Biomimetic synthesis of enamel-like hydroxyapatite on self-assembled monolayers

Alendronate as Bioactive Coating on Titanium Surfaces: An Investigation of CaP-Alendronate Interactions

The surface modification of dental implants plays an important role in establishing a successful interaction of the implant with the surrounding tissue, as the bioactivity and osseointegration properties are strongly dependent on the physicochemical properties of the implant surface. A surface coating with bioactive molecules that stimulate the formation of a mineral calcium phosphate (CaP) layer has a positive effect on the bone bonding process, as biomineralization is crucial for improving the osseointegration process and rapid bone ingrowth. In this work, the spontaneous deposition of calcium phosphate on the titanium surface covered with chemically stable and covalently bound alendronate molecules was investigated using an integrated experimental and theoretical approach. The initial nucleation of CaP was investigated using quantum chemical calculations at the density functional theory (DFT) level. Negative Gibbs free energies show a spontaneous nucleation of CaP on the biomolecule-covered titanium oxide surface. The deposition of calcium and phosphate ions on the alendronate-modified titanium oxide surface is governed by Ca2+-phosphonate (-PO3H) interactions and supported by hydrogen bonding between the phosphate group of CaP and the amino group of the alendronate molecule. The morphological and structural properties of CaP deposit were investigated using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and attenuated total reflectance Fourier transform infrared spectroscopy. This integrated experimental-theoretical study highlights the spontaneous formation of CaP on the alendronate-coated titanium surface, confirming the bioactivity ability of the alendronate coating. The results provide valuable guidance for the promising forthcoming advancements in the development of biomaterials and surface modification of dental implants.

Electrodeposition of hydroxyapatite coating on Mg alloy modified with organic acid self-assembled monolayers

Calcium phosphate coatings are used in orthopedics due to their excellent bioactivity, which improves the bonding between the metal implant and the bone. The use of self-assembling monolayers of long-chain organic acids can induce calcium phosphate growth. In this article, the self-assembling monolayers of stearic acid and octadecylphosphonic acid formed on the Mg alloy surface were additionally modified with electrodeposited hydroxyapatite coating to increase the bioactivity and biocompatibility of the Mg alloy in a physiological solution. Hydroxyapatite coating was prepared by a two-step reaction: hydrogen phosphate formed by electrodeposition at constant potential was converted into hydroxyapatite coating through an acid–base reaction. The results obtained by voltammetry and electrochemical impedance spectroscopy have shown a beneficial effect of organic acid self-assembling monolayer and especially of organic acid self-assembling monolayer modification by hydroxyapatite electrodeposition on the corrosion properties of Mg alloy in physiological solution. Fourier transform infrared spectroscopy and scanning electron microscopy were used to verify the existence of the organic acid SAM|HAp film on the Mg alloy surface and their morphology.

Rapid and area-specific coating of fluoride-incorporated apatite layers by a laser-assisted biomimetic process for tooth surface functionalization

Surface functionalization of teeth with fluoride-incorporated apatite layers displays great potential in treatments and prevention of dental disorders. In this study, we used a sintered hydroxyapatite (sHA) substrate as a model material of teeth, and established a rapid and area-specific coating technique of fluoride-incorporated apatite layers by using a laser-assisted biomimetic (LAB) process. In this technique, a sHA substrate was irradiated on the surface with a Nd:YAG pulsed UV laser for 30 min in supersaturated calcium phosphate (CaP) solutions with various fluoride concentrations. The fluoride concentration in the CaP solution was varied to control morphology, crystalline structure, and fluoride content of the resulting layers. Without fluoride in the CaP solution, an octacalcium phosphate (OCP) layer with a flake-like structure was formed on the laser-irradiated surface of the substrate. The addition of fluoride (1000 µM and 3000 µM) to the CaP solution led to the formation of fluoride-incorporated apatite layers with an enamel-like needle-like nanostructure. The fluoride-incorporated apatite layers adhered firmly to the sHA surface and reduced acid dissolution of the sHA substrate by acting as a protective covering. Additionally, the layers released fluoride ions for more than 24 h, and exhibited antibacterial activity relative to a caries-causing bacterium, namely Streptococcus mutans. Thus, our LAB process can potentially act as a new tool for functionalization of tooth surfaces.

