Nanofiber arrangement regulates peripheral nerve regeneration through differential modulation of macrophage phenotypes

Therapeutic effect of exosomes derived from Schwann cells in the repair of peripheral nerve injury

Peripheral nerve injury (PNI) can cause nerve demyelination, neuronal apoptosis, axonal atrophy, inflammatory infiltration, glial scar formation, and other pathologies that can lead to sensory and motor dysfunction and seriously affect the psychosomatic health of patients. There is currently no effective treatment method, so exploring a promising treatment method is of great significance. Several studies have revealed the therapeutic roles of Schwann cells (SCs) and their exosomes in nerve injury repair. Exosomes are extracellular nanovesicles secreted by cells that act as key molecules in intercellular communication. Progress has been made in understanding the role of exosomes derived from SCs (SC-EXOs) in peripheral nerve regeneration, including the promotion of axonal regeneration and myelin formation, anti-inflammation, vascular regeneration, neuroprotection, and neuroregulation. Therefore, in this paper, we summarize the functional characteristics of SC-EXOs and discuss their potential therapeutic effects on PNI repair as well as some existing problems and future challenges.

Immunomodulatory Biomaterials: Tailoring Surface Properties to Mitigate Foreign Body Reaction and Enhance Tissue Regeneration

The immune cells have demonstrated the ability to promote tissue repair by removing debris, breaking down the extracellular matrix, and regulating cytokine secretion profile. If the behavior of immune cells is not well directed, chronic inflammation and foreign body reaction (FBR) will lead to scar formation and loss of biomaterial functionality. The immunologic response toward tissue repair or chronic inflammation after injury and implantation can be modulated by manipulating the surface properties of biomaterials. Tailoring surface properties of biomaterials enables the regulation of immune cell fate such as adhesion, proliferation, recruitment, polarization, and cytokine secretion profile. This review begins with an overview of the role of immune cells in tissue healing and their interactions with biomaterials. It then discusses how the surface properties of biomaterials influence immune cell behavior. The core focus is reviewing surface modification methods to create innovative materials that reduce foreign body reactions and enhance tissue repair and regeneration by modulating immune cell activities. The review concludes with insights into future advancements in surface modification techniques and the associated challenges.

Electrospun Fiber Surface Roughness Modulates Human Monocyte-Derived Macrophage Phenotype

Injuries to fibrous connective tissues have very little capacity for self-renewal and exhibit poor healing after injury. Phenotypic shifts in macrophages play a vital role in mediating the healing response, creating an opportunity to design immunomodulatory biomaterials which control macrophage polarization and promote regeneration. In this study, electrospun poly(-caprolactone) fibers with increasing surface roughness (SR) were produced by increasing relative humidity and inducing vapor-induced phase separation during the electrospinning process. The impact of surface roughness on macrophage phenotype was assessed using human monocyte-derived macrophages in vitro and in vivo using B6.Cg-Tg(Csf1r-EGFP)1Hume/J (MacGreen) mice. In vitro experiments showed that macrophages cultured on mesh with increasing SR exhibited decreased release of both pro- and anti-inflammatory cytokines potentially driven by increased protein adsorption and biophysical impacts on the cells. Further, increasing SR led to an increase in the expression of the pro-regenerative cell surface marker CD206 relative to the pro-inflammatory marker CD80. Mesh with increasing SR were implanted subcutaneously in MacGreen mice, again showing an increase in the ratio of cells expressing CD206 to those expressing CD80 visualized by immunofluorescence. SR on implanted biomaterials is sufficient to drive macrophage polarization, demonstrating a simple feature to include in biomaterial design to control innate immunity.

Reactive Oxygen Species-Responsive Composite Fibers Regulate Oxidative Metabolism through Internal and External Factors to Promote the Recovery of Nerve Function

It is challenging to sufficiently regulate endogenous neuronal reactive oxygen species (ROS) production, reduce neuronal apoptosis, and reconstruct neural networks under spinal cord injury conditions. Here, hydrogel surface grafting and microsol electrospinning are used to construct a composite biomimetic scaffold with "external-endogenous" dual regulation of ROS. The outer hydrogel enhances local autophagy through responsive degradation and rapid release of rapamycin (≈80% within a week), neutralizing extracellular ROS and inhibiting endogenous ROS production, further reducing neuronal apoptosis. The inner directional fibers continuously supply brain-derived neurotrophic factors to guide axonal growth. The results of in vitro co-culturing show that the dual regulation of oxidative metabolism by the composite scaffold approximately doubles the neuronal autophagy level, reduces 60% of the apoptosis induced by oxidative stress, and increases the differentiation of neural stem cells into neuron-like cells by ≈2.5 times. The in vivo results show that the composite fibers reduce the ROS levels by ≈80% and decrease the formation of scar tissue. RNA sequencing results show that composite scaffolds upregulate autophagy-associated proteins, antioxidase genes, and axonal growth proteins. The developed composite biomimetic scaffold represents a therapeutic strategy to achieve neurofunctional recovery through programmed and accurate bidirectional regulation of the ROS cascade response.

Electrospun Poly-l-Lactic Acid Membranes Promote M2 Macrophage Polarization by Regulating the PCK2/AMPK/mTOR Signaling Pathway

Electrospun membranes are widely used in tissue engineering. Regretfully, there is limited research on how its morphological characteristics precisely regulate macrophage activation and immune response. Therefore, electrospun poly-l-lactic acid (PLLA) membranes with different alignments (align and random) and diameters (nanoscale and microscale) are prepared to investigate the effects of different surface morphologies on M2 macrophage polarization. Additionally, transcriptome, proteome, and phosphoproteome sequencings are combined to examine the underlying regulatory mechanisms. The results show that the electrospun PLLA membranes with different surface morphologies have good biocompatibility and can regulate the phenotype and function of macrophages by changing the micromorphology of the matrix surface. Especially, macrophages cultured on the electrospun membranes of the A600 group exhibit higher M2 macrophage polarization than the other three groups. Furthermore, the findings demonstrate that electrospun PLLA membranes enhance AMP-activated protein kinase (AMPK)/ mammalian target of rapamycin (mTOR) signaling activation by upregulating the expression of integrin phosphoenolpyruvate carboxykinase 2 (PCK2), which is critical for M2 macrophage polarization. Taken together, electrospun PLLA membranes promote M2 macrophage polarization by regulating the PCK2/AMPK/mTOR signaling pathway. This research can provide further theoretical bases for scaffold design, immunoregulatory mechanisms, and clinical application based on electrospinning technology in the future.

Towards the development of sensation-enabled skin substitutes

Recent advances in cell and biofabrication technologies have contributed to the development of complex human organs. In particular, several skin substitutes are being generated using tissue engineering and regenerative medicine (TERM) technologies. However, recent studies mainly focus on the restoration of the dermis and epidermis layers rather than the regeneration of a fully functional innervated skin organ. Innervation is a critical step in functional tissue repair which has been overlooked in the current TERM studies. In the current study, we highlight the importance of sensation in the skin as the largest sensory organ in the human body. In large non-healing skin wounds, the skin sensation is severely diminished or completely lost and ultimately lead to chronic pain and wound healing process interruption. Current therapeutics for restoring skin sensation after trauma are limited. Recent regenerative medicine-based studies could successfully induce neural networks in skin substitutes, but the effectiveness of these technologies in enhancing sensory capability needs further investigation.

Neutrophil peptide 1 accelerates the clearance of degenerative axons during Wallerian degeneration by activating macrophages after peripheral nerve crush injury

JOURNAL/nrgr/04.03/01300535-202408000-00036/figure1/v/2023-12-16T180322Z/r/image-tiff Macrophages play an important role in peripheral nerve regeneration, but the specific mechanism of regeneration is still unclear. Our preliminary findings indicated that neutrophil peptide 1 is an innate immune peptide closely involved in peripheral nerve regeneration. However, the mechanism by which neutrophil peptide 1 enhances nerve regeneration remains unclear. This study was designed to investigate the relationship between neutrophil peptide 1 and macrophages in vivo and in vitro in peripheral nerve crush injury. The functions of RAW 264.7 cells were elucidated by Cell Counting Kit-8 assay, flow cytometry, migration assays, phagocytosis assays, immunohistochemistry and enzyme-linked immunosorbent assay. Axonal debris phagocytosis was observed using the CUBIC (Clear, Unobstructed Brain/Body Imaging Cocktails and Computational analysis) optical clearing technique during Wallerian degeneration. Macrophage inflammatory factor expression in different polarization states was detected using a protein chip. The results showed that neutrophil peptide 1 promoted the proliferation, migration and phagocytosis of macrophages, and CD206 expression on the surface of macrophages, indicating M2 polarization. The axonal debris clearance rate during Wallerian degeneration was enhanced after neutrophil peptide 1 intervention. Neutrophil peptide 1 also downregulated inflammatory factors interleukin-1α, -6, -12, and tumor necrosis factor-α in vivo and in vitro. Thus, the results suggest that neutrophil peptide 1 activates macrophages and accelerates Wallerian degeneration, which may be one mechanism by which neutrophil peptide 1 enhances peripheral nerve regeneration.

Targeting Macrophage Polarization for Reinstating Homeostasis following Tissue Damage

Tissue regeneration and remodeling involve many complex stages. Macrophages are critical in maintaining micro-environmental homeostasis by regulating inflammation and orchestrating wound healing. They display high plasticity in response to various stimuli, showing a spectrum of functional phenotypes that vary from M1 (pro-inflammatory) to M2 (anti-inflammatory) macrophages. While transient inflammation is an essential trigger for tissue healing following an injury, sustained inflammation (e.g., in foreign body response to implants, diabetes or inflammatory diseases) can hinder tissue healing and cause tissue damage. Modulating macrophage polarization has emerged as an effective strategy for enhancing immune-mediated tissue regeneration and promoting better integration of implantable materials in the host. This article provides an overview of macrophages' functional properties followed by discussing different strategies for modulating macrophage polarization. Advances in the use of synthetic and natural biomaterials to fabricate immune-modulatory materials are highlighted. This reveals that the development and clinical application of more effective immunomodulatory systems targeting macrophage polarization under pathological conditions will be driven by a detailed understanding of the factors that regulate macrophage polarization and biological function in order to optimize existing methods and generate novel strategies to control cell phenotype.

Periodic Lamellae-Based Nanofibers for Precise Immunomodulation to Treat Inflammatory Bone Loss in Periodontitis

Periodontitis is a common oral disease accompanied by inflammatory bone loss. The pathological characteristics of periodontitis usually accompany an imbalance in the periodontal immune microenvironment, leading to difficulty in bone regeneration. Therefore, effective treatment strategies are needed to modulate the immune environment in order to treat periodontitis. Here, highly-oriented periodic lamellae poly(ε-caprolactone) electrospun nanofibers (PLN) are developed by surface-directed epitaxial crystallization. The in vitro result shows that the PLN can precisely modulate macrophage polarization toward the M2 phenotype. Macrophages polarized by PLN significantly enhance the migration and osteogenic differentiation of Bone marrow stromal cells. Notably, results suggest that the topographical cues presented by PLN can modulate macrophage polarization by activating YAP, which reciprocally inhibits the NF-κB signaling pathway. The in vivo results indicate that PLN can inhibit inflammatory bone loss and facilitate bone regeneration in periodontitis. The authors' findings suggest that topographical nanofibers with periodic lamellae is a promising strategy for modulating immune environment to treat inflammatory bone loss in periodontitis.

Biomaterials-Based Technologies in Skeletal Muscle Tissue Engineering

For many clinically prevalent severe injuries, the inherent regenerative capacity of skeletal muscle remains inadequate. Skeletal muscle tissue engineering (SMTE) seeks to meet this clinical demand. With continuous progress in biomedicine and related technologies including micro/nanotechnology and 3D printing, numerous studies have uncovered various intrinsic mechanisms regulating skeletal muscle regeneration and developed tailored biomaterial systems based on these understandings. Here, the skeletal muscle structure and regeneration process are discussed and the diverse biomaterial systems derived from various technologies are explored in detail. Biomaterials serve not merely as local niches for cell growth, but also as scaffolds endowed with structural or physicochemical properties that provide tissue regenerative cues such as topographical, electrical, and mechanical signals. They can also act as delivery systems for stem cells and bioactive molecules that have been shown as key participants in endogenous repair cascades. To achieve bench-to-bedside translation, the typical effect enabled by biomaterial systems and the potential underlying molecular mechanisms are also summarized. Insights into the roles of biomaterials in SMTE from cellular and molecular perspectives are provided. Finally, perspectives on the advancement of SMTE are provided, for which gene therapy, exosomes, and hybrid biomaterials may hold promise to make important contributions.

The Role of Macrophages in Nerve Regeneration: Polarization and Combination with Tissue Engineering

Peripheral nerve regeneration after trauma poses a substantial clinical challenge that has already been investigated for many years. Infiltration of immune cells is a critical step in the response to nerve damage that creates a supportive microenvironment for regeneration. In this work, we focus on a special type of immune cell, macrophage, in addressing the problem of neuronal regeneration. We discuss the complex endogenous mechanisms of peripheral nerve injury and regrowth vis-à-vis macrophages, including their recruitment, polarization, and interplay with Schwann cells post-trauma. Furthermore, we elucidate the underlying mechanisms by which exogenous stimuli govern the above events. Finally, we summarize the necessary roles of macrophages in peripheral nerve lesions and reconstruction. There are many challenges in controlling macrophage functions to achieve complete neuronal regeneration, even though considerable progress has been made in understanding the connection between these cells and peripheral nerve damage.

An injectable hydrogel dressing for controlled release of hydrogen sulfide pleiotropically mediates the wound microenvironment

The healing of scalded wounds faces many challenges such as chronic inflammation, oxidative stress, wound infection, and difficulties in vascular and nerve regeneration. Treating a single problem cannot effectively coordinate the complex regenerative microenvironment of scalded wounds, limiting the healing and functional recovery of the skin. Therefore, there is a need to develop a multi-effect treatment plan that can adaptively address the issues at each stage of wound healing. In this study, we propose a scheme for on-demand release of hydrogen sulfide (H2S) based on the concentration of reactive oxygen species (ROS) in the wound microenvironment. This is achieved by encapsulating peroxythiocarbamate (PTCM) in the ROS-responsive polymer poly(ethylene glycol)-poly(L-methionine) (PMet) to form nanoparticles, which are loaded into a thermosensitive injectable hydrogel, F127-poly(L-aspartic acid-N-hydroxysuccinimide) (F127-P(Asp-NHS)), to create a scald dressing. The H2S released by the hydrogel dressing on demand regulates the wound microenvironment by alleviating infection, reducing oxidative stress, and remodeling inflammation, thereby accelerating the healing of full-thickness scalded wounds. This hydrogel dressing for the adaptive release of H2S has great potential in addressing complex scalded wounds associated with infection and chronic inflammation.

PXL01 alters macrophage response with no effect on axonal outgrowth or Schwann cell response after nerve repair in rats

Background: Adjunctive pharmacological treatment may improve nerve regeneration. We investigated nerve regeneration processes of PXL01 - a lactoferrin-derived peptide - after repair of the sciatic nerve in healthy Wistar rats.Materials & methods: PXL01, sodium hyaluronate (carrier) or sodium chloride was administered around the repair. After 6 days axonal outgrowth, Schwann cell response, pan- (CD68) and pro-healing (CD206) macrophages in sciatic nerve, sensory neuronal response in dorsal root ganglia (DRG) and expression of heat shock protein 27 (HSP27) in sciatic nerves and DRGs were analyzed.Results: Despite a lower number of pan-macrophages, other investigated variables in sciatic nerves or DRGs did not differ between the treatment groups.Conclusion: PLX01 applied locally inhibits inflammation through pan-macrophages in repaired sciatic nerves without any impact on nerve regeneration or pro-healing macrophages.

Janus Nanofibrous Patch with In Situ Grown Superlubricated Skin for Soft Tissue Repair with Inhibited Postoperative Adhesion

The patch with a superlubricated surface shows great potential for the prevention of postoperative adhesion during soft tissue repair. However, the existing patches suffer from the destruction of topography during superlubrication coating and lack of pro-healing capability. Herein, we demonstrate a facile and versatile strategy to develop a Janus nanofibrous patch (J-NFP) with antiadhesion and reactive oxygen species (ROS) scavenging functions. Specifically, sequential electrospinning is performed with initiators and CeO2 nanoparticles (CeNPs) embedded on the different sides, followed by subsurface-initiated atom transfer radical polymerization for grafting zwitterionic polymer brushes, introducing superlubricated skin on the surface of single nanofibers. The poly(sulfobetaine methacrylate) brush-grafted patch retains fibrous topography and shows a coefficient of friction of around 0.12, which is reduced by 77% compared with the pristine fibrous patch. Additionally, a significant reduction in protein, platelet, bacteria, and cell adhesion is observed. More importantly, the CeNPs-embedded patch enables ROS scavenging as well as inhibits pro-inflammatory cytokine secretion and promotes anti-inflammatory cytokine levels. Furthermore, the J-NFP can inhibit tissue adhesion and promote repair of both rat skin wounds and intrauterine injuries. The present strategy for developing the Janus patch exhibits enormous prospects for facilitating soft tissue repair.

