the formation of new oriented and aligned muscle fibers. Moreover, after in vivo orthotopic implantation in a surgical CDH mouse model, mechanically stimulated muscles maintained the presence of human cells within myofibers and hernia recurrence did not occur, suggesting the value of this approach for treating diaphragm defects.

Decellularization Strategies for Regenerating Cardiac and Skeletal Muscle Tissues

Cardiovascular disease is the leading cause of death worldwide and is associated with approximately 17.9 million deaths each year. Musculoskeletal conditions affect more than 1.71 billion people globally and are the leading cause of disability. These two areas represent a massive global health burden that is perpetuated by a lack of functionally restorative treatment options. The fields of regenerative medicine and tissue engineering offer great promise for the development of therapies to repair damaged or diseased tissues. Decellularized tissues and extracellular matrices are cornerstones of regenerative biomaterials and have been used clinically for decades and many have received FDA approval. In this review, we first discuss and compare methods used to produce decellularized tissues and ECMs from cardiac and skeletal muscle. We take a focused look at how different biophysical properties such as spatial topography, extracellular matrix composition, and mechanical characteristics influence cell behavior and function in the context of regenerative medicine. Lastly, we describe emerging research and forecast the future high impact applications of decellularized cardiac and skeletal muscle that will drive novel and effective regenerative therapies.

Expanding the Repertoire of Electrospinning: New and Emerging Biopolymers, Techniques, and Applications

Electrospinning has emerged as a versatile and accessible technology for fabricating polymer fibers, particularly for biological applications. Natural polymers or biopolymers (including synthetically derivatized natural polymers) represent a promising alternative to synthetic polymers, as materials for electrospinning. Many biopolymers are obtained from abundant renewable sources, are biodegradable, and possess inherent biological functions. This review surveys recent literature reporting new fibers produced from emerging biopolymers, highlighting recent developments in the use of sulfated polymers (including carrageenans and glycosaminoglycans), tannin derivatives (condensed and hydrolyzed tannins, tannic acid), modified collagen, and extracellular matrix extracts. The proposed advantages of these biopolymer-based fibers, focusing on their biomedical applications, are also discussed to highlight the use of new and emerging biopolymers (or new modifications to well-established ones) to enhance or achieve new properties for electrospun fiber materials.

Key concepts in muscle regeneration: muscle "cellular ecology" integrates a gestalt of cellular cross-talk, motility, and activity to remodel structure and restore function

This review identifies some key concepts of muscle regeneration, viewed from perspectives of classical and modern research. Early insights noted the pattern and sequence of regeneration across species was similar, regardless of the type of injury, and differed from epimorphic limb regeneration. While potential benefits of exercise for tissue repair was debated, regeneration was not presumed to deliver functional restoration, especially after ischemia-reperfusion injury; muscle could develop fibrosis and ectopic bone and fat. Standard protocols and tools were identified as necessary for tracking injury and outcomes. Current concepts vastly extend early insights. Myogenic regeneration occurs within the environment of muscle tissue. Intercellular cross-talk generates an interactive system of cellular networks that with the extracellular matrix and local, regional, and systemic influences, forms the larger gestalt of the satellite cell niche. Regenerative potential and adaptive plasticity are overlain by epigenetically regionalized responsiveness and contributions by myogenic, endothelial, and fibroadipogenic progenitors and inflammatory and metabolic processes. Muscle architecture is a living portrait of functional regulatory hierarchies, while cellular dynamics, physical activity, and muscle-tendon-bone biomechanics arbitrate regeneration. The scope of ongoing research-from molecules and exosomes to morphology and physiology-reveals compelling new concepts in muscle regeneration that will guide future discoveries for use in application to fitness, rehabilitation, and disease prevention and treatment.

Nanofiber configuration affects biological performance of decellularized meniscus extracellular matrix incorporated electrospun scaffolds

Electrospinning represents the simplest approach to fabricate nanofiber scaffolds that approximate the heterogeneous fibrous structure of the meniscus. More effort is needed to understand the relationship between scaffold properties and cell responses to determine the appropriate scaffolds supporting meniscus tissue repair and regeneration. In this study, we investigate the influence of nanofiber configuration of electrospun scaffolds on phenotype and matrix production of meniscus cells, as well as on scaffold degradation behaviors and biocompatibility. Twisting electrospun nanofibers into yarns not only recapitulates the major collagen bundles of the meniscus but also increases the pore size and porosity of resultant scaffolds. The yarn scaffold significantly regulated expression levels of meniscus-associated genes and promoted extracellular matrix production compared with conventional electrospun scaffolds with random or aligned nanofiber orientation. Additionally, the yarn scaffold allowed considerable cell infiltration and experienced faster degradation and tissue remodeling upon subcutaneous implantation in a rat model. These results suggest that nanofiber configuration dictates cell interactions, scaffold degradation and integration with host tissue, providing design parameters of porosity and pore size of electrospun scaffolds toward meniscus repair.

