In Vivo Printing of Nanoenabled Scaffolds for the Treatment of Skeletal Muscle Injuries

Extremity skeletal muscle injuries result in substantial disability. Current treatments fail to recoup muscle function, but properly designed and implemented tissue engineering and regenerative medicine techniques can overcome this challenge. In this study, a nanoengineered, growth factor-eluting bioink that utilizes Laponite nanoclay for the controlled release of vascular endothelial growth factor (VEGF) and a GelMA hydrogel for a supportive and adhesive scaffold that can be crosslinked in vivo is presented. The bioink is delivered with a partially automated handheld printer for the in vivo formation of an adhesive and 3D scaffold. The effect of the controlled delivery of VEGF alone or paired with adhesive, supportive, and fibrilar architecture has not been studied in volumetric muscle loss (VML) injuries. Upon direct in vivo printing, the constructs are adherent to skeletal muscle and sustained release of VEGF. The in vivo printing of muscle ink in a murine model of VML injury promotes functional muscle recovery, reduced fibrosis, and increased anabolic response compared to untreated mice. The in vivo construction of a therapeutic-eluting 3D scaffold paves the way for the immediate treatment of a variety of soft tissue traumas.

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

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

Semisynthetic Hyaluronic Acid-Based Hydrogel Promotes Recovery of the Injured Tibialis Anterior Skeletal Muscle Form and Function

Volumetric muscle loss (VML) injuries are characterized by a degree of tissue loss that exceeds the endogenous regenerative capacity of muscle, resulting in permanent structural and functional deficits. Such injuries are a consequence of trauma, as well as a host of congenital and acquired diseases and disorders. Despite significant preclinical research with diverse biomaterials, as well as early clinical studies with implantation of decellularized extracellular matrices, there are still significant barriers to more complete restoration of muscle form and function following repair of VML injuries. In fact, identification of novel biomaterials with more advantageous regenerative profiles is a critical limitation to the development of improved therapeutics. As a first step in this direction, we evaluated a novel semisynthetic hyaluronic acid-based (HyA) hydrogel that embodies material features more favorable for robust muscle regeneration. This HyA-based hydrogel is composed of an acrylate-modified HyA (AcHyA) macromer, an AcHyA macromer conjugated with the bsp-RGD(15) peptide sequence to enhance cell adhesion, a high-molecular-weight heparin to sequester growth factors, and a matrix metalloproteinase-cleavable cross-linker to allow for cell-dependent remodeling. In a well-established, clinically relevant rat tibialis anterior VML injury model, we report observations of robust functional recovery, accompanied by volume reconstitution, muscle regeneration, and native-like vascularization following implantation of the HyA-based hydrogel at the site of injury. These findings have important implications for the development and clinical application of the improved biomaterials that will be required for stable and complete functional recovery from diverse VML injuries.

Ionic Silicon Protects Oxidative Damage and Promotes Skeletal Muscle Cell Regeneration

Volumetric muscle loss injuries overwhelm the endogenous regenerative capacity of skeletal muscle, and the associated oxidative damage can delay regeneration and prolong recovery. This study aimed to investigate the effect of silicon-ions on C2C12 skeletal muscle cells under normal and excessive oxidative stress conditions to gain insights into its role on myogenesis during the early stages of muscle regeneration. In vitro studies indicated that 0.1 mM Si-ions into cell culture media significantly increased cell viability, proliferation, migration, and myotube formation compared to control. Additionally, MyoG, MyoD, Neurturin, and GABA expression were significantly increased with addition of 0.1, 0.5, and 1.0 mM of Si-ion for 1 and 5 days of C2C12 myoblast differentiation. Furthermore, 0.1-2.0 mM Si-ions attenuated the toxic effects of H2O2 within 24 h resulting in increased cell viability and differentiation. Addition of 1.0 mM of Si-ions significantly aid cell recovery and protected from the toxic effect of 0.4 mM H2O2 on cell migration. These results suggest that ionic silicon may have a potential effect in unfavorable situations where reactive oxygen species is predominant affecting cell viability, proliferation, migration, and differentiation. Furthermore, this study provides a guide for designing Si-containing biomaterials with desirable Si-ion release for skeletal muscle regeneration.

