A 3D, Magnetically Actuated, Aligned Collagen Fiber Hydrogel Platform Recapitulates Physical Microenvironment of Myoblasts for Enhancing Myogenesis

Recent trends in 3D bioprinting technology for skeletal muscle regeneration

Skeletal muscle is a pro-regenerative tissue, that utilizes a tissue-resident stem cell system to effect repair upon injury. Despite the demonstrated efficiency of this system in restoring muscle mass after many acute injuries, in conditions of severe trauma such as those evident in volumetric muscle loss (VML) (>20 % by mass), this self-repair capability is unable to restore tissue architecture, requiring interventions which currently are largely surgical. As a possible alternative, the generation of artificial muscle using tissue engineering approaches may also be of importance in the treatment of VML and muscle diseases such as dystrophies. Three-dimensional (3D) bioprinting has been identified as a promising technique for regeneration of the complex architecture of skeletal muscle. This review discusses existing treatment strategies following muscle damage, recent progress in bioprinting techniques, the bioinks used for muscle regeneration, the immunogenicity of scaffold materials, and in vitro and in vivo maturation techniques for 3D bio-printed muscle constructs. The pros and cons of these bioink formulations are also highlighted. Finally, we present the current limitations and challenges in the field and critical factors to consider for bioprinting approaches to become more translationa and to produce clinically relevant engineered muscle. STATEMENT OF SIGNIFICANCE: This review discusses the physiopathology of muscle injuries and existing clinical treatment strategies for muscle damage, the types of bioprinting techniques that have been applied to bioprinting of muscle, and the bioinks commonly used for muscle regeneration. The pros and cons of these bioinks are highlighted. We present a discussion of existing gaps in the literature and critical factors to consider for the translation of bioprinting approaches and to produce clinically relevant engineered muscle. Finally, we provide insights into what we believe will be the next steps required before the realization of the application of tissue-engineered muscle in humans. We believe this manuscript is an insightful, timely, and instructive review that will guide future muscle bioprinting research from a fundamental construct creation approach, down a translational pathway to achieve the desired impact in the clinic.

Matrix Nonlinear Viscoelasticity Regulates Skeletal Myogenesis through MRTF Nuclear Localization and Nuclear Mechanotransduction

Mechanically sensitive tissues (e.g., skeletal muscles) greatly need mechanical stimuli during the development and maturation. The extracellular matrix (ECM) mediates these signals through nonlinear viscoelasticity of collagen networks that are predominant components of the ECM. However, the interactions between cells and ECM form a feedback loop, and it has not yet been possible to determine the degree to which, if any, of the features of matrix nonlinear viscoelasticity affect skeletal muscle development and regeneration. In this study, a nonlinear viscoelastic feature (i.e., strain-enhanced stress relaxation (SESR)) in normal skeletal muscles is observed, which however is almost absent in diseased muscles from Duchenne muscular dystrophy mice. It is recapitulated such SESR feature in vitro and separated the effects of mechanical strain and ECM viscoelasticity on myoblast response by developing a collagen-based hydrogel platform. Both strain and stress relaxation induce myogenic differentiation and myotube formation by C2C12 myoblasts, and myogenesis is more promoted by applying SESR. This promotion can be explained by the effects of SESR on actin polymerization-mediated myocardin related transcription factor (MRTF) nuclear localization and nuclear mechanotransduction. This study represents the first attempt to investigate the SESR phenomenon in skeletal muscles and reveal underlying mechanobiology, which will provide new opportunities for the tissue injury treatments.

A well plate-based GelMA photo-crosslinking system with tunable hydrogel mechanical properties to regulate the PTH-mediated osteogenic fate

Versatile and efficient regulation of the mechanical properties of the extracellular matrix is crucial not only for understanding the dynamic changes in biological systems, but also for obtaining precise and effective cellular responses in drug testing. In this study, we developed a well plate-based hydrogel photo-crosslinking system to effectively control the mechanical properties of hydrogels and perform high-throughput assays. We improved cell biocompatibility by using gelatin methacryloyl (GelMA) with a visible light photo-crosslinking method. Multiple cell-laden GelMA hydrogels were simultaneously and uniformly created using multi-arrayed 520 nm light-emitting diodes in a well plate format. The elastic modulus of the hydrogels can be widely adjusted (0.5-30 kPa) using a photo-crosslinking system capable of independently controlling the light intensity or exposure time for multiple samples. We demonstrate the feasibility of our system by observing enhanced bone differentiation of human mesenchymal stem cells (hMSCs) cultured on stiffer hydrogels. Additionally, we observed that the osteogenic fate of hMSCs, affected by the different mechanical properties of the gel, was regulated by parathyroid hormone (PTH). Notably, in response to PTH, hMSCs in a high-stiffness microenvironment upregulate osteogenic differentiation while exhibiting increased proliferation in a low-stiffness microenvironment. Overall, the developed system enables the generation of multiple cell-laden three-dimensional cell culture models with diverse mechanical properties and holds significant potential for expansion into drug testing.

