In situ bone tissue engineering using gene delivery nanocomplexes

Engineered 3D mesenchymal stem cell aggregates with multifunctional prowess for bone regeneration: Current status and future prospects

BACKGROUND

Impaired efficacy of in vitro expanded mesenchymal stem cells (MSCs) is a universal and thorny situation, which cast a shadow on further clinical translation of exogenous MSCs. Moreover, the relatively lengthy healing process, host metabolic heterogeneity and the sophisticated cell recognition and crosstalk pose rigorous challenges towards MSC-based bone regeneration strategies. Three-dimensional (3D) cell aggregates facilitate more robust intercellular communications and cell-extracellular matrix (ECM) interactions, providing a better mimicry of microarchitectures and biochemical milieus in vivo, which is conducive for stemness maintenance and downstream bone formation.

AIM OF REVIEW

This review enunciates the phenotypic features of MSCs in aggregates, which deepens the knowledge of the MSC fate determination in 3D microenvironment. By summarizing current empowerment methods and biomaterial-combined techniques for establishing functionalized MSC aggregates, this review aims to spark innovative and promising solutions for exalting the translational value of MSCs and improve their therapeutic applications in bone tissue repair.

KEY SCIENTIFIC CONCEPTS OF REVIEW

3D aggregates optimize regenerative behaviors of in vitro cultured MSCs including cell adhesion, viability, proliferation, pluripotency and immunoregulation capacity, etc. Biomaterials hybridization endows MSC aggregates with tailored mechanical and biological properties, which offers more possibilities in adapting various clinical scenarios.

Development of a dual-responsive injectable GelMA/F127DA hydrogel for enhanced cartilage regeneration in osteoarthritis: Harnessing MMP-triggered and mechanical stress-induced release of therapeutic agents

Osteoarthritis (OA) presents a significant challenge in clinical settings due to the limited self-renewal capability of cartilage tissue. To address this, engineered biomaterials employing biomimetic strategies have been developed to modulate and enhance cell-microenvironment interactions, facilitating cartilage regeneration. Nonetheless, excessive mechanical stress on joint structures can induce inflammatory responses, thereby impeding the process of cartilage repair. In this study, we focus on the OA microenvironment, characterized by the overexpression of matrix metalloproteinases (MMPs), and the mechanical stimuli due to joint movement. We engineered a dual-responsive injectable hydrogel: a blend of MMP-responsive, thermo-sensitive GelMA and mechanically robust, reverse thermo-sensitive F127DA. This hydrogel was designed to deliver TGF-β and KGN in a controlled manner via simple temperature modulation. The hydrophilic properties of GelMA and the hydrophobic nature of F127DA allow for efficient intra-articular delivery of diverse drug types, optimizing their therapeutic effects. Photocrosslinking the hydrogel in situ effectively seals cartilage defects and prevents further degradation. The overexpressed MMP in the OA environment triggers the release of TGF-β, recruiting bone marrow-derived stem cells (BMSCs), while mechanical pressure from joint movements releases KGN, promoting chondrogenic differentiation and mitigating inflammation. In summary, our injectable hydrogel, responsive to both the OA microenvironment and mechanical stress, shows promise in enhancing cartilage regeneration in OA. This approach holds significant potential for advancing the field of OA cartilage tissue engineering.

Matrix vesicle-inspired delivery system based on nanofibrous chitosan microspheres for enhanced bone regeneration

Inspired by the initial mineralization process with bone matrix vesicles (MVs), this study innovatively developed a delivery system to mediate mineralization during bone regeneration. The system comprises nanofibrous chitosan microspheres (NCM) and poly (allylamine hydrochloride)-stabilized amorphous calcium phosphate (PAH-ACP), which is thereafter referred to as NCMP. NCM is synthesized through the thermal induction of chitosan molecular chains, serving as the carrier, while PAH-ACP functions as the mineralization precursor. Additionally, the nanofibrous network of NCMP mimics the architecture of natural extracellular matrix (ECM), creating an optimal niche for the active adhesion of stem cells to its surface, exhibiting good biocompatibility, immunoregulation, and osteogenic performance. In vivo, NCMP effectively recruits cells and mineralizes collagen, modulates cell behavior and differentiation, and promotes in situ biomineralization in rat calvarial defects. These results underscore the dual efficacy of NCMP not only as an effective delivery system for mineralization precursors but also as ECM-mimicking bio-blocks, offering a promising avenue for enhancing the repair and regeneration of bone defects.