Regenerative rehabilitation of catastrophic extremity injury in military conflicts and a review of recent developmental efforts

AIM OF THE REVIEW

This review aims to describe the current state of regenerative rehabilitation of severe military extremity injuries, and promising new therapies on the horizon.

DISCUSSION

The nature of warfare is rapidly shifting with information operations, autonomous weapons, and the threat of full-scale peer adversary conflicts threatening to create contested environments with delayed medical evacuation to definitive care. More destructive weapons will lead to more devastating injuries, creating new challenges for limb repair and restoration. Current paradigms of delayed rehabilitation following initial stabilization, damage control surgery, and prolonged antibiotic therapy will need to shift. Advances in regenerative medicine technologies offer the possibility of treatment along the continuum of care. Regenerative rehabilitation will begin at the point of injury and require a holistic, organ-systems approach.

CONCLUSIONS

Both technological improvements and a rapidly advancing understanding of injury pathophysiology will contribute to improved limb-salvage outcomes, and shift the calculus away from early limb amputation.

Regeneration of lingual musculature in rats using myoblasts over porcine bladder acellular matrix

OBJECTIVES

To use tissue engineering muscle repair (TEMR) for regenerating the lingual musculature of hemiglossectomized rats using neonatal myoblasts (NM) on porcine acellular urinary bladder matrix (AUBM).

MATERIAL AND METHODS

The study used 80 male rats. A volumetric muscle loss (VML) injury was created on the left side of the tongue. The rats were randomized into four groups: Group 1 (AUBM + myoblasts); Group 2 (AUBM); Group 3 (myoblasts); and Group 4 (control). NM were obtained from neonatal rats. The animals were weighed on day 0 and just before euthanasia. Five rats in each group were euthanized at days 2, 14, 28, and 42; the tongues were prepared for morphometric analysis, postoperative left hemitongue weight, and immunohistochemical analysis (desmin, CD-31, and anti-neurofilament).

RESULTS

The weight gain from greatest to least was as follows: AUBM + myoblasts > myoblasts > AUBM > control. The tongue dorsum occupied by VML, and difference in mg between control side and intervened side from least to great was as follows: AUBM + myoblasts < myoblasts < AUBM < control. The order from highest to lowest antibody positivity was as follows: AUBM + myoblasts > myoblasts > AUBM > control.

CONCLUSION

The use of porcine AUBM and NM for the regeneration of lingual musculature was found to be an effective TEMR treatment for repairing tongue VML injury.

Information-Driven Design as a Potential Approach for 3D Printing of Skeletal Muscle Biomimetic Scaffolds

Severe muscle injuries are a real clinical issue that still needs to be successfully addressed. Tissue engineering can represent a potential approach for this aim, but effective healing solutions have not been developed yet. In this regard, novel experimental protocols tailored to a biomimetic approach can thus be defined by properly systematizing the findings acquired so far in the biomaterials and scaffold manufacturing fields. In order to plan a more comprehensive strategy, the extracellular matrix (ECM), with its properties stimulating neomyogenesis and vascularization, should be considered as a valuable biomaterial to be used to fabricate the tissue-specific three-dimensional structure of interest. The skeletal muscle decellularized ECM can be processed and printed, e.g., by means of stereolithography, to prepare bioactive and biomimetic 3D scaffolds, including both biochemical and topographical features specifically oriented to skeletal muscle regenerative applications. This paper aims to focus on the skeletal muscle tissue engineering sector, suggesting a possible approach to develop instructive scaffolds for a guided healing process.

The Effects of Engineered Skeletal Muscle on Volumetric Muscle Loss in The Tibialis Anterior Of Rat After Three Months In Vivo