Electrical aligned polyurethane nerve guidance conduit modulates macrophage polarization and facilitates immunoregulatory peripheral nerve regeneration

Biomaterials can modulate the local immune microenvironments to promote peripheral nerve regeneration. Inspired by the spatial orderly distribution and endogenous electric field of nerve fibers, we aimed to investigate the synergistic effects of electrical and topological cues on immune microenvironments of peripheral nerve regeneration. Nerve guidance conduits (NGCs) with aligned electrospun nanofibers were fabricated using a polyurethane copolymer containing a conductive aniline trimer and degradable L-lysine (PUAT). In vitro experiments showed that the aligned PUAT (A-PUAT) membranes promoted the recruitment of macrophages and induced their polarization towards the pro-healing M2 phenotype, which subsequently facilitated the migration and myelination of Schwann cells. Furthermore, NGCs fabricated from A-PUAT increased the proportion of pro-healing macrophages and improved peripheral nerve regeneration in a rat model of sciatic nerve injury. In conclusion, this study demonstrated the potential application of NGCs in peripheral nerve regeneration from an immunomodulatory perspective and revealed A-PUAT as a clinically-actionable strategy for peripheral nerve injury.

Artificial periosteum promotes bone regeneration through synergistic immune regulation of aligned fibers and BMSC-recruiting phages

Given the crucial role of periosteum in bone repair, the use of artificial periosteum to induce spontaneous bone healing instead of using bone substitutes has become a potential strategy. Also, the proper transition from pro-inflammatory signals to anti-inflammatory signals is pivotal for achieving optimal repair outcomes. Hence, we designed an artificial periosteum loaded with a filamentous bacteriophage clone named P11, featuring an aligned fiber morphology. P11 endowed the artificial periosteum with the capacity to recruit bone marrow mesenchymal stem cells (BMSCs). The artificial periosteum also regulated the immune microenvironment at the bone injury site through the synergistic effects of biochemical factors and topography. Specifically, the inclusion of P11 preserved inflammatory signaling in macrophages and additionally facilitated the migration of BMSCs. Subsequently, aligned fibers stimulated macrophages, inducing alterations in cytoskeletal and metabolic activities, resulting in the polarization into the M2 phenotype. This progression encouraged the osteogenic differentiation of BMSCs and promoted vascularization. In vivo experiments showed that the new bone generated in the AP group exhibited the most efficient healing pattern. Overall, the integration of biochemical factors with topographical considerations for sequential immunomodulation during bone repair indicates a promising approach for artificial periosteum development. STATEMENT OF SIGNIFICANCE: The appropriate transition of macrophages from a pro-inflammatory to an anti-inflammatory phenotype is pivotal for achieving optimal bone repair outcomes. Hence, we designed an artificial periosteum featuring an aligned fiber morphology and loaded with specific phage clones. The artificial periosteum not only fostered the recruitment of BMSCs but also achieved sequential regulation of the immune microenvironment through the synergistic effects of biochemical factors and topography, and improved the effect of bone repair. This study indicates that the integration of biochemical factors with topographical considerations for sequential immunomodulation during bone repair is a promising approach for artificial periosteum development.

Extracellular Matrix-Bioinspired Anisotropic Topographical Cues of Electrospun Nanofibers: A Strategy of Wound Healing through Macrophage Polarization

The skin serves as the body's outermost barrier and is the largest organ, providing protection not only to the body but also to various internal organs. Owing to continuous exposure to various external factors, it is susceptible to damage that can range from simple to severe, including serious types of wounds such as burns or chronic wounds. Macrophages play a crucial role in the entire wound-healing process and contribute significantly to skin regeneration. Initially, M1 macrophages infiltrate to phagocytose bacteria, debris, and dead cells in fresh wounds. As tissue repair is activated, M2 macrophages are promoted, reducing inflammation and facilitating restoration of the dermis and epidermis to regenerate the tissue. This suggests that extracellular matrix (ECM) promotes cell adhesion, proliferation, migrationand macrophage polarization. Among the numerous strategies, electrospinning is a versatile technique for obtaining ECM-mimicking structures with anisotropic and isotropic topologies of micro/nanofibers. Various electrospun biomaterials influence macrophage polarization based on their isotropic or anisotropic topologies. Moreover, these fibers possess a high surface-area-to-volume ratio, promoting the effective exchange of vital nutrients and oxygen, which are crucial for cell viability and tissue regeneration. Micro/nanofibers with diverse physical and chemical properties can be tailored to polarize macrophages toward skin regeneration and wound healing, depending on specific requirements. This review describes the significance of micro/nanostructures for activating macrophages and promoting wound healing.

Electrospun aligned tacrolimus-loaded polycaprolactone biomaterials for peripheral nerve repair

Background: Efficacious repair of peripheral nerve injury is an unmet clinical need. The implantation of biomaterials containing neurotrophic drugs at the injury site could promote nerve regeneration and improve outcomes for patients. Materials & methods: Random and aligned electrospun poly-ε-caprolactone scaffolds containing encapsulated tacrolimus were fabricated, and the gene expression profile of Schwann cells (SCs) cultured on the surface was elucidated. On aligned fibers, the morphology of SCs and primary rat neurons was investigated. Results: Both scaffold types exhibited sustained release of drug, and the gene expression of SCs was modulated by both nanofibrous topography and the presence of tacrolimus. Aligned fibers promoted the alignment of SCs and orientated outgrowth from neurons. Conclusion: Electrospun PCL scaffolds with tacrolimus hold promise for the repair of peripheral nerve injury.

Electrospun Composite PLLA-PPSB Nanofiber Nerve Conduits for Peripheral Nerve Defects Repair and Regeneration

Peripheral nerve injury (PNI) is a common clinical problem and regenerating peripheral nerve defects remain a significant challenge. Poly(polyol sebacate) (PPS) polymers are developed as promising materials for biomedical applications due to their biodegradability, biocompatibility, elastomeric properties, and ease of production. However, the application of PPS-based biomaterials in nerve tissue engineering, especially in PNI repair, is limited. In this study, PPS-based composite nanofibers poly(l-lactic acid)-poly(polycaprolactone triol-co-sebacic acid-co-N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid sodium salt) (PLLA-PPSB) are aimed to construct through electrospinning and assess their in vitro biocompatibility with Schwann cells (SCs) and in vivo repair capabilities for peripheral nerve defects. For the first time, the biocompatibility and bioactivity of PPS-based nanomaterial are examined at the molecular, cellular, and animal levels for PNI repair. Electrospun PLLA-PPSB nanofibers display favorable physicochemical properties and biocompatibility, providing an effective interface for the proliferation, glial expression, and adhesion of SCs in vitro. In vivo experiments using a 10-mm rat sciatic nerve defect model show that PLLA-PPSB nanofiber nerve conduits enhance myelin formation, axonal regeneration, angiogenesis, and functional recovery. Transcriptome analysis and biological validation indicate that PLLA-PPSB nanofibers may promote SC proliferation by activating the PI3K/Akt signaling pathway. This suggests the promising potential of PLLA-PPSB nanomaterial for PNI repair.

3D-printed bredigite scaffolds with ordered arrangement structures promote bone regeneration by inducing macrophage polarization in onlay grafts

Bone tissue engineering scaffolds may provide a potential strategy for onlay bone grafts for oral implants. For determining the fate of scaffold biomaterials and osteogenesis effects, the host immune response is crucial. In the present study, bredigite (BRT) bioceramic scaffolds with an ordered arrangement structure (BRT-O) and a random morphology (BRT-R) were fabricated. The physicochemical properties of scaffolds were first characterized by scanning electron microscopy, mechanical test and micro-Fourier transform infrared spectroscopy. In addition, their osteogenic and immunomodulatory properties in an onlay grafting model were investigated. In vitro, the BRT-O scaffolds facilitated the macrophage polarization towards a pro-regenerative M2 phenotype, which subsequently facilitated the migration and osteogenic differentiation of bone marrow-derived mesenchymal stem cells. In vivo, an onlay grafting model was successfully established in the cranium of rabbits. In addition, the BRT-O scaffolds grafted on rabbit cranium promoted bone regeneration and CD68 + CD206 + M2 macrophage polarization. In conclusion, the 3D-printed BRT-O scaffold presents as a promising scaffold biomaterial for onlay grafts by regulating the local immune microenvironment.

