Hydrothermally processed 1D hydroxyapatite: Mechanism of formation and biocompatibility studies

Humidity-Responsive Amorphous Calcium-Magnesium Pyrophosphate/Cassava Starch Scaffold for Enhanced Neurovascular Bone Regeneration

Developing a neurovascular bone repair scaffold with an appropriate mechanical strength remains a challenge. Calcium phosphate (CaP) is similar to human bone, but its scaffolds are inherently brittle and inactive, which require recombination with active ions and polymers for bioactivity and suitable strength. This work discussed the synthesis of amorphous magnesium-calcium pyrophosphate (AMCP) and the subsequent development of a humidity-responsive AMCP/cassava starch (CS) scaffold. The scaffold demonstrated enhanced mechanical properties by strengthening the intermolecular hydrogen bonds and ionic bonds between AMCP and CS during the gelatinization and freeze-thawing processes. The release of active ions was rapid initially and stabilized into a long-term stable release after 3 days, which is well-matched with new bone growth. The release of pyrophosphate ions endowed the scaffold with antibacterial properties. At the cellular level, the released active ions simultaneously promoted the proliferation and mineralization of osteoblasts, the proliferation and migration of endothelial cells, and the proliferation of Schwann cells. At the animal level, the scaffold was demonstrated to promote vascular growth and peripheral nerve regeneration in a rat skull defect experiment, ultimately resulting in the significant and rapid repair of bone defects. The construction of the AMCP/CS scaffold offers practical suggestions and references for neurovascular bone repair.

High-aspect-ratio nanostructured hydroxyapatite: towards new functionalities for a classical material

Hydroxyapatite-based materials have been widely used in countless applications, such as bone regeneration, catalysis, air and water purification or protein separation. Recently, much interest has been given to controlling the aspect ratio of hydroxyapatite crystals from bulk samples. The ability to exert control over the aspect ratio may revolutionize the applications of these materials towards new functional materials. Controlling the shape, size and orientation of HA crystals allows obtaining high aspect ratio structures, improving several key properties of HA materials such as molecule adsorption, ion exchange, catalytic reactions, and even overcoming the well-known brittleness of ceramic materials. Regulating the morphogenesis of HA crystals to form elongated oriented fibres has led to flexible inorganic synthetic sponges, aerogels, membranes, papers, among others, with applications in sustainability, energy and catalysis, and especially in the biomedical field.

Enhancing bioactivity and stability of polymer-based material-tissue interface through coupling multiscale interfacial interactions with atomic-thin TiO2 nanosheets

Stable and bioactive material-tissue interface (MTF) basically determines the clinical applications of biomaterials in wound healing, sustained drug release, and tissue engineering. Although many inorganic nanomaterials have been widely explored to enhance the stability and bioactivity of polymer-based biomaterials, most are still restricted by their stability and biocompatibility. Here we demonstrate the enhanced bioactivity and stability of polymer-matrix bio-composite through coupling multiscale material-tissue interfacial interactions with atomically thin TiO2 nanosheets. Resin modified with TiO2 nanosheets displays improved mechanical properties, hydrophilicity, and stability. Also, we confirm that this resin can effectively stimulate the adhesion, proliferation, and differentiation into osteogenic and odontogenic lineages of human dental pulp stem cells using in vitro cell-resin interface model. TiO2 nanosheets can also enhance the interaction between demineralized dentinal collagen and resin. Our results suggest an approach to effectively up-regulate the stability and bioactivity of MTFs by designing biocompatible materials at the sub-nanoscale.

Electronic Supplementary Material

Supplementary material (further details of fabrication and characterization of TiO2 NSs and TiO2-ARCs, the bioactivity evaluation of TiO2-ARCs on hDPSCs, and the measurement of interaction with demineralized dentin collagen) is available in the online version of this article at 10.1007/s12274-022-5153-1.

