Silica- and zirconia-hybridized amorphous calcium phosphate: effect on transformation to hydroxyapatite.

The goal of this study was to determine the effect that silica and zirconia have on the stability of bioactive amorphous calcium phosphate (ACP) mineral, i.e., in retarding its transformation to hydroxyapatite (HAP). The glass-forming agents, tetraethoxysilane and zirconyl chloride, were introduced individually during the low-temperature preparation of ACP. These hybrid ACPs (Si-ACP and Zr-ACP, respectively) as well as the control, unhybridized ACP (u-ACP), were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, specific surface area measurements, and chemical analysis (Ca/PO(4) ratio of the solids) before being dispersed in one of the following four test solutions: N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES)-buffered (pH = 7.40) saline solutions with 0 microg/g fluoride (test solution A1), 1 microg/g fluoride (test solution A2), and 10 microg/g fluoride (test solution A3), or a lactic acid-containing solution (pH = 5.10, adjusted with NaOH; test solution B). Aliquots were taken at predetermined time intervals for solution Ca and PO(4) analysis. Solids isolated after 30 and 90 min exposure to solution B as well as the final dissolution/transformation products from all four solution experiments were analyzed by Fourier transform infrared spectroscopy and X-ray diffraction. Regardless of the type of experimental solution used, slower conversion to HAP was observed with the hybrid ACPs compared with u-ACP. The retarding effect of the Si or Zr species in the hybridized ACPs is probably due to these ions specifically blocking, by adsorption, potential sites for HAP nucleation and growth. The stability of ACP toward HAP conversion increased in the following order: u-ACP < Si-ACP < Zr-ACP. Hybrid ACP fillers, especially Zr-ACP, could be utilized in applications in which it is desired to enhance performance of composites, sealants, and/or adhesives in preventing demineralization or actively promoting remineralization.     

Study on the oxidation form of adenine in phosphate buffer solution

The oxidation of adenine in phosphate buffer solution is investigated using square-wave voltammetry and in situ UV spectroelectrochemistry. The geometry of adenine and the derivatives optimized at DFTB3LYP-6-31G (d, p)-PCM level is in agreement with the crystal structure, and the imitated UV spectra of adenine and the product at electrode are consistent with the in situ UV spectra. The relationship between the electrochemical property and the molecular structure is also discussed. The experimental and theoretical results show that the adenine oxidation origins from the neutral adenine.     

Mechanism for converting Al2O3-containing borate glass to hydroxyapatite in aqueous phosphate solution

The effect of replacing varying amounts (0–2.5 mol.%) of B₂O₃ with Al₂O₃ in a borate glass on (1) the conversion of the glass to HA in an aqueous phosphate solution and (2) the compressive strength of the as-formed HA product was investigated. Samples of each glass (10 × 10 × 8 mm) were placed in 0.25 M K₂HPO₄ solution at 60 °C, and the conversion kinetics to HA were determined from the weight loss of the glass and the pH of the solution. The structure and composition of the solid reaction products were characterized using X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy. While the conversion rate of the glass to HA decreased considerably with increasing Al₂O₃ content, the microstructure of the HA product became denser and the compressive strength of the HA product increased. The addition of SiO₂ to the Al₂O₃-containing borate glass reversed the deterioration of the conversion rate, and produced a further improvement in the strength of the HA product. The compressive strength of the HA formed from the borate glass with 2.5 mol.% Al₂O₃ and 5 mol.% SiO₂ was 11.1 ± 0.2 MPa, which is equal to the highest strengths reported for trabecular bone. The results indicated that simultaneous additions of Al₂O₃ and SiO₂ could be used to control the bioactivity of the borate glass and to enhance the mechanical strength of the HA product. Furthermore, the HA product formed from the glass containing both SiO₂ and Al₂O₃ could be applied to bone repair.     

Transformation of nacre coatings into apatite coatings in phosphate buffer solution at low temperature

Nacre coatings were deposited on Ti6Al4V substrates by electrophoretic technique, and subsequently converted into apatite coatings with hierarchical porous structures by treatment with a phosphate buffer solution. The samples were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, inductively coupled plasma optical emission spectroscopy, X-ray photoelectron spectroscopy (XPS), and N(2) adsorption-desorption isotherms. The results show that the nacre coatings are converted into the plate-like apatite coatings via a dissolution-precipitation reaction, while the organic components of the nacre are reserved. The mesopores with pore size of 4.4 nm are formed within the plate-like structure, and the macropores are formed among the plate-like structure. Simulated body fluid (SBF) immersion tests reveal that the apatite coatings have a good in vitro bioactivity. Bone-like apatite crystals are formed on the surfaces of the apatite coatings after soaking in SBF for 12 h, and fill up the macropores on the coatings with increasing the soaking time. In addition, XPS indicates that a TiO(x) layer and PO(4) (3-) ions appear on the substrate surfaces by pretreatment with a H(3)PO(4)/HF solution. The TiO(x) layer and PO(4) (3-) ions can induce the formation of apatite crystals, resulting in a composition gradient from the oxide layer to the external apatite layer.     

The study of surface transformation of pulsed laser deposited hydroxyapatite coatings

Hydroxyapatite (HA) coatings generally exhibit very good biocompatibility owing to their compositional resemblance to the natural hard tissue and to bioactive properties that are directly related to surface transformations in physiological fluids. In this study, two types of porous HA coatings produced with pulsed laser deposition were tested with respect to their dissolution/reprecipitation in a semidynamic simulated physiological solution. Coatings with higher porosity produced with a 355-nm wavelength laser exhibited significant reprecipitation earlier than those produced with a 266-nm wavelength laser. The dissolution of the non-HA phases played a major role in the reprecipitation of HA-like material as indicated by X-ray diffraction (XRD). The coatings' Ca/P ratio became closer to the theoretical value of HA. The newly formed HA had imperfect crystal structure and/or small crystal size as suggested by XRD. The reprecipitation resulted in a very dense morphology as shown by scanning electron microscopy, suggesting a mechanically strong structure after reprecipitation. Despite undergoing dissolution and reprecipitation, the coatings showed sufficient stability in the solution, as XRD and energy-dispersive X-ray studies indicated no significant loss of the coatings. The stability of these HA coatings and their ability to cause reprecipitation of HA in the simulated physiological solution showed the potential of these coatings for clinical applications.     

Effect of albumin on brushite transformation to hydroxyapatite

Brushite (CaHPO(4) x 2H(2)O) is a precursor to hydroxyapatite [HA, Ca(5)(PO(4))(3)OH]. It has been shown that a modified form of brushite, with potassium substituting for calcium at specific sites, demonstrated accelerated transformation to HA when exposed to nonproteinaceous Hanks' balanced aqueous salt solutions (HBSS). The biocompatibility of a transforming material is related to cellular response to the process, which is initiated by protein adsorption. The effect of adsorbed protein on the kinetics and chemistry of brushite transformation to HA, when exposed to HBSS containing bovine serum albumin (BSA), was examined using Fourier transform IR spectroscopy, X-ray diffraction, and energy dispersive spectrometry techniques. The effect of solution pH was also studied. Results show that, in the presence of a protein-free environment, transformation is faster in buffered medium than in nonbuffered medium. Moreover, curve fitting and second derivatives of the IR spectra show that some bands shift depending on whether the brushite transforms in a buffered or nonbuffered medium. Therefore, variation in pH affects both transformation rate and the associated chemistry. The presence of BSA in either buffered or nonbuffered medium retards the transformation in comparison to the corresponding BSA-free medium. The extent of this retardation increases with the increase in bulk concentration of BSA but does not alter the transformation chemistry. This suggests the retardation on the transformation rate is due to BSA adsorption coverage on the calcium phosphate ceramic. This may be due to the shielding of Ca(2+) and PO(4)(-3) sites, preventing their interaction with the HBSS.     

45S5 bioactive glass surface charge variations and the formation of a surface calcium phosphate layer in a solution containing fibronectin

This study investigated the effect of fibronectin adsorption on surface charge variations and calcium phosphate (Ca-P) layer formation kinetics on the surface of 45S5 bioactive glass (BG). We hypothesize that the adsorption of fibronectin on BG changes the surface charge and alters the kinetics of Ca-P layer formation on the glass surface. The charge at a material's surface modulates surface chemistry, protein adsorption, and interactions with bone cells. The zeta potential of BG in a solution containing human plasma fibronectin (TE-FN) was measured as a function of time by particle electrophoresis, and Ca-P layer formation was characterized using SEM, EDXA, and FTIR. Si, Ca, and P solution concentrations also were determined. It was found that the adsorption of fibronectin reduced the initial electronegativity of the BG surface and delayed the formation of both the amorphous and the crystalline Ca-P layers. The delayed formation of these surface layers may be attributed to the competitive binding of Ca2+ ions by the fibronectin molecule. In addition, the formation of an amorphous Ca-P layer correlated with the reversal from a negatively to a positively charged surface, independent of the presence of fibronectin. The addition of a single protein (in this case fibronectin) can significantly alter material surface parameters, such as charge, and subsequently affect the formation of a surface Ca-P layer. Furthermore, the formation of an amorphous Ca-P layer is an important event in the reactions leading to bioactive behavior, and proteins such as FN are actively involved in the transformation of the surface into a Ca-P layer.     

Critical surface energy of composite cement containing MDP (10-methacryloyloxydecyl dihydrogen phosphate) and chemical bonding to hydroxyapatite

Self-adhesive composite cements are increasingly used for cementing inlays/onlays, intraradicular posts, crowns and laminate veneers. Wider clinical acceptance is driven by simpler and faster handling procedures, much like observed for self-etching adhesives. 10-Methacryloyloxydecyl dihydrogen phosphate (MDP) is a bi-functional monomer incorporated as the reactive ingredient in a contemporary self-adhesive cement. We have examined the surface free energy parameters of this cement and studied the mode of action of the cement on dentine substrate by contact angle measurements to determine the critical surface energy of the cement. Retention of the infrared absorption bands characteristic of the acrylate moieties on the surface of hydroxyapatite particles suggests that MDP contributes to the overall bonding to dentine by forming ionic chemical bonds with surface calcium ions in dentine crystalites.     

Binding affinity of surface functionalized gold nanoparticles to hydroxyapatite

Gold nanoparticles (Au NPs) have been investigated for a number of biomedical applications, including drug and gene delivery vehicles, thermal ablation therapy, diagnostic sensors, and imaging contrast agents. Surface functionalization with molecular groups exhibiting calcium affinity can enable targeted delivery of Au NPs to calcified tissue, including damaged bone tissue. Therefore, the objective of this study was to investigate the binding affinity of functionalized Au NPs for targeted delivery to bone mineral, using hydroxyapatite (HA) crystals as a synthetic analog in vitro. Au NPs were synthesized to a mean particle size of 10-15 nm and surface functionalized with either L-glutamic acid, 2-aminoethylphosphonic acid, or alendronate, which exhibit a primary amine for binding gold opposite carboxylate, phosphonate, or bisphosphonate groups, respectively, for targeting calcium. Bisphosphonate functionalized Au NPs exhibited the most rapid binding kinetics and greatest binding affinity to HA, followed by glutamic acid and phosphonic acid. All functional groups reached complete binding after 24 h. Equilibrium binding constants in de-ionized water, determined by nonlinear regression of Langmuir isotherms, were 3.40, 0.69, and 0.25 mg/L for bisphosphonate, carboxylate, and phosphonate functionalized Au NPs, respectively. Functionalized Au NPs exhibited lower overall binding in fetal bovine serum compared to de-ionized water, but relative differences between functional groups were similar.     

The formation of an antibacterial agent-apatite composite coating on a polymer surface using a metastable calcium phosphate solution

A percutaneous device with antibacterial activity and good biocompatibility is desired for clinical applications. Three types of antibacterial agent: lactoferrin (LF), tetracycline (TC), and gatifloxacin (GFLX) were immobilized on the surface of an ethylene-vinyl alcohol copolymer (EVOH) using a liquid phase coating process. In this process, an EVOH plate was alternately dipped in calcium and phosphate ion solutions, and then immersed in a metastable calcium phosphate solution supplemented with 4, 40, or 400 microg/mL of the antibacterial agent. As a result, the antibacterial agent was immobilized on the EVOH surface in the form of an antibacterial agent-apatite composite layer. The amount of immobilized antibacterial agent increased with increasing absorption affinity for apatite in the order: GFLX相似文献   

Electrodeposition of brushite coatings and their transformation to hydroxyapatite in aqueous solutions.

In this study we examine the potential for using brushite (dicalcium phosphate dihydrate) as a coating for orthopedic implants. It was found that brushite is transformed into a more stable calcium phosphate (hydroxyapatite), regardless of the aqueous transforming media utilized. The transformation was carried out at room temperature in deionized water, deionized water with added calcium ions, and modified Hank's type solution without calcium and magnesium ions, modified Hank's type solution with calcium and magnesium ions, and modified Hank's type solution with added calcium ions. In several of the transformation systems it was noted that brushite provided an increased amount of calcium ions to the transforming media through solubility. The process was monitored using X-ray diffraction, X-ray photoelectron spectroscopy, and dissolution studies. Scanning electron microscopy and energy dispersive spectroscopy were also utilized for characterization. The brushite coatings can be easily achieved electrolytically on conducting substrates. The process is non line of sight, simple, and inexpensive; and when using an electrolyte that can sustain currents of approximately 100 mA/cm2, the necessary coating can be completed in about 3 min.     

Acidification of phosphate buffer in cortical tubules of rat kidney

Summary Rate of tubular hydrogen ion secretion was measured by a new method combining stationary microperfusion methods and the continuous measurement of pH changes of phosphate buffer with an antimony electrode. Significant differences in rate of acidification and transepithelial steady-state hydrogen ion gradients were observed in proximal and distal tubules in various acid-base abnormalities.     

Comparison of plasma-sprayed hydroxyapatite coatings and hydroxyapatite/tricalcium phosphate composite coatings: in vivo study

This study aimed to compare biological properties, including osteoconduction, osseointegration, and shear strength, between plasma-sprayed hydroxyapatite (HA) and HA/tricalcium phosphate (TCP) coatings, using a transcortical implant model in the femora of canines. After 3 and 12 weeks of implantation, the implants with surrounding bone were assessed histologically in undecalcified sections in backscattered electron images (BEIs) under a scanning electron microscope (SEM). After short-term (3 week) follow-up, both coatings conducted new bone formation and revealed direct bone-to-coating contact. The HA/TCP coating could not enhance early host-to-coating responses. At 12 weeks, serious dissolution of the HA/TCP coatings evidently occurred. By the new bone healing index (NBHI) and apposition index (AI), we found no significant difference between HA/TCP-coated implants and HA-coated implants throughout all implant periods. At 12 weeks of implantation, some particles dissociated from the HA/TCP coating were found within the remodeling canal. After push-out measurements, the shear strength and failure mode of HA/TCP-coated implants were similar to those of HA-coated implants, and no statistical differences were found between either coating. Consequently, this study indicates that HA/TCP coatings have excellent biological response and may be considered suitable bioactive ceramic coatings for short-term clinical use.     

FTIR/ATR study of protein adsorption and brushite transformation to hydroxyapatite

Previous study has demonstrated that brushite (CaHPO4 x 2H2O), modified by partial potassium substitution for calcium, can transform quickly into hydroxyapatite (HA, Ca5(PO4)3OH) when exposed to aqueous salt solutions at room temperature. Analyses techniques used in those studies required sample retrieval from solution, which may alter the sample surface. In this work FTIR/ ATR was used in analysis, enabling in situ study of the transformation within the aqueous environment. To test the biocompatibility of this brushite, cellular response to the transformation needs to be understood. Cellular response was initiated by bovine serum albumin adsorption on the brushite surface. The response was studied by monitoring the conformation of the adsorbed protein, which is critical to cellular reaction. This required monitoring the brushite transformation and surface adsorbed protein conformation simultaneously which can be realized using FTIR/ATR. Based on band fitting and second derivative results from the spectra it was found that the conformation of the adsorbed BSA changes during the brushite transformation to HA. This study also demonstrated that the deposition of the brushite could be monitored in real time which offers the possibility for studying surface bonding during electrodeposition.     

Spontaneous growth of a laminin-apatite nano-composite in a metastable calcium phosphate solution

We have previously reported that a laminin-apatite composite layer is formed on an ethylene-vinyl alcohol copolymer (EVOH) in a laminin-containing calcium phosphate (LCP) solution. In this work, the stability of the LCP solution and growth process of the laminin-apatite composite layer have been investigated. Dynamic light scattering technique revealed that the LCP solution was stable for periods as long as 24 h; it did not induce homogeneous precipitation of laminin or calcium phosphates in the solution. Analysis of the EVOH surface and the LCP solution showed that the laminin-apatite composite layer was formed via coprecipitation of laminin and apatite on the EVOH plate, i.e., spontaneous growing of apatite and simultaneous immobilization of laminin molecules or laminin-calcium phosphate nano-complexes onto its surface. Transmission electron microscopy also revealed that the laminin molecules in the resulting composite layer were not localized or aggregated, but were dispersed on a nano-scale in the entire layer. Because of this nano-composite structure, a large number of laminin molecules were stably immobilized on the EVOH plate. This may be responsible for the excellent cell adhesion properties of this type of composite material.     

Behaviour of tricalcium phosphate and hydroxyapatite granules in sheep bone defects

Granules of hydroxyapatite (HA) and tricalcium phosphate (TCP) were implanted in separate holes drilled in mandibular bone of sheep to check the bone growth and in vivo behaviour of the materials. The experiment was performed in three sheep, killed respectively at 4, 8, 12 month. Samples of bone with the materials were explanted, microradiographed and sectioned to evaluate the interface under optical and electron scanning electron microscope. The hole, left empty as a reference, showed no full repair; whereas 4 month after implantation the TCP granules induce total repair of the hole. HA granules crumbled and no new bone induction was seen even 12 month after implantation.     

Bone-bonding ability of a hydroxyapatite coated zirconia-alumina nanocomposite with a microporous surface

Using a combination of hydroxyapatite (HA) coating and microporous surface treatment, bone-bonding ability was given to composites of ceria-stabilized tetragonal zirconia and alumina (CZA), which possesses excellent mechanical and wear properties and phase stability. Four types of CZA plates (2 x 10 x 15 mm3) were prepared for this study, which were CZA with a polished surface (group 1), a microporous surface prepared by hydrofluoric acid and heat treatment (group 2), a microporous surface with a submicron HA coating prepared by alternately soaking the plate from group 2 in aqueous CaCl2/HCl and Na2HPO4 solutions (group 3), and a microporous surface with a 4-microm HA coating prepared by the biomimetic method, where the plates from group 3 were soaked in simulated body fluid (group 4). Plates were implanted into rabbit tibia, and after 4, 8, and 16 weeks, tensile testing and histological examination of the bone-implant interface were conducted. At 4 weeks, group 4 had superior bone-bonding ability compared with other implants, which was maintained at the later postimplantation times. This HA-coated CZA with a microporous surface has the possibility of clinical use as a bearing material in cