Advances in musculoskeletal tissue engineering: moving towards therapy

Extrusion-Based 3D Bioprinting of Bioactive and Piezoelectric Scaffolds as Potential Therapy for Treating Critical Soft Tissue Wounds

Objective: This study focuses on developing bioactive piezoelectric scaffolds that could deliver bioelectrical cues to potentially treat injuries to soft tissues such as skeletal muscles and promote active regeneration. Approach: To address the underexplored aspect of bioelectrical cues in skeletal muscle tissue engineering (SMTE), we developed piezoelectric bioink based on natural bioactive materials such as sodium alginate, gelatin, and chitosan. Extrusion-based 3D bioprinting was utilized to develop scaffolds that mimic muscle stiffness and generate electrical stimulation (E-stim) when subjected to forces. The biocompatibility of these scaffolds was tested with the C2C12 muscle cell line. Results: The bioink demonstrated suitable rheological properties for 3D bioprinting, resulting in high-resolution composite sodium alginate-gelatin-chitosan scaffolds with good structural fidelity. The scaffolds exhibited a 42-60 kPa stiffness, similar to muscle. When a controlled force of 5N was applied to the scaffolds at a constant frequency of 4 Hz, they generated electrical fields and impulses (charge), indicating their suitability as a stand-alone scaffold to generate E-stim and instill bioelectrical cues in the wound region. The cell viability and proliferation test results confirm the scaffold's biocompatibility with C2C12s and the benefit of piezoelectricity in promoting muscle cell growth kinetics. Our study indicates that our piezoelectric bioink and scaffolds offer promise as autonomous E-stim-generating regenerative therapy for SMTE. Innovation: A novel approach for treating skeletal muscle wounds was introduced by developing a bioactive electroactive scaffold capable of autonomously generating E-stim without stimulators and electrodes. This scaffold offers a unique approach to enhancing skeletal muscle regeneration through bioelectric cues, addressing a major gap in the SMTE, that is, fibrotic tissue formation due to delayed muscle regeneration. Conclusion: A piezoelectric scaffold was developed, providing a promising solution for promoting skeletal muscle regeneration. This development can potentially address skeletal muscle injuries and offers a unique approach to facilitating skeletal muscle wound healing.

Harvest of Cell-Only Muscle Fibers Using Thermally Expandable Hydrogels with Adhesive Patterns

Muscle tissue engineering has been the focus of extensive research due to its potential for numerous medical applications, including ex vivo actuator development and clinical treatments. In this study, we developed a method for harvesting muscle fiber in a floatable and translocatable manner utilizing thermally expandable hydrogels with a chemically patterned polydopamine (PD) layer generated by microcontact printing (μCP). The μCP of PD on the hydrogel facilitated the formation of stripe patterns with varying widths of printed/nonprinted area (50/50, 100/100, and 200/200 μm). The spatially controlled adhesion of C2C12 myoblasts on the PD patterns produced clearly distinguishable muscle fibers, and translocated muscle fibers exhibited preserved extracellular matrix and junction proteins. Furthermore, the development of anisotropic arrangements and mature myotubes within the fibers suggests the potential for functional control of engineered muscle tissues. Overall, the muscle fiber harvesting method developed herein is suitable for both translocation and floating and is a promising technique for muscle tissue engineering as it mimics the structure-function relationship of natural tissue.

Bioprinted hASC-laden cell constructs with mechanically stable and cell alignment cue for tenogenic differentiation

3D bioprinting is a technology that enables the precise and controlled deposition of cells and an artificial extracellular matrix (ECM) to create functional tissue constructs. However, current 3D bioprinting methods still struggle to obtain mechanically stable and unique cell-morphological structures, such as fully aligned cells. In this study, we propose a new 3D bioprinting approach that utilizes a high concentration of bioink without cells to support mechanical properties and drag flow to fully align cells in a thin bath filled with cell-laden bioink, resulting in a hybrid cell-laden construct with a mechanical stable and fully aligned cell structure. To demonstrate the feasibility of this approach, we used it to fabricate a cell-laden construct using human adipose stem cells (hASCs) for tendon tissue engineering. To achieve appropriate processing conditions, various factors such as the bioink concentration, nozzle moving speed, and volume flow rate were considered. To enhance the biocompatibility of the cell-laden construct, we used porcine decellularized tendon ECM.In vitrocellular responses, including tenogenic differentiation of the fabricated hybrid cell structures with aligned or randomly distributed cells, were evaluated using hASCs. In addition, the mechanical properties of the hybrid cell-laden construct could be adjusted by controlling the concentration of the mechanically reinforcing strut using methacrylated tendon-decellularized extracellular matrix. Based on these results, the hybrid cell-laden structure has the potential to be a highly effective platform for the alignment of musculoskeletal tissues.