Volumetric muscle loss (VML) is traumatic, degenerative, or surgical loss of skeletal muscle that exceeds the regenerative capacity of the remaining muscle, thus resulting in impaired muscle function. In humans, the loss of 30% or more mass of any one muscle will result in permanent structural and functional loss. Current VML repair treatments are limited by donor site morbidity and graft tissue availability, necessitating alternative muscle graft sources. To address this need, our lab has fabricated tissue-engineered skeletal muscle units (SMUs) for implantation into a 30 % VML model in the tibialis anterior (TA) muscle of rat. Previous results showed that after 28 days in vivo, muscle with a 30% VML repaired with our SMUs produced significantly more force than muscle with acute VML. But repair with our SMU did not fully restore muscle force production to that of native muscle. Thus, we hypothesized that more time for in vivo tissue regeneration would allow for greater force recovery. Therefore, the purpose of this study was to examine the long-term (3-month) effects of our SMUs on a 30% VML repair. We also assessed the effects of reinnervation by redirecting a branch of the peroneal nerve to the repair site. Thirty-nine, 2-month old female F344 rats were separated into a nonsurgical control group (n=5) and four surgical experimental groups (VML Only, n=5; VML+Nerve Redirect, n=6; VML+SMU, n=5; VML+SMU+ Nerve Redirect, n=8). Experimental rats were allowed a 3-month recovery period post-surgery before undergoing in situ force testing of the surgical (left) TA. The left TA of the control animals also underwent in situ force testing. Finally, the surgical (left) and contralateral (right) TAs of the experimental animals, as well as the left TA of the control animals, were explanted for histological analysis. Results for specific force showed that while all groups recovered specific forces similar to that of native muscle, the two SMU groups had significantly higher specific forces, on average, compared to the uninjured control group. Histological staining showed small muscle fibers in the repair site in animals that received an SMU. The average cross-sectional area of the native fibers just outside the area of repair (or the equivalent area in control animals) was not significantly different between groups, indicating that hypertrophy of remaining fibers did not contribute to the recovery of force following the VML. Our results suggest that following a 30% VML of the TA muscle, all surgical groups were able to recover TA mass, maximum tetanic and specific force production. Thus, creating a 30% VML in the TA in a rat model is not enough a sufficient VML to produce the sustained VML seen in humans following similar 30% loss of muscle volume.

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.

Skeletal Muscle Tissue Engineering: Biomaterials-Based Strategies for the Treatment of Volumetric Muscle Loss

Millions of Americans suffer from skeletal muscle injuries annually that can result in volumetric muscle loss (VML), where extensive musculoskeletal damage and tissue loss result in permanent functional deficits. In the case of small-scale injury skeletal muscle is capable of endogenous regeneration through activation of resident satellite cells (SCs). However, this is greatly reduced in VML injuries, which remove native biophysical and biochemical signaling cues and hinder the damaged tissue's ability to direct regeneration. The current clinical treatment for VML is autologous tissue transfer, but graft failure and scar tissue formation leave patients with limited functional recovery. Tissue engineering of instructive biomaterial scaffolds offers a promising approach for treating VML injuries. Herein, we review the strategic engineering of biophysical and biochemical cues in current scaffold designs that aid in restoring function to these preclinical VML injuries. We also discuss the successes and limitations of the three main biomaterial-based strategies to treat VML injuries: acellular scaffolds, cell-delivery scaffolds, and in vitro tissue engineered constructs. Finally, we examine several innovative approaches to enhancing the design of the next generation of engineered scaffolds to improve the functional regeneration of skeletal muscle following VML injuries.

Recent Trends in Injury Models to Study Skeletal Muscle Regeneration and Repair

Skeletal muscle injuries that occur from traumatic incidents, such as those caused by car accidents or surgical resections, or from injuries sustained on the battlefield, result in the loss of functionality of the injured muscle. To understand skeletal muscle regeneration and to better treat these large scale injuries, termed volumetric muscle loss (VML), in vivo injury models exploring the innate mechanisms of muscle injury and repair are essential for the creation of clinically applicable treatments. While the end result of a muscle injury is often the destruction of muscle tissue, the manner in which these injuries are induced as well as the response from the innate repair mechanisms found in muscle in each animal models can vary. This targeted review describes injury models that assess both skeletal muscle regeneration (i.e., the response of muscle to myotoxin or ischemic injury) and skeletal muscle repair (i.e., VML injury). We aimed to summarize the injury models used in the field of skeletal muscle tissue engineering, paying particular attention to strategies to induce muscle damage and how to standardize injury conditions for future experiments.