Polymeric Nanotechnologies for the Treatment of Periodontitis: A Chronological Review

Periodontitis is a chronic infectious and inflammatory disease of periodontal tissues estimated to affect 70 - 80 % of all adults. At the same time, periodontium, the site of periodontal pathologies, is an extraordinarily complex plexus of soft and hard tissues, the regeneration of which using even the most advanced forms of tissue engineering continues to be a challenge. Nanotechnologies, meanwhile, have provided exquisite tools for producing biomaterials and pharmaceutical formulations capable of elevating the efficacies of standard pharmacotherapies and surgical approaches to whole new levels. A bibliographic analysis provided here demonstrates a continuously increasing research output of studies on the use of nanotechnologies in the management of periodontal disease, even when they are normalized to the total output of studies on periodontitis. The great majority of biomaterials used to tackle periodontitis, including those that pioneered this interesting field, have been polymeric. In this article, a chronological review of polymeric nanotechnologies for the treatment of periodontitis is provided, focusing on the major conceptual innovations since the late 1990s, when the first nanostructures for the treatment of periodontal diseases were fabricated. In the opening sections, the etiology and pathogenesis of periodontitis and the anatomical and histological characteristics of the periodontium are being described, along with the general clinical manifestations of the disease and the standard means of its therapy. The most prospective chemistries in the design of polymers for these applications are also elaborated. It is concluded that the amount of innovation in this field is on the rise, despite the fact that most studies are focused on the refinement of already established paradigms in tissue engineering rather than on the development of revolutionary new concepts.

Surface Properties of Graphene Functionalized TiO2/nHA Hybrid Coatings Made on Ti6Al7Nb Alloys via Plasma Electrolytic Oxidation (PEO)

Nano-hydroxyapatite (nHA)-matrix coatings containing graphene nanosheets (GNS)-nHA were coated on Ti6Al7Nb alloys by plasma electrolytic oxidation (PEO) treatment for the improvement of their surface properties. Crystallographic properties, functional groups, and elemental analysis of coatings were characterized by XRD, ATR-FTIR, and EDS analysis. Surface morphological changes of the coated surfaces were investigated by AFM and SEM. The electrochemical corrosion behavior of the coatings was examined by using the potentiodynamic scanning (PDS) tests under in-vitro conditions in simulated body fluid (SBF). The results showed that the GNS was successfully deposited in ceramic matrix coatings on Ti6Al7Nb alloys. Also, the microstructural observations revealed that the coatings have a porous and rough structure. The XRD and ATR-FTIR quantitative analysis have proved the appearance of HA and GNS in the coating layers. An increase in the coating thickness, surface hardness, and anatase/rutile transformation rate was determined, while the GNS ratio in the coating layers was increased. The microhardness of the nHA coating reinforced with 1.5 wt% GNS was measured at 862 HV, which was significantly higher than that of GNS-free (only nHA) coating (584 HV). The best in-vitro resistance to corrosion in SBF was observed in the nHA/1.5GNS wt% coating.

Emerging marine derived nanohydroxyapatite and their composites for implant and biomedical applications

Implant materials must mimic natural human bones with biocompatibility, osteoconductivity and mechanical stability to successfully replace damaged or disease-affected bones. Synthetic hydroxyapatite was incorporated with bioglass to mimic natural bones for replacing conventional implant materials which has led to certain toxicity issues. Hence, hydroxyapatite (HAp) are recently gaining applicational importance as they are resembling the structure and function of natural bones. Further, nanosized HAp is under extensive research to utilize them as a potential replacement for traditional implants with several exclusive properties. However, chemical synthesis of nano-HAp exhibited toxicity towards normal and healthy cells. Recently, biogenic Hap synthesis from marine and animal sources are introduced as a next generation implant materials, due to their mineral ion and significant porous architecture mediated biocompatibility and bone bonding ability, compared to synthetic HAp. Thus, the purpose of the paper is to give a bird's eye view into the conventional approaches for fabricating nano-HAp, its limitations and the significance of using marine organisms and marine food wastes as a precursor for biogenic nano-Hap production. Moreover, in vivo and in vitro analyses of marine source derived nano-HAp and their potential biomedical applications were also discussed.

Hydrothermal synthesis of nanocrystalline hydroxyapatite-graphene nanosheet on Ti-6Al-7Nb: mechanical and in vitro corrosion performance

The hybrid coatings containing the graphene nano-sheet (GNS) and nano-hydroxyapatite (nHA) phases have been successfully synthesized on Ti6Al7Nb alloys by a one-step hydrothermal method. The hydrothermal reaction was carried out for 24 h at 200 °C. The GNS ratio has been altered as 1, 3, 5 and 7 wt.% in the coatings and, the results have compared with non- GNS containing coatings. The effect of the GNS ratio on the microstructure, hardness, and in vitro corrosion responses has been investigated in detail. The characterizations of the coatings were carried out by SEM, EDS, AFM, XRD and, FTIR. The corrosion behavior of the hybrid coatings was compared in Kokubo's solution at 37 °C by using potentiodynamic polarization tests. The results showed that the hydroxyapatite phases were deposed on the graphene layers with nano-size nucleation with its Ca/P stoichiometric ratio. The best hydrophilicity (~52°) property has been obtained in nHA/3GNS coatings. In addition, the corrosion rates of coatings increased in the following order: nHA/3GNS < nHA/1GNS < nHA/7GNS < nHA/5GNS < only nHA.

Pourbaix-Guided Mineralization and Site-Selective Photoluminescence Properties of Rare Earth Substituted B-Type Carbonated Hydroxyapatite Nanocrystals

Rare-earth labeling in biological apatite could provide critical information for the pathologic transition (osteoclastic) and physiologic regeneration (osteogenesis) of bone and teeth because of their characteristic site-sensitive fluorescence in different coordinative conditions of various tissues in many biological processes. However, the rare-earth labeling method for biological apatites, i.e., carbonated-hydroxyapatite, has been rarely found in the literature. In this paper, we report a Pourbaix-diagram guided mineralizing strategy to controllable carbonation and doping of rare-earth ions in the hydroxyapatite (HA) lattice. The carbonation process of hydroxyapatite was achieved by controllable mineralization in hydrothermal condition with K₂CO₃ as the carbonate source, which results into the pure B-type carbonated hydroxyapatite (CHA) with tunable carbonate substitution degree. All of the as-synthesized materials crystalized into P63/m (No. 176) space group with the lattice parameter of a decreases and c increases with the increasing of carbonate content in the reactants. Structural refinement results revealed that the substitution of planar CO₃²⁻ is superimposed on one of the faces of PO₄³⁻ tetrahedral sub-units with a rotation angle of 30° in reference to c-axis. All of the hydrothermally synthesized CHA nanocrystals show hexagonal rod-like morphology with the length of 70–110 nm and diameter of 21–35 nm, and the decreasing length/diameter ratio from 3.61 to 2.96 from low to high carbonated level of the samples. Five rare-earth cations, of Pr³⁺, Sm³⁺, Eu³⁺, Tb³⁺, and Ho³⁺, were used as possible probe ions that can be doped into either HA or CHA lattice. The site-preference of Tb³⁺ doping is the same in the crystallographic site of HA and CHA according to characteristic emission peaks of ⁵D₄–⁷F_j (j = 3–6) transitions in their photoluminescent spectroscopy. Our work provides a controllable carbonation method for rare-earth labeling hydroxyapatite nanomaterials with potential biologically active implant powders for bone repair and tissue regeneration.

Hydroxyapatite and Titanium Dioxide Nanoparticles: Radiolabelling and In Vitro Stability of Prospective Theranostic Nanocarriers for 223Ra and 99mTc

Hydroxyapatite and titanium dioxide are widely used materials in a broad spectrum of branches. Due to their appropriate properties such as a large specific surface area, radiation stability or relatively low toxicity, they could be potentially used as nanocarriers for medicinal radionuclides for diagnostics and therapy. Two radiolabelling strategies of both nanomaterials were carried out by 99mTc for diagnostic purposes and by 223Ra for therapeutic purposes. The first one was the radionuclide sorption on ready-made nanoparticles and the second one was direct radionuclide incorporation into the structure of the nanoparticles. Achieved labelling yields were higher than 94% in all cases. Afterwards, in vitro stability tests were carried out in several solutions: physiological saline, bovine blood plasma, bovine blood serum, 1% and 5% human albumin solutions. In vitro stability studies were performed as short-term (59 h for 223Ra and 31 h for 99mTc) and long-term experiments (five half-lives of 223Ra, approx. 55 days). Both radiolabelled nanoparticles with 99mTc have shown similar released activities (about 20%) in all solutions. The best results were obtained for 223Ra radiolabelled titanium dioxide nanoparticles, where overall released activities were under 6% for 59 h study in all matrices and under 3% for 55 days in a long-term perspective.