STATEMENT OF SIGNIFICANCE

We used a sintered hydroxyapatite (sHA) substrate as a model material of teeth, and established a rapid and area-specific coating technique of fluoride-incorporated apatite layers on the sHA surface by using our laser-assisted biomimetic (LAB) process. In this process, pulsed laser was utilized to accelerate seeded crystal growth in supersaturated calcium phosphate solutions supplemented with NaF. The thus-fabricated fluoride-incorporated apatite layers consisted of enamel-like needle-like nanocrystals with c-axis orientation. These fluoride-incorporated apatite layers adhered firmly to the sHA surface, reduced acid dissolution of the sHA substrate by acting as a protective covering, and exhibited antibacterial activity against Streptococcus mutans through the fluoride release. Thus, our LAB process can potentially act as a new tool for functionalization of tooth surfaces.

Hierarchical Biomineralization: from Nature's Designs to Synthetic Materials for Regenerative Medicine and Dentistry

Biomineralization is a highly dynamic, yet controlled, process that many living creatures employ to develop functional tissues such as tooth enamel, bone, and others. A major goal in materials science is to create bioinspired functional structures based on the precise organization of building blocks across multiple length scales. Therefore, learning how nature has evolved to use biomineralization could inspire new ways to design and develop synthetic hierarchical materials with enhanced functionality. Toward this goal, this review dissects the current understanding of structure-function relationships of dental enamel and bone using a materials science perspective and discusses a wide range of synthetic technologies that aim to recreate their hierarchical organization and functionality. Insights into how these strategies could be applied for regenerative medicine and dentistry are also provided.

Anisotropic Growth of Hydroxyapatite in Stretched Double Network Hydrogel

Bone tissues possess excellent mechanical properties such as compatibility between strength and flexibility and load bearing owing to the hybridization of organic/inorganic matters with anisotropic structure. To synthetically mimic such an anisotropic structure of natural organic/inorganic hybrid materials, we carried out hydroxyapatite (HAp) mineralization in stretched tough double network (DN) hydrogels. Anisotropic mineralization of HAp took place in stretched hydrogels, as revealed by high brightness synchrotron X-ray scattering and transmission electron microscopic observation. The c-axis of mineralized HAp aligned along the stretching direction, and the orientation degree S calculated from scattering profiles increased with increasing in the elongation ratio λ of the DN gel, and S at λ = 4 became comparable to that of rabbit tibial bones. The morphology of HAp polycrystal gradually changed from spherical to unidirectional rod-like shape with increased elongation ratio. A possible mechanism for the anisotropic mineralization is proposed, which would be one of the keys to develop mechanically anisotropic organic/inorganic hybrid materials.

The role of amino acids in hydroxyapatite mineralization

Polar and charged amino acids (AAs) are heavily expressed in non-collagenous proteins (NCPs), and are involved in hydroxyapatite (HA) mineralization in bone. Here, we review what is known on the effect of single AAs on HA precipitation. Negatively charged AAs, such as aspartic acid, glutamic acid (Glu) and phosphoserine are largely expressed in NCPs and play a critical role in controlling HA nucleation and growth. Positively charged ones such as arginine (Arg) or lysine (Lys) are heavily involved in HA nucleation within extracellular matrix proteins such as collagen. Glu, Arg and Lys intake can also increase bone mineral density by stimulating growth hormone production. In vitro studies suggest that the role of AAs in controlling HA precipitation is affected by their mobility. While dissolved AAs are able to inhibit HA precipitation and growth by chelating Ca2+ and PO43- ions or binding to nuclei of calcium phosphate and preventing their further growth, AAs bound to surfaces can promote HA precipitation by attracting Ca2+ and PO43- ions and increasing the local supersaturation. Overall, the effect of AAs on HA precipitation is worth being investigated more, especially under conditions closer to the physiological ones, where the presence of other factors such as collagen, mineralization inhibitors, and cells heavily influences HA precipitation. A deeper understanding of the role of AAs in HA mineralization will increase our fundamental knowledge related to bone formation, and could lead to new therapies to improve bone regeneration in damaged tissues or cure pathological diseases caused by excessive mineralization in tissues such as cartilage, blood vessels and cardiac valves.