Effect of Boron-Doped Mesoporous Bioactive Glass Nanoparticles on C2C12 Cell Viability and Differentiation: Potential for Muscle Tissue Application

Mesoporous bioactive glasses (MBGs) exhibit a high surface area and a highly ordered mesoporous structure. MBGs have potential for both hard and soft tissue engineering applications. MBGs may be doped with biologically active ions to tailor their biological activity. Boron is being widely studied as a dopant of bioactive glasses. Recently, research has demonstrated the potential of boron-containing bioactive glasses for muscle regeneration. In this study, boron-containing MBGs, 10B-MBG and 18B-MBG nanoparticles, were produced by a microemulsion-assisted sol-gel approach for potential muscle regeneration applications. First, X-ray diffraction (XRD), Fourier transform infrared (FTIR), and energy-dispersive X-ray spectroscopy (EDX) analyses were conducted to study the chemical structure and composition of the nanoparticles. To examine the nanoparticle morphology, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were analyzed. Both SEM images and particle size distribution determined by dynamic light scattering (DLS) indicated a decrease of the average particle size after boron doping. TEM images indicated a slit-shaped mesoporous structure of nanoparticles for all compositions. The ζ potential was measured, and a negative surface charge was found for all study groups due to the presence of silanol groups. Cytocompatibility and fluorescence microscopy studies were also carried out. The results indicated that low concentrations (0.1 and 1 mg mL^-1) of all MBG nanoparticles led to high viability of C2C12 cells. Fluorescence microscopy images indicated that at lower nanoparticle concentrations (0.1 and 1 mg mL^-1), C2C12 cells appeared to differentiate into myotubes, which was indicated by a spindle-shaped morphology. For 10 mg mL^-1 concentration of nanoparticles, C2C12 cells had a lower aspect ratio (estimated qualitatively by inspection of the images), which implied a lower degree of differentiation. Boron-doped MBG nanoparticles in reduced concentrations are suitable to induce differentiation of C2C12 cells into myotubes, indicating their potential for applications in muscle tissue repair.

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.

Tuning Myogenesis by Controlling Gelatin Hydrogel Properties through Hydrogen Peroxide-Mediated Cross-Linking and Degradation

Engineering skeletal muscle tissue in vitro is important to study the mechanism of myogenesis, which is crucial for regenerating muscle cells. The physicochemical properties of the cellular microenvironment are known to govern various cell behaviours. Yet, most studies utilised synthetic materials to model the extracellular matrix that suffers from cytotoxicity to the cells. We have previously reported that the physicochemical property of hydrogels obtained from horseradish peroxidase (HRP)-catalysed cross-linking could be controlled by a simple adjustment to the exposure time to air containing H₂O₂. In this study, we evaluated the influence of physicochemical properties dynamics in the gelatin possessing phenol groups (Gelatin-Ph) hydrogel to regulate the myogenesis in vitro. We controlled the Young's modulus of the Gelatin-Ph hydrogel by tuning the air containing 16 ppm H₂O₂ exposure time for 15-60 min. Additionally, prolonged exposure to air containing H₂O₂ also induced Gelatin-Ph degradation. Myoblasts showed higher adhesion and myotube formation on stiff hydrogel (3.53 kPa) fabricated through 30 min of exposure to air containing H₂O₂ compared to those on softer hydrogel (0.77-2.79 kPa) fabricated through 15, 45, and 60 min of the exposure. These results demonstrate that the myogenesis can be tuned by changes in the physicochemical properties of Gelatin-Ph hydrogel mediated by H₂O₂.

Controllable assembly of skeletal muscle-like bundles through 3D bioprinting

3D printing is an effective technology for recreating skeletal muscle tissuein vitro. To achieve clinical skeletal muscle injury repair, relatively large volumes of highly aligned skeletal muscle cells are required; obtaining these is still a challenge. It is currently unclear how individual skeletal muscle cells and their neighbouring components co-ordinate to establish anisotropic architectures in highly homogeneous orientations. Here, we demonstrated a 3D printing strategy followed by sequential culture processes to engineer skeletal muscle tissue. The effects of confined printing on the skeletal muscle during maturation, which impacted the myotube alignment, myogenic gene expression, and mechanical forces, were observed. Our findings demonstrate the dynamic changes of skeletal muscle tissue duringin vitro3D construction and reveal the role of physical factors in the orientation and maturity of muscle fibres.

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.

Specifiable biomimetic microsponges for timed release of crystal entrapped biomolecules useful in bone repair

Most marine materials, by nature, contain crystals of inorganic matter with specific structures that allow the loading, release, and delivery of biomolecules that can be utilized in clinical applications. These structures can be biomimetically synthesized. Aggregates of inorganic particles generated by biomimetic microsponges may provide surfaces and structures for cell attachment, organization, and promotion of matrix synthesis. Biomimetic microsponges have been developed with tunable release profiles differing by the rate (speed over distance), velocity (rate of change in direction), and the quantity discharged over time, according to biomolecular species. Specifically, the types of proteins involved guide and regulate cells in physical contact with the microsponges, for instance, reprogramming somatic cells, the switching phenotypes, or specifying stem cell differentiation. Applications for these microsponges include gene transfection of localized cells and promotion of bone matrix synthesis by the externalized display of RGD cell adhesive peptides and the release of crystal entrapped, occluded, adsorbed and infused rhBMP-2 and plasmid. A requirement for de novo bone formation is a solid structure to enable osteocytes to lay new bone tissue. In this study, biomimetic microsponges highlight tremendous potential as osteoconductive packing material in bone repair with parallel influence on regeneration. Majorly, microsponges offer pronounced osteoinductivity, unlike many other bone particulates, by solid-state integration of active regenerative biological molecules through the prism of the biomineral crystalline structure.