3D-printed bioink loading with stem cells and cellular vesicles for periodontitis-derived bone defect repair

The suitable microenvironment of bone regeneration is critically important for periodontitis-derived bone defect repair. Three major challenges in achieving a robust osteogenic reaction are the exist of oral inflammation, pathogenic bacteria invasion and unaffluent seed cells. Herein, a customizable and multifunctional 3D-printing module was designed with glycidyl methacrylate (GMA) modified epsilon-poly-L-lysine (EPLGMA) loading periodontal ligament stem cells (PDLSCs) and myeloid-derived suppressive cells membrane vesicles (MDSCs-MV) bioink (EPLGMA/PDLSCs/MDSCs-MVs, abbreviated as EPM) for periodontitis-derived bone defect repair. The EPM showed excellent mechanical properties and physicochemical characteristics, providing a suitable microenvironment for bone regeneration.In vitro, EPMs presented effectively kill the periodontopathic bacteria depend on the natural antibacterial properties of the EPL. Meanwhile, MDSCs-MV was confirmed to inhibit T cells through CD73/CD39/adenosine signal pathway, exerting an anti-inflammatory role. Additionally, seed cells of PDLSCs provide an adequate supply for osteoblasts. Moreover, MDSCs-MV could significantly enhance the mineralizing capacity of PDLSCs-derived osteoblast. In the periodontal bone defect rat model, the results of micro-CT and histological staining demonstrated that the EPM scaffold similarly had an excellent anti-inflammatory and bone regeneration efficacyin vivo. This biomimetic and multifunctional 3D-printing bioink opens new avenues for periodontitis-derived bone defect repair and future clinical application.

Efficient Myogenic Activities Achieved through Blade-Casting-Assisted Bioprinting of Aligned Myoblasts Laden in Collagen Bioink

This study investigated mechanical stimulation combined with three-dimensional (3D) bioprinting as a new approach for introducing biophysical and biological cues for tissue regeneration. A blade-casting method in conjunction with bioprinting was employed to fabricate bioengineered skeletal muscle constructs using a bioink composed of C2C12 myoblasts and collagen type-I. Various printing process parameters were selected and optimized to achieve a highly organized cell alignment within the constructs. The resulting cell-aligned constructs demonstrated remarkable improvement in actin filament alignment and cell proliferation compared with conventionally printed cell-laden constructs. This improvement can be attributed to the synergistic effects of mechanotransduction, facilitating the cellular response to mechanical cues and the alignment of fibrillated collagen, which plays a significant role in modulating cellular functions and promoting muscle tissue regeneration. Furthermore, we assessed the impact of blade casting combined with 3D bioprinting on gene expression. The expression levels of myogenesis-related genes were substantially upregulated, with an approximately 1.6-fold increase compared to the constructs fabricated without the blade-casting technique. The results demonstrated the effectiveness of combining mechanical stimulation through blade casting with 3D bioprinting in promoting aligned cell structures, enhancing cellular functions, and driving muscle tissue regeneration.

A Platform for Assessing Cellular Contractile Function Based on Magnetic Manipulation of Magnetoresponsive Hydrogel Films

Despite significant advancements in in vitro cardiac modeling approaches, researchers still lack the capacity to obtain in vitro measurements of a key indicator of cardiac function: contractility, or stroke volume under specific loading conditions-defined as the pressures to which the heart is subjected prior to and during contraction. This work puts forward a platform that creates this capability, by providing a means of dynamically controlling loading conditions in vitro. This dynamic tissue loading platform consists of a thin magnetoresponsive hydrogel cantilever on which 2D engineered myocardial tissue is cultured. Exposing the cantilever to an external magnetic field-generated by positioning magnets at a controlled distance from the cantilever-causes the hydrogel film to stretch, creating tissue load. Next, cell contraction is induced through electrical stimulation, and the force of the contraction is recorded, by measuring the cantilever's deflection. Force-length-based measurements of contractility are then derived, comparable to clinical measurements. In an illustrative application, the platform is used to measure contractility both in untreated myocardial tissue and in tissue exposed to an inotropic agent. Clear differences are observed between conditions, suggesting that the proposed platform has significant potential to provide clinically relevant measurements of contractility.

