The Influence of Physicochemical Properties of Biomimetic Hydroxyapatite on the In Vitro Behavior of Endothelial Progenitor Cells and Their Interaction with Mesenchymal Stem Cells

A Multifunctional Scaffold for Bone Infection Treatment by Delivery of microRNA Therapeutics Combined With Antimicrobial Nanoparticles

Treating bone infections and ensuring bone repair is one of the greatest global challenges of modern orthopedics, made complex by antimicrobial resistance (AMR) risks due to long-term antibiotic treatment and debilitating large bone defects following infected tissue removal. An ideal multi-faceted solution would will eradicate bacterial infection without long-term antibiotic use, simultaneously stimulating osteogenesis and angiogenesis. Here, a multifunctional collagen-based scaffold that addresses these needs by leveraging the potential of antibiotic-free antimicrobial nanoparticles (copper-doped bioactive glass, CuBG) to combat infection without contributing to AMR in conjunction with microRNA-based gene therapy (utilizing an inhibitor of microRNA-138) to stimulate both osteogenesis and angiogenesis, is developed. CuBG scaffolds reduce the attachment of gram-positive bacteria by over 80%, showcasing antimicrobial functionality. The antagomiR-138 nanoparticles induce osteogenesis of human mesenchymal stem cells in vitro and heal a large load-bearing defect in a rat femur when delivered on the scaffold. Combining both promising technologies results in a multifunctional antagomiR-138-activated CuBG scaffold inducing hMSC-mediated osteogenesis and stimulating vasculogenesis in an in vivo chick chorioallantoic membrane model. Overall, this multifunctional scaffold catalyzes killing mechanisms in bacteria while inducing bone repair through osteogenic and angiogenic coupling, making this platform a promising multi-functional strategy for treating and repairing complex bone infections.

3D printing of hierarchical porous biomimetic hydroxyapatite scaffolds: Adding concavities to the convex filaments

Porosity plays a key role on the osteogenic performance of bone scaffolds. Direct Ink Writing (DIW) allows the design of customized synthetic bone grafts with patient-specific architecture and controlled macroporosity. Being an extrusion-based technique, the scaffolds obtained are formed by arrays of cylindrical filaments, and therefore have convex surfaces. This may represent a serious limitation, as the role of surface curvature and more specifically the stimulating role of concave surfaces in osteoinduction and bone growth has been recently highlighted. Hence the need to design strategies that allow the introduction of concave pores in DIW scaffolds. In the current study, we propose to add gelatin microspheres as a sacrificial material in a self-setting calcium phosphate ink. Neither the phase transformation responsible for the hardening of the scaffold nor the formation of characteristic network of needle-like hydroxyapatite crystals was affected by the addition of gelatin microspheres. The partial dissolution of the gelatin resulted in the creation of spherical pores throughout the filaments and exposed on the surface, increasing filament porosity from 0.2 % to 67.9 %. Moreover, the presence of retained gelatin proved to have a significant effect on the mechanical properties, reducing the strength but simultaneously giving the scaffolds an elastic behavior, despite the high content of ceramic as a continuous phase. Notwithstanding the inherent difficulty of in vitro cultures with this highly reactive material an enhancement of MG-63 cell proliferation, as well as better spreading of hMSCs was recorded on the developed scaffolds. STATEMENT OF SIGNIFICANCE: Recent studies have stressed the role that concave surfaces play in tissue regeneration and, more specifically, in osteoinduction and osteogenesis. Direct ink writing enables the production of patient-specific bone grafts with controlled architecture. However, besides many advantages, it has the serious limitation that the surfaces obtained are convex. In this article, for the first time we develop a strategy to introduce concave pores in the printed filaments of biomimetic hydroxyapatite by incorporation and partial dissolution of gelatin microspheres. The retention of part of the gelatin results in a more elastic behavior compared to the brittleness of hydroxyapatite scaffolds, while the needle-shaped nanostructure of biomimetic hydroxyapatite is maintained and gelatin-coated concave pores on the surface of the filaments enhance cell spreading.

Maturation of biomimetic hydroxyapatite in physiological fluids: a physicochemical and proteomic study