Nanoparticle-Mediated Gene Delivery for Bone Tissue Engineering

Critical-sized bone defects represent an urgent clinical problem, necessitating innovative treatment approaches. Gene-activated grafts for bone tissue engineering have emerged as a promising solution. However, traditional gene delivery methods are constrained by limited osteogenic efficacy and safety concerns. Recently, organic and inorganic nanoparticle (NP) vectors have attracted significant attention in bone tissue engineering for their safe, stable, and controllable gene delivery. Targeted gene delivery guided by insights into bone healing mechanisms, coupled with the multifunctional design of NPs, is crucial for enhancing therapeutic outcomes. Here, the theoretical foundations underlying NP-mediated gene therapy for enhancing bone healing across different histological stages are elucidated. Furthermore, the distinct attributes of functionalized NP vectors are discussed, and cutting-edge strategies aimed at optimizing gene delivery efficiency throughout the therapeutic process are highlighted. Additionally, the review addresses the unresolved challenges and prospects of this technology. This review may contribute to the continued development and clinical application of NP-mediated gene delivery for treating critical-sized bone defects.

Recent Strategies and Advances in Hydrogel-Based Delivery Platforms for Bone Regeneration

Bioactive molecules have shown great promise for effectively regulating various bone formation processes, rendering them attractive therapeutics for bone regeneration. However, the widespread application of bioactive molecules is limited by their low accumulation and short half-lives in vivo. Hydrogels have emerged as ideal carriers to address these challenges, offering the potential to prolong retention times at lesion sites, extend half-lives in vivo and mitigate side effects, avoid burst release, and promote adsorption under physiological conditions. This review systematically summarizes the recent advances in the development of bioactive molecule-loaded hydrogels for bone regeneration, encompassing applications in cranial defect repair, femoral defect repair, periodontal bone regeneration, and bone regeneration with underlying diseases. Additionally, this review discusses the current strategies aimed at improving the release profiles of bioactive molecules through stimuli-responsive delivery, carrier-assisted delivery, and sequential delivery. Finally, this review elucidates the existing challenges and future directions of hydrogel encapsulated bioactive molecules in the field of bone regeneration.

Nordihydroguaiaretic Acid-Cross-Linked Phenylboronic Acid-Functionalized Polyplex Micelles for Anti-angiogenic Gene Therapy of Orthotopic and Metastatic Tumors

Polyplexes are required to be equipped with multiple functionalities to accomplish adequate structure stability and gene transfection efficacy for gene therapy. Herein, a 4-carboxy-3-fluorophenylboronic acid (FPBA)-functionalized block copolymer of PEG-b-PAsp(DET/FBA) and PAsp(DET/FBA) (abbreviated as PB and HB) was synthesized and applied for engineering functional polyplex micelles (PMs) through ionic complexation with pDNA followed by strategic cross-linking with nordihydroguaiaretic acid (NDGA) in respect to the potential linkage of polyphenol and FPBA moieties. In relation to polyplex micelles void of cross-linking, the engineered multifunctional polyplex micelles (PBHBN-PMs) were determined to possess improved structural tolerability against the exchange reaction with charged species. Besides, the FPBA/NDGA cross-linking appeared to be selectively cleaved in the acidic endosomal compartments but not the neutral milieu. Furthermore, the PBHB-PMs with the optimal FPBA/NDGA cross-linking degree were identified to possess appreciable cellular uptake and endosomal escape activities, eliciting a significantly high level of gene expression relative to P-PMs and PB-PMs. Eventually, in vivo antitumor therapy by our proposed multifunctional PMs appeared to be capable of facilitating expression of the antiangiogenic genomic payloads (sFlt-1 pDNA) via systemic administration. The enriched antiangiogenic sFlt-1 in the tumors could silence the activities of angiogenic cytokines for the inhibited neo-vasculature and the suppressed growth of orthotopic 4T1 tumors. Of note, the persistent expression of the antiangiogenic sFlt-1 is also presumed to migrate into the blood circulation, thereby accounting for an overall antiangiogenic environment in preventing the potential pulmonary metastasis. Hence, our elaborated multifaceted PMs inspired fascinating potential as an intriguing gene delivery system for the treatment of clinical solid tumors and metastasis.

Smart Hydrogels for Tissue Regeneration

The rapid growth in the portion of the aging population has led to a consequent increase in demand for biomedical hydrogels, together with an assortment of challenges that need to be overcome in this field. Smart hydrogels can autonomously sense and respond to the physiological/pathological changes of the tissue microenvironment and continuously adapt the response according to the dynamic spatiotemporal shifts in conditions. This along with other favorable properties, make smart hydrogels excellent materials for employing toward improving the precision of treatment for age-related diseases. The key factor during the smart hydrogel design is on accurately identifying the characteristics of natural tissues and faithfully replicating the composition, structure, and biological functions of these tissues at the molecular level. Such hydrogels can accurately sense distinct physiological and external factors such as temperature and biologically active molecules, so they may in turn actively and promptly adjust their response, by regulating their own biological effects, thereby promoting damaged tissue repair. This review summarizes the design strategies employed in the creation of smart hydrogels, their response mechanisms, as well as their applications in field of tissue engineering; and concludes by briefly discussing the relevant challenges and future prospects.