Elastin-Like Recombinamer Hydrogels for Improved Skeletal Muscle Healing Through Modulation of Macrophage Polarization

Large skeletal muscle injuries, such as a volumetric muscle loss (VML), often result in an incomplete regeneration due to the formation of a non-contractile fibrotic scar tissue. This is, in part, due to the outbreak of an inflammatory response, which is not resolved over time, meaning that type-1 macrophages (M1, pro-inflammatory) involved in the initial stages of the process are not replaced by pro-regenerative type-2 macrophages (M2). Therefore, biomaterials that promote the shift from M1 to M2 are needed to achieve optimal regeneration in VML injuries. In this work, we used elastin-like recombinamers (ELRs) as biomaterials for the formation of non- (physical) and covalently (chemical) crosslinked bioactive and biodegradable hydrogels to fill the VML created in the tibialis anterior (TA) muscles of rats. These hydrogels promoted a higher infiltration of M2 within the site of injury in comparison to the non-treated control after 2 weeks (p<0.0001), indicating that the inflammatory response resolves faster in the presence of both types of ELR-based hydrogels. Moreover, there were not significant differences in the amount of collagen deposition between the samples treated with the chemical ELR hydrogel at 2 and 5 weeks, and this same result was found upon comparison of these samples with healthy tissue after 5 weeks, which implies that this treatment prevents fibrosis. The macrophage modulation also translated into the formation of myofibers that were morphologically more similar to those present in healthy muscle. Altogether, these results highlight that ELR hydrogels provide a friendly niche for infiltrating cells that biodegrades over time, leaving space to new muscle tissue. In addition, they orchestrate the shift of macrophage population toward M2, which resulted in the prevention of fibrosis in the case of the chemical hydrogel treatment and in a more healthy-like myofiber phenotype for both types of hydrogels. Further studies should focus in the assessment of the regeneration of skeletal muscle in larger animal models, where a more critical defect can be created and additional methods can be used to evaluate the functional recovery of skeletal muscle.

Bioprinting on sheet-based scaffolds applied to the creation of implantable tissue-engineered constructs with potentially diverse clinical applications: Tissue-Engineered Muscle Repair (TEMR) as a representative testbed

Purpose: This report explores the overlooked potential of bioprinting to automate biomanufacturing of simple tissue structures, such as the uniform deposition of (mono)layers of progenitor cells on sheetlike decellularized extracellular matrices (dECM). In this scenario, dECM serves as a biodegradable celldelivery matrix to provide enhanced regenerative microenvironments for tissue repair. The Tissue-Engineered Muscle Repair (TEMR) technology-where muscle progenitor cells are seeded onto a porcine bladder acellular matrix (BAM), serves as a representative testbed for bioprinting applications. Previous work demonstrated that TEMR implantation improved functional outcomes following VML injury in biologically relevant rodent models.Materials and Methods: In the described bioprinting system, a cell-laden hydrogel bioink is used to deposit high cell densities (1.4 × 105-3.5 × 105 cells/cm²), onto both sides of the bladder acellular matrix as proof-of-concept.Results: These bioprinting methods achieve a reproducible and homogeneous distribution of cells, on both sides of the BAM scaffold, after just 24hrs, with cell viability as high as 98%. These preliminary results suggest bioprinting allows for improved dual-sided cell coverage compared to manual-seeding.Conclusions: Bioprinting can enable automated fabrication of TEMR constructs with high fidelity and scalability, while reducing biomanufacturing costs and timelines. Such bioprinting applications are underappreciated, yet critical, to expand the overall biomanufacturing paradigm for tissue engineered medical products. In addition, biofabrication of sheet-like implantable constructs, with cells deposited on both sides, is a process that is both scaffold and cell-type agnostic, and furthermore, is amenable to many geometries, and thus, additional tissue engineering applications beyond skeletal muscle.

Repairing Volumetric Muscle Loss in the Ovine Peroneus Tertius Following a 3-Month Recovery