Extracellular matrix extraction techniques and applications in biomedical engineering

The concept of tissue engineering involves regeneration and repair of damaged tissue and organs using various combinations of cells, growth factors and scaffolds. The extracellular matrix (ECM) forms the integral part of the scaffold to induce cell proliferation thereby leading to new tissue formation. Decellularization technique provides decellularized ECM (dECM), free of cells while preserving the in vivo biomolecules. In this review, we focus on the detailed methodology of diverse decellularization techniques for various organs of different animals, and the biomedical applications employing the dECM. A culmination of different methods of decellularization is optimized, which offers a suitable microenvironment mimicking the native in vivo topography for in vitro organ regeneration. A detailed assessment of the dECM provides information on the microarchitecture, presence of ECM proteins, biocompatibility and cell proliferation. dECM has also been processed as scaffolds and drug-delivery vehicles, and utilized for regeneration.

Photoreactive hydrogel stiffness influences volumetric muscle loss repair

Volumetric muscle loss (VML) injuries are characterized by permanent loss of muscle mass, structure, and function. Hydrogel biomaterials provide an attractive platform for skeletal muscle tissue engineering due to the ability to easily modulate their biophysical and biochemical properties to match a range of tissue characteristics. In this work we successfully developed a mechanically tunable hyaluronic acid (HA) hydrogel system to investigate the influence of hydrogel stiffness on VML repair. HA was functionalized with photoreactive norbornene groups to create hydrogel networks that rapidly crosslink via thiol-ene click chemistry with tailored mechanics. Mechanical properties were controlled by modulating the amount of matrix metalloproteinase (MMP)-degradable peptide crosslinker to produce hydrogels with increasing elastic moduli of 1.1 ± 0.002, 3.0 ± 0.002, and 10.6 ± 0.006 kPa mimicking a relevant range of developing and mature muscle stiffnesses. Functional muscle recovery was assessed following implantation of the HA hydrogels by in situ photopolymerization into rat latissimus dorsi (LD) VML defects at 12 and 24 weeks post-injury. After 12 weeks, muscles treated with medium stiffness (3.0 kPa) hydrogels produced maximum isometric forces most similar to contralateral healthy LD muscles. This trend persisted at 24 weeks post-injury, suggestive of sustained functional recovery. Histological analysis revealed a significantly larger zone of regeneration with more de novo muscle fibers following implantation of medium stiffness hydrogels in VML-injured muscles compared to other experimental groups. Lower (low and medium) stiffness hydrogels also appeared to attenuate the chronic inflammatory response characteristic of VML injuries, displaying similar levels of macrophage infiltration and polarization to healthy muscle. Together these findings illustrate the importance of hydrogel mechanical properties in supporting functional repair of VML injuries.

Towards bioengineered skeletal muscle: recent developments in vitro and in vivo

Skeletal muscle is a functional tissue that accounts for approximately 40% of the human body mass. It has remarkable regenerative potential, however, trauma and volumetric muscle loss, progressive disease and aging can lead to significant muscle loss that the body cannot recover from. Clinical approaches to address this range from free-flap transfer for traumatic events involving volumetric muscle loss, to myoblast transplantation and gene therapy to replace muscle loss due to sarcopenia and hereditary neuromuscular disorders, however, these interventions are often inadequate. The adoption of engineering paradigms, in particular materials engineering and materials/tissue interfacing in biology and medicine, has given rise to the rapidly growing, multidisciplinary field of bioengineering. These methods have facilitated the development of new biomaterials that sustain cell growth and differentiation based on bionic biomimicry in naturally occurring and synthetic hydrogels and polymers, as well as additive fabrication methods to generate scaffolds that go some way to replicate the structural features of skeletal muscle. Recent advances in biofabrication techniques have resulted in significant improvements to some of these techniques and have also offered promising alternatives for the engineering of living muscle constructs ex vivo to address the loss of significant areas of muscle. This review highlights current research in this area and discusses the next steps required towards making muscle biofabrication a clinical reality.

Aligned and electrically conductive 3D collagen scaffolds for skeletal muscle tissue engineering

Skeletal muscle is characterized by its three-dimensional (3D) anisotropic architecture composed of highly aligned and electrically-excitable muscle fibers that enable normal movement. Biomaterial-based tissue engineering approaches to repair skeletal muscle are limited due to difficulties combining 3D structural alignment (to guide cell/matrix organization) and electrical conductivity (to enable electrically-excitable myotube assembly and maturation). In this work we successfully produced aligned and electrically conductive 3D collagen scaffolds using a freeze-drying approach. Conductive polypyrrole (PPy) nanoparticles were synthesized and directly mixed into a suspension of type I collagen and chondroitin sulfate followed by directional lyophilization. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and confocal microscopy showed that directional solidification resulted in scaffolds with longitudinally aligned pores with homogeneously-distributed PPy content. Chronopotentiometry verified that PPy incorporation resulted in a five-fold increase in conductivity compared to non-PPy-containing collagen scaffolds without detrimentally affecting myoblast metabolic activity. Furthermore, the aligned scaffold microstructure provided contact guidance cues that directed myoblast growth and organization. Incorporation of PPy also promoted enhanced myotube formation and maturation as measured by myosin heavy chain (MHC) expression and number of nuclei per myotube. Together these data suggest that aligned and electrically conductive 3D collagen scaffolds could be useful for skeletal muscle tissue engineering.

Advances in 3D bioprinting technology for cardiac tissue engineering and regeneration

Cardiovascular disease is still one of the leading causes of death in the world, and heart transplantation is the current major treatment for end-stage cardiovascular diseases. However, because of the shortage of heart donors, new sources of cardiac regenerative medicine are greatly needed. The prominent development of tissue engineering using bioactive materials has creatively laid a direct promising foundation. Whereas, how to precisely pattern a cardiac structure with complete biological function still requires technological breakthroughs. Recently, the emerging three-dimensional (3D) bioprinting technology for tissue engineering has shown great advantages in generating micro-scale cardiac tissues, which has established its impressive potential as a novel foundation for cardiovascular regeneration. Whether 3D bioprinted hearts can replace traditional heart transplantation as a novel strategy for treating cardiovascular diseases in the future is a frontier issue. In this review article, we emphasize the current knowledge and future perspectives regarding available bioinks, bioprinting strategies and the latest outcome progress in cardiac 3D bioprinting to move this promising medical approach towards potential clinical implementation.

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.

From cells-on-a-chip to organs-on-a-chip: scaffolding materials for 3D cell culture in microfluidics

It is an emerging research area to integrate scaffolding materials in microfluidic devices for 3D cell culture (organs-on-a-chip). The technology of organs-on-a-chip holds the potential to obviate the gaps between pre-clinical and clinical studies. As accumulating evidence shows the importance of extracellular matrix in in vitro cell culture, significant efforts have been made to integrate 3D ECM/scaffolding materials in microfluidics. There are two families of materials that are commonly used for this purpose: hydrogels and electrospun fibers. In this review, we briefly discuss the properties of the materials, and focus on the various technologies to obtain the materials (e.g. extraction of collagen from animal tissues) and to include the materials in microfluidic devices. Challenges and potential solutions of the current materials and technologies were also thoroughly discussed. At the end, we provide a perspective on future efforts to make these technologies more translational to broadly benefit pharmaceutical and pathophysiological research.

The Role of Innate and Adaptive Immune Cells in Skeletal Muscle Regeneration

Skeletal muscle regeneration is highly dependent on the inflammatory response. A wide variety of innate and adaptive immune cells orchestrate the complex process of muscle repair. This review provides information about the various types of immune cells and biomolecules that have been shown to mediate muscle regeneration following injury and degenerative diseases. Recently developed cell and drug-based immunomodulatory strategies are highlighted. An improved understanding of the immune response to injured and diseased skeletal muscle will be essential for the development of therapeutic strategies.

Recapitulating pathophysiology of skeletal muscle diseases in vitro using primary mouse myoblasts on a nanofibrous platform

Tissue engineering approaches are used to mimic the microenvironment of the skeletal muscle in vitro. However, the validation of a bioengineered muscle as a model to study diseases is inadequate. Here, we present polycaprolactone nanofibers as a robust platform that mimics cellular organization and recapitulates critical functions of the myotubes observed in vivo. We isolated myoblasts from mice following a simplified protocol and cultured them on aligned nanofibers. Myotubes grown on aligned nanofibers maintained alignment for 14 days and exhibited a time-dependent increase in levels of p-AKT upon insulin stimulation. Treatment with matrix-assisted integrin inhibitor led to reduction in p-AKT levels, underscoring the critical role of environment on the biological processes. We demonstrate the suitability of myotubes grown on nanofibrous platform to study corticosteroid-induced muscle degeneration. This study, thus, demonstrates that aligned nanofibers retain myotubes in culture for longer duration and recapitulate the functions of skeletal muscle under pathophysiological conditions.

3D Printing Decellularized Extracellular Matrix to Design Biomimetic Scaffolds for Skeletal Muscle Tissue Engineering

Functional engineered muscles are still a critical clinical issue to be addressed, although different strategies have been considered so far for the treatment of severe muscular injuries. Indeed, the regenerative capacity of skeletal muscle (SM) results inadequate for large-scale defects, and currently, SM reconstruction remains a complex and unsolved task. For this aim, tissue engineered muscles should provide a proper biomimetic extracellular matrix (ECM) alternative, characterized by an aligned/microtopographical structure and a myogenic microenvironment, in order to promote muscle regeneration. As a consequence, both materials and fabrication techniques play a key role to plan an effective therapeutic approach. Tissue-specific decellularized ECM (dECM) seems to be one of the most promising material to support muscle regeneration and repair. 3D printing technologies, on the other side, enable the fabrication of scaffolds with a fine and detailed microarchitecture and patient-specific implants with high structural complexity. To identify innovative biomimetic solutions to develop engineered muscular constructs for the treatment of SM loss, the more recent (last 5 years) reports focused on SM dECM-based scaffolds and 3D printing technologies for SM regeneration are herein reviewed. Possible design inputs for 3D printed SM dECM-based scaffolds for muscular regeneration are also suggested.

Composite Membranes with Nanofibrous Cross-Hatched Supports for Reverse Osmosis Desalination

A novel membrane structure composed of cross-hatched electrospun nanofibers is developed. We illustrate that this novel structure allows for much higher water permeability when used as a support for reverse osmosis thin-film composite membranes. Reinforcement and lamination of the aligned nanofibers generates mechanically robust structures that retain very high porosity and low tortuosity when applied to high pressure desalination operations. The cross-hatched nanofiber layers support the polyamide active layer firmly and reduce resistance to water flow due to the high porosity, low tortuosity, high mechanical strength, and minimal thickness of the structures. The nanofiber composite membrane gives a water flux significantly greater than when a traditional support layer is used, at 99 ± 5 m^-2 h^-1 with NaCl rejection of 98.7% at 15.5 bar.

Next Stage Approach to Tissue Engineering Skeletal Muscle

Large-scale muscle injury in humans initiates a complex regeneration process, as not only the muscular, but also the vascular and neuro-muscular compartments have to be repaired. Conventional therapeutic strategies often fall short of reaching the desired functional outcome, due to the inherent complexity of natural skeletal muscle. Tissue engineering offers a promising alternative treatment strategy, aiming to achieve an engineered tissue close to natural tissue composition and function, able to induce long-term, functional regeneration after in vivo implantation. This review aims to summarize the latest approaches of tissue engineering skeletal muscle, with specific attention toward fabrication, neuro-angiogenesis, multicellularity and the biochemical cues that adjuvate the regeneration process.

Smart ECM-Based Electrospun Biomaterials for Skeletal Muscle Regeneration

The development of smart and intelligent regenerative biomaterials for skeletal muscle tissue engineering is an ongoing challenge, owing to the requirement of achieving biomimetic systems able to communicate biological signals and thus promote optimal tissue regeneration. Electrospinning is a well-known technique to produce fibers that mimic the three dimensional microstructural arrangements, down to nanoscale and the properties of the extracellular matrix fibers. Natural and synthetic polymers are used in the electrospinning process; moreover, a blend of them provides composite materials that have demonstrated the potential advantage of supporting cell function and adhesion. Recently, the decellularized extracellular matrix (dECM), which is the noncellular component of tissue that retains relevant biological cues for cells, has been evaluated as a starting biomaterial to realize composite electrospun constructs. The properties of the electrospun systems can be further improved with innovative procedures of functionalization with biomolecules. Among the various approaches, great attention is devoted to the "click" concept in constructing a bioactive system, due to the modularity, orthogonality, and simplicity features of the "click" reactions. In this paper, we first provide an overview of current approaches that can be used to obtain biofunctional composite electrospun biomaterials. Finally, we propose a design of composite electrospun biomaterials suitable for skeletal muscle tissue regeneration.