Scaffold-based non-viral CRISPR delivery platform for efficient and prolonged gene activation to accelerate tissue regeneration

Clustered regularly interspaced short palindromic repeat activation (CRISPRa) technology has emerged as a precise genome editing tool for activating endogenous transgene expression. While it holds promise for precise cell modification, its translation into tissue engineering has been hampered by biosafety concerns and suboptimal delivery methods. To address these challenges, we have developed a CRISPRa non-viral gene delivery platform by immobilizing non-viral CRISPRa complexes into a biocompatible hydrogel/nanofiber (Gel/NF) composite scaffold. The Gel/NF scaffold facilitates the controlled and sustained release of CRISPRa complexes and also promotes cell recruitment to the scaffold for efficient and localized transfection. As a proof of concept, we employed this CRISPRa delivery platform to activate the vascular endothelial growth factor (VEGF) gene in a rat model with full-thickness skin defects. Our results demonstrate sustained upregulation of VEGF expression even at 21 days post-implantation, resulting in enhanced angiogenesis and improved skin regeneration. These findings underscore the potential of the Gel/NF scaffold-based CRISPRa delivery platform as an efficient and durable strategy for gene activation, offering promising prospects for tissue regeneration. STATEMENT OF SIGNIFICANCE: Translation of clustered regularly interspaced short palindromic repeat activation (CRISPRa) therapy to tissue engineering is limited by biosafety concerns and unsatisfactory delivery strategy. To solve this issue, we have developed a CRISPRa non-viral gene delivery platform by immobilizing non-viral CRISPRa complexes into a biocompatible hydrogel/nanofiber (Gel/NF) composite scaffold. This scaffold enables controlled and sustained release of CRISPRa and can induce cell recruitment for localized transfection. As a proof of concept, we activated vascular endothelial growth factor (VEGF) in a rat model with full-thickness skin defects, leading to sustained upregulation of VEGF expression, enhanced angiogenesis and improved skin regeneration in vivo. These findings demonstrate the potential of this platform for gene activation, thereby offering promising prospects for tissue regeneration.

Botulinum toxin A dissolving microneedles for hyperhidrosis treatment: design, formulation and in vivo evaluation

Multiple periodic injections of botulinum toxin A (BTX-A) are the standard treatment of hyperhidrosis which causes excessive sweating. However, BTX-A injections can create problems, including incorrect and painful injections, the risk of drug entry into the bloodstream, the need for medical expertise, and waste disposal problems. New drug delivery systems can substantially reduce these problems. Transdermal delivery is an effective alternative to conventional BTX-A injections. However, BTX-A's large molecular size and susceptibility to degradation complicate transdermal delivery. Dissolving microneedle patches (DMNPs) encapsulated with BTX-A (BTX-A/DMNPs) are a promising solution that can penetrate the dermis painlessly and provide localized translocation of BTX-A. In this study, using high-precision 3D laser lithography and subsequent molding, DMNPs were prepared based on a combination of biocompatible polyvinylpyrrolidone and hyaluronic acid polymers to deliver BTX-A with ultra-sharp needle tips of 1.5 ± 0.5 µm. Mechanical, morphological and histological assessments of the prepared DMNPs were performed to optimize their physicochemical properties. Furthermore, the BTX-A release and diffusion kinetics across the skin layers were investigated. A COMSOL simulation was conducted to study the diffusion process. The primary stability analysis reported significant stability for three months. Finally, the functionality of the BTX-A/DMNPs for the suppression of sweat glands was confirmed on the hyperhidrosis mouse footpad, which drastically reduced sweat gland activity. The results demonstrate that these engineered DMNPs can be an effective, painless, inexpensive alternative to hypodermic injections when treating hyperhidrosis.

Biomedical applications of nanomaterials in the advancement of nucleic acid therapy: Mechanistic challenges, delivery strategies, and therapeutic applications

In the past few decades, substantial advancement has been made in nucleic acid (NA)-based therapies. Promising treatments include mRNA, siRNA, miRNA, and anti-sense DNA for treating various clinical disorders by modifying the expression of DNA or RNA. However, their effectiveness is limited due to their concentrated negative charge, instability, large size, and host barriers, which make widespread application difficult. The effective delivery of these medicines requires safe vectors that are efficient & selective while having non-pathogenic qualities; thus, nanomaterials have become an attractive option with promising possibilities despite some potential setbacks. Nanomaterials possess ideal characteristics, allowing them to be tuned into functional bio-entity capable of targeted delivery. In this review, current breakthroughs in the non-viral strategy of delivering NAs are discussed with the goal of overcoming challenges that would otherwise be experienced by therapeutics. It offers insight into a wide variety of existing NA-based therapeutic modalities and techniques. In addition to this, it provides a rationale for the use of non-viral vectors and a variety of nanomaterials to accomplish efficient gene therapy. Further, it discusses the potential for biomedical application of nanomaterials-based gene therapy in various conditions, such as cancer therapy, tissue engineering, neurological disorders, and infections.