Immune-cell-mediated tissue engineering strategies for peripheral nerve injury and regeneration

During the process of peripheral nerve repair, there are many complex pathological and physiological changes, including multi-cellular responses and various signaling molecules, and all these events establish a dynamic microenvironment for axon repair, regeneration, and target tissue/organ reinnervation. The immune system plays an indispensable role in the process of nerve repair and function recovery. An effective immune response not only involves innate-immune and adaptive-immune cells but also consists of chemokines and cytokines released by these immune cells. The elucidation of the orchestrated interplay of immune cells with nerve regeneration and functional restoration is meaningful for the exploration of therapeutic strategies. This review mainly enumerates the general immune cell response to peripheral nerve injury and focuses on their contributions to functional recovery. The tissue engineering-mediated strategies to regulate macrophages and T cells through physical and biochemical factors combined with scaffolds are discussed. The dynamic immune responses during peripheral nerve repair and immune-cell-mediated tissue engineering methods are presented, which provide a new insight and inspiration for immunomodulatory therapies in peripheral nerve regeneration.

Understanding exosomes: Part 2-Emerging leaders in regenerative medicine

Exosomes are the smallest subset of extracellular signaling vesicles secreted by most cells with the ability to communicate with other tissues and cell types over long distances. Their use in regenerative medicine has gained tremendous momentum recently due to their ability to be utilized as therapeutic options for a wide array of diseases/conditions. Over 5000 publications are currently being published yearly on this topic, and this number is only expected to dramatically increase as novel therapeutic strategies continue to be developed. Today exosomes have been applied in numerous contexts including neurodegenerative disorders (Alzheimer's disease, central nervous system, depression, multiple sclerosis, Parkinson's disease, post-traumatic stress disorders, traumatic brain injury, peripheral nerve injury), damaged organs (heart, kidney, liver, stroke, myocardial infarctions, myocardial infarctions, ovaries), degenerative processes (atherosclerosis, diabetes, hematology disorders, musculoskeletal degeneration, osteoradionecrosis, respiratory disease), infectious diseases (COVID-19, hepatitis), regenerative procedures (antiaging, bone regeneration, cartilage/joint regeneration, osteoarthritis, cutaneous wounds, dental regeneration, dermatology/skin regeneration, erectile dysfunction, hair regrowth, intervertebral disc repair, spinal cord injury, vascular regeneration), and cancer therapy (breast, colorectal, gastric cancer and osteosarcomas), immune function (allergy, autoimmune disorders, immune regulation, inflammatory diseases, lupus, rheumatoid arthritis). This scoping review is a first of its kind aimed at summarizing the extensive regenerative potential of exosomes over a broad range of diseases and disorders.

DLP fabrication of HA scaffold with customized porous structures to regulate immune microenvironment and macrophage polarization for enhancing bone regeneration

The immune microenvironment plays a pivotal role in osteoanagenesis. Biomaterials can modulate osteogenic efficacy by inducing specific local immune reactions. As 3D-printing technology advances, digital light projection printing has emerged as a promising method for creating large scale, high-precision biomaterial scaffolds. By adjusting the solid content and the sintering conditions during printing, the pore size of biomaterials can be meticulously controlled. Yet, the systematic influence of pore size on the immune microenvironment remains uncharted. We fabricated 3D-printed hydroxyapatite bioceramic scaffolds with three distinct pore sizes: 400 μm, 600 μm, and 800 μm. Our study revealed that scaffolds with a pore size of 600 μm promote macrophage M2 polarization, which is achieved by upregulating interferon-beta and HIF-1α production. When these materials were implanted subcutaneously in rats and within rabbit skulls, we observed that the 600 μm scaffolds notably improved the long-term inflammatory response, fostered vascular proliferation, and augmented new bone growth. This research paves the way for innovative therapeutic strategies for treating large segmental bone defects in clinical settings.

Fiber configuration determines foreign body response of electrospun scaffolds:in vitroandin vivoassessments

Biomaterial scaffolds boost tissue repair and regeneration by providing physical support, delivering biological signals and/or cells, and recruiting endogenous cells to facilitate tissue-material integration and remodeling. Foreign body response (FBR), an innate immune response that occurs immediately after biomaterial implantation, is a critical factor in determining the biological outcomes of biomaterial scaffolds. Electrospinning is of great simplicity and cost-effectiveness to produce nanofiber scaffolds with well-defined physicochemical properties and has been used in a variety of regenerative medicine applications in preclinical trials and clinical practice. A deep understanding of causal factors between material properties and FBR of host tissues is beneficial to the optimal design of electrospun scaffolds with favorable immunomodulatory properties. We herein prepared and characterized three electrospun scaffolds with distinct fiber configurations and investigated their effects on FBR in terms of immune cell-material interactions and host responses. Our results show that electrospun yarn scaffold results in greater cellular immune reactions and elevated FBR inin vivoassessments. Although the yarn scaffold showed aligned fiber bundles, it failed to induce cell elongation of macrophages due to its rough surface and porous grooves between yarns. In contrast, the aligned scaffold showed reduced FBR compared to the yarn scaffold, indicating a smooth surface is also a contributor to the immunomodulatory effects of the aligned scaffold. Our study suggests that balanced porousness and smooth surface of aligned fibers or yarns should be the key design parameters of electrospun scaffolds to modulate host responsein vivo.

Guiding the Path to Healing: CuO2 -Laden Nanocomposite Membrane for Diabetic Wound Treatment

Diabetic chronic wounds pose significant clinical challenges due to their characteristic features of impaired extracellular matrix (ECM) function, diminished angiogenesis, chronic inflammation, and increased susceptibility to infection. To tackle these challenges and provide a comprehensive therapeutic approach for diabetic wounds, the first coaxial electrospun nanocomposite membrane is developed that incorporates multifunctional copper peroxide nanoparticles (n-CuO2 ). The membrane's nanofiber possesses a unique "core/sheath" structure consisting of n-CuO2 +PVP (Polyvinylpyrrolidone)/PCL (Polycaprolactone) composite sheath and a PCL core. When exposed to the wound's moist environment, PVP within the sheath gradually disintegrates, releasing the embedded n-CuO2 . Under a weakly acidic microenvironment (typically diabetic and infected wounds), n-CuO2 decomposes to release H2 O2 and Cu2+ ions and subsequently produce ·OH through chemodynamic reactions. This enables the anti-bacterial activity mediated by reactive oxygen species (ROS), suppressing the inflammation while enhancing angiogenesis. At the same time, the dissolution of PVP unveils unique nano-grooved surface patterns on the nanofibers, providing desirable cell-guiding function required for accelerated skin regeneration. Through meticulous material selection and design, this study pioneers the development of functional nanocomposites for multi-modal wound therapy, which holds great promise in guiding the path to healing for diabetic wounds.

An injectable and adaptable hydrogen sulfide delivery system for modulating neuroregenerative microenvironment

Peripheral nerve regeneration is a complex physiological process. Single-function nerve scaffolds often struggle to quickly adapt to the imbalanced regenerative microenvironment, leading to slow nerve regeneration and limited functional recovery. In this study, we demonstrate a "pleiotropic gas transmitter" strategy based on endogenous reactive oxygen species (ROS), which trigger the on-demand H2S release at the defect area for transected peripheral nerve injury (PNI) repair through concurrent neuroregeneration and neuroprotection processing. This H2S delivery system consists of an H2S donor (peroxyTCM) encapsulated in a ROS-responsive polymer (mPEG-PMet) and loaded into a temperature-sensitive poly (amino acid) hydrogel (mPEG-PA-PP). This multi-effect combination strategy greatly promotes the regeneration of PNI, attributed to the physiological effects of H2S. These effects include the inhibition of inflammation and oxidative stress, protection of nerve cells, promotion of angiogenesis, and the restoration of normal mitochondrial function. The adaptive release of pleiotropic messengers to modulate the tissue regeneration microenvironment offers promising peripheral nerve repair and tissue engineering opportunities.