Synthesis of nanostructured methotrexate/hydroxyapatite: Morphology control, growth mechanism, and bioassay explore

In this study, a new structure of methotrexate/hydroxyapatite (MTX/HAp) nanorods via a facile hydrothermal route was reported. The as-synthesized samples were then characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), thermogravimetric (TG) and differential scanning calorimetry (DSC) analysis. In order to explore the formation mechanism, the effects of reaction time, MTX concentrations and addition of ethylene glycol (PEG) were emphatically examined. The results indicated that, with the increase in reaction time, the fibrous nanoparticles turned to needle-like and then to rod-like. Our study also proved that reaction time of 12h was enough for the full-growth of the nanostructure. Drug-loading capacities (AIn) rose dramatically in the first 12h and reached a plateau afterwards. Importantly, MTX played a critical role in the longitudinal growth of MTX/HAp nanostructure and polyethylene glyco (PEG) was a good dispersing agent to improve the monodispersity. As expected, the functional agent of MTX was served as both the target anticancer drug loaded in HAp and effective complex agents to modify and control the morphologies of MTX/HAp. Lastly, in vitro bioassay tests gave us evidence that obvious tumor inhibition can be achieved when MTX was hybridized with HAp.

Bioactivity response of Ta1-xOx coatings deposited by reactive DC magnetron sputtering

The use of dental implants is sometimes accompanied by failure due to periimplantitis disease and subsequently poor esthetics when soft-hard tissue margin recedes. As a consequence, further research is needed for developing new bioactive surfaces able to enhance the osseous growth. Tantalum (Ta) is a promising material for dental implants since, comparing with titanium (Ti), it is bioactive and has an interesting chemistry which promotes the osseointegration. Another promising approach for implantology is the development of implants with oxidized surfaces since bone progenitor cells interact with the oxide layer forming a diffusion zone due to its ability to bind with calcium which promotes a stronger bond. In the present report Ta-based coatings were deposited by reactive DC magnetron sputtering onto Ti CP substrates in an Ar+O2 atmosphere. In order to assess the osteoconductive response of the studied materials, contact angle and in vitro tests of the samples immersed in Simulated Body Fluid (SBF) were performed. Structural results showed that oxide phases where achieved with larger amounts of oxygen (70 at.% O). More compact and smooth coatings were deposited by increasing the oxygen content. The as-deposited Ta coating presented the most hydrophobic character (100°); with increasing oxygen amount contact angles progressively diminished, down to the lowest measured value, 63°. The higher wettability is also accompanied by an increase on the surface energy. Bioactivity tests demonstrated that highest O-content coating, in good agreement with wettability and surface energy values, showed an increased affinity for apatite adhesion, with higher Ca/P ratio formation, when compared to the bare Ti substrates.

Advances in synthesis of calcium phosphate crystals with controlled size and shape