The Delivery and Activation of Growth Factors Using Nanomaterials for Bone Repair

Bone regeneration is a comprehensive process that involves different stages, and various growth factors (GFs) play crucial roles in the entire process. GFs are currently widely used in clinical settings to promote bone repair; however, the direct application of GFs is often limited by their fast degradation and short local residual time. Additionally, GFs are expensive, and their use may carry risks of ectopic osteogenesis and potential tumor formation. Nanomaterials have recently shown great promise in delivering GFs for bone regeneration, as they can protect fragile GFs and control their release. Moreover, functional nanomaterials can directly activate endogenous GFs, modulating the regeneration process. This review provides a summary of the latest advances in using nanomaterials to deliver exogenous GFs and activate endogenous GFs to promote bone regeneration. We also discuss the potential for synergistic applications of nanomaterials and GFs in bone regeneration, along with the challenges and future directions that need to be addressed.

Nanofabrication Technologies to Control Cell and Tissue Function in Three-Dimension

In the 2000s, advances in cellular micropatterning using microfabrication contributed to the development of cell-based biosensors for the functional evaluation of newly synthesized drugs, resulting in a revolutionary evolution in drug screening. To this end, it is essential to utilize cell patterning to control the morphology of adherent cells and to understand contact and paracrine-mediated interactions between heterogeneous cells. This suggests that the regulation of the cellular environment by means of microfabricated synthetic surfaces is not only a valuable endeavor for basic research in biology and histology, but is also highly useful to engineer artificial cell scaffolds for tissue regeneration. This review particularly focuses on surface engineering techniques for the cellular micropatterning of three-dimensional (3D) spheroids. To establish cell microarrays, composed of a cell adhesive region surrounded by a cell non-adherent surface, it is quite important to control a protein-repellent surface in the micro-scale. Thus, this review is focused on the surface chemistries of the biologically inspired micropatterning of two-dimensional non-fouling characters. As cells are formed into spheroids, their survival, functions, and engraftment in the transplanted site are significantly improved compared to single-cell transplantation. To improve the therapeutic effect of cell spheroids even further, various biomaterials (e.g., fibers and hydrogels) have been developed for spheroid engineering. These biomaterials not only can control the overall spheroid formation (e.g., size, shape, aggregation speed, and degree of compaction), but also can regulate cell-to-cell and cell-to-matrix interactions in spheroids. These important approaches to cell engineering result in their applications to tissue regeneration, where the cell-biomaterial composite is injected into diseased area. This approach allows the operating surgeon to implant the cell and polymer combinations with minimum invasiveness. The polymers utilized in hydrogels are structurally similar to components of the extracellular matrix in vivo, and are considered biocompatible. This review will provide an overview of the critical design to make hydrogels when used as cell scaffolds for tissue engineering. In addition, the new strategy of injectable hydrogel will be discussed as future directions.

Functional Approaches in Promoting Vascularization and Angiogenesis in Bone Critical-Sized Defects via Delivery of Cells, Growth Factors, Drugs, and Particles

Critical-sized bone defects, or CSDs, are defined as bone defects that cannot be regenerated by themselves and require surgical intervention via employing specific biomaterials and a certain regenerative strategy. Although a variety of approaches can be used to treat CSDs, poor angiogenesis and vascularization remain an obstacle in these methods. The complex biological healing of bone defects depends directly on the function of blood flow to provide sufficient oxygen and nutrients and the removal of waste products from the defect site. The absence of vascularization can lead to non-union and delayed-union defect development. To overcome this challenge, angiogenic agents can be delivered to the site of injury to stimulate vessel formation. This review begins by introducing the treatment methods for CSDs. The importance of vascularization in CSDs is subsequently highlighted. Delivering angiogenesis agents, including relevant growth factors, cells, drugs, particles, cell secretion substances, their combination, and co-delivery to CSDs are fully explored. Moreover, the effects of such agents on new bone formation, followed by vessel formation in defect areas, are evaluated.

Genetically modified cell spheroids for tissue engineering and regenerative medicine

Cell spheroids offer cell-to-cell interactions and show advantages in survival rate and paracrine effect to solve clinical and biomedical inquiries ranging from tissue engineering and regenerative medicine to disease pathophysiology. Therefore, cell spheroids are ideal vehicles for gene delivery. Genetically modified spheroids can enhance specific gene expression to promote tissue regeneration. Gene deliveries to cell spheroids are via viral vectors or non-viral vectors. Some new technologies like CRISPR/Cas9 also have been used in genetically modified methods to deliver exogenous gene to the host chromosome. It has been shown that genetically modified cell spheroids had the potential to differentiate into bone, cartilage, vascular, nerve, cardiomyocytes, skin, and skeletal muscle as well as organs like the liver to replace the diseased organ in the animal and pre-clinical trials. This article reviews the recent articles about genetically modified spheroid cells and explains the fabrication, applications, development timeline, limitations, and future directions of genetically modified cell spheroid.