Self-templated microwave-assisted hydrothermal synthesis of two-dimensional holey hydroxyapatite nanosheets for efficient heavy metal removal

Heavy metals have caused serious environmental problems and threat to human health. Ultrathin and holey two-dimensional (2D) nanosheets have recently drawn significant attention as superb adsorbent material to remove heavy metal ions due to their unique physicochemical properties. Herein, we report a self-template-directed ultrafast reaction route to synthesis porous hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) nanosheets via a microwave-assisted hydrothermal method using poly(allylamine hydrochloride) as an additive. The resulting hydroxyapatite nanosheets showed a high specific surface area (92.9 m² g^-1) and excellent adsorption performance for various heavy metal ions including Pb(II), Cu(II), and Cd(II), with maximum adsorption capacities of 210.5, 31.6, and 24.9 mg g^-1, respectively. The adsorption kinetics fitted well with the pseudo-second-order equation and the equilibrium data showed a high correlation coefficient with the Langmuir model. Based on the experimental results and analysis, we can conclude that the sorption of heavy metal ions with the hydroxyapatite nanosheets mainly attributes to surface complexation and cation exchange. The present synthetic strategy allows the fast and massive production of porous hydroxyapatite ultrathin nanosheets and may also potentially be applicable to the fabrication of other metal phosphates with assembled or hierarchical porous structures towards various applications such as water purification.

Preparation of Micro/Nano-Structure Copper-Substituted Hydroxyapatite Scaffolds with Improved Angiogenesis Capacity for Bone Regeneration

The surface microstructures of calcium phosphate ceramics play an essential role in determining bone regeneration. However, it is difficult to produce micro/nano-structures on the surface of the porous hydroxyapatite (HA) scaffolds. In this study, we successfully developed and fabricated various micro/nano-structured surfaces on the HA scaffolds in copper ion (Cu²⁺)-containing solutions under hydrothermal conditions. The micro/nano-structures on the surface of the HA scaffolds were controlled by modulating the Cu²⁺ concentrations during the hydrothermal process. With an increase in the Cu²⁺ concentration, the surface morphology of the HA scaffolds changed significantly from sphere-like to flower-like, before becoming nano-structures. These findings indicated that the Cu²⁺ concentration affects the morphologies of calcium phosphate coatings that grow on the HA scaffolds. In vitro endothelial cell (EC) cultures showed that the cell proliferation was significantly enhanced when cultured on the flower-like morphology compared with other morphologies. Furthermore, an in vivo test in New Zealand rabbits demonstrated that the HA scaffold with the flower-like surface resulted in more angiogenesis compared with the control scaffold. This copper-assisted hydrothermal deposition process provides a simple and controllable route for engineering a micro/nano-structured surface on the HA scaffolds, which has benefits in terms of angiogenesis and bone regeneration.

Surface grafting of rare-earth ions doped hydroxyapatite nanorods (HAp:Ln(Eu/Tb)) with hydrophilic copolymers based on ligand exchange reaction: Biological imaging and cancer treatment