Regenerative medicine for skeletal muscle loss: a review of current tissue engineering approaches

Skeletal muscle is capable of regeneration following minor damage, more significant volumetric muscle loss (VML) however results in permanent functional impairment. Current multimodal treatment methodologies yield variable functional recovery, with reconstructive surgical approaches restricted by limited donor tissue and significant donor morbidity. Tissue-engineered skeletal muscle constructs promise the potential to revolutionise the treatment of VML through the regeneration of functional skeletal muscle. Herein, we review the current status of tissue engineering approaches to VML; firstly the design of biocompatible tissue scaffolds, including recent developments with electroconductive materials. Secondly, we review the progenitor cell populations used to seed scaffolds and their relative merits. Thirdly we review in vitro methods of scaffold functional maturation including the use of three-dimensional bioprinting and bioreactors. Finally, we discuss the technical, regulatory and ethical barriers to clinical translation of this technology. Despite significant advances in areas, such as electroactive scaffolds and three-dimensional bioprinting, along with several promising in vivo studies, there remain multiple technical hurdles before translation into clinically impactful therapies can be achieved. Novel strategies for graft vascularisation, and in vitro functional maturation will be of particular importance in order to develop tissue-engineered constructs capable of significant clinical impact.

3D printed oxidized alginate-gelatin bioink provides guidance for C2C12 muscle precursor cell orientation and differentiation via shear stress during bioprinting

Biofabrication can be a tool to three-dimensionally (3D) print muscle cells embedded inside hydrogel biomaterials, ultimately aiming to mimic the complexity of the native muscle tissue and to create in-vitro muscle analogues for advanced repair therapies and drug testing. However, to 3D print muscle analogues of high cell alignment and synchronous contraction, the effect of biofabrication process parameters on myoblast growth has to be understood. A suitable biomaterial matrix is required to provide 3D printability as well as matrix degradation to create space for cell proliferation, matrix remodelling capacity, and cell differentiation. We demonstrate that by the proper selection of nozzle size and extrusion pressure, the shear stress during extrusion-bioprinting of mouse myoblast cells (C2C12) can achieve cell orientation when using oxidized alginate-gelatin (ADA-GEL) hydrogel bionk. The cells grow in the direction of printing, migrate to the hydrogel surface over time, and differentiate into ordered myotube segments in areas of high cell density. Together, our results show that ADA-GEL hydrogel can be a simple and cost-efficient biodegradable bioink that allows the successful 3D bioprinting and cultivation of C2C12 cells in-vitro to study muscle engineering.

Polymer- and Hybrid-Based Biomaterials for Interstitial, Connective, Vascular, Nerve, Visceral and Musculoskeletal Tissue Engineering

In this review, materials based on polymers and hybrids possessing both organic and inorganic contents for repairing or facilitating cell growth in tissue engineering are discussed. Pure polymer based biomaterials are predominantly used to target soft tissues. Stipulated by possibilities of tuning the composition and concentration of their inorganic content, hybrid materials allow to mimic properties of various types of harder tissues. That leads to the concept of "one-matches-all" referring to materials possessing the same polymeric base, but different inorganic content to enable tissue growth and repair, proliferation of cells, and the formation of the ECM (extra cellular matrix). Furthermore, adding drug delivery carriers to coatings and scaffolds designed with such materials brings additional functionality by encapsulating active molecules, antibacterial agents, and growth factors. We discuss here materials and methods of their assembly from a general perspective together with their applications in various tissue engineering sub-areas: interstitial, connective, vascular, nervous, visceral and musculoskeletal tissues. The overall aims of this review are two-fold: (a) to describe the needs and opportunities in the field of bio-medicine, which should be useful for material scientists, and (b) to present capabilities and resources available in the area of materials, which should be of interest for biologists and medical doctors.

iPSCs: A powerful tool for skeletal muscle tissue engineering

Both volumetric muscle loss (VML) and muscle degenerative diseases lead to an important decrease in skeletal muscle mass, condition that nowadays lacks an optimal treatment. This issue has driven towards an increasing interest in new strategies in tissue engineering, an emerging field that can offer very promising approaches. In addition, the discovery of induced pluripotent stem cells (iPSCs) has completely revolutionized the actual view of personalized medicine, and their utilization in skeletal muscle tissue engineering could, undoubtedly, add myriad benefits. In this review, we want to provide a general vision of the basic aspects to consider when engineering skeletal muscle tissue using iPSCs. Specifically, we will focus on the three main pillars of tissue engineering: the scaffold designing, the selection of the ideal cell source and the addition of factors that can enhance the resemblance with the native tissue.

Progress in three-dimensional printing with growth factors

Incorporation of growth factors in biomedical constructs can encourage cellular activities necessary for tissue regeneration within an implant system. Three-dimensional printing offers a capacity for spatial dictation and dosage control of incorporated growth factors which promises to minimize complications from the supraphysiologic doses and burst release involved in current growth factor delivery systems. Successful implementation of three-dimensional printing with growth factors requires preservation of the bioactivity of printed growth factors, spatial localization of growth factors within the construct architecture during printing, and controlled release of growth factors after printing. This review describes demonstrated approaches for addressing each of these goals, including direct inclusion of growth factors with the biomaterial during printing, or intermediary encapsulation of growth factors in delivery vehicles such as microparticles or nanoparticles.