Combinatorial extracellular matrix cues with mechanical strain induce differential effects on myogenesis in vitro

Skeletal muscle regeneration remains a clinical unmet need for volumetric muscle loss and atrophy where muscle function cannot be restored to prior capacity. Current experimental approaches do not account for the complex microenvironmental factors that modulate myogenesis. In this study we developed a biomimetic tissue chip platform to systematically study the combined effects of the extracellular matrix (ECM) microenvironment and mechanical strain on myogenesis of murine myoblasts. Using stretchable tissue chips composed of collagen I (C), fibronectin (F) and laminin (L), as well as their combinations thereof, we tested the addition of mechanical strain regimens on myogenesis at the transcriptomic and translational levels. Our results show that ECMs have a significant effect on myotube formation in C2C12 murine myoblasts. Under static conditions, laminin substrates induced the longest myotubes, whereas fibronectin produced the widest myotubes. Combinatorial ECMs showed non-intuitive effects on myotube formation. Genome-wide analysis revealed the upregulation in actin cytoskeletal related genes that are suggestive of myogenesis. When mechanical strain was introduced to C + F + L combinatorial ECM substrates in the form of constant or intermittent uniaxial strain at low (5%) and high (15%) levels, we observed synergistic enhancements in myotube width, along with transcriptomic upregulation in myosin heavy chain genes. Together, these studies highlight the complex role of microenvironmental factors such as ECM interactions and strain on myotube formation and the underlying signaling pathways.

Hierarchical hydrogel scaffolds with a clustered and oriented structure

Hydrogel scaffolds play a critical role in tissue engineering due to their hydrophilic network structure and good biocompatibility. Constructing anisotropic scaffolds geometrically similar to injured tissues is conducive to promoting the generation of tissue and organ equivalents, or to guiding and enhancing the regeneration of injured tissues. In this study, we developed polyvinyl alcohol (PVA)/alginate hierarchical hydrogel scaffolds with a clustered and oriented structure using a method that combines directional freezing and drying under stretching. Our hydrogel scaffolds with an adjustable modulus (50 kPa-20 MPa) can match different types of injured tissues. The clustered and oriented structure successfully guided the alignment and orientation of fibroblasts and chondrocytes. This work provides a new idea for constructing hydrogels with hierarchical and anisotropic microstructures, which have promising applications in tissue regeneration.

Dynamic Stimulations with Bioengineered Extracellular Matrix-Mimicking Hydrogels for Mechano Cell Reprogramming and Therapy

Cells interact with their surrounding environment through a combination of static and dynamic mechanical signals that vary over stimulus types, intensity, space, and time. Compared to static mechanical signals such as stiffness, porosity, and topography, the current understanding on the effects of dynamic mechanical stimulations on cells remains limited, attributing to a lack of access to devices, the complexity of experimental set-up, and data interpretation. Yet, in the pursuit of emerging translational applications (e.g., cell manufacturing for clinical treatment), it is crucial to understand how cells respond to a variety of dynamic forces that are omnipresent in vivo so that they can be exploited to enhance manufacturing and therapeutic outcomes. With a rising appreciation of the extracellular matrix (ECM) as a key regulator of biofunctions, researchers have bioengineered a suite of ECM-mimicking hydrogels, which can be fine-tuned with spatiotemporal mechanical cues to model complex static and dynamic mechanical profiles. This review first discusses how mechanical stimuli may impact different cellular components and the various mechanobiology pathways involved. Then, how hydrogels can be designed to incorporate static and dynamic mechanical parameters to influence cell behaviors are described. The Scopus database is also used to analyze the relative strength in evidence, ranging from strong to weak, based on number of published literatures, associated citations, and treatment significance. Additionally, the impacts of static and dynamic mechanical stimulations on clinically relevant cell types including mesenchymal stem cells, fibroblasts, and immune cells, are evaluated. The aim is to draw attention to the paucity of studies on the effects of dynamic mechanical stimuli on cells, as well as to highlight the potential of using a cocktail of various types and intensities of mechanical stimulations to influence cell fates (similar to the concept of biochemical cocktail to direct cell fate). It is envisioned that this progress report will inspire more exciting translational development of mechanoresponsive hydrogels for biomedical applications.