Calcium phosphate (CaP) materials have a wide range of applications, including biomaterials, adsorbents, chemical engineering materials, catalysts and catalyst supports and mechanical reinforcements. The size and shape of CaP crystals and aggregates play critical roles in their applications. The main inorganic building blocks of human bones and teeth are nanocrystalline CaPs; recently, much progress has been made in the application of CaP nanocrystals and their composites for clinical repair of damaged bone and tooth. For example, CaPs with special micro- and nanostructures can better imitate the biomimetic features of human bone and tooth, and this offers significantly enhanced biological performances. Therefore, the design of CaP nano-/microcrystals, and the shape and hierarchical structures of CaPs, have great potential to revolutionize the field of hard tissue engineering, starting from bone/tooth repair and augmentation to controlled drug delivery devices. Previously, a number of reviews have reported the synthesis and properties of CaP materials, especially for hydroxyapatite (HAp). However, most of them mainly focused on the characterizations and physicochemical and biological properties of HAp particles. There are few reviews about the control of particle size and size distribution of CaPs, and in particular the control of nano-/microstructures on bulk CaP ceramic surfaces, which is a big challenge technically and may have great potential in tissue engineering applications. This review summarizes the current state of the art for the synthesis of CaP crystals with controlled sizes from the nano- to the macroscale, and the diverse shapes including the zero-dimensional shapes of particles and spheres, the one-dimensional shapes of rods, fibers, wires and whiskers, the two-dimensional shapes of sheets, disks, plates, belts, ribbons and flakes and the three-dimensional (3-D) shapes of porous, hollow, and biomimetic structures similar to biological bone and tooth. In addition, this review will also summarize studies on the controlled formation of nano-/microstructures on the surface of bulk ceramics, and the preparation of macroscopical bone grafts with 3-D architecture nano-/microstructured surfaces. Moreover, the possible directions of future research and development in this field, such as the detailed mechanisms behind the size and shape control in various strategies, the importance of theoretical simulation, self-assembly, biomineralization and sacrificial precursor strategies in the fabrication of biomimetic bone-like and enamel-like CaP materials are proposed.

Calcium phosphate growth at electropolished titanium surfaces

This work investigated the ability of electropolished Ti surface to induce Hydroxyapatite (HA) nucleation and growth in vitro via a biomimetic method in Simulated Body Fluid (SBF). The HA induction ability of Ti surface upon electropolishing was compared to that of Ti substrates modified with common chemical methods including alkali, acidic and hydrogen peroxide treatments. Our results revealed the excellent ability of electropolished Ti surfaces in inducing the formation of bone-like HA at the Ti/SBF interface. The chemical composition, crystallinity and thickness of the HA coating obtained on the electropolished Ti surface was found to be comparable to that achieved on the surface of alkali treated Ti substrate, one of the most effective and popular chemical treatments. The surface characteristics of electropolished Ti contributing to HA growth were discussed thoroughly.

Photoluminescence in the characterization and early detection of biomimetic bone-like apatite formation on the surface of alkaline-treated titanium implant: state of the art

Photoluminescence (PL) property is particularly important in the characterization of materials that contain significant proportions of noncrystalline components, multiple phases, or low concentrations of mineral phases. In this research, the ability of biomimetic bone-like apatite deposition on the surface of titanium alloy (Ti6Al4V) substrates in simulated body fluid (SBF) right after alkaline-treatment and subsequent heat-treatment was studied by the inherent luminescence properties of apatite. For this purpose, the metallic substrates were treated in 5 M NaOH solution at 60 °C. Subsequently, the substrates were heat-treated at 600 °C for 1 h for consolidation of the sodium titanate hydrogel layer. Then, they were soaked in SBF for different periods of time. Finally, the possibility to use of PL monitoring as an effective method and early detection tool is discussed. According to the obtained results, it was concluded that the PL emission peak did not have any significant shift to the shorter or higher wavelengths, and the PL intensity increased as the exposure time increased. This research proved that the observed inherent PL of the newly formed apatite coatings might be of specific interest for histological probing and bone remodelling monitoring.

Development of fluorapatite cement for dental enamel defects repair

In order to restore the badly carious lesion of human dental enamel, a crystalline paste of fluoride substituted apatite cement was synthesized by using the mixture of tetracalcium phosphate (TTCP), dicalcium phosphate anhydrous (DCPA) and ammonium fluoride. The apatite cement paste could be directly filled into the enamel defects (cavities) to repair damaged dental enamel. The results indicated that the hardened cement was fluorapatite [Ca(10)(PO(4))(6)F(2), FA] with calcium to phosphorus atom molar ratio (Ca/P) of 1.67 and Ca/F ratio of 5. The solubility of FA cement in Tris-HCl solution (pH = 5) was slightly lower than the natural enamel, indicating the FA cement was much insensitive to the weakly acidic solutions. The FA cement was tightly combined with the enamel surface, and there was no obvious difference of the hardness between the FA cement and natural enamel. The extracts of FA cement caused no cytotoxicity on L929 cells, which satisfied the relevant criterion on dental biomaterials, revealing good cytocompatibility. In addition, the results showed that the FA cement had good mechanical strength, hydrophilicity, and anti-bacterial adhesion properties. The study suggested that using FA cement was simple and promising approach to effectively and conveniently restore enamel defects.

A new approach in biomimetic synthesis of calcium phosphate coatings using lactic acid-Na lactate buffered body fluid solution

The main objective of this study was to investigate calcium phosphate (CaP) coatings on Ti6Al4V substrates by using the biomimetic technique. To this purpose, a new solution was developed to coat CaP on Ti6Al4V alloy substrates. The newly formulated body fluid (Lac-SBF) contained appropriate amounts of sodium lactate (NaL) and lactic acid (HL), as well as all the other ionic constituents of the human blood plasma. The inorganic ion concentrations of the Lac-SBF solutions were identical with those of human blood plasma. The new Lac-SBF solution of this study eliminated the need for using Tris/HCl or Hepes/NaOH buffers. Prior to coating, Ti6Al4V substrates were chemically treated in NaOH and/or NaOH+H(2)O(2) solutions as an alternative route and then heated at 600 degrees C for 1h in air. In the previous applications, the Cl(-) ion concentration was found to be higher than blood plasma 103mM, which exists in human blood plasma as a result of Tris/HCl which are used to prevent precipitation and to keep the pH level at certain values. In this study, instead of using Tris/HCl, HL/NaL which are generated by human body and do not show any toxic behavior, are used and Cl(-) concentration was kept at 103mM value for the first time. The prepared Lac-SBF was shown to have similar concentration to human blood plasma in terms of all inorganic ions for the first time. Solution properties were evaluated by using turbidimeter, pH meter and rheometer. The coatings were characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and a scratch tester. The obtained results are presented and discussed.

Kinetic model for hydroxyapatite precipitation on human enamel surface by electrolytic deposition

The electrolytic deposition (ELD) of hydroxyapatite (HAP) coating on human enamel surface for different loading times at varied temperatures (ranging from 37 degrees C to 85 degrees C) and varied current densities (ranging from 0.05 mA cm(-2) to 10 mA cm(-2)) was investigated in this study. Thin film x-ray diffraction, Fourier transform infrared and micro-Raman spectra analysis, as well as an environmental scanning electron microscope, were used to characterize the coating. The results showed that only the HAP phase occurred on the enamel surface after ELD experiments. The contents of HAP deposits on the enamel surface linearly changed proportional to the square root of the loading time, which was in good agreement with the kinetic model of ELD of HAP coating based on one-dimensional diffusion. The induction periods were observed on all the regression lines, and the rate of the HAP coating formation on enamel showed a linear relationship with the current density. It was implied that the diffusion process was the rate-determining step in the ELD of the HAP coating on human enamel.

Preparation and in vitro characterization of scaffolds of poly(L-lactic acid) containing bioactive glass ceramic nanoparticles

Porous nanocomposite scaffolds of poly(l-lactic acid) (PLLA) containing different quantities of bioactive glass ceramic (BGC) nanoparticles (SiO(2):CaO:P(2)O(5) approximately 55:40:5 (mol)) were prepared by a thermally induced phase-separation method. Dioxane was used as the solvent for PLLA. Introduction of less than 20wt.% of BGC nanoparticles did not remarkably affect the porosity of PLLA foam. However, as the BGC content increased to 30wt.%, the porosity of the composite was observed to decrease rapidly. The compressive modulus of the scaffolds increased from 5.5 to 8.0MPa, while the compressive strength increased from 0.28 to 0.35MPa as the BGC content increased from 0 to 30wt.%. The in vitro bioactivity and biodegradability of nanocomposites were investigated by incubation in simulated body fluid (SBF) and phosphate-buffered saline, respectively. Scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy and X-ray diffraction were employed to monitor the surface variation of neat PLLA and PLLA/BGC porous scaffolds during incubation. PLLA/(20wt.%)BGC composite exhibited the best mineralization property in SBF, while the PLLA/(10wt.%)BGC composite showed the highest water absorption ability.