Nanotopographic cues and stiffness control of tendon-derived stem cells from diverse conditions

Background

Tendon-derived stem cells (TDSCs) are key factors associated with regeneration and healing in tendinopathy. The aim of this study was to investigate the effects of mechanical stiffness and topographic signals on the differentiation of TDSCs depending on age and pathological conditions.

Materials and methods

We compared TDSCs extracted from normal tendon tissues with TDSCs from tendinopathic Achilles tendon tissues of Sprague Dawley rats in vitro and TDSCs cultured on nanotopographic cues and substrate stiffness to determine how to control the TDSCs. The tendinopathy model was created using a chemical induction method, and the tendon injury model was created via an injury-and-overuse method. Norland Optical Adhesive 86 (NOA86) substrate with 2.48 GPa stiffness with and without 800 nm-wide nanogrooves and a polyurethane substrate with 800 nm-wide nanogrooves were used.

Results

TDSCs from 5-week-old normal tendon showed high expression of type III collagen on the flat NOA86 substrate. In the 15-week normal tendon model, expression of type III collagen was high in TDSCs cultured on the 800 nm NOA86 substrates. However, in the 15-week tendon injury model, expression of type III collagen was similar irrespective of nanotopographic cues or substrate stiffness. The expression of type I collagen was also independent of nanotopographic cues and substrate stiffness in the 15-week normal and tendon injury models. Gene expression of scleraxis was increased in TDSCs cultured on the flat NOA86 substrate in the 5-week normal tendon model (P=0.001). In the 15-week normal tendon model, scleraxis was highly expressed in TDSCs cultured on the 800 nm and flat NOA86 substrate (P=0.043). However, this gene expression was not significantly different between the substrates in the 5-week tendinopathy and 15-week tendon injury models.

Conclusion

Development and maturation of tendon are enhanced when TDSCs from normal tendons were cultured on stiff surface, but not when the TDSCs came from pathologic models. Therapeutic applications of TDSCs need to be flexible based on tendon age and tendinopathy.

Satellite cells delivered in their niche efficiently generate functional myotubes in three-dimensional cell culture

Biophysical/biochemical cues from the environment contribute to regulation of the regenerative capacity of resident skeletal muscle stem cells called satellites cells. This can be observed in vitro, where muscle cell behaviour is influenced by the particular culture substrates and whether culture is performed in a 2D or 3D environment, with changes including morphology, nuclear shape and cytoskeletal organization. To create a 3D skeletal muscle model we compared collagen I, Fibrin or PEG-Fibrinogen with different sources of murine and human myogenic cells. To generate tension in the 3D scaffold, biomaterials were polymerised between two flexible silicone posts to mimic tendons. This 3D culture system has multiple advantages including being simple, fast to set up and inexpensive, so providing an accessible tool to investigate myogenesis in a 3D environment. Immortalised human and murine myoblast lines, and primary murine satellite cells showed varying degrees of myogenic differentiation when cultured in these biomaterials, with C2 myoblasts in particular forming large multinucleated myotubes in collagen I or Fibrin. However, murine satellite cells retained in their niche on a muscle fibre and embedded in 3D collagen I or Fibrin gels generated aligned, multinucleated and contractile myotubes.

Modelling multi-scale cell-tissue interaction of tissue-engineered muscle constructs

Expectation on engineered tissue substitute continues to grow, and for an effective development of a functional tissue and to control its quality, cellular mechanoresponse plays a key role. Although the mechanoresponse – in terms of cell–tissue interaction across scales – has been understood better in recent years, there are still technical limitations to quantitatively monitor the processes involved in the development of both native and engineered tissues. Computational (in silico) studies have been utilised to complement the experimental limitations and successfully applied to the prediction of tissue growth. We here review recent activities in the area of combined experimental and computational analyses of tissue growth, especially in the tissue engineering context, and highlight the advantages of such an approach for the future of the tissue engineering, using our own case study of predicting musculoskeletal tissue engineering construct development.

Gelatin-genipin-based biomaterials for skeletal muscle tissue engineering

Skeletal muscle engineering aims at tissue reconstruction to replace muscle loss following traumatic injury or in congenital muscle defects. Skeletal muscle can be engineered by using biodegradable and biocompatible scaffolds that favor myogenic cell adhesion and subsequent tissue organization. In this study, we characterized scaffolds made of gelatin cross-linked with genipin, a natural derived cross-linking agent with low cytotoxicity and high biocompatibility, for tissue engineering of skeletal muscle. We generated gelatin-genipin hydrogels with a stiffness of 13 kPa to reproduce the mechanical properties characteristic of skeletal muscle and we show that their surface can be topographically patterned through soft lithography to drive myogenic cells differentiation and unidirectional orientation. Furthermore, we demonstrate that these biomaterials can be successfully implanted in vivo under dorsal mouse skin, showing good biocompatibility and slow biodegradation rate. Moreover, the grafting of this biomaterial in partially ablated tibialis anterior muscle does not impair muscle regeneration, supporting future applications of gelatin-genipin biomaterials in the field of skeletal muscle tissue repair. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2763-2777, 2018.

