Dual growth factor-loaded core-shell polymer microcapsules can promote osteogenesis and angiogenesis

Cartilage and bone injectable hydrogels: A review of injectability methods and treatment strategies for repair in tissue engineering

Cartilage and bone are crucial tissues causing disability in the elderly population, often requiring prolonged treatment and surgical intervention due to limited regenerative capacity. Injectable hydrogels that closely mimic the extracellular matrix (ECM) of native hard tissue have attracted attention due to their minimally invasive application and ability to conform to irregular defect sites. These hydrogels facilitate key biological processes such as cell migration, chondrogenesis in cartilage repair, osteoinduction, angiogenesis, osteoconduction, and mineralization in bone repair. This review analyzes in-vitro and in-vivo biomedical databases over the past decade to identify advancements in hydrogel formulations, crosslinking mechanisms, and biomaterial selection for cartilage and bone tissue engineering. The review emphasizes the effectiveness of injectable hydrogels as carriers for cells, growth factors, and drugs, offering additional therapeutic benefits. The relevance of these findings is discussed in the context of their potential to serve as a robust alternative to current surgical and non-surgical treatments. This review also examines the advantages of injectable hydrogels, such as ease of administration, reduced patient recovery time, and enhanced bioactivity, thereby emphasizing their potential in clinical applications for cartilage and bone regeneration with emphasis on addressing the shortcomings of current treatments.

Engineering of an Osteoinductive and Growth Factor-Free Injectable Bone-Like Microgel for Bone Regeneration

Bone autografts remain the gold standard for bone grafting surgeries despite having increased donor site morbidity and limited availability. Bone morphogenetic protein-loaded grafts represent another successful commercial alternative. However, the therapeutic use of recombinant growth factors has been associated with significant adverse clinical outcomes. This highlights the need to develop biomaterials that closely approximate the structure and composition of bone autografts, which are inherently osteoinductive and biologically active with embedded living cells, without the need for added supplements. Here, injectable growth factor-free bone-like tissue constructs are developed, that closely approximate the cellular, structural, and chemical composition of bone autografts. It is demonstrated that these micro-constructs are inherently osteogenic, and demonstrate the ability to stimulate mineralized tissue formation and regenerate bone in critical-sized defects in-vivo. Furthermore, the mechanisms that allow human mesenchymal stem cells (hMSCs) to be highly osteogenic in these constructs, despite the lack of osteoinductive supplements, are assessed, whereby Yes activated protein (YAP) nuclear localization and adenosine signaling appear to regulate osteogenic cell differentiation. The findings represent a step toward a new class of minimally invasive, injectable, and inherently osteoinductive scaffolds, which are regenerative by virtue of their ability to mimic the tissue cellular and extracellular microenvironment, thus showing promise for clinical applications in regenerative engineering.

Triple growth factor delivery promotes functional bone regeneration following composite musculoskeletal trauma

Successful bone healing in severe trauma depends on early revascularization to restore oxygen, nutrient, growth factor, and progenitor cell supply to the injury. Therapeutic angiogenesis strategies have therefore been investigated to promote revascularization following severe bone injuries; however, results have been inconsistent. This is the first study investigating the effects of dual angiogenic growth factors (VEGF and PDGF) with low-dose bone morphogenetic protein-2 (BMP-2; 2.5 µg) on bone healing in a clinically challenging composite bone-muscle injury model. Our hydrogel-based delivery systems demonstrated a more than 90% protein entrapment efficiency and a controlled simultaneous release of three growth factors over 28 days. Co-stimulation of microvascular fragment constructs with VEGF and PDGF promoted vascular network formation in vitro compared to VEGF or PDGF alone. In an in vivo model of segmental bone and volumetric muscle loss injury, combined VEGF (5 µg) and PDGF (7.5 µg or 15 µg) delivery with a low dose of BMP-2 significantly enhanced regeneration of vascularized bone compared to BMP-2 treatment alone. Notably, the regenerated bone mechanics reached ~60% of intact bone, a value that was previously only achieved by delivery of high-dose BMP-2 (10 µg) in this injury model. Overall, sustained delivery of VEGF, PDFG, and BMP-2 is a promising strategy to promote functional vascularized bone tissue regeneration following severe composite musculoskeletal injury. Although this study is conducted in a clinically relevant composite injury model in rats using a simultaneous release strategy, future studies are necessary to test the regenerative potential of spatiotemporally controlled delivery of triple growth factors on bone healing using large animal models. STATEMENT OF SIGNIFICANCE: Volumetric muscle loss combined with delayed union or non-union bone defect causes deleterious effects on bone regeneration even with the supplementation of bone morphogenetic protein-2 (BMP-2). In this study, the controlled delivery of dual angiogenic growth factors (vascular endothelial growth factor [VEGF] + Platelet-derived growth factor [PDGF]) increases vascular growth in vitro. Co-delivering VEGF+PDGF significantly increase the bone formation efficacy of low-dose BMP-2 and improves the mechanics of regenerated bone in a challenging composite bone-muscle injury model.