Neuro-bone tissue engineering: emerging mechanisms, potential strategies, and current challenges

The skeleton is a highly innervated organ in which nerve fibers interact with various skeletal cells. Peripheral nerve endings release neurogenic factors and sense skeletal signals, which mediate bone metabolism and skeletal pain. In recent years, bone tissue engineering has increasingly focused on the effects of the nervous system on bone regeneration. Simultaneous regeneration of bone and nerves through the use of materials or by the enhancement of endogenous neurogenic repair signals has been proven to promote functional bone regeneration. Additionally, emerging information on the mechanisms of skeletal interoception and the central nervous system regulation of bone homeostasis provide an opportunity for advancing biomaterials. However, comprehensive reviews of this topic are lacking. Therefore, this review provides an overview of the relationship between nerves and bone regeneration, focusing on tissue engineering applications. We discuss novel regulatory mechanisms and explore innovative approaches based on nerve-bone interactions for bone regeneration. Finally, the challenges and future prospects of this field are briefly discussed.

Macroporous Granular Hydrogels Functionalized with Aligned Architecture and Small Extracellular Vesicles Stimulate Osteoporotic Tendon-To-Bone Healing

Osteoporotic tendon-to-bone healing (TBH) after rotator cuff repair (RCR) is a significant orthopedic challenge. Considering the aligned architecture of the tendon, inflammatory microenvironment at the injury site, and the need for endogenous cell/tissue infiltration, there is an imminent need for an ideal scaffold to promote TBH that has aligned architecture, ability to modulate inflammation, and macroporous structure. Herein, a novel macroporous hydrogel comprising sodium alginate/hyaluronic acid/small extracellular vesicles from adipose-derived stem cells (sEVs) (MHA-sEVs) with aligned architecture and immunomodulatory ability is fabricated. When implanted subcutaneously, MHA-sEVs significantly improve cell infiltration and tissue integration through its macroporous structure. When applied to the osteoporotic RCR model, MHA-sEVs promote TBH by improving tendon repair through macroporous aligned architecture while enhancing bone regeneration by modulating inflammation. Notably, the biomechanical strength of MHA-sEVs is approximately two times higher than the control group, indicating great potential in reducing postoperative retear rates. Further cell-hydrogel interaction studies reveal that the alignment of microfiber gels in MHA-sEVs induces tenogenic differentiation of tendon-derived stem cells, while sEVs improve mitochondrial dysfunction in M1 macrophages (Mφ) and inhibit Mφ polarization toward M1 via nuclear factor-kappaB (NF-κb) signaling pathway. Taken together, MHA-sEVs provide a promising strategy for future clinical application in promoting osteoporotic TBH.

How Advancing is Peripheral Nerve Regeneration Using Nanofiber Scaffolds? A Comprehensive Review of the Literature

Peripheral nerve injuries present significant challenges in regenerative medicine, primarily due to inherent limitations in the body's natural healing processes. In response to these challenges and with the aim of enhancing peripheral nerve regeneration, nanofiber scaffolds have emerged as a promising and advanced intervention. However, a deeper understanding of the underlying mechanistic foundations that drive the favorable contributions of nanofiber scaffolds to nerve regeneration is essential. In this comprehensive review, we make an exploration of the latent potential of nanofiber scaffolds in augmenting peripheral nerve regeneration. This exploration includes a detailed introduction to the fabrication methods of nanofibers, an analysis of the intricate interactions between these scaffolds and cellular entities, an examination of strategies related to the controlled release of bioactive agents, an assessment of the prospects for clinical translation, an exploration of emerging trends, and thorough considerations regarding biocompatibility and safety. By comprehensively elucidating the intricate structural attributes and multifaceted functional capacities inherent in nanofiber scaffolds, we aim to offer a prospective and effective strategy for the treatment of peripheral nerve injury.

M2 macrophage-derived cathepsin S promotes peripheral nerve regeneration via fibroblast-Schwann cell-signaling relay

BACKGROUND

Although peripheral nerves have an intrinsic self-repair capacity following damage, functional recovery is limited in patients. It is a well-established fact that macrophages accumulate at the site of injury. Numerous studies indicate that the phenotypic shift from M1 macrophage to M2 macrophage plays a crucial role in the process of axon regeneration. This polarity change is observed exclusively in peripheral macrophages but not in microglia and CNS macrophages. However, the molecular basis of axonal regeneration by M2 macrophage is not yet fully understood. Herein, we aimed to identify the M2 macrophage-derived axon regeneration factor.

METHODS

We established a peripheral nerve injury model by transection of the inferior alveolar nerve (IANX) in Sprague-Dawley rats. Transcriptome analysis was performed on the injured nerve. Recovery from sensory deficits in the mandibular region and histological reconnection of IAN after IANX were assessed in rats with macrophage depletion by clodronate. We investigated the effects of adoptive transfer of M2 macrophages or M2-derived cathepsin S (CTSS) on the sensory deficit. CTSS initiating signaling was explored by western blot analysis in IANX rats and immunohistochemistry in co-culture of primary fibroblasts and Schwann cells (SCs).

RESULTS

Transcriptome analysis revealed that CTSS, a macrophage-selective lysosomal protease, was upregulated in the IAN after its injury. Spontaneous but partial recovery from a sensory deficit in the mandibular region after IANX was abrogated by macrophage ablation at the injured site. In addition, a robust induction of c-Jun, a marker of the repair-supportive phenotype of SCs, after IANX was abolished by macrophage ablation. As in transcriptome analysis, CTSS was upregulated at the injured IAN than in the intact IAN. Endogenous recovery from hypoesthesia was facilitated by supplementation of CTSS but delayed by pharmacological inhibition or genetic silencing of CTSS at the injured site. Adoptive transfer of M2-polarized macrophages at this site facilitated sensory recovery dependent on CTSS in macrophages. Post-IANX, CTSS caused the cleavage of Ephrin-B2 in fibroblasts, which, in turn, bound EphB2 in SCs. CTSS-induced Ephrin-B2 cleavage was also observed in human sensory nerves. Inhibition of CTSS-induced Ephrin-B2 signaling suppressed c-Jun induction in SCs and sensory recovery.

CONCLUSIONS

These results suggest that M2 macrophage-derived CTSS contributes to axon regeneration by activating SCs via Ephrin-B2 shedding from fibroblasts.

Implantable smart hyperthermia nanofibers for cancer therapy: Challenges and opportunities

Nanofibers (NFs) with practical drug-loading capacities, high stability, and controllable release have caught the attention of investigators due to their potential applications in on-demand drug delivery devices. Developing novel and efficient multidisciplinary management of locoregional cancer treatment through the design of smart NF-based systems integrated with combined chemotherapy and hyperthermia could provide stronger therapeutic advantages. On the other hand, implanting directly at the tumor area is a remarkable benefit of hyperthermia NF-based drug delivery approaches. Hence, implantable smart hyperthermia NFs might be very hopeful for tumor treatment in the future and provide new avenues for developing highly efficient localized drug delivery systems. Indeed, features of the smart NFs lead to the construction of a reversibly flexible nanostructure that enables hyperthermia and facile switchable release of antitumor agents to eradicate cancer cells. Accordingly, this study covers recent updates on applications of implantable smart hyperthermia NFs regarding their current scope and future outlook. This article is categorized under: Implantable Materials and Surgical Technologies > Nanomaterials and Implants.

The Porous Structure of Peripheral Nerve Guidance Conduits: Features, Fabrication, and Implications for Peripheral Nerve Regeneration

Porous structure is an important three-dimensional morphological feature of the peripheral nerve guidance conduit (NGC), which permits the infiltration of cells, nutrients, and molecular signals and the discharge of metabolic waste. Porous structures with precisely customized pore sizes, porosities, and connectivities are being used to construct fully permeable, semi-permeable, and asymmetric peripheral NGCs for the replacement of traditional nerve autografts in the treatment of long-segment peripheral nerve injury. In this review, the features of porous structures and the classification of NGCs based on these characteristics are discussed. Common methods for constructing 3D porous NGCs in current research are described, as well as the pore characteristics and the parameters used to tune the pores. The effects of the porous structure on the physical properties of NGCs, including biodegradation, mechanical performance, and permeability, were analyzed. Pore structure affects the biological behavior of Schwann cells, macrophages, fibroblasts, and vascular endothelial cells during peripheral nerve regeneration. The construction of ideal porous structures is a significant advancement in the regeneration of peripheral nerve tissue engineering materials. The purpose of this review is to generalize, summarize, and analyze methods for the preparation of porous NGCs and their biological functions in promoting peripheral nerve regeneration to guide the development of medical nerve repair materials.