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

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

Regenerative medicine solutions in congenital diaphragmatic hernia

Congenital diaphragmatic hernia (CDH) remains a major challenge and associated mortality is still significant. Patients have benefited from current therapeutic options, but most severe cases are still associated to poor outcome. Regenerative medicine is emerging as a valid option in many diseases and clinical trials are currently happening for various conditions in children and adults. We report here the advancement in the field which will help both in the understanding of further CDH development and in offering new treatment options for the difficult situations such as repair of large diaphragmatic defects and lung hypoplasia. The authors believe that advancements in regenerative medicine may lead to increase of CDH patients׳ survival.

Cell and Growth Factor-Loaded Keratin Hydrogels for Treatment of Volumetric Muscle Loss in a Mouse Model

Wounds to the head, neck, and extremities have been estimated to account for ∼84% of reported combat injuries to military personnel. Volumetric muscle loss (VML), defined as skeletal muscle injuries in which tissue loss results in permanent functional impairment, is common among these injuries. The present standard of care entails the use of muscle flap transfers, which suffer from the need for additional surgery when using autografts or the risk of rejection when cadaveric grafts are used. Tissue engineering (TE) strategies for skeletal muscle repair have been investigated as a means to overcome current therapeutic limitations. In that regard, human hair-derived keratin (KN) biomaterials have been found to possess several favorable properties for use in TE applications and, as such, are a viable candidate for use in skeletal muscle repair. Herein, KN hydrogels with and without the addition of skeletal muscle progenitor cells (MPCs) and/or insulin-like growth factor 1 (IGF-1) and/or basic fibroblast growth factor (bFGF) were implanted in an established murine model of surgically induced VML injury to the latissimus dorsi (LD) muscle. Control treatments included surgery with no repair (NR) as well as implantation of bladder acellular matrix (BAM). In vitro muscle contraction force was evaluated at two months postsurgery through electrical stimulation of the explanted LD in an organ bath. Functional data indicated that implantation of KN+bFGF+IGF-1 (n = 8) enabled a greater recovery of contractile force than KN+bFGF (n = 8)***, KN+MPC (n = 8)**, KN+MPC+bFGF+IGF-1 (n = 8)**, BAM (n = 8)*, KN+IGF-1 (n = 8)*, KN+MPCs+bFGF (n = 9)*, or NR (n = 9)**, (*p < 0.05, **p < 0.01, ***p < 0.001). Consistent with the physiological findings, histological evaluation of retrieved tissue revealed much more extensive new muscle tissue formation in groups with greater functional recovery (e.g., KN+IGF-1+bFGF) when compared with observations in tissue from groups with lower functional recovery (i.e., BAM and NR). Taken together, these findings further indicate the general utility of KN biomaterials in TE and, moreover, specifically highlight their potential application in the treatment of VML injuries.

Natural polymeric hydrogel evaluation for skeletal muscle tissue engineering

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

Fabrication and Applications of Micro/Nanostructured Devices for Tissue Engineering

Nanotechnology allows the realization of new materials and devices with basic structural unit in the range of 1-100 nm and characterized by gaining control at the atomic, molecular, and supramolecular level. Reducing the dimensions of a material into the nanoscale range usually results in the change of its physiochemical properties such as reactivity, crystallinity, and solubility. This review treats the convergence of last research news at the interface of nanostructured biomaterials and tissue engineering for emerging biomedical technologies such as scaffolding and tissue regeneration. The present review is organized into three main sections. The introduction concerns an overview of the increasing utility of nanostructured materials in the field of tissue engineering. It elucidates how nanotechnology, by working in the submicron length scale, assures the realization of a biocompatible interface that is able to reproduce the physiological cell-matrix interaction. The second, more technical section, concerns the design and fabrication of biocompatible surface characterized by micro- and submicroscale features, using microfabrication, nanolithography, and miscellaneous nanolithographic techniques. In the last part, we review the ongoing tissue engineering application of nanostructured materials and scaffolds in different fields such as neurology, cardiology, orthopedics, and skin tissue regeneration.

Extensive Characterization and Comparison of Endothelial Cells Derived from Dermis and Adipose Tissue: Potential Use in Tissue Engineering

Tissue-engineered constructs need to become quickly vascularized in order to ensure graft take. One way of achieving this is to incorporate endothelial cells (EC) into the construct. The adipose tissue stromal vascular fraction (adipose-SVF) might provide an alternative source for endothelial cells as adipose tissue can easily be obtained by liposuction. Since adipose-EC are now gaining more interest in tissue engineering, we aimed to extensively characterize endothelial cells from adipose tissue (adipose-EC) and compare them with endothelial cells from dermis (dermal-EC). The amount of endothelial cells before purification varied between 4–16% of the total stromal population. After MACS selection for CD31 positive cells, a >99% pure population of endothelial cells was obtained within two weeks of culture. Adipose- and dermal-EC expressed the typical endothelial markers PECAM-1, ICAM-1, Endoglin, VE-cadherin and VEGFR2 to a similar extent, with 80–99% of the cell population staining positive. With the exception of CXCR4, which was expressed on 29% of endothelial cells, all other chemokine receptors (CXCR1, 2, 3, and CCR2) were expressed on less than 5% of the endothelial cell populations. Adipose-EC proliferated similar to dermal-EC, but responded less to the mitogens bFGF and VEGF. A similar migration rate was found for both adipose-EC and dermal-EC in response to bFGF. Sprouting of adipose-EC and dermal-EC was induced by bFGF and VEGF in a 3D fibrin matrix. After stimulation of adipose-EC and dermal-EC with TNF-α an increased secretion was seen for PDGF-BB, but not uPA, PAI-1 or Angiopoietin-2. Furthermore, secretion of cytokines and chemokines (IL-6, CCL2, CCL5, CCL20, CXCL1, CXCL8 and CXCL10) was also upregulated by both adipose- and dermal-EC. The similar characteristics of adipose-EC compared to their dermal-derived counterpart make them particularly interesting for skin tissue engineering. In conclusion, we show here that adipose tissue provides for an excellent source of endothelial cells for tissue engineering purposes, since they are readily available, and easily isolated and amplified.

Biomaterials for hollow organ tissue engineering

Tissue engineering is a rapidly advancing field that is likely to transform how medicine is practised in the near future. For hollow organs such as those found in the cardiovascular and respiratory systems or gastrointestinal tract, tissue engineering can provide replacement of the entire organ or provide restoration of function to specific regions. Larger tissue-engineered constructs often require biomaterial-based scaffold structures to provide support and structure for new tissue growth. Consideration must be given to the choice of material and manufacturing process to ensure the de novo tissue closely matches the mechanical and physiological properties of the native tissue. This review will discuss some of the approaches taken to date for fabricating hollow organ scaffolds and the selection of appropriate biomaterials.

Biologic-free mechanically induced muscle regeneration

Severe skeletal muscle injuries are common and can lead to extensive fibrosis, scarring, and loss of function. Clinically, no therapeutic intervention exists that allows for a full functional restoration. As a result, both drug and cellular therapies are being widely investigated for treatment of muscle injury. Because muscle is known to respond to mechanical loading, we investigated instead whether a material system capable of massage-like compressions could promote regeneration. Magnetic actuation of biphasic ferrogel scaffolds implanted at the site of muscle injury resulted in uniform cyclic compressions that led to reduced fibrous capsule formation around the implant, as well as reduced fibrosis and inflammation in the injured muscle. In contrast, no significant effect of ferrogel actuation on muscle vascularization or perfusion was found. Strikingly, ferrogel-driven mechanical compressions led to enhanced muscle regeneration and a ∼threefold increase in maximum contractile force of the treated muscle at 2 wk compared with no-treatment controls. Although this study focuses on the repair of severely injured skeletal muscle, magnetically stimulated bioagent-free ferrogels may find broad utility in the field of regenerative medicine.

In vitro myogenesis induced by human recombinant elastin-like proteins

Mammalian adult skeletal muscle has a limited ability to regenerate after injury, usage or trauma. A promising strategy for successful regenerative technology is the engineering of bio interfaces that mimic the characteristics of the extracellular matrix. Human elastin-like polypeptides (HELPs) have been synthesized as biomimetic materials that maintain some peculiar properties of the native protein. We developed a novel Human Elastin Like Polypeptide obtained by fusing the elastin-like backbone to a domain present in the α2 chain of type IV collagen, containing two RGD motives. We employed this peptide as adhesion substrate for C2C12 myoblasts and compared its effects to those induced by two other polypeptides of the HELP series. Myoblast adhered to all HELPs coatings, where they assumed morphology and cytoarchitecture that depended on the polypeptide structure. Adhesion to HELPs stimulated at a different extent cell proliferation and differentiation, the expression of Myosin Heavy Chain and the fusion of aligned fibers into multinucleated myotubes. Adhesion substrates significantly altered myotubes stiffness, measured by Atomic Force Microscopy, and differently affected the cells Ca(2+) handling capacity and the maturation of excitation-contraction coupling machinery, evaluated by Ca(2+) imaging. Overall, our findings indicate that the properties of HELP biopolymers can be exploited for dissecting the molecular connections underlying myogenic differentiation and for designing novel substrates for skeletal muscle regeneration.

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

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

Biomaterial-based delivery for skeletal muscle repair

Skeletal muscle possesses a remarkable capacity for regeneration in response to minor damage, but severe injury resulting in a volumetric muscle loss can lead to extensive and irreversible fibrosis, scarring, and loss of muscle function. In early clinical trials, the intramuscular injection of cultured myoblasts was proven to be a safe but ineffective cell therapy, likely due to rapid death, poor migration, and immune rejection of the injected cells. In recent years, appropriate therapeutic cell types and culturing techniques have improved progenitor cell engraftment upon transplantation. Importantly, the identification of several key biophysical and biochemical cues that synergistically regulate satellite cell fate has paved the way for the development of cell-instructive biomaterials that serve as delivery vehicles for cells to promote in vivo regeneration. Material carriers designed to spatially and temporally mimic the satellite cell niche may be of particular importance for the complete regeneration of severely damaged skeletal muscle.