Recent advances in gene therapy for bone tissue engineering

Autografting, a major treatment for bone fractures, has potential risks related to the required surgery and disease transmission. Bone morphogenetic proteins (BMPs) are the most common osteogenic factors used for bone-healing applications. However, BMP delivery can have shortcomings such as a short half-life and the high cost of manufacturing the recombinant proteins. Gene delivery methods have demonstrated promising alternative strategies for producing BMPs or other osteogenic factors using engineered cells. These approaches can also enable temporal overexpression and local production of the therapeutic genes in the target tissues. This review addresses recent progress on engineered viral, non-viral, and RNA-mediated gene delivery systems that are being used for bone repair and regeneration. Advances in clustered regularly interspaced short palindromic repeats/Cas9 genome engineering for bone tissue regeneration also is discussed.

Intraoperative Creation of Tissue-Engineered Grafts with Minimally Manipulated Cells: New Concept of Bone Tissue Engineering In Situ

Transfer of regenerative approaches into clinical practice is limited by strict legal regulation of in vitro expanded cells and risks associated with substantial manipulations. Isolation of cells for the enrichment of bone grafts directly in the Operating Room appears to be a promising solution for the translation of biomedical technologies into clinical practice. These intraoperative approaches could be generally characterized as a joint concept of tissue engineering in situ. Our review covers techniques of intraoperative cell isolation and seeding for the creation of tissue-engineered grafts in situ, that is, directly in the Operating Room. Up-to-date, the clinical use of tissue-engineered grafts created in vitro remains a highly inaccessible option. Fortunately, intraoperative tissue engineering in situ is already available for patients who need advanced treatment modalities.

Chitosan-based scaffolds as drug delivery systems in bone tissue engineering

The bone tissue engineering approach for treating large bone defects becomes necessary when the tissue damage surpasses the threshold of the inherent regenerative ability of the human body. A myriad of natural biodegradable polymers and scaffold fabrication techniques have emerged in the last decade. Chitosan (CS) is especially attractive as a bone scaffold material to support cell attachment and proliferation and mineralization of the bone matrix. The primary amino groups in CS are responsible for properties such as controlled drug release, mucoadhesion, in situ gelation, and transfection. CS-based smart drug delivery scaffolds that respond to environmental stimuli have been reported to have a localized sustained delivery of drugs in the large bone defect area. This review outlines the recent advances in the fabrication of CS-based scaffolds as a pharmaceutical carrier to deliver drugs such as antibiotics, growth factors, nucleic acids, and phenolic compounds for bone tissue regeneration.

Engineered cells along with smart scaffolds: critical factors for improving tissue engineering approaches

In this review, gene delivery and its applications are discussed in tissue engineering (TE); also, new techniques such as the CRISPR-Cas9 system, synthetics biology and molecular dynamics simulation to improve the efficiency of the scaffolds have been studied. CRISPR-Cas9 is expected to make significant advances in TE in the future. The fundamentals of synthetic biology have developed powerful and flexible methods for programming cells via artificial genetic circuits. The combination of regenerative medicine and artificial biology allows the engineering of cells and organisms for use in TE, biomaterials, bioprocessing and scaffold development. The dynamics of protein adsorption at the scaffold surface at the atomic level can provide valuable guidelines for the future design of TE scaffolds /implants.

Chitosan-Based Scaffolds for Facilitated Endogenous Bone Re-Generation

Facilitated endogenous tissue engineering, as a facile and effective strategy, is emerging for use in bone tissue regeneration. However, the development of bioactive scaffolds with excellent osteo-inductivity to recruit endogenous stem cells homing and differentiation towards lesion areas remains an urgent problem. Chitosan (CS), with versatile qualities including good biocompatibility, biodegradability, and tunable physicochemical and biological properties is undergoing vigorously development in the field of bone repair. Based on this, the review focus on recent advances in chitosan-based scaffolds for facilitated endogenous bone regeneration. Initially, we introduced and compared the facilitated endogenous tissue engineering with traditional tissue engineering. Subsequently, the various CS-based bone repair scaffolds and their fabrication methods were briefly explored. Furthermore, the functional design of CS-based scaffolds in bone endogenous regeneration including biomolecular loading, inorganic nanomaterials hybridization, and physical stimulation was highlighted and discussed. Finally, the major challenges and further research directions of CS-based scaffolds were also elaborated. We hope that this review will provide valuable reference for further bone repair research in the future.

Functionalization of Electrospun Nanofiber for Bone Tissue Engineering

Bone-tissue engineering is an alternative treatment for bone defects with great potential in which scaffold is a critical factor to determine the effect of bone regeneration. Electrospun nanofibers are widely used as scaffolds in the biomedical field for their similarity with the structure of the extracellular matrix (ECM). Their unique characteristics are: larger surface areas, porosity and processability; these make them ideal candidates for bone-tissue engineering. This review briefly introduces bone-tissue engineering and summarizes the materials and methods for electrospining. More importantly, how to functionalize electrospun nanofibers to make them more conducive for bone regeneration is highlighted. Finally, the existing deficiencies of functionalized electrospun nanofibers for promoting osteogenesis are proposed. Such a summary can lay the foundation for the clinical practice of functionalized electrospun nanofibers.