Electrospun Nanofiber Membranes with Various Structures for Wound Dressing

Electrospun nanofiber membranes (NFMs) have high porosity and a large specific surface area, which provide a suitable environment for the complex and dynamic wound healing process and a large number of sites for carrying wound healing factors. Further, the design of the nanofiber structure can imitate the structure of the human dermis, similar to the natural extracellular matrix, which better promotes the hemostasis, anti-inflammatory and healing of wounds. Therefore, it has been widely studied in the field of wound dressing. This review article overviews the development of electrospinning technology and the application of electrospun nanofibers in wound dressings. It begins with an introduction to the history, working principles, and transformation of electrospinning, with a focus on the selection of electrospun nanofiber materials, incorporation of functional therapeutic factors, and structural design of nanofibers and nanofiber membranes. Moreover, the wide application of electrospun NFMs containing therapeutic factors in wound healing is classified based on their special functions, such as hemostasis, antibacterial and cell proliferation promotion. This article also highlights the structural design of electrospun nanofibers in wound dressing, including porous structures, bead structures, core-shell structures, ordered structures, and multilayer nanofiber membrane structures. Finally, their advantages and limitations are discussed, and the challenges faced in their application for wound dressings are analyzed to promote further research in this field.

Tuning foreign body response with tailor-engineered nanoscale surface modifications: fundamentals to clinical applications

Biomaterials are omnipresent in today's healthcare services and are employed in various applications, including implants, sensors, healthcare accessories, and drug delivery systems. Unfavorable host immunological responses frequently jeopardize the efficacy of biomaterials. As a result, surface modification has received much attention in controlling inflammatory responses since it helps camouflage the biomaterial from the host immune system, influencing the foreign body response (FBR) from protein adsorption to fibrous capsule formation. Surfaces with controlled nanotopography and chemistry, among other surface modification methodologies, have effectively altered the immune response to biomaterials. However, the field is still in its early stages, with only a few studies showing a synergistic effect of surface chemistry and nanotopography on inflammatory and wound healing pathways. Therefore, this review will concentrate on the individual and synergistic effects of surface chemistry and nanotopography on FBR modulation and the molecular processes known to modulate these responses. This review will also provide insights into crucial research gaps and advancements in various tactics for modulating FBR, opening new paths for future research. This will further aid in improving our understanding of the immune response to biomaterials, developing advanced surface modification techniques, designing immunomodulatory biomaterials, and translating discoveries into clinical applications.

Evaluation of the Aging Effect on Peripheral Nerve Regeneration: A Systematic Review

INTRODUCTION

Peripheral nerve injuries have been associated with increased healthcare costs and decreased patients' quality of life. Aging represents one factor that slows the speed of peripheral nervous system (PNS) regeneration. Since cellular homeostasis imbalance associated with aging lead to an increased failure in nerve regeneration in mammals of advanced age, this systematic review aims to determine the main molecular and cellular mechanisms involved in peripheral nerve regeneration in aged murine models after a peripheral nerve injuries.

METHODS

Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, a literature search of 4 databases was conducted in July 2022 for studies comparing the peripheral nerve regeneration capability between young and aged murine models.

RESULTS

After the initial search yielded 744 publications, ten articles fulfilled the inclusion criteria. These studies show that age-related changes such as chronic inflammatory state, delayed macrophages' response to injury, dysfunctional Schwann Cells (SCs), and microenvironment alterations cause a reduction in the regenerative capability of the PNS in murine models. Furthermore, identifying altered gene expression patterns of SC after nerve damage can contribute to the understanding of physiological modifications produced by aging.

CONCLUSIONS

The interaction between macrophages and SC plays a crucial role in the nerve regeneration of aged models. Therefore, studies aimed at developing new and promising therapies for nerve regeneration should focus on these cellular groups to enhance the regenerative capabilities of the PNS in elderly populations.

Sandwich Biomimetic Scaffold Based Tendon Stem/Progenitor Cell Alignment in a 3D Microenvironment for Functional Tendon Regeneration

Tendon injuries are some of the most commonly diagnosed musculoskeletal diseases. Tendon regeneration is sensitive to the topology of the substitute as it affects the cellular microenvironment and homeostasis. To bionic in vivo three-dimensional (3D) aligned microenvironment, an ordered 3D sandwich model was used to investigate the cell response in the tendon. First, high-resolution 3D printing provided parallel-grooved topographical cues on the hydrogel surface. Then the cells were seeded on its surface to acquire a 2D model. Afterward, an additional hydrogel coating layer was applied to the cells to create the 3D model. The interaction between cells and order structures in three-dimensions is yet to be explored. The study found that the tendon stem/progenitor cells (TSPCs) still maintain their ordering growth in the 3D model as in the 2D model. The study also found that the 3D-aligned TSPCs exhibited enhanced tenogenic differentiation through the PI3K-AKT signaling pathway and presented a less inflammatory phenotype than those in the 2D model. The in vivo implantation of such a 3D-aligned TSPC composite promoted tendon regeneration and mitigated heterotopic ossification in an Achilles defect model. These findings demonstrated that 3D-aligned TSPCs within a biomimetic topology environment are promising for functional tendon regeneration.

Constructing high-strength nano-micro fibrous woven scaffolds with native-like anisotropic structure and immunoregulatory function for tendon repair and regeneration

Tendon injuries are common debilitating musculoskeletal diseases with high treatment expenditure in sports medicine. The development of tendon-biomimetic scaffolds may be promising for improving the unsatisfactory clinical outcomes of traditional therapies. In this study, we combined an advanced electrospun nanofiber yarn-generating technique with a traditional textile manufacturing strategy to fabricate innovative nano-micro fibrous woven scaffolds with tendon-like anisotropic structure and high-strength mechanical properties for the treatment of large-size tendon injury. Electrospun nanofiber yarns made from pure poly L-lactic acid (PLLA) or silk fibroin (SF)/PLLA blend were fabricated, and their mechanical properties matched and even exceeded those of commercial PLLA microfiber yarns. The PLLA or SF/PLLA nanofiber yarns were then employed as weft yarns interlaced with commercial PLLA microfiber yarns as warp yarns to generate two new types of nanofibrous scaffolds (nmPLLA and nmSF/PLLA) with a plain-weaving structure. Woven scaffolds made from pure PLLA microfiber yarns (both weft and warp directions) (mmPLLA) were used as controls.In vitroexperiments showed that the nmSF/PLLA woven scaffold with aligned fibrous topography significantly promoted cell adhesion, elongation, proliferation, and phenotypic maintenance of tenocytes compared with mmPLLA and nmPLLA woven scaffolds. Moreover, the nmSF/PLLA woven scaffold exhibited the strongest immunoregulatory functions and effectively modulated macrophages towards the M2 phenotype.In vivoexperiments revealed that the nmSF/PLLA woven scaffold notably facilitated Achilles tendon regeneration with improved structure by macroscopic, histological, and ultrastructural observations six months after surgery, compared with the other two groups. More importantly, the regenerated tissue in the nmSF/PLLA group had excellent biomechanical properties comparable to those of the native tendon. Overall, our study provides an innovative biological-free strategy with ready-to-use features, which presents great potential for clinical translation for damaged tendon repair.

Evaluation of peripheral nerve regeneration in Murphy Roths Large mouse strain following transection injury

Aim: Murphy Roths Large (MRL/MpJ) mice have demonstrated the ability to heal with minimal or no scar formation in several tissue types. In order to identify a novel animal model, this study sought to evaluate whether this attribute applies to peripheral nerve regeneration. Materials & methods: This was a two-phase study. 6-week-old male mice were divided into two interventional groups: nerve repair and nerve graft. The MRL/MpJ was compared with the C57BL/6J strain for evaluation of both functional and histological outcomes. Results: MRL/MpJ strain demonstrated superior axon myelination and less scar formation, however functional outcomes did not show significant difference between strains. Conclusion: Superior histological outcomes did not translate into superior peripheral nerve regeneration in MRL/MpJ strain.

The 3D-Printed Ordered Bredigite Scaffold Promotes Pro-Healing of Critical-Sized Bone Defects by Regulating Macrophage Polarization

Background

Repairing critical-sized bone defects secondary to traumatic or tumorous damage is a complex conundrum in clinical practice; in this case, artificial scaffolds exhibited preferable outcomes. Bredigite (BRT, Ca₇MgSi₄O₁₆) bioceramic possesses excellent physicochemical properties and biological activity as a promising candidate for bone tissue engineering.