Rare-earth ions doped hydroxyapatite nanoparticles (HAp:Ln NPs) have demonstrated to be very promising candidates for biological imaging applications owing to their small size and chemical compositions similar to bone. However, these HAp:Ln NPs with controllable size and morphology should be prepared under hydrothermal treatment with hydrophobic molecules as the protective layers. The hydrophobic nature of these luminescent HAp:Ln NPs largely impeded their applications in biomedical fields. In this study, a novel and effective strategy has been developed for the surface modification of HAp:Ln nanorods through the combination of surface ligand exchange reaction and reversible-addition fragmentation chain transfer (RAFT) polymerization using 2-methacryloyloxyethyl phosphorylcholine (MPC) and itaconic acid (IA) as the monomers. Herein, a small molecule adenosine 5'-monophosphate disodium salt (AMP) that contains a phosphate group and two hydroxyl groups was used to displace the hydrophobic oleic acid on pristine HAp NPs through surface ligand exchange reaction owing to its stronger interaction with HAp NPs. On the other hand, the MPC and IA were introduced on HAp NPs through RAFT polymerization after the chain transfer agent was immobilized on the HAp NPs through the esterification reaction. The poly(IA-MPC) could not only endow the high water dispersibility but also be used for loading anticancer agent cisplatin (CDDP) through coordination interaction. To evaluate their potential biomedical applications, the cell uptake behavior, drug loading capacity and release behavior as well as cell viability of HAp:Ln-AMP-poly(IA-MPC) polymeric composites were examined. We demonstrated that the method developed in this work is very effective for introduction of functional polymers onto HAp:Ln nanorods. The HAp:Ln-AMP-poly(IA-MPC) composites are promising for cell imaging and controlled delivery of CDDP.

Biomimetically grown apatite spheres from aggregated bioglass nanoparticles with ultrahigh porosity and surface area imply potential drug delivery and cell engineering applications

Here we communicate the generation of biomimetically grown apatite spheres from aggregated bioglass nanoparticles and the potential properties applicable for drug delivery and cell/tissue engineering. Ion releasing nanoparticulates of bioglass (85%SiO₂-15%CaO) in a mineralizing medium show an intriguing dynamic phenomenon - aggregation, mineralization to apatite, integration and growth into micron-sized (1.5-3μm) spheres. During the progressive ionic dissolution/precipitation reactions, nano-to-micro-morphology, glass-to-crystal composition, and the physico-chemical properties (porosity, surface area, and charge) change dynamically. With increasing reaction period, the apatite becomes more crystallized with increased crystallinity and crystal size, and gets a composition closer to the stoichiometry. The developed microspheres exhibit hierarchical surface nanostructure, negative charge (ς-potential of -20mV), and ultrahigh mesoporosity (mesopore size of 6.1nm, and the resultant surface area of 63.7m²/g and pore volume of 0.153cm³/g) at 14days of mineralization, which are even higher than those of its precursor bioglass nanoparticles. Thanks to these properties, the biomimetic mineral microspheres take up biological molecules effectively, i.e., loading capacity of positive-charged protein is over 10%. Of note, the release is highly sustainable at a constant rate, i.e., profiling almost 'zero-order' kinetics for 4weeks, suggesting the potential usefulness as protein delivery systems. The biomimetic mineral microspheres hold some remnant Si in the core region, and release calcium, phosphate, and silicate ions over the test period, implying the long-term ionic-related therapeutic functions. The mesenchymal stem cells favour the biomimetic spheres with an excellent viability. Due to the merit of sizes (a few micrometers), the spheres can be intercalated into cells, mediating cellular interactions in 3D cell-spheroid engineering, and also can stimulate osteogenic differentiation of cells when incorporated into cell-laden gels. The intriguing properties observed in this study, including biomimetic composition, high mesoporosity, release of therapeutic ions, effective loading and long-term release of proteins, and diverse yet favorable 3D cellular interactions, suggest great potential of the newly developed biomimetic microspheres in biomedical applications, such as drug delivery and cell/tissue engineering.

STATEMENT OF SIGNIFICANCE

This work reports the generation of apatite spheres with a few micrometers in size biomimetically grown from bioactive glass nanoparticles, through a series of intriguing yet unprecedented phenomenon involving aggregation of nanoparticles, mineralization and sphere growth. The mineral microspheres possess some unique physico-chemical properties including mesoporosity, ultrahigh surface area, and therapeutic ionic release. Furthermore, the spheres show excellent loading and delivery capacity of protein molecules, and mediate favorable cellular interactions in 2D and 3D culture conditions, demonstrating a future multifunctional microcarrier platform for the therapeutics delivery and cell/tissue engineering.