Extracellular matrix-derived biomaterials in engineering cell function

Extracellular matrix (ECM) derived components are emerging sources for the engineering of biomaterials that are capable of inducing desirable cell-specific responses. This review explores the use of biomaterials derived from naturally occurring ECM proteins and their derivatives in approaches that aim to regulate cell function. Biomaterials addressed are grouped into six categories: purified single ECM proteins, combinations of purified ECM proteins, cell-derived ECM, tissue-derived ECM, diseased and modified ECM, and ECM-polymer coupled biomaterials. Purified ECM proteins serve as a material coating for enhanced cell adhesion and biocompatibility. Cell-derived and tissue-derived ECM, generated by cell isolation and decellularization technologies, can capture the native state of the ECM environment and guide cell migration and alignment patterns as well as stem cell differentiation. We focus primarily on recent advances in the fields of soft tissue, cardiac, and dermal repair, and explore the utilization of ECM proteins as biomaterials to engineer cell responses.

Pre-Clinical Cell Therapeutic Approaches for Repair of Volumetric Muscle Loss

Extensive damage to skeletal muscle tissue due to volumetric muscle loss (VML) is beyond the inherent regenerative capacity of the body, and results in permanent functional debilitation. Current clinical treatments fail to fully restore native muscle function. Recently, cell-based therapies have emerged as a promising approach to promote skeletal muscle regeneration following injury and/or disease. Stem cell populations, such as muscle stem cells, mesenchymal stem cells and induced pluripotent stem cells (iPSCs), have shown a promising capacity for muscle differentiation. Support cells, such as endothelial cells, nerve cells or immune cells, play a pivotal role in providing paracrine signaling cues for myogenesis, along with modulating the processes of inflammation, angiogenesis and innervation. The efficacy of cell therapies relies on the provision of instructive microenvironmental cues and appropriate intercellular interactions. This review describes the recent developments of cell-based therapies for the treatment of VML, with a focus on preclinical testing and future trends in the field.

Mechanical Stimulation of Adhesion Receptors Using Light-Responsive Nanoparticle Actuators Enhances Myogenesis

The application of cyclic strain is known to enhance myoblast differentiation and muscle growth in vitro and in vivo. However, current techniques apply strain to full tissues or cell monolayers, making it difficult to evaluate whether mechanical stimulation at the subcellular or single-cell scales would drive myoblast differentiation. Here, we report the use of optomechanical actuator (OMA) particles, comprised of a ∼0.6 μm responsive hydrogel coating a gold nanorod (100 × 20 nm) core, to mechanically stimulate the integrin receptors in myoblasts. When illuminated with near-infrared (NIR) light, OMA nanoparticles rapidly collapse, exerting mechanical forces to cell receptors bound to immobilized particles. Using a pulsed illumination pattern, we applied cyclic integrin forces to C2C12 myoblasts cultured on a monolayer of OMA particles and then measured the cellular response. We found that 20 min of OMA actuation resulted in cellular elongation in the direction of the stimulus and enhancement of nuclear YAP1 accumulation, an effector of ERK phosphorylation. Cellular response was dependent on direct conjugation of RGD peptides to the OMA particles. Repeated OMA mechanical stimulation for 5 days led to enhanced myogenesis as quantified using cell alignment, fusion, and sarcomeric myosin expression in myotubes. OMA-mediated myogenesis was sensitive to the geometry of stimulation but not to MEK1/2 inhibition. Finally, we found that OMA stimulation in regions proximal to the nucleus resulted in localization of the transcription activator YAP-1 to the nucleus, further suggesting the role of YAP1 in mechanotransduction in C2C12 cells. These findings demonstrate OMAs as a novel tool for studying the role of spatially localized forces in influencing myogenesis.