Biomimetic calcium-deficient hydroxyapatite (CDHA) as a bioactive material exhibits exceptional intrinsic osteoinductive and osteogenic properties because of its nanostructure and composition, which promote a favorable microenvironment. Its high reactivity has been hypothesized to play a relevant role in the in vivo performance, mediated by the interaction with the biological fluids, which is amplified by its high specific surface area. Paradoxically, this high reactivity is also behind the in vitro cytotoxicity of this material, especially pronounced in static conditions. The present work explores the structural and physicochemical changes that CDHA undergoes in contact with physiological fluids and to investigate its interaction with proteins. Calcium-deficient hydroxyapatite discs with different micro/nanostructures, coarse (C) and fine (F), were exposed to cell-free complete culture medium over extended periods of time: 1, 7, 14, 21, 28, and 50 days. Precipitate formation was not observed in any of the materials in contact with the physiological fluid, which would indicate that the ionic exchanges were linked to incorporation into the crystal structure of CDHA or in the hydrated layer. In fact, CDHA experienced a maturation process, with a progressive increase in crystallinity and the Ca/P ratio, accompanied by an uptake of Mg and a B-type carbonation process, with a gradual propagation into the core of the samples. However, the reactivity of biomimetic hydroxyapatite was highly dependent on the specific surface area and was amplified in nanosized needle-like crystal structures (F), whereas in coarse specimens the ionic exchanges were restricted to the surface, with low penetration in the material bulk. In addition to showing a higher protein adsorption on F substrates, the proteomics study revealed the existence of protein selectivity toward F or C microstructures, as well as the capability of CDHA, and more remarkably of F-CDHA, to concentrate specific proteins from the culture medium. Finally, a substantial improvement in the material's ability to support cell proliferation was observed after the CDHA maturation process.

Inflammation and biomaterials: role of the immune response in bone regeneration by inorganic scaffolds

The regulatory role of the immune system in maintaining bone homeostasis and restoring its functionality, when disturbed due to trauma or injury, has become evident in recent years. The polarization of macrophages, one of the main constituents of the immune system, into the pro-inflammatory or anti-inflammatory phenotype has great repercussions for cellular crosstalk and the subsequent processes needed for proper bone regeneration such as angiogenesis and osteogenesis. In certain scenarios, the damaged osseous tissue requires the placement of synthetic bone grafts to facilitate the healing process. Inorganic biomaterials such as bioceramics or bioactive glasses are the most widely used due to their resemblance to the mineral phase of bone and superior osteogenic properties. The immune response of the host to the inorganic biomaterial, which is of an exogenous nature, might determine its fate, leading either to active bone regeneration or its failure. Therefore, various strategies have been employed, like the modification of structural/chemical features or the incorporation of bioactive molecules, to tune the interplay with the immune cells. Understanding how these particular modifications impact the polarization of macrophages and further osteogenic and osteoclastogenic events is of great interest in view of designing a new generation of osteoimmunomodulatory materials that support the regeneration of osseous tissue during all stages of bone healing.

A Biphasic Calcium Phosphate Cement Enhances Dentin Regeneration by Dental Pulp Stem Cells and Promotes Macrophages M2 Phenotype In Vitro

Calcium phosphate cement (CPC) is promising for bone and dentin repair and regeneration. However, there has been no report of biphasic CPC for inducing dentin regeneration. The aim of this study was to develop a novel biphasic CPC containing β-tricalcium phosphate (β-TCP), and investigate its effects on odontogenic differentiation of human dental pulp stem cells (hDPSCs) and macrophage polarization. New biphasic CPC was formulated with different ratios of β-TCP to an equimolar mixture of tetracalcium phosphate and dicalcium phosphate anhydrous. Mechanical properties, biocompatibility, and odontogenic differentiation induction ability of the cements and the inflammatory reaction to the cements were examined. A series of CPC containing β-TCP were developed. CPC with 20% β-TCP exhibited homogeneity and injectability, an acceptable setting time, and a twofold increase in compressive strength. Significant increases in hDPSCs' alkaline phosphatase activity, mineral deposit, DMP1 and DSPP gene, and protein expressions were obtained for 20% TCP-CPC, compared with traditional CPC (p < 0.01). The addition of β-TCP did not promote macrophage polarization to the proinflammation phenotype. The addition of 10% and 20% β-TCP promoted macrophage polarization to the anti-inflammatory phenotype. In conclusion, a biphasic β-TCP-modified CPC was developed for the first time, demonstrating substantially increased dentin regeneration capability, while promoting macrophages to an anti-inflammation phenotype. The novel biphasic CPC is promising for tooth tissue engineering and dentin regeneration applications.

Immobilization of BMP-2 and VEGF within Multilayered Polydopamine-Coated Scaffolds and the Resulting Osteogenic and Angiogenic Synergy of Co-Cultured Human Mesenchymal Stem Cells and Human Endothelial Progenitor Cells

We have previously reported the fabrication of a polycaprolactone and hydroxyapatite composite scaffold incorporating growth factors to be used for bone regeneration. Two growth factors were incorporated employing a multilayered coating based on polydopamine (PDA). In particular, Bone morphogenetic protein-2 (BMP-2) was bound onto the inner PDA layer while vascular endothelial growth factor (VEGF) was immobilized onto the outer one. Herein, the in vitro release of both growth factors is evaluated. A fastest VEGF delivery followed by a slow and more sustained release of BMP-2 was demonstrated, thus fitting the needs for bone tissue engineering applications. Due to the relevance of the crosstalk between bone-promoting and vessel-forming cells during bone healing, the functionalized scaffolds are further assessed on a co-culture setup of human mesenchymal stem cells and human endothelial progenitor cells. Osteogenic and angiogenic gene expression analysis indicates a synergistic effect between the growth factor-loaded scaffolds and the co-culture conditions. Taken together, these results indicate that the developed scaffolds hold great potential as an efficient platform for bone-tissue applications.