Electrospun-Fibrous-Architecture-Mediated Non-Viral Gene Therapy Drug Delivery in Regenerative Medicine

Gene-based therapy represents the latest advancement in medical biotechnology. The principle behind this innovative approach is to introduce genetic material into specific cells and tissues to stimulate or inhibit key signaling pathways. Although enormous progress has been achieved in the field of gene-based therapy, challenges connected to some physiological impediments (e.g., low stability or the inability to pass the cell membrane and to transport to the desired intracellular compartments) still obstruct the exploitation of its full potential in clinical practices. The integration of gene delivery technologies with electrospun fibrous architectures represents a potent strategy that may tackle the problems of stability and local gene delivery, being capable to promote a controlled and proficient release and expression of therapeutic genes in the targeted cells, improving the therapeutic outcomes. This review aims to outline the impact of electrospun-fibrous-architecture-mediated gene therapy drug delivery, and it emphatically discusses the latest advancements in their formulation and the therapeutic outcomes of these systems in different fields of regenerative medicine, along with the main challenges faced towards the translation of promising academic results into tangible products with clinical application.

Highlights on Advancing Frontiers in Tissue Engineering

The field of tissue engineering continues to advance, sometimes in exponential leaps forward, but also sometimes at a rate that does not fulfill the promise that the field imagined a few decades ago. This review is in part a catalog of success in an effort to inform the process of innovation. Tissue engineering has recruited new technologies and developed new methods for engineering tissue constructs that can be used to mitigate or model disease states for study. Key to this antecedent statement is that the scientific effort must be anchored in the needs of a disease state and be working toward a functional product in regenerative medicine. It is this focus on the wildly important ideas coupled with partnered research efforts within both academia and industry that have shown most translational potential. The field continues to thrive and among the most important recent developments are the use of three-dimensional bioprinting, organ-on-a-chip, and induced pluripotent stem cell technologies that warrant special attention. Developments in the aforementioned areas as well as future directions are highlighted in this article. Although several early efforts have not come to fruition, there are good examples of commercial profitability that merit continued investment in tissue engineering. Impact statement Tissue engineering led to the development of new methods for regenerative medicine and disease models. Among the most important recent developments in tissue engineering are the use of three-dimensional bioprinting, organ-on-a-chip, and induced pluripotent stem cell technologies. These technologies and an understanding of them will have impact on the success of tissue engineering and its translation to regenerative medicine. Continued investment in tissue engineering will yield products and therapeutics, with both commercial importance and simultaneous disease mitigation.

Advances in Electrospun Hybrid Nanofibers for Biomedical Applications

Electrospun hybrid nanofibers, based on functional agents immobilized in polymeric matrix, possess a unique combination of collective properties. These are beneficial for a wide range of applications, which include theranostics, filtration, catalysis, and tissue engineering, among others. The combination of functional agents in a nanofiber matrix offer accessibility to multifunctional nanocompartments with significantly improved mechanical, electrical, and chemical properties, along with better biocompatibility and biodegradability. This review summarizes recent work performed for the fabrication, characterization, and optimization of different hybrid nanofibers containing varieties of functional agents, such as laser ablated inorganic nanoparticles (NPs), which include, for instance, gold nanoparticles (Au NPs) and titanium nitride nanoparticles (TiNPs), perovskites, drugs, growth factors, and smart, inorganic polymers. Biocompatible and biodegradable polymers such as chitosan, cellulose, and polycaprolactone are very promising macromolecules as a nanofiber matrix for immobilizing such functional agents. The assimilation of such polymeric matrices with functional agents that possess wide varieties of characteristics require a modified approach towards electrospinning techniques such as coelectrospinning and template spinning. Additional focus within this review is devoted to the state of the art for the implementations of these approaches as viable options for the achievement of multifunctional hybrid nanofibers. Finally, recent advances and challenges, in particular, mass fabrication and prospects of hybrid nanofibers for tissue engineering and biomedical applications have been summarized.

Hydrogels for Tissue Engineering: Addressing Key Design Needs Toward Clinical Translation

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Biocompatible and Electroconductive Nanocomposite Scaffolds with Improved Piezoelectric Response for Bone Tissue Engineering

Electroactive scaffolds are relatively new tools in tissue engineering that open new avenue in repairing damaged soft and hard tissues. These scaffolds can induce electrical signaling while providing an ECM-like microenvironment. However, low biocompatibility and lack of biodegradability of piezoelectric and conductive polymers limits their clinical translation. In the current study, we have developed highly biocompatible, electroconductive nanofibrous scaffolds based on poly-L-lactic acid/polyaniline/carbon nanotube (PLLA/polyaniline/CNT). Physical and chemical properties of fabricated scaffolds were tested using various techniques. Biological characteristics of the scaffolds are also examined to check cellular attachment as well as differentiation of cultured (progenitor) cells. Scaffolds were optimized to direct osteogenic differentiation of mesenchymal stem cells. Such scaffolds can offer new strategies for the regeneration of damaged/lost bone.