Methods

Structurally ordered BRT (BRT-O) scaffolds were fabricated by a three-dimensional (3D) printing technique, and the random BRT (BRT-R) scaffolds and clinically available β-tricalcium phosphate (β-TCP) scaffolds were compared as control groups. Their physicochemical properties were characterized, and RAW 264.7 cells, bone marrow mesenchymal stem cells (BMSCs), and rat cranial critical-sized bone defect models were utilized for evaluating macrophage polarization and bone regeneration.

Results

The BRT-O scaffolds exhibited regular morphology and homogeneous porosity. In addition, the BRT-O scaffolds released higher concentrations of ionic products based on coordinated biodegradability than the β-TCP scaffolds. In vitro, the BRT-O scaffolds facilitated RWA264.7 cells polarization to pro-healing M2 macrophage phenotype, whereas the BRT-R and β-TCP scaffolds stimulated more pro-inflammatory M1-type macrophages. A conditioned medium derived from macrophages seeding on the BRT-O scaffolds notably promoted the osteogenic lineage differentiation of BMSCs in vitro. The cell migration ability of BMSCs was significantly enhanced under the BRT-O-induced immune microenvironment. Moreover, in rat cranial critical-sized bone defect models, the BRT-O scaffolds group promoted new bone formation with a higher proportion of M2-type macrophage infiltration and expression of osteogenesis-related markers. Therefore, in vivo, BRT-O scaffolds play immunomodulatory roles in promoting critical-sized bone defects by enhancing the polarization of M2 macrophages.

Conclusion

3D-printed BRT-O scaffolds can be a promising option for bone tissue engineering, at least partly through macrophage polarization and osteoimmunomodulation.

Micropatterned photothermal double-layer periosteum with angiogenesis-neurogenesis coupling effect for bone regeneration

The abundant neurovascular network in the periosteal fibrous layer is essential for regulating bone homeostasis and repairing bone defects. However, the majority of the current studies only focus on the structure or function, and most of them merely involve osteogenesis and angiogenesis, lacking an in-depth study of periosteal neurogenesis. In this study, a photothermal double-layer biomimetic periosteum with neurovascular coupling was proposed. The outer layer of biomimetic periosteum is a conventional electrospinning membrane to prevent soft tissue invasion, and the inner layer is an oriented nanofiber membrane to promote cell recruitment and angiogenesis. From the perspective of functional bionics, based on the whitlockite (WH) similar to bone composition, we doped Nd (the trivalent form of neodymium element) in it as the inducing element of photothermal response to prepare photothermal whitlockite (Nd@WH). The sustained release of Mg²⁺ in Nd@WH can effectively promote the up-regulation of nerve growth factor (NGF) and vascular endothelial growth factor (VEGF). The release of Ca²⁺ and PO₄ ^3- ions and photothermal osteogenesis jointly promote bone regeneration. Under the combined effect of structure and function, the formation of nerves, blood vessels, and related collagens greatly simulates the microenvironment of extracellular matrix and periosteum regeneration and ultimately promotes bone regeneration. In this study, physical and chemical characterization proved that the bionic periosteum has good flexibility and operability. The in vitro cell experiment and in vivo calvarial defect model verified that PPCL/Nd@WH biomimetic periosteum had excellent bone tissue regeneration function compared with other groups. Finally, PPCL/Nd@WH provides a new idea for the design of bionic periosteum.

Recent advances in PLLA-based biomaterial scaffolds for neural tissue engineering: Fabrication, modification, and applications

Repairing and regenerating injured neural tissue remains a worldwide challenge. Tissue engineering (TE) has been highlighted as a potential solution to provide functional substitutes for damaged organs or tissue. Among the biocompatible and biodegradable materials, poly-L-lactic-acid (PLLA) has been widely investigated in the TE field because of its tunable mechanical properties and tailorable surface functionalization. PLLA-based biomaterials can be engineered as scaffolds that mimic neural tissue extracellular matrix and modulate inflammatory responses. With technological advances, PLLA-based scaffolds can also have well-controlled three-dimensional sizes and structures to facilitate neurite extension. Furthermore, PLLA-based scaffolds have the potential to be used as drug-delivery carriers with controlled release. Moreover, owing to the good piezoelectric properties and capacity to carry conductive polymers, PLLA-based scaffolds can be combined with electrical stimulation to maintain stemness and promote axonal guidance. This mini-review summarizes and discusses the fabrication and modification techniques utilized in the PLLA-based biomaterial scaffolds for neural TE. Recent applications in peripheral nerve and spinal cord regeneration are also presented, and it is hoped that this will guide the future development of more effective and multifunctional PLLA-based nerve scaffolds.

Topographic Orientation of Scaffolds for Tissue Regeneration: Recent Advances in Biomaterial Design and Applications

Tissue engineering to develop alternatives for the maintenance, restoration, or enhancement of injured tissues and organs is gaining more and more attention. In tissue engineering, the scaffold used is one of the most critical elements. Its characteristics are expected to mimic the native extracellular matrix and its unique topographical structures. Recently, the topographies of scaffolds have received increasing attention, not least because different topographies, such as aligned and random, have different repair effects on various tissues. In this review, we have focused on various technologies (electrospinning, directional freeze-drying, magnetic freeze-casting, etching, and 3-D printing) to fabricate scaffolds with different topographic orientations, as well as discussed the physicochemical (mechanical properties, porosity, hydrophilicity, and degradation) and biological properties (morphology, distribution, adhesion, proliferation, and migration) of different topographies. Subsequently, we have compiled the effect of scaffold orientation on the regeneration of vessels, skin, neural tissue, bone, articular cartilage, ligaments, tendons, cardiac tissue, corneas, skeletal muscle, and smooth muscle. The compiled information in this review will facilitate the future development of optimal topographical scaffolds for the regeneration of certain tissues. In the majority of tissues, aligned scaffolds are more suitable than random scaffolds for tissue repair and regeneration. The underlying mechanism explaining the various effects of aligned and random orientation might be the differences in “contact guidance”, which stimulate certain biological responses in cells.

Recent advances in nanomedicines for regulation of macrophages in wound healing

Macrophages are essential immune cells and play a major role in the immune response as pro-inflammatory or anti-inflammatory agents depending on their plasticity and functions. Infiltration and activation of macrophages are usually involved in wound healing. Herein, we first described macrophage polarization and their critical functions in wound healing process. It is addressed how macrophages collaborate with other immune cells in the wound microenvironment. Targeting macrophages by manipulating or re-educating macrophages in inflammation using nanomedicines is a novel and feasible strategy for wound management. We discussed the design and physicochemical properties of nanomaterials and their functions for macrophages activation and anti-inflammatory signaling during wound therapy. The mechanism of action of the strategies and appropriate examples are also summarized to highlight the pros and cons of those approaches. Finally, the potential of nanomedicines to modulate macrophage polarization for skin regeneration is discussed.

Efficacy of Nerve-Derived Hydrogels to Promote Axon Regeneration Is Influenced by the Method of Tissue Decellularization

Injuries to large peripheral nerves are often associated with tissue defects and require reconstruction using autologous nerve grafts, which have limited availability and result in donor site morbidity. Peripheral nerve-derived hydrogels could potentially supplement or even replace these grafts. In this study, three decellularization protocols based on the ionic detergents sodium dodecyl sulfate (P1) and sodium deoxycholate (P2), or the organic solvent tri-n-butyl phosphate (P3), were used to prepare hydrogels. All protocols resulted in significantly decreased amounts of genomic DNA, but the P2 hydrogel showed the best preservation of extracellular matrix proteins, cytokines, and chemokines, and reduced levels of sulfated glycosaminoglycans. In vitro P1 and P2 hydrogels supported Schwann cell viability, secretion of VEGF, and neurite outgrowth. Surgical repair of a 10 mm-long rat sciatic nerve gap was performed by implantation of tubular polycaprolactone conduits filled with hydrogels followed by analyses using diffusion tensor imaging and immunostaining for neuronal and glial markers. The results demonstrated that the P2 hydrogel considerably increased the number of axons and the distance of regeneration into the distal nerve stump. In summary, the method used to decellularize nerve tissue affects the efficacy of the resulting hydrogels to support regeneration after nerve injury.