Myoconductive and osteoinductive free-standing polysaccharide membranes

Free-standing (FS) membranes have increasing applications in the biomedical field as drug delivery systems for wound healing and tissue engineering. Here, we studied the potential of free-standing membranes made by the layer-by-layer assembly of chitosan and alginate to be used as a simple biomimetic system of the periosteum. The design of a periosteum-like membrane implies the elaboration of a thick membrane suitable for both muscle and bone formation. Our aim was to produce well-defined ∼50 μm thick polysaccharide membranes that could be easily manipulated, were mechanically resistant, and would enable both myogenesis and osteogenesis in vitro and in vivo. The membranes were chemically crosslinked to improve their mechanical properties. Crosslinking chemistry was followed via Fourier transform infrared spectroscopy and the mechanical properties of the membranes were assessed using dynamic mechanical analysis. The loading and release of the potent osteoinductive growth factor bone morphogenetic protein 2 (BMP-2) inside and outside of the FS membrane was followed by fluorescence spectroscopy in a physiological buffer over 1 month. The myogenic and osteogenic potentials of the membranes in vitro were assessed using BMP-2-responsive skeletal myoblasts. Finally, their osteoinductive properties in vivo were studied in a preliminary experiment using a mouse ectopic model. Our results showed that the more crosslinked FS membranes enabled a more efficient myoblast differentiation in myotubes. In addition, we showed that a tunable amount of BMP-2 can be loaded into and subsequently released from the membranes, depending on the crosslinking degree and the initial BMP-2 concentration in solution. Only the more crosslinked membranes were found to be osteoinductive in vivo. These polysaccharide-based membranes have strong potential as a periosteum-mimetic scaffold for bone tissue regeneration.

Myogenic Precursors from iPS Cells for Skeletal Muscle Cell Replacement Therapy

The use of adult myogenic stem cells as a cell therapy for skeletal muscle regeneration has been attempted for decades, with only moderate success. Myogenic progenitors (MP) made from induced pluripotent stem cells (iPSCs) are promising candidates for stem cell therapy to regenerate skeletal muscle since they allow allogenic transplantation, can be produced in large quantities, and, as compared to adult myoblasts, present more embryonic-like features and more proliferative capacity in vitro, which indicates a potential for more self-renewal and regenerative capacity in vivo. Different approaches have been described to make myogenic progenitors either by gene overexpression or by directed differentiation through culture conditions, and several myopathies have already been modeled using iPSC-MP. However, even though results in animal models have shown improvement from previous work with isolated adult myoblasts, major challenges regarding host response have to be addressed and clinically relevant transplantation protocols are lacking. Despite these challenges we are closer than we think to bringing iPSC-MP towards clinical use for treating human muscle disease and sporting injuries.

Organ bioengineering for the newborn

Regenerative medicine has recently been established as an emerging interdisciplinary field focused on the repair; replacement or regeneration of cells, tissues and organs. It involves various disciplines, which are focused on different aspects of the regeneration process such as cell biology, gene therapy, bioengineering, material science and pharmacology. In this article, we will outline progress on tissue engineering of specific tissues and organs relevant to paediatric surgery.

Engineering muscle constructs for the creation of functional engineered musculoskeletal tissue

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

[New perspectives in skeletal muscle tissue engineering]

Due to the enormous expansion of knowledge in the fields of stem cell research and biomaterials, skeletal muscle tissue engineering represents a rapidly developing field of biomedical research. This article provides a general overview of skeletal muscle tissue engineering, including a discussion of recent findings and future research perspectives. Additionally, the results of myogenic differentiation of human mesenchymal stem cells and satellite cells are presented.

3D hydrogel environment rejuvenates aged pericytes for skeletal muscle tissue engineering

Skeletal muscle tissue engineering is a promising approach for the treatment of muscular disorders. However, the complex organization of muscle, combined with the difficulty in finding an appropriate source of regenerative cells and in providing an adequate blood supply to the engineered tissue, makes this a hard task to face. In the present work, we describe an innovative approach to rejuvenate adult skeletal muscle-derived pericytes (MP) based on the use of a PEG-based hydrogel scaffold. MP were isolated from young (piglet) and adult (boar) pigs to assess whether aging affects tissue regeneration efficiency. In vitro, MP from boars had similar morphology and colony forming capacity to piglet MP, but an impaired ability to form myotubes and capillary-like structures. However, the use of a PEG-based hydrogel to support adult MP significantly improved their myogenic differentiation and angiogenic potentials in vitro and in vivo. Thus, PEG-based hydrogel scaffolds may provide a progenitor cell “niche” that promotes skeletal muscle regeneration and blood vessel growth, and together with pericytes may be developed for use in regenerative applications.

Biomimetic engineered muscle with capacity for vascular integration and functional maturation in vivo

Tissue-engineered skeletal muscle can serve as a physiological model of natural muscle and a potential therapeutic vehicle for rapid repair of severe muscle loss and injury. Here, we describe a platform for engineering and testing highly functional biomimetic muscle tissues with a resident satellite cell niche and capacity for robust myogenesis and self-regeneration in vitro. Using a mouse dorsal window implantation model and transduction with fluorescent intracellular calcium indicator, GCaMP3, we nondestructively monitored, in real time, vascular integration and the functional state of engineered muscle in vivo. During a 2-wk period, implanted engineered muscle exhibited a steady ingrowth of blood-perfused microvasculature along with an increase in amplitude of calcium transients and force of contraction. We also demonstrated superior structural organization, vascularization, and contractile function of fully differentiated vs. undifferentiated engineered muscle implants. The described in vitro and in vivo models of biomimetic engineered muscle represent enabling technology for novel studies of skeletal muscle function and regeneration.