Designing electrospun fiber platforms for efficient delivery of genetic material and genome editing tools

Electrospun fibers are versatile biomaterial platforms with great potential to support regeneration. Electrospun fiber characteristics such as fiber diameter, degree of alignment, rate of degradation, and surface chemistry enable the creation of unique, tunable scaffolds for various drug or gene delivery applications. The delivery of genetic material and genome editing tools via viral and non-viral vectors are approaches to control cellular protein production. However, immunogenicity, off-target effects, and low delivery efficiencies slow the progression of gene delivery strategies to clinical settings. The delivery of genetic material from electrospun fibers overcomes such limitations by allowing for localized, tunable delivery of genetic material. However, the process of electrospinning is harsh, and care must be taken to retain genetic material bioactivity. This review presents an up-to-date summary of strategies to incorporate genetic material onto or within electrospun fiber platforms to improve delivery efficiency and enhance the regenerative potential of electrospun fibers for various tissue engineering applications.

Advancements in nucleic acids-based techniques for bone regeneration

The dynamic biology of bone involving an enormous magnitude of cellular interactions and signaling transduction provides ample biomolecular targets, which can be enhanced or repressed to mediate a rapid regeneration of the impaired bone tissue. The delivery of nucleic acids such as DNA and RNA can enhance the expression of osteogenic proteins. Members of the RNA interference pathway such as miRNA and siRNA can repress negative osteoblast differentiation regulators. Advances in nanomaterials have provided researchers with a plethora of delivery modules that can ensure proper transfection. Combining the nucleic acid carrying vectors with bone scaffolds has met with tremendous success in accomplishing bone formation. Recent years have witnessed the advent of CRISPR and DNA nanostructures in regenerative medicine. This review focuses on the delivery of nucleic acids and touches upon the prospect of CRISPR and DNA nanostructures for bone tissue engineering, emphasizing their potential in treating bone defects.

In situ bone regeneration with sequential delivery of aptamer and BMP2 from an ECM-based scaffold fabricated by cryogenic free-form extrusion

In situ tissue engineering is a powerful strategy for the treatment of bone defects. It could overcome the limitations of traditional bone tissue engineering, which typically involves extensive cell expansion steps, low cell survival rates upon transplantation, and a risk of immuno-rejection. Here, a porous scaffold polycaprolactone (PCL)/decellularized small intestine submucosa (SIS) was fabricated via cryogenic free-form extrusion, followed by surface modification with aptamer and PlGF-2123-144*-fused BMP2 (pBMP2). The two bioactive molecules were delivered sequentially. The aptamer Apt19s, which exhibited binding affinity to bone marrow-derived mesenchymal stem cells (BMSCs), was quickly released, facilitating the mobilization and recruitment of host BMSCs. BMP2 fused with a PlGF-2123-144 peptide, which showed "super-affinity" to the ECM matrix, was released in a slow and sustained manner, inducing BMSC osteogenic differentiation. In vitro results showed that the sequential release of PCL/SIS-pBMP2-Apt19s promoted cell migration, proliferation, alkaline phosphatase activity, and mRNA expression of osteogenesis-related genes. The in vivo results demonstrated that the sequential release system of PCL/SIS-pBMP2-Apt19s evidently increased bone formation in rat calvarial critical-sized defects compared to the sequential release system of PCL/SIS-BMP2-Apt19s. Thus, the novel delivery system shows potential as an ideal alternative for achieving cell-free scaffold-based bone regeneration in situ.

Graphene oxide-functionalized nanocomposites promote osteogenesis of human mesenchymal stem cells via enhancement of BMP-SMAD1/5 signaling pathway

Biomaterials that can harness the intrinsic osteogenic potential of stem cells offer a promising strategy to accelerate bone regeneration and repair. Previously, we had used methacrylated gelatin (GelMA)-based scaffolds to achieve bone formation from human mesenchymal stem cells (hMSCs). In this study, we aimed to further enhance hMSC osteogenesis by incorporating graphene oxide (GO)-based nanosheets into GelMA. In vitro results showed high viability and metabolic activities in hMSCs encapsulated in the newly developed nanocomposites. Incorporation of GO markedly increased mineralization within hMSC-laden constructs, which was further increased by replacing GO with silica-coated graphene oxide (SiGO). Mechanistic analysis revealed that the nanosheet enhanced the production, retention, and biological activity of endogenous bone morphogenetic proteins (BMPs), resulting in robust osteogenesis in the absence of exogenous osteoinductive growth factors. Specifically, the osteoinductive effect of the nanosheets was abolished by inhibiting the BMP signaling pathway with LDN-193189 treatment. The bone formation potential of the technology was further tested in vivo using a mouse subcutaneous implantation model, where hMSCs-laden GO/GelMA and SiGO/GelMA samples resulted in bone volumes 108 and 385 times larger, respectively, than the GelMA control group. Taken together, these results demonstrate the biological activity and mechanism of action of GO-based nanosheets in augmenting the osteogenic capability of hMSCs, and highlights the potential of leveraging nanomaterials such as GO and SiGO for bone tissue engineering applications.