Prevascularized hydrogels with mature vascular networks promote the regeneration of critical-size calvarial bone defects in vivo

Adequate vascularization of scaffolds is a prerequisite for successful repair and regeneration of lost and damaged tissues. It has been suggested that the maturity of engineered vascular capillaries, which is largely determined by the presence of functional perivascular mural cells (or pericytes), plays a vital role in maintaining vessel integrity during tissue repair and regeneration. Here, we investigated the role of pericyte-supported-engineered capillaries in regenerating bone in a critical-size rat calvarial defect model. Prior to implantation, human umbilical vein endothelial cells and human bone marrow stromal cells (hBMSCs) were cocultured in a collagen hydrogel to induce endothelial cell morphogenesis into microcapillaries and hBMSC differentiation into pericytes. Upon implantation into the calvarial bone defects (8 mm), the prevascularized hydrogels showed better bone formation than either untreated controls or defects treated with autologous bone grafts (positive control). Bone formation parameters such as bone volume, coverage area, and vascularity were significantly better in the prevascularized hydrogel group than in the autologous bone group. Our results demonstrate that tissue constructs engineered with pericyte-supported vascular capillaries may approximate the regenerative capacity of autologous bone, despite the absence of osteoinductive or vasculogenic growth factors.

Embedding cells within nanoscale, rapidly mineralizing hydrogels: A new paradigm to engineer cell-laden bone-like tissue

Bone mineralization is a highly specific and dynamic nanoscale process that has been studied extensively from a structural, chemical, and biological standpoint. Bone tissue, therefore, may be defined by the interplay of its intricately mineralized matrix and the cells that regulate its biological function. However, the far majority of engineered bone model systems and bone replacement materials have been unable to replicate this key characteristic of bone tissue; that is, the ability of cells to be gradually and rapidly embedded in a three-dimensional (3D) heavily calcified matrix material. Here we review the characteristics that define the bone matrix from a nanostructural perspective. We then revisit the benefits and challenges of existing model systems and engineered bone replacement materials, and discuss recent efforts to replicate the biological, cellular, mechanical, and materials characteristics of bone tissue on the nano- to microscale. We pay particular attention to a recently proposed method developed by our group, which seeks to replicate key aspects of the entrapment of bone cells within a mineralized matrix with precisions down to the level of individual nano-crystallites, inclusive of the bone vasculature, and osteogenic differentiation process. In summary, this paper discusses existing and emerging evidence pointing towards future developments bridging the gap between the fields of biomineralization, structural biology, stem cells, and tissue engineering, which we believe will hold the key to engineer truly functional bone-like tissue in the laboratory.

Cell-modified bioprinted microspheres for vascular regeneration

Cell therapy is a promising strategy in which living cells or cellular materials are delivered to treat a variety of diseases. Here, we developed an electrospray bioprinting method to rapidly generate cell-laden hydrogel microspheres, which limit the migration of the captured cells and provide an immunologically privileged microenvironment for cell survival in vivo. Currently, therapeutic angiogenesis aims to induce collateral vessel formation after limb ischemia. However, the clinical application of gene and cell therapy has been impeded by concerns regarding its inefficacy, as well as the associated risk of immunogenicity and oncogenicity. In this study, hydrogel microspheres encapsulating VEGF-overexpressing HEK293T cells showed good safety via subcutaneously injecting into male C57BL/6 mice. In addition, these cell-modified microspheres effectively promoted angiogenesis in a mouse hind-limb ischemia model. Therefore, we demonstrated the great therapeutic potential of this approach to induce angiogenesis in limb ischemia, indicating that bioprinting has a bright future in cell therapy.

Dual functional approaches for osteogenesis coupled angiogenesis in bone tissue engineering

Bone fracture healing is a multistep and overlapping process of inflammation, angiogenesis and osteogenesis. It is initiated by inflammation, causing the release of various cytokines and growth factors. It leads to the recruitment of stem cells and formation of vasculature resulting in the functional bone formation. This combined phenomenon is used by bone tissue engineers from past few years to address the problem of vasculature and osteogenic differentiation during bone regeneration. In this review, we have discussed all major studies reporting the dual functioning approach to promote osteogenesis coupled angiogenesis using various scaffolds. These scaffolds are broadly classified into four types based on the nature of their structural and functional components. The functionality of the scaffold is either due to the structural components or the loaded cargo which conducts or induces the coupled functionality. Dual delivery system for osteoinductive and angioinductive factors ensures the co-delivery of two different types of molecules to induce osteogenesis and angiogenesis. Single delivery scaffold for angioinductive and osteoinductive molecule releases single type of molecules which could induce both angiogenesis and osteogenesis. Osteoconductive scaffold consisted of bone constituents releases angioinductive factors. Osteoconductive and angioconductive scaffold composed of components which provide the native substrate features for osteogenesis and angiogenesis. This review article also discusses the studies highlighting the synergism of physico-chemical stimuli as dual functioning feature to enhance angiogenesis and osteogenesis simultaneously. In addition, this article covers one of the least discussed area of the bone regeneration i.e. 'cartilage formation as a median between angiogenesis and osteogenesis'.

Materials Science and Design Principles of Growth Factor Delivery Systems in Tissue Engineering and Regenerative Medicine

Growth factors (GFs) are signaling molecules that direct cell development by providing biochemical cues for stem cell proliferation, migration, and differentiation. GFs play a key role in tissue regeneration, but one major limitation of GF-based therapies is dosage-related adverse effects. Additionally, the clinical applications and efficacy of GFs are significantly affected by the efficiency of delivery systems and other pharmacokinetic factors. Hence, it is crucial to design delivery systems that provide optimal activity, stability, and tunable delivery for GFs. Understanding the physicochemical properties of the GFs and the biomaterials utilized for the development of biomimetic GF delivery systems is critical for GF-based regeneration. Many different delivery systems have been developed to achieve tunable delivery kinetics for single or multiple GFs. The identification of ideal biomaterials with tunable properties for spatiotemporal delivery of GFs is still challenging. This review characterizes the types, properties, and functions of GFs, the materials science of widely used biomaterials, and various GF loading strategies to comprehensively summarize the current delivery systems for tunable spatiotemporal delivery of GFs aimed for tissue regeneration applications. This review concludes by discussing fundamental design principles for GF delivery vehicles based on the interactive physicochemical properties of the proteins and biomaterials.

Electrospun polymeric micro/nanofibrous scaffolds for long-term drug release and their biomedical applications

Electrospun polymeric micro/nanofibrous scaffolds have been investigated extensively as drug delivery platforms capable of controlled and sustained release of therapeutic agents in situ. Such scaffolds exhibit excellent physicochemical and biological properties and can encapsulate and release various drugs in a controlled fashion. This article reviews recent advances in the design and manufacture of electrospun scaffolds for long-term drug release, placing particular emphasis on polymer selection, types of incorporated drugs and the latest drug-loading techniques. Finally, applications of such devices in traumatic or disease states requiring effective and sustained drug action are discussed and critically appraised in their biomedical context.

Osteogenic/angiogenic dual growth factor delivery microcapsules for regeneration of vascularized bone tissue

Growth factors (GFs) are major biochemical cues for tissue regeneration. Herein, a novel dual GF delivery system is designed composed of poly(lactic-co-glycolic acid) (PLGA) nanoparticles (NPs) and alginate microcapsules (MCs) via an electrodropping method. While bone morphogenetic protein (BMP)-2 is encapsulated in the PLGA NPs, vascular endothelial growth factor (VEGF) is included in the alginate MCs, where BMP-2-loaded PLGA NPs are entrapped together in the fabrication process. The initial loading efficiencies of BMP-2 and VEGF are 78% ± 3.6% and 43% ± 1.7%, respectively. When our dual GF-loaded MCs are assessed for in vitro osteogenesis of umbilical cord blood-derived mesenchymal stem cells (UCB-MSCs) on 2D and 3D environment, MCs contribute to much better UCB-MSCs osteogenesis as confirmed by von Kossa staining, immunofluorescence (osteocalcin, collagen 1), calcium content measurement, and osteogenic markers expression. In addition, when dual GF-encapsulated MCs are combined with collagen and then applied to 8 mm diameter rat calvarial defect model, the positive effects on vascularized bone regeneration are much more pronounced; micro computed tomography (CT) and histology analyses exhibit 82.3% bone healing coupled with 12.6% vessel occupied area. Put together, current study indicates a synergistic effect of BMP-2/VEGF and highlights the great potential of dual GF delivery modality (PLGA NPs-in-MC) for regeneration of vascularized bone.

The scope and sequence of growth factor delivery for vascularized bone tissue regeneration

Bone regeneration is a complex process, that in vivo, requires the highly coordinated presentation of biochemical cues to promote the various stages of angiogenesis and osteogenesis. Taking inspiration from the natural healing process, a wide variety of growth factors are currently being released within next generation tissue engineered scaffolds (in a variety of ways) in order to heal non-union fractures and bone defects. This review will focus on the delivery of multiple growth factors to the bone regeneration niche, specifically 1) dual growth factor delivery signaling and crosstalk, 2) the importance of growth factor timing and temporal separation, and 3) the engineering of delivery systems that allow for temporal control over presentation of soluble growth factors. Alternative methods for growth factor presentation, including the use of gene therapy and platelet-rich plasma scaffolds, are also discussed.