Innovative Strategies for Hair Regrowth and Skin Visualization

Today, about 50% of men and 15-30% of women are estimated to face hair-related problems, which create a significant psychological burden. Conventional treatments, including drug therapy and transplantation, remain the main strategies for the clinical management of these problems. However, these treatments are hindered by challenges such as drug-induced adverse effects and poor drug penetration due to the skin's barrier. Therefore, various efforts have been undertaken to enhance drug permeation based on the mechanisms of hair regrowth. Notably, understanding the delivery and diffusion of topically administered drugs is essential in hair loss research. This review focuses on the advancement of transdermal strategies for hair regrowth, particularly those involving external stimulation and regeneration (topical administration) as well as microneedles (transdermal delivery). Furthermore, it also describes the natural products that have become alternative agents to prevent hair loss. In addition, given that skin visualization is necessary for hair regrowth as it provides information on drug localization within the skin's structure, this review also discusses skin visualization strategies. Finally, it details the relevant patents and clinical trials in these areas. Together, this review highlights the innovative strategies for skin visualization and hair regrowth, aiming to provide novel ideas to researchers studying hair regrowth in the future.

Removal of GAGs Regulates Mechanical Properties, Collagen Fiber Formation, and Alignment in Tissue Engineered Meniscus

The complex fibrillar architecture of native meniscus is essential for proper function and difficult to recapitulate in vitro. In the native meniscus, proteoglycan content is low during the development of collagen fibers and progressively increases with aging. In vitro, fibrochondrocytes produce glycosaminoglycans (GAGs) early in culture, in contrast to native tissue, where they are deposited after collagen fibers have formed. This difference in the timing of GAG production hinders the formation of a mature fiber network in such in vitro models. In this study, we removed GAGs from collagen gel-based tissue engineered constructs using chondroitinase ABC (cABC) and evaluated the effect on the formation and alignment of collagen fibers and the subsequent effect on tensile and compressive mechanical properties. Removal of GAGs during maturation of in vitro constructs improved collagen fiber alignment in tissue engineered meniscus constructs. Additionally, removal of GAGs during maturation improved fiber alignment without compromising compressive strength, and this removal improved not only fiber alignment and formation but also tensile properties. The increased fiber organization in cABC-treated groups also appeared to influence the size, shape, and location of defects in these constructs, suggesting that treatment may prevent the propagation of large defects under loading. This data gives another method of modulating the ECM for improved collagen fiber formation and mechanical properties in tissue engineered constructs.

Strain Gradient Programming in 3D Fibrous Hydrogels to Direct Graded Cell Alignment

Biological tissues experience various stretch gradients which act as mechanical signaling from the extracellular environment to cells. These mechanical stimuli are sensed by cells, triggering essential signaling cascades regulating cell migration, differentiation, and tissue remodeling. In most previous studies, a simple, uniform stretch to 2D elastic substrates has been applied to analyze the response of living cells. However, induction of nonuniform strains in controlled gradients, particularly in biomimetic 3D hydrogels, has proven challenging. In this study, 3D fibrin hydrogels of manipulated geometry are stretched by a silicone carrier to impose programmable strain gradients along different chosen axes. The resulting strain gradients are analyzed and compared to finite element simulations. Experimentally, the programmed strain gradients result in similar gradient patterns in fiber alignment within the gels. Additionally, temporal changes in the orientation of fibroblast cells embedded in the stretched fibrin gels correlate to the strain and fiber alignment gradients. The experimental and simulation data demonstrate the ability to custom-design mechanical gradients in 3D biological hydrogels and to control cell alignment patterns. It provides a new technology for mechanobiology and tissue engineering studies.