Gene- and RNAi-activated scaffolds for bone tissue engineering: Current progress and future directions

Large bone defects are usually managed by replacing lost bone with non-biological prostheses or with bone grafts that come from the patient or a donor. Bone tissue engineering, as a field, offers the potential to regenerate bone within these large defects without the need for grafts or prosthetics. Such therapies could provide improved long- and short-term outcomes in patients with critical-sized bone defects. Bone tissue engineering has long relied on the administration of growth factors in protein form to stimulate bone regeneration, though clinical applications have shown that using such proteins as therapeutics can lead to concerning off-target effects due to the large amounts required for prolonged therapeutic action. Gene-based therapies offer an alternative to protein-based therapeutics where the genetic material encoding the desired protein is used and thus loading large doses of protein into the scaffolds is avoided. Gene- and RNAi-activated scaffolds are tissue engineering devices loaded with nucleic acids aimed at promoting local tissue repair. A variety of different approaches to formulating gene- and RNAi-activated scaffolds for bone tissue engineering have been explored, and include the activation of scaffolds with plasmid DNA, viruses, RNA transcripts, or interfering RNAs. This review will discuss recent progress in the field of bone tissue engineering, with specific focus on the different approaches employed by researchers to implement gene-activated scaffolds as a means of facilitating bone tissue repair.

Mesenchymal Stem Cells as a Gene Delivery Tool: Promise, Problems, and Prospects

The cell-based approach in gene therapy arises as a promising strategy to provide safe, targeted, and efficient gene delivery. Owing to their unique features, as homing and tumor-tropism, mesenchymal stem cells (MSCs) have recently been introduced as an encouraging vehicle in gene therapy. Nevertheless, non-viral transfer of nucleic acids into MSCs remains limited due to various factors related to the main stakeholders of the process (e.g., nucleic acids, carriers, or cells). In this review, we have summarized the main types of nucleic acids used to transfect MSCs, the pros and cons, and applications of each. Then, we have emphasized on the most efficient lipid-based carriers for nucleic acids to MSCs, their main features, and some of their applications. While a myriad of studies have demonstrated the therapeutic potential for engineered MSCs therapy in various illnesses, optimization for clinical use is an ongoing challenge. On the way of improvement, genetically modified MSCs have been combined with various novel techniques and tools (e.g., exosomes, spheroids, 3D-Bioprinting, etc.,) aiming for more efficient and safe applications in biomedicine.

Oral Bone Tissue Regeneration: Mesenchymal Stem Cells, Secretome, and Biomaterials

In the last few decades, tissue engineering has become one of the most studied medical fields. Even if bone shows self-remodeling properties, in some cases, due to injuries or anomalies, bone regeneration can be required. In particular, oral bone regeneration is needed in the dentistry field, where the functional restoration of tissues near the tooth represents a limit for many dental implants. In this context, the application of biomaterials and mesenchymal stem cells (MSCs) appears promising for bone regeneration. This review focused on in vivo studies that evaluated bone regeneration using biomaterials with MSCs. Different biocompatible biomaterials were enriched with MSCs from different sources. These constructs showed an enhanced bone regenerative power in in vivo models. However, we discussed also a future perspective in tissue engineering using the MSC secretome, namely the conditioned medium and extracellular vesicles. This new approach has already shown promising results for bone tissue regeneration in experimental models.

Recent Trends in the Development of Bone Regenerative Biomaterials

The goal of a biomaterial is to support the bone tissue regeneration process at the defect site and eventually degrade in situ and get replaced with the newly generated bone tissue. Biomaterials that enhance bone regeneration have a wealth of potential clinical applications from the treatment of non-union fractures to spinal fusion. The use of bone regenerative biomaterials from bioceramics and polymeric components to support bone cell and tissue growth is a longstanding area of interest. Recently, various forms of bone repair materials such as hydrogel, nanofiber scaffolds, and 3D printing composite scaffolds are emerging. Current challenges include the engineering of biomaterials that can match both the mechanical and biological context of bone tissue matrix and support the vascularization of large tissue constructs. Biomaterials with new levels of biofunctionality that attempt to recreate nanoscale topographical, biofactor, and gene delivery cues from the extracellular environment are emerging as interesting candidate bone regenerative biomaterials. This review has been sculptured around a case-by-case basis of current research that is being undertaken in the field of bone regeneration engineering. We will highlight the current progress in the development of physicochemical properties and applications of bone defect repair materials and their perspectives in bone regeneration.