Evaluation and management of hamstring injuries

Muscle injuries are the most common injuries in sports, with hamstring injuries accounting for 29% of all injuries in athletes. These injuries lead to prolonged impairment and have a reinjury risk of 12% to 31%. They range from mild muscle damage without loss of structural integrity to complete muscle tearing with fiber disruption. Novel MRI scores are increasingly being used and allow a more precise prediction of return to sport. In this article, the authors review the history, mechanisms of injury, and classification systems for hamstring injuries as well as present the latest evidence related to the management of hamstring injuries, including intramuscular and both proximal and distal insertional injuries. Indications for surgical treatment of certain proximal and distal avulsions, biological augmentation to the nonoperative treatment of midsubstance injuries, and advances in risk reduction and injury prevention are discussed.

Directed in vitro myogenesis of human embryonic stem cells and their in vivo engraftment

Development of human embryonic stem cell (hESC)-based therapy requires derivation of in vitro expandable cell populations that can readily differentiate to specified cell types and engraft upon transplantation. Here, we report that hESCs can differentiate into skeletal muscle cells without genetic manipulation. This is achieved through the isolation of cells expressing a mesodermal marker, platelet-derived growth factor receptor-α (PDGFRA), following embryoid body (EB) formation. The ESC-derived cells differentiated into myoblasts in vitro as evident by upregulation of various myogenic genes, irrespective of the presence of serum in the medium. This result is further corroborated by the presence of sarcomeric myosin and desmin, markers for terminally differentiated cells. When transplanted in vivo, these pre-myogenically committed cells were viable in tibialis anterior muscles 14 days post-implantation. These hESC-derived cells, which readily undergo myogenic differentiation in culture medium containing serum, could be a viable cell source for skeletal muscle repair and tissue engineering to ameliorate various muscle wasting diseases.

A contactless electrical stimulator: application to fabricate functional skeletal muscle tissue

Engineered skeletal muscle tissues are ideal candidates for applications in drug screening systems, bio-actuators, and as implantable constructs in tissue engineering. Electrical field stimulation considerably improves the differentiation of muscle cells to muscle myofibers. Currently used electrical stimulators often use direct contact of electrodes with tissue constructs or their culture medium, which may cause hydrolysis of the culture medium, joule heating of the medium, contamination of the culture medium due to products of electrodes corrosion, and surface fouling of electrodes. Here, we used an interdigitated array of electrodes combined with an isolator coverslip as a contactless platform to electrically stimulate engineered muscle tissue, which eliminates the aforementioned problems. The effective stimulation of muscle myofibers using this device was demonstrated in terms of contractile activity and higher maturation as compared to muscle tissues without applying the electrical field. Due to the wide array of potential applications of electrical stimulation to two- and three-dimensional (2D and 3D) cell and tissue constructs, this device could be of great interest for a variety of biological applications as a tool to create noninvasive, safe, and highly reproducible electric fields.

Skeletal muscle tissue engineering: which cell to use?

Tissue-engineered skeletal muscle is urgently required to treat a wide array of devastating congenital and acquired conditions. Selection of the appropriate cell type requires consideration of several factors which amongst others include, accessibility of the cell source, in vitro myogenicity at high efficiency with the ability to maintain differentiation over extended periods of time, susceptibility to genetic manipulation, a suitable mode of delivery and finally in vivo differentiation giving rise to restoration of structural morphology and function. Potential stem-progenitor cell sources include and are not limited to satellite cells, myoblasts, mesoangioblasts, pericytes, muscle side-population cells, CD133(+) cells, in addition to embryonic stem cells, mesenchymal stem cells, amniotic fluid stem cells and induced pluripotent stem (iPS) cells. The relative merits and inherent limitations of these cell types within the field of tissue-engineering are discussed in the light of current research. Recent advances in the field of iPS cells should bear the fruits for some exciting developments within the field in the forthcoming years.

Engineering skeletal muscle repair

Healthy skeletal muscle has a remarkable capacity for regeneration. Even at a mature age, muscle tissue can undergo a robust rebuilding process that involves the formation of new muscle cells and extracellular matrix and the re-establishment of vascular and neural networks. Understanding and reverse-engineering components of this process is essential for our ability to restore loss of muscle mass and function in cases where the natural ability of muscle for self-repair is exhausted or impaired. In this article, we will describe current approaches to restore the function of diseased or injured muscle through combined use of myogenic stem cells, biomaterials, and functional tissue-engineered muscle. Furthermore, we will discuss possibilities for expanding the future use of human cell sources toward the development of cell-based clinical therapies and in vitro models of human muscle disease.

Free-standing polyelectrolyte membranes made of chitosan and alginate

Free-standing films have increasing applications in the biomedical field as drug delivery systems for wound healing and tissue engineering. Here, we prepared free-standing membranes by the layer-by-layer assembly of chitosan and alginate, two widely used biomaterials. Our aim was to produce a thick membrane and to study the permeation of model drugs and the adhesion of muscle cells. We first defined the optimal growth conditions in terms of pH and alginate concentration. The membranes could be easily detached from polystyrene or polypropylene substrate without any postprocessing step. The dry thickness was varied over a large range from 4 to 35 μm. A 2-fold swelling was observed by confocal microscopy when they were immersed in PBS. In addition, we quantified the permeation of model drugs (fluorescent dextrans) through the free-standing membrane, which depended on the dextran molecular weight. Finally, we showed that myoblast cells exhibited a preferential adhesion on the alginate-ending membrane as compared to the chitosan-ending membrane or to the substrate side.

Electrical stimulation as a biomimicry tool for regulating muscle cell behavior

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