Much effort has been made to fabricate engineered tissues on a scale that is clinically relevant to humans; however, scale-up remains one of the most significant technological challenges of tissue engineering to date. To address this limitation, our laboratory has developed tissue-engineered skeletal muscle units (SMUs) and engineered neural conduits (ENCs), and modularly scaled them to clinically relevant sizes for the treatment of volumetric muscle loss (VML). The goal of this study was to evaluate the SMUs and ENCs in vitro, and to test the efficacy of our SMUs and ENCs in restoring muscle function in a clinically relevant large animal (sheep) model. The animals received a 30% VML injury to the peroneus tertius muscle and were allowed to recover for 3 months. The animals were divided into three experimental groups: VML injury without a repair (VML only), repair with an SMU (VML+SMU), or repair with an SMU and ENC (VML+SMU+ENC). We evaluated the SMUs before implantation and found that our single scaled-up SMUs were characterized by the presence of contracting myotubes, linearly aligned extracellular matrix proteins, and Pax7⁺ satellite cells. Three months after implantation, we found that the repair groups (VML+SMU and VML+SMU+ENC) had restored muscle mass and tetanic force production to a level that was statistically indistinguishable from the uninjured contralateral muscle after 3 months in vivo. Furthermore, we demonstrated the ability of our ENCs to effectively bridge the gap between native nerve and the repair site by eliciting a muscle contraction through direct electrical stimulation of the re-routed nerve. Impact statement The fabrication of tissues of clinically relevant sizes is one of the largest obstacles preventing engineered tissues from achieving widespread use in the clinic. This study aimed to combat this limitation by developing a fabrication method to scale-up tissue-engineered skeletal muscle for the treatment of volumetric muscle loss in a large animal (sheep) model and evaluating the efficacy of the tissue-engineered constructs after a 3-month recovery.

Long-Term Evaluation of Functional Outcomes Following Rat Volumetric Muscle Loss Injury and Repair

Volumetric muscle loss (VML) injuries, by definition, exceed the endogenous repair capacity of skeletal muscle resulting in permanent structural and functional deficits. VML injuries present a significant burden for both civilian and military medicine. Despite progress, there is still considerable room for therapeutic improvement. In this regard, tissue-engineered constructs show promise for VML repair, as they provide an opportunity to introduce both scaffolding and cellular components. We have pioneered the development of a tissue-engineered muscle repair (TEMR) technology created by seeding muscle progenitor cells onto a porcine-derived bladder acellular matrix followed by cyclic stretch preconditioning before implantation. Our work to date has demonstrated significant functional repair (60-90% functional recovery) in progressively larger rodent models of VML injury following TEMR implantation. Notwithstanding this success, TEMR implantation in cylindrically shaped VML injuries in the tibialis anterior (TA) muscle was associated with more variable functional outcomes than has been observed in sheet-like muscles such as the latissimus dorsi. In fact, previous observations documented a dichotomy of responses following TEMR implantation in a rodent TA VML injury model; with an ≈61% functional improvement observed in fewer than half (46%) of TEMR-implanted animals at 12 weeks postinjury. This current study builds directly from those observations as we modified the geometry of both the VML injury and the TEMR construct to determine if improved matching of the implanted TEMR construct to the surgically created VML injury resulted in increased functional recovery posttreatment. Following these modifications, we observed a comparable degree of functional improvement in a larger proportion of animals (≈67%) that was durable up to 24 weeks post-TEMR implantation. Moreover, in ≈25% of all TEMR-implanted animals, functional recovery was virtually complete (TEMR max responders), and furthermore, the functional recovery in all 67% of responding animals was accompanied by the presence of native-like muscle properties within the repaired TA muscle, including fiber cross-sectional area, fiber type, vascularization, and innervation. This study emphasizes the importance of tuning the application of tissue engineering technology platforms to the specific requirements of diverse VML injuries to improve functional outcomes. Impact Statement This report confirms and extends previous observations with our implantable tissue-engineered technology platform for repair of volumetric muscle loss (VML) injuries. Based on our prior work, we addressed factors hypothesized to be responsible for significant outcome variability following treatment of VML injuries in a rat tibialis anterior model. Through customization of the muscle repair technology to a specific VML injury, we were able to significantly increase the frequency at which functional recovery occurred, and furthermore, demonstrate durability out to 6 months. In addition, the enhanced biomimetic qualities of repaired muscle tissue were associated with the most robust functional outcomes.

Vascularized and Innervated Skeletal Muscle Tissue Engineering

Volumetric muscle loss (VML) is a devastating loss of muscle tissue that overwhelms the native regenerative properties of skeletal muscle and results in lifelong functional deficits. There are currently no treatments for VML that fully recover the lost muscle tissue and function. Tissue engineering presents a promising solution for VML treatment and significant research has been performed using tissue engineered muscle constructs in preclinical models of VML with a broad range of defect locations and sizes, tissue engineered construct characteristics, and outcome measures. Due to the complex vascular and neural anatomy within skeletal muscle, regeneration of functional vasculature and nerves is vital for muscle recovery following VML injuries. This review aims to summarize the current state of the field of skeletal muscle tissue engineering using 3D constructs for VML treatment with a focus on studies that have promoted vascular and neural regeneration within the muscle tissue post-VML.

Myogenic differentiation of human amniotic mesenchymal cells and its tissue repair capacity on volumetric muscle loss

Stem cell-based tissue engineering therapy is the most promising method for treating volumetric muscle loss. Human amniotic mesenchymal cells possess characteristics similar to those of embryonic stem cells. In this study, we verified the stem cell characteristics of human amniotic mesenchymal cells by the flow cytometry analysis, and osteogenic and adipogenic differentiation. Through induction with the DNA demethylating agent 5-azacytidine, human amniotic mesenchymal cells can undergo myogenic differentiation and express skeletal muscle cell-specific markers such as desmin and MyoD. The Wnt/β-catenin signaling pathway also plays an important role. After 5-azacytidine-induced human amniotic mesenchymal cells were implanted into rat tibialis anterior muscle with volumetric muscle loss, we observed increased angiogenesis and improved local tissue repair. We believe that human amniotic mesenchymal cells can serve as a potential source of cells for skeletal muscle tissue engineering.

In Silico and In Vivo Studies Detect Functional Repair Mechanisms in a Volumetric Muscle Loss Injury

IMPACT STATEMENT

Despite medical advances, volumetric muscle loss (VML) injuries to craniofacial muscles represent an unmet clinical need. We report an implantable tissue-engineered construct that leads to substantial tissue regeneration and functional recovery in a preclinical model of VML injury that is dimensionally relevant to unilateral cleft lip repair, and a series of corresponding computational models that provide biomechanical insight into mechanism(s) responsible for the VML-induced functional deficits and recovery following tissue-engineered muscle repair implantation. This unique combined approach represents a critical first step toward establishing a crucial biomechanical basis for the development of efficacious regenerative technologies, considering the spectrum of VML injuries.

The Effect of Laminin-111 Hydrogels on Muscle Regeneration in a Murine Model of Injury

IMPACT STATEMENT

Extremity injuries make up the most common survivable injuries in vehicular accidents and modern military conflicts. A majority of these injuries involve volumetric muscle loss (VML). The potential for donor site morbidity may limit the clinical use of autologous muscle grafts for VML injuries. Treatments that can improve the regeneration of functional muscle tissue are critically needed to improve limb salvage and reduce the rate of delayed amputations. The development of a laminin-111-enriched fibrin hydrogel will offer a potentially transformative and "off-the-shelf" clinically relevant therapy for functional skeletal muscle regeneration.

Analysis and Modeling of Rat Gait Biomechanical Deficits in Response to Volumetric Muscle Loss Injury

There is currently a substantial volume of research underway to develop more effective approaches for the regeneration of functional muscle tissue as treatment for volumetric muscle loss (VML) injury, but few studies have evaluated the relationship between injury and the biomechanics required for normal function. To address this knowledge gap, the goal of this study was to develop a novel method to quantify the changes in gait of rats with tibialis anterior (TA) VML injuries. This method should be sensitive enough to identify biomechanical and kinematic changes in response to injury as well as during recovery. Control rats and rats with surgically-created VML injuries were affixed with motion capture markers on the bony landmarks of the back and hindlimb and were recorded walking on a treadmill both prior to and post-surgery. Data collected from the motion capture system was exported for post-hoc analysis in OpenSim and Matlab. In vivo force testing indicated that the VML injury was associated with a significant deficit in force generation ability. Analysis of joint kinematics showed significant differences at all three post-surgical timepoints and gait cycle phase shifting, indicating augmented gait biomechanics in response to VML injury. In conclusion, this method identifies and quantifies key differences in the gait biomechanics and joint kinematics of rats with VML injuries and allows for analysis of the response to injury and recovery. The comprehensive nature of this method opens the door for future studies into dynamics and musculoskeletal control of injured gait that can inform the development of regenerative technologies focused on the functional metrics that are most relevant to recovery from VML injury.

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.

Engineering Biomimetic Materials for Skeletal Muscle Repair and Regeneration

Although skeletal muscle is highly regenerative following injury or disease, endogenous self-regeneration is severely impaired in conditions of volume traumatic muscle loss. Consequently, tissue engineering approaches are a promising means to regenerate skeletal muscle. Biological scaffolds serve as not only structural support for the promotion of cellular ingrowth but also impart potent modulatory signaling cues that may be beneficial for tissue regeneration. In this work, the progress of tissue engineering approaches for skeletal muscle engineering and regeneration is overviewed, with a focus on the techniques to create biomimetic engineered tissue using extracellular cues. These factors include mechanical and electrical stimulation, geometric patterning, and delivery of growth factors or other bioactive molecules. The progress of evaluating the therapeutic efficacy of these approaches in preclinical models of muscle injury is further discussed.

Determination of a Critical Size Threshold for Volumetric Muscle Loss in the Mouse Quadriceps

IMPACT STATEMENT

The goal of this study was to determine the threshold for a critically sized, nonhealing muscle defect by characterizing key components in the balance between fibrosis and regeneration as a function of injury size in the mouse quadriceps. There is currently limited understanding of what leads to a critically sized muscle defect and which muscle regenerative components are functionally impaired. With the substantial increase in preclinical VML models as testbeds for tissue engineering therapeutics, defining the critical threshold for VML injuries will be instrumental in characterizing therapeutic efficacy and potential for subsequent translation.

Encapsulation of bone marrow-MSCs in PRP-derived fibrin microbeads and preliminary evaluation in a volumetric muscle loss injury rat model: modular muscle tissue engineering

Repair of volumetric muscle loss (VML) injuries is a complicated endeavour which necessitates the collaborative use of different regenerative approaches and technologies. Herein is proposed the development of fibrin-based microbeads (FMs) alone or as a bone marrow mesenchymal stem cell (MSC) encapsulation matrix for modular muscle engineering. FMs were generated through the ionotropic gelation of alginate and fibrinogen obtained from the platelet-rich plasma of whole blood, and then removing the alginate by citrate treatment. FMs were first characterized by FT-IR, SEM and water uptake tests. Then, the stability of FMs and the mitochondrial dehydrogenase activity of the MSCs encapsulated in FMs were evaluated under in vitro culture conditions. Eventually, the regenerative capacity of the cell-devoid and MSCs-encapsulated FMs was evaluated in a rat VML injury model involving 8 × 4×4 mm³-size bilateral defects in the biceps femoris muscles. The histochemical, immunohistochemical and semi-quantitative histomorphological scoring results retrieved at 30, 60 and 180 days demonstrated that the cell-devoid FMs supported muscle regeneration to a great extent. Moreover, MSCs-encapsulated FMs were more effective in shortening the regeneration period of the injured tissue of the rat VML, resulting in good myofibre orientation, while the Sham group resulted in incomplete repair with fibrotic scar tissue formations.

Biomimetic sponges for regeneration of skeletal muscle following trauma

Skeletal muscle is inept in regenerating after traumatic injuries due to significant loss of basal lamina and the resident satellite cells. To improve regeneration of skeletal muscle, we have developed biomimetic sponges composed of collagen, gelatin, and laminin (LM)-111 that were crosslinked with 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC). Collagen and LM-111 are crucial components of the muscle extracellular matrix and were chosen to impart bioactivity whereas gelatin and EDC were used to provide mechanical strength to the scaffold. Morphological and mechanical evaluation of the sponges showed porous structure, water-retention capacity and a compressive modulus of 590-808 kPa. The biomimetic sponges supported the infiltration and viability of C₂ C₁₂ myoblasts over 5 days of culture. The myoblasts produced higher levels of myokines such as VEGF, IL-6, and IGF-1 and showed higher expression of myogenic markers such as MyoD and myogenin on the biomimetic sponges. Biomimetic sponges implanted in a mouse model of volumetric muscle loss (VML) supported satellite, endothelial, and inflammatory cell infiltration but resulted in limited myofiber regeneration at 2 weeks post-injury. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 92-103, 2019.