Graft alignment impacts the regenerative response of skeletal muscle after volumetric muscle loss in a rat model

A key event in the etiology of volumetric muscle loss (VML) injury is the bulk loss of structural cues provided by the underlying extracellular matrix (ECM). To re-establish the lost cues, there is broad consensus within the literature supporting the utilization of implantable scaffolding. However, while scaffold based regenerative medicine strategies have shown potential, there remains a significant amount of outcome variability observed across the field. We suggest that an overlooked source of outcome variability is differences in scaffolding architecture. The goal of this study was to test the hypothesis that implant alignment has a significant impact on genotypic and phenotypic outcomes following the repair of VML injuries. Using a rat VML model, outcomes across three autograft implant treatment groups (aligned implants, 45° misaligned, and 90° misaligned) and two recovery time points (2 weeks and 12 weeks) were examined (n = 6-8/group). At 2 weeks post-repair there were no significant differences in muscle mass and torque recovery between the treatment groups, however we did observe a significant upregulation of MyoD (2.5 fold increase) and Pax7 (2 fold increase) gene expression as well as the presence of immature myofibers at the implant site for those animals repaired with aligned autografts. By 12 weeks post-repair, functional and structural differences between the treatment groups could be detected. Aligned autografts had significantly greater mass and torque recovery (77 ± 10% of normal) when compared to 45° and 90° misaligned autografts (64 ± 10% and 61 ± 11%, respectively). Examination of tissue structure revealed extensive fibrosis and a significant increase in non-contractile tissue area fraction for only those animals treated using misaligned autografts. When taken together, the results suggest that implant graft orientation has a significant impact on in-vivo outcomes and indicate that the effect of graft alignment on muscle phenotype may be mediated through genotypic changes to myogenesis and fibrosis at the site of injury and repair. STATEMENT OF SIGNIFICANCE: A key event in the etiology of volumetric muscle loss injury is the bulk loss of architectural cues provided by the underlying extracellular matrix. To re-establish the lost cues, there is broad consensus within the literature supporting the utilization of implantable scaffolding. Yet, although native muscle is a highly organized tissue with network and cellular alignment in the direction of contraction, there is little evidence within the field concerning the importance of re-establishing native architectural alignment. The results of this study suggest that critical interactions exist between implant and native muscle alignment cues during healing, which influence the balance between myogenesis and fibrosis. Specifically, it appears that alignment of implant architectural cues with native muscle cues is necessary to create a pro-myogenic environment and contractile force recovery. The results also suggest that misaligned cues may be pathological, leading to fibrosis and poor contractile force recovery.

Regulation of Myogenic Activity by Substrate and Electrical Stimulation In Vitro

Skeletal muscle has a remarkable regenerative capacity in response to mild injury. However, when muscle is severely injured, muscle regeneration is impaired due to the loss of muscle-resident stem cells, known as satellite cells. Fibrotic tissue, primarily comprising collagen I (COL), is deposited with this critical loss of muscle. In recent studies, supplementation of laminin (LM)-111 has been shown to improve skeletal muscle regeneration in several models of disease and injury. Additionally, electrical stimulation (E-stim) has been investigated as a possible rehabilitation therapy to improve muscle's functional recovery. This study investigated the role of E-stim and substrate in regulating myogenic response. C2C12 myoblasts were allowed to differentiate into myotubes on COL- and LM-coated polydimethylsiloxane molds. The myotubes were subjected to E-stim and compared with nonstimulated controls. While E-stim resulted in increased myogenic activity, irrespective of substrate, LM supported increased proliferation and uniform distribution of C2C12 myoblasts. In addition, C2C12 myoblasts cultured on LM showed higher Sirtuin 1, mammalian target of rapamycin, desmin, nitric oxide, and vascular endothelial growth factor expression. Taken together, these results suggest that an LM substrate is more conducive to myoblast growth and differentiation in response to E-stim in vitro.

Biomimetic Scaffolds in Skeletal Muscle Regeneration

Skeletal muscle tissue has inherent capacity for regeneration in response to minor injuries. However, in the case of severe trauma, tumor ablations, or in congenital muscle defects, these myopathies can cause irreversible loss of muscle mass and function, a condition referred to as volumetric muscle loss (VML). The natural muscle repair mechanisms are overwhelmed, prompting the search for new muscle regenerative strategies, such as using biomaterials that can provide regenerative signals to either transplanted or host muscle cells. Recent studies involve the use of suitable biomaterials which may be utilized as a template to guide tissue reorganization and ultimately provide optimum micro-environmental conditions to cells. These strategies range from approaches that utilize biomaterials alone to those that combine materials with exogenous growth factors, and ex vivo cultured cells. A number of scaffold materials have been used in the development of grafts to treat VML. In this brief review, we outline the natural skeletal regeneration process, available treatments used in the clinic for muscle injury and promising tissue bioengineering and regenerative approaches for muscle loss treatment.