Ultrasoft and Biocompatible Magnetic-Hydrogel-Based Strain Sensors for Wireless Passive Biomechanical Monitoring

Implantable flexible mechanical sensors have exhibited great potential in health monitoring and disease diagnosis due to continuous and real-time monitoring capability. However, the wires and power supply required in current devices cause inconvenience and potential risks. Magnetic-based devices have demonstrated advantages in wireless and passive sensing, but the mismatched mechanical properties, poor biocompatibility, and insufficient sensitivity have limited their applications in biomechanical monitoring. Here, a wireless and passive flexible magnetic-based strain sensor based on a gelatin methacrylate/Fe₃O₄ magnetic hydrogel has been fabricated. The sensor exhibits ultrasoft mechanical properties, strong magnetic properties, and long-term stability in saline solution and can monitor strains down to 50 μm. A model of the sensing process is established to identify the optimal detection location and the relation between the relative magnetic permeability and the sensitivity of the sensors. Moreover, an in vitro tissue model is developed to investigate the potential of the sensor in detecting subtle biomechanical signals and avoiding interference with bioactivities. Furthermore, a real-time and high-throughput biomonitoring platform is built and implements passive wireless monitoring of the drug response and cultural status of the cardiomyocytes. This work demonstrates the potential of applying magnetic sensing for biomechanical monitoring and provides ideas for the design of wireless and passive implantable devices.

Three-Dimensional Collagen Topology Shapes Cell Morphology, beyond Stiffness

Cellular heterogeneity is associated with many physiological processes, including pathological ones, such as morphogenesis and tumorigenesis. The extracellular matrix (ECM) is a key player in the generation of cellular heterogeneity. Advances in our understanding rely on our ability to provide relevant in vitro models. This requires obtainment of the characteristics of the tissues that are essential for controlling cell fate. To do this, we must consider the diversity of tissues, the diversity of physiological contexts, and the constant remodeling of the ECM along these processes. To this aim, we have fabricated a library of ECM models for reproducing the scaffold of connective tissues and the basement membrane by using different biofabrication routes based on the electrospinning and drop casting of biopolymers from the ECM. Using a combination of electron microscopy, multiphoton imaging, and AFM nanoindentation, we show that we can vary independently protein composition, topology, and stiffness of ECM models. This in turns allows one to generate the in vivo complexity of the phenotypic landscape of ovarian cancer cells. We show that, while this phenotypic shift cannot be directly correlated with a unique ECM feature, the three-dimensional collagen fibril topology patterns cell shape, beyond protein composition and stiffness of the ECM. On this line, this work is a further step toward the development of ECM models recapitulating the constantly remodeled environment that cells face and thus provides new insights for cancer model engineering and drug testing.

Nanotopography in directing osteogenic differentiation of mesenchymal stem cells: potency and future perspective

Severe bone injuries can result in disabilities and thus affect a person's quality of life. Mesenchymal stem cells (MSCs) can be an alternative for bone healing by growing them on nanopatterned substrates that provide mechanical signals for differentiation. This review aims to highlight the role of nanopatterns in directing or inducing MSC osteogenic differentiation, especially in bone tissue engineering. Nanopatterns can upregulate the expression of osteogenic markers, which indicates a faster differentiation process. Combined with growth factors, nanopatterns can further upregulate osteogenic markers, but with fewer growth factors needed, thereby reducing the risks and costs involved. Nanopatterns can be applied in scaffolds for tissue engineering for their lasting effects, even in vivo, thus having great potential for future bone treatment.

Magnetomechanical force: an emerging paradigm for therapeutic applications

Mechanical forces, which play an profound role in cell fate regulation, have prompted the rapid development and popularization of mechanobiology. More recently, magnetic fields in combination with intelligent materials featuring...

Analysis of the effects of magnetic levitation to simulate microgravity environment on the Arp2/3 complex pathway in macrophage

With dwindling natural resources on earth, current and future generations will need to explore space to new planets that will require travel under no or varying gravity conditions. Hence, long-term space missions and anticipated impacts on human biology such as changes in immune function are of growing research interest. Here, we reported new findings on mechanisms of immune response to microgravity with a focus on macrophage as a cellular model. We employed a superconducting magnet to generate a simulated microgravity environment and evaluated the effects of simulated microgravity on RAW 264.7 mouse macrophage cell line in three time frames: 8, 24, and 48 h. As study endpoints, we measured cell viability, phagocytosis, and used next-generation sequencing to explore its changing mechanism. Macrophage cell viability and phagocytosis both showed a marked decrease under microgravity. The differentially expressed genes (DEG) were identified in two ways: (1) gravity-dependent DEG, compared μg samples and 1 g samples at each time point; (2) time-dependent DEG, compared time-point samples within the same gravitational field. Through transcriptome analysis and confirmed by molecular biological verification, our findings firstly suggest that microgravity might affect macrophage phagocytosis by targeting Arp2/3 complex involved cytoskeleton synthesis and causing macrophage immune dysfunction. Our findings contribute to an emerging body of scholarship on biological effects of space travel.