Introduction of an interface layer on hydroxyapatite whisker/poly(L-lactide) composite and its contribution for improved bioactivity and mechanical properties

A hydroxyapatite whisker (w-HA) was synthesized via dissolution-precipitation by forming calcium-ethylene diamine tetra acetic acid (Ca-EDTA) complexing. The hydroxyapatite whisker was formed with precipitation of Ca²⁺ along the c-axis due to the space inhibition of Ca-EDTA complex during refluxing. The op-w-HA (oligomeric poly(lactic acid) modified w-HA), p-w-HA (poly(L-lactide) modified w-HA) and pc-w-HA (poly(L-lactide) and cyclodextrin modified w-HA) were obtained via the surface modification of w-HA. The particle size, surface charge and biocompatibility of theses modified w-HA particles were successfully adjusted. Among these materials, pc-w-HA exhibited nearly no toxicity, better adhesion to mesenchymal stem cells (MSCs) (5 times better than w-HA) and greater osteoinductivity among the obtained materials (40% of mineralized extracellular matrix higher than w-HA) due to better surface properties. Different kinds of powders (w-HA, p-w-HA and pc-w-HA) were blended with PLLA (poly(L-Lactide)) to form a composite material, respectively. The pc-w-HA/PLLA composite showed better mechanical properties (tensile strength of the pc-w-HA/PLLA composite was 22.3% higher than that of w-HA/PLLA), which could be attributed to mainly two factors including the structure preservation of w-HA bundles and pseudorotaxane linkage between PLA-cyclodextrin and PLLA. The MSCs adhesion of the pc-w-HA/PLLA composite was much better due to balanced hydrophilicity/hydrophobicity and surface roughness. This surface modification method could provide a new and effective strategy for the preparation of bioresorbable composite material with great bioactivity and mechanical property, which has great potential in the medical device industry.

The effect of biomimetic calcium deficient hydroxyapatite and sintered β-tricalcium phosphate on osteoimmune reaction and osteogenesis

Biomaterial implantation triggers inflammatory reactions. Understanding the effect of physicochemical features of biomaterials on the release of inflammatory cytokines from immune cells would be of great interest in view of designing bone graft materials to enhance the healing of bone defects. The present work investigated the interactions of two chemically and texturally different calcium phosphate (CaPs) substrates with macrophages, one of the main innate immune cells, and its further impact on osteogenic differentiation of bone forming cells. The behaviour of macrophages seeded on biomimetic calcium deficient hydroxyapatite (CDHA) and sintered β-tricalcium phosphate (β-TCP) was assessed in terms of the release of inflammatory cytokines and osteoclastogenic factors. The osteogenic differentiation of bone progenitor cells (bone marrow stromal cells (BMSCs) and osteoblastic cell line (SaOS-2)) were subsequently studied by incubating with the conditioned medium induced by macrophage-CaPs interaction in order to reveal the effect of immune cell reaction to CaPs on osteogenic differentiation. It was found that the incubation of macrophages with CaPs substrates caused a decrease of pro-inflammatory cytokines, more pronounced for β-TCP compared with CDHA showing significantly decreased IL-6, TNF-a, and iNOS. However, the macrophage-CDHA interaction resulted in a more favourable environment for osteogenic differentiation of osteoblasts with more collagen type I production and osteogenic genes (Runx2, BSP) expression, suggesting that osteogenic differentiation of bone cells is not only determined by the nature of biomaterials, but also significantly influenced by the inflammatory environment generated by the interaction of immune cells and biomaterials. STATEMENT OF SIGNIFICANCE: The field of osteoimmunology highlights the importance of the cross-talk between immune and bone cells for effective bone regeneration. This tight interaction opens the door to new strategies that encompass the development of smart cell-instructive biomaterials which performance covers the events from early inflammation to osteogenesis. The present work links the anti-inflammatory and osteoimmunomodulatory features of synthetic bone grafts to their chemistry and texture, focussing on the cross-talk between macrophages and two major orchestrators of bone healing, namely primary mesenchymal stem cells and osteoblasts. The results emphasize the importance of the microenvironment created through the interaction between the substrate and the immune cells as it can stimulate osteogenic events and subsequently foster bone healing.