Aligned electrospun poly(L-lactide) nanofibers facilitate wound healing by inhibiting macrophage M1 polarization via the JAK-STAT and NF-κB pathways

Delayed wound healing remains a challenge, and macrophages play an important role in the inflammatory process of wound healing. Morphological changes in macrophages can affect their phenotype, but little is known about the underlying mechanism. Aligned electrospun nanofibers have natural advantages in modulating cell morphology. Therefore, the current study constructed aligned electrospun nanofibers that could transform macrophages into elongated shapes. Our results demonstrated that aligned nanofibers without exogenous cytokines could downregulate the proinflammatory M1 phenotype and upregulate the prohealing M2 phenotype in an inflammatory environment. Importantly, our study revealed that aligned electrospun nanofibers could inhibit macrophage M1 polarization via the JAK-STAT and NF-κB pathways. Furthermore, the conditioned medium from macrophages cultured on aligned nanofibers could encourage fibroblast migration, proliferation and collagen secretion. In vivo, aligned nanofibers alleviated the inflammatory microenvironment, promoted angiogenesis and accelerated wound healing in mouse skin defects by modulating macrophage phenotypes. Collectively, aligned electrospun nanofibers can influence macrophage polarization via the JAK-STAT and NF-κB pathways and attenuate the local inflammatory response in skin wounds. This study provides a potential strategy to modulate macrophage polarization and promote wound healing by controlling the topology of biomaterials and offers a new perspective for the application of nanotechnology in wound healing.

Dual-Action Icariin-Containing Thermosensitive Hydrogel for Wound Macrophage Polarization and Hair-Follicle Neogenesis

Bone morphogenetic protein (BMP) pathway is essential for M2 macrophage polarization and hair-follicle neogenesis. Icariin, a flavonoid derived from Epimedium, is a mediator of the BMP pathway. Here, we develop a hydrogel formulation functionalized with icariin for regulation of macrophage polarization to accelerate wound healing and hair-follicle neogenesis. Compared to skin defects without icariin treatment, those treated with icariin+PEG hydrogel healed faster and had new hair follicles. Results in vivo showed that icariin+PEG hydrogel induced a higher level of M2 phenotypic transformation of macrophages. Moreover, icariin+PEG hydrogel significantly accelerated wound-repair process by reducing the invasion of inflammation, excessive deposition of collagen, immoderate activation of myofibroblasts, and increasing the regeneration of hair follicles. Furthermore, studies in vitro demonstrated that the icariin+PEG hydrogel induced macrophages to polarize to the M2 phenotype and dermal papilla cell to hair follicles. Finally, molecular analysis demonstrated that the icariin+PEG hydrogel increased the expression of BMP4 and Smad1/5 phosphorylation in skin wounds. These results demonstrate the therapeutic potential of icariin-containing thermosensitive hydrogels for inducing M2 macrophage polarization to accelerate wound healing and promote hair-follicle neogenesis by regulating the BMP pathway.

Biomimetic Asymmetric Composite Dressing by Electrospinning with Aligned Nanofibrous and Micropatterned Structures for Severe Burn Wound Healing

The surface structure and topography of biomaterials play a crucial role in directing cell behaviors and fates. Meanwhile, asymmetric dressings that mimic the natural skin structure have been identified as an effective strategy for enhancing wound healing. Inspired by the skin structure and the superhydrophobic structure of the lotus leaf, an asymmetric composite dressing was obtained by constructing an asymmetric structure and wettability surface modification on both sides of the sponge based on electrospinning. Among them, the collagen and quaternized chitosan sponge was fabricated by freeze-drying, followed by an aligned poly(ε-caprolactone) (PCL)/gelatin nanofiber hydrophilic inner layer and hierarchical micronanostructure PCL/polystyrene microsphere highly hydrophobic outer layer constructed on each side of the sponge. The proposed asymmetric composite dressing combines topological morphology with the material's properties to effectively prevent bacterial colonization/infection and promote wound healing by directing cellular behavior. In vitro experimental results confirmed that the aligned nanofiber inner layer effectively promotes cell adhesion, proliferation, directed cell growth, and migration. Meanwhile, the sponge has good water absorption and antibacterial properties, while the biomimetic hydrophobic outer layer exhibits strong mechanical properties and resistance to bacterial adhesion. In vivo results showed that the composite dressing can reduce inflammatory response, prevent infection, accelerate angiogenesis and epithelial regeneration, and significantly accelerate the healing of severe burns. Thus, the proposed bionic asymmetric dressing is expected to be a promising candidate for severe burn wound healing.

Biomechanical microenvironment in peripheral nerve regeneration: from pathophysiological understanding to tissue engineering development

Peripheral nerve injury (PNI) caused by trauma, chronic disease and other factors may lead to partial or complete loss of sensory, motor and autonomic functions, as well as neuropathic pain. Biological activities are always accompanied by mechanical stimulation, and biomechanical microenvironmental homeostasis plays a complicated role in tissue repair and regeneration. Recent studies have focused on the effects of biomechanical microenvironment on peripheral nervous system development and function maintenance, as well as neural regrowth following PNI. For example, biomechanical factors-induced cluster gene expression changes contribute to formation of peripheral nerve structure and maintenance of physiological function. In addition, extracellular matrix and cell responses to biomechanical microenvironment alterations after PNI directly trigger a series of cascades for the well-organized peripheral nerve regeneration (PNR) process, where cell adhesion molecules, cytoskeletons and mechanically gated ion channels serve as mechanosensitive units, mechanical effector including focal adhesion kinase (FAK) and yes-associated protein (YAP)/transcriptional coactivator with PDZ-binding motif (TAZ) as mechanotransduction elements. With the rapid development of tissue engineering techniques, a substantial number of PNR strategies such as aligned nerve guidance conduits, three-dimensional topological designs and piezoelectric scaffolds emerge expected to improve the neural biomechanical microenvironment in case of PNI. These tissue engineering nerve grafts display optimized mechanical properties and outstanding mechanomodulatory effects, but a few bottlenecks restrict their application scenes. In this review, the current understanding in biomechanical microenvironment homeostasis associated with peripheral nerve function and PNR is integrated, where we proposed the importance of balances of mechanosensitive elements, cytoskeletal structures, mechanotransduction cascades, and extracellular matrix components; a wide variety of promising tissue engineering strategies based on biomechanical modulation are introduced with some suggestions and prospects for future directions.

Electrospun fiber-based strategies for controlling early innate immune cell responses: Towards immunomodulatory mesh designs that facilitate robust tissue repair

Electrospun fibrous meshes are widely used for tissue repair due to their ability to guide a host of cell responses including phenotypic differentiation and tissue maturation. A critical factor determining the eventual biological outcomes of mesh-based regeneration strategies is the early innate immune response following implantation. The natural healing process involves a sequence of tightly regulated, temporally varying and delicately balanced pro-/anti-inflammatory events which together promote mesh integration with host tissue. Matrix designs that do not account for the immune milieu can result in dysregulation, chronic inflammation and fibrous capsule formation, thus obliterating potential therapeutic outcomes. In this review, we provide systematic insights into the effects of specific fiber/mesh properties and mechanical stimulation on the responses of early innate immune modulators viz., neutrophils, monocytes and macrophages. We identify matrix characteristics that promote anti-inflammatory immune phenotypes, and we correlate such responses with pro-regenerative in vivo outcomes. We also discuss recent advances in 3D fabrication technologies, bioactive functionalization approaches and biomimetic/bioinspired immunomodulatory mesh design strategies for tissue repair and wound healing. The mechanobiological insights and immunoregulatory strategies discussed herein can help improve the translational outcomes of fiber-based regeneration and may also be leveraged for intervention in degenerative diseases associated with dysfunctional immune responses. STATEMENT OF SIGNIFICANCE: The crucial role played by immune cells in promoting biomaterial-based tissue regeneration is being increasingly recognized. In this review focusing on the interactions of innate immune cells (primarily neutrophils, monocytes and macrophages) with electrospun fibrous meshes, we systematically elucidate the effects of the fiber microenvironment and mechanical stimulation on biological responses, and build upon these insights to inform the rational design of immunomodulatory meshes for effective tissue repair. We discuss state-of-the-art fabrication methods and mechanobiological advances that permit the orchestration of temporally controlled phenotypic switches in immune cells during different phases of healing. The design strategies discussed herein can also be leveraged to target several complex autoimmune and inflammatory diseases.