Development and Characterization of a Bioinspired Bone Matrix with Aligned Nanocrystalline Hydroxyapatite on Collagen Nanofibers

The Effect of Pore Directionality of Collagen Scaffolds on Cell Differentiation and In Vivo Osteogenesis

Although many bone substitutes have been designed and produced, the development of bone tissue engineering products that mimic the microstructural characteristics of native bone remains challenging. It has been shown that pore orientation within collagen scaffolds influences bone matrix formation by the endochondral route. In addition, that the unidirectional orientation of the scaffolds can limit the growth of blood vessels. However, a comparison between the amount of bone that can be formed in scaffolds with different pore orientations in addition to analyzing the effect of loading osteogenic and proangiogenic factors is still required. In this work we fabricated uni- and multidirectional collagen sponges and evaluated their microstructural, physicochemical, mechanical and biological characteristics. Although the porosity and average pore size of the uni- and multidirectional scaffolds was similar (94.5% vs. 97.1% and 260 µm vs. 269 µm, respectively) the unidirectional sponges had a higher tensile strength, Young's modulus and capacity to uptake liquids than the multidirectional ones (0.271 MPa vs. 0.478 MPa, 9.623 MPa vs. 3.426 MPa and 8000% mass gain vs. 4000%, respectively). Culturing of rat bone marrow mesenchymal stem cells demonstrated that these scaffolds support cell growth and osteoblastic differentiation in the presence of BMP-2 in vitro, although the pore orientation somehow affected cell attachment and differentiation. The evaluation of the ability of the scaffolds to support bone growth when loaded with BMP-2 or BMP-2 + VEGF in an ectopic rat model showed that they both supported bone formation. Histological analysis and quantification of mineralized matrix revealed that the pore orientation of the collagen scaffolds influenced the osteogenic process.

Mechanical Enhancement of Cytocompatible 3D Scaffolds, Consisting of Hydroxyapatite Nanocrystals and Natural Biomolecules, Through Physical Cross-Linking

Bioinspired scaffolds mimicking natural bone-tissue properties holds great promise in tissue engineering applications towards bone regeneration. Within this work, a way to reinforce mechanical behavior of bioinspired bone scaffolds was examined by applying a physical crosslinking method. Scaffolds consisted of hydroxyapatite nanocrystals, biomimetically synthesized in the presence of collagen and l-arginine. Scaffolds were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), microcomputed tomography, and nanoindentation. Results revealed scaffolds with bone-like nanostructure and composition, thus an inherent enhanced cytocompatibility. Evaluation of porosity proved the development of interconnected porous network with bimodal pore size distribution. Mechanical reinforcement was achieved through physical crosslinking with riboflavin irradiation, and nanoindentation tests indicated that within the experimental conditions of 45% humidity and 37 °C, photo-crosslinking led to an increase in the scaffold’s mechanical properties. Elastic modulus and hardness were augmented, and specifically elastic modulus values were doubled, approaching equivalent values of trabecular bone. Cytocompatibility of the scaffolds was assessed using MG63 human osteosarcoma cells. Cell viability was evaluated by double staining and MTT assay, while attachment and morphology were investigated by SEM. The results suggested that scaffolds provided a cell friendly environment with high levels of viability, thus supporting cell attachment, spreading and proliferation.

Surface Treatment of Acetabular Cups with a Direct Deposition of a Composite Nanostructured Layer Using a High Electrostatic Field

A composite nanofibrous layer containing collagen and hydroxyapatite was deposited on selected surface areas of titanium acetabular cups. The layer was deposited on the irregular surface of these 3D objects using a specially developed electrospinning system designed to ensure the stability of the spinning process and to produce a layer approximately 100 micrometers thick with an adequate thickness uniformity. It was verified that the layer had the intended nanostructured morphology throughout its entire thickness and that the prepared layer sufficiently adhered to the smooth surface of the model titanium implants even after all the post-deposition sterilization and stabilization treatments were performed. The resulting layers had an average thickness of (110 ± 30) micrometers and an average fiber diameter of (170 ± 49) nanometers. They were produced using a relatively simple and cost-effective technology and yet they were verifiably biocompatible and structurally stable. Collagen- and hydroxyapatite-based composite nanostructured surface modifications represent promising surface treatment options for metal implants.

Polylactide Composite Pins Reinforced with Bioresorbable Continuous Glass Fibers Demonstrating Bone-like Apatite Formation and Spiral Delamination Degradation

The emergence of polylactide composites reinforced with bioresorbable silicate glass fibers has allowed for the long-term success of biodegradable polymers in load-bearing orthopedic applications. However, few studies have reported on the degradation behavior and bioactivity of such biocomposites. The aim of this work was to investigate the degradation behavior and in vitro bioactivity of a novel biocomposite pin composed of bioresorbable continuous glass fibers and poly-L-D-lactide in simulated body fluid for 78 weeks. As the materials degraded, periodic spiral delamination formed microtubes and funnel-shaped structures in the biocomposite pins. It was speculated that the direction of degradation, from both ends towards the middle of the fibers and from the surface through to the bulk of the polymer matrix, could facilitate bone healing. Following immersion in simulated body fluid, a bone-like apatite layer formed on the biocomposite pins which had a similar composition and structure to natural bone. The sheet- and needle-like apatite nanostructure was doped with sodium, magnesium, and carbonate ions, which acted to lower the Ca/P atomic ratio to less than the stoichiometric apatite and presented a calcium-deficient apatite with low crystallinity. These findings demonstrated the bioactivity of the new biocomposite pins in vitro and their excellent potential for load-bearing applications.

Harnessing Nanotopography of Electrospun Nanofibrous Nerve Guide Conduits (NGCs) for Neural Tissue Engineering

The anatomical recovery of nerve defects with their neurological functions after an injury caused by diseases or accidents is an important clinical issue. The most efficient surgical technique so far to the nerve defects, which are unrepairable by direct end-to-end suture, can be autograft transplantation. The autograft transplantation, however, has disadvantages including multiple rounds of surgery, a shortage of nerve donor, and function loss at the donor site. Tissue-engineered nerve guide conduits (TENGCs) have emerged as a potential alternative to autologous nerve grafts for nerve regeneration and functional recovery. Various TENGCs researches are being carried out to improve characteristics and enhance functionality such as material selection, biomimetic, topography, and enhancement by the biomolecules additions. Among them, the customizable surface nanotopography of aligned fibrous TENGCs foster neural repair by providing a cell-friendly environment, permissiveness, guidance cues, and directional growth of the cells. Fibrous nerve guide conduits (NGCs) made of longitudinally ordered fibers is a promising candidate for nerve tissue engineering.

Enzyme-crosslinked gene-activated matrix for the induction of mesenchymal stem cells in osteochondral tissue regeneration

The development of osteochondral tissue engineering is an important issue for the treatment of traumatic injury or aging associated joint disease. However, the different compositions and mechanical properties of cartilage and subchondral bone show the complexity of this tissue interface, making it challenging for the design and fabrication of osteochondral graft substitute. In this study, a bilayer scaffold is developed to promote the regeneration of osteochondral tissue within a single integrated construct. It has the capacity to serve as a gene delivery platform to promote transfection of human mesenchymal stem cells (hMSCs) and the functional osteochondral tissues formation. For the subchondral bone layer, the bone matrix with organic (type I collagen, Col) and inorganic (hydroxyapatite, Hap) composite scaffold has been developed through mineralization of hydroxyapatite nanocrystals oriented growth on collagen fibrils. We also prepare multi-shell nanoparticles in different layers with a calcium phosphate core and DNA/calcium phosphate shells conjugated with polyethyleneimine to act as non-viral vectors for delivery of plasmid DNA encoding BMP2 and TGF-β3, respectively. Microbial transglutaminase is used as a cross-linking agent to crosslink the bilayer scaffold. The ability of this scaffold to act as a gene-activated matrix is demonstrated with successful transfection efficiency. The results show that the sustained release of plasmids from gene-activated matrix can promote prolonged transgene expression and stimulate hMSCs differentiation into osteogenic and chondrogenic lineages by spatial and temporal control within the bilayer composite scaffold. This improved delivery method may enhance the functionalized composite graft to accelerate healing process for osteochondral tissue regeneration.

STATEMENT OF SIGNIFICANCE

In this study, a gene-activated matrix (GAM) to promote the growth of both cartilage and subchondral bone within a single integrated construct is developed. It has the capacity to promote transfection of human mesenchymal stem cells (hMSCs) and the functional osteochondral tissues formation. The results show that the sustained release of plasmids including TGF-beta and BMP-2 from GAM could promote prolonged transgene expression and stimulate hMSCs differentiation into the osteogenic and chondrogenic lineages by spatial control manner. This improved delivery method should enhance the functionalized composite graft to accelerate healing process in vitro and in vivo for osteochondral tissue regeneration.

One-dimensional hydroxyapatite materials: preparation and applications

As one of the biominerals, hydroxyapatite (HAP) plays important roles in biology, and inspires researchers to investigate HAP-based materials for the applications in various biomedical fields. Among them, one-dimensional (1-D) micro-/nanostructured HAP materials have attracted great interest in the last decades. This review summarizes the preparation and applications of 1-D HAP materials, and discusses different aspects of 1-D HAP materials. Various synthetic methods have been developed to prepare 1-D HAP materials with different morphologies, sizes, surface properties and crystallinities. In addition, elements-substituted 1-D HAP materials and composites have also been prepared. Surfactants and additives are usually adopted to control the nucleation and growth of 1-D HAP materials, but the related mechanisms are not very clear yet. The applications of 1-D HAP materials have been widely investigated, and the biomedical applications show great prospect but still need further improvements. A new kind of highly flexible fire-resistant inorganic paper made of ultralong HAP nanowires has been developed and is a promising alternative of the traditional cellulose paper for valuable archives and important documents. Regardless of the advances, further studies should be made for preparing 1-D HAP materials with controlled structures, sizes and morphologies and for boosting their various applications.

Biocompatibility and Corrosion Protection Behaviour of Hydroxyapatite Sol-Gel-Derived Coatings on Ti6Al4V Alloy

The aim of this work was to prepare hydroxyapatite coatings (HAp) by a sol-gel method on Ti6Al4V alloy and to study the bioactivity, biocompatibility and corrosion protection behaviour of these coatings in presence of simulated body fluids (SBFs). Thermogravimetric/Differential Thermal Analyses (TG/DTA) and X-ray Diffraction (XRD) have been applied to obtain information about the phase transformations, mass loss, identification of the phases developed, crystallite size and degree of crystallinity of the obtained HAp powders. Fourier Transformer Infrared Spectroscopy (FTIR) has been utilized for studying the functional groups of the prepared structures. The surface morphology of the resulting HAp coatings was studied by Scanning Electron Microscopy (SEM). The bioactivity was evaluated by soaking the HAp-coatings/Ti6Al4V system in Kokubo's Simulated Body Fluid (SBF) applying Inductively Coupled Plasma (ICP) spectrometry. 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) and Alamar blue cell viability assays were used to study the biocompatibility. Finally, the corrosion behaviour of HAp-coatings/Ti6Al4V system was researched by means of Electrochemical Impedance Spectroscopy (EIS). The obtained results showed that the prepared powders were nanocrystalline HAp with little deviations from that present in the human bone. All the prepared HAp coatings deposited on Ti6Al4V showed well-behaved biocompatibility, good bioactivity and corrosion protection properties.

Biodegradable polymeric nanostructures in therapeutic applications: opportunities and challenges

Biodegradable polymeric nanostructures (BPNs) have shown great promise in different therapeutic applications such as diagnosis, imaging, drug delivery, cosmetics, organ implants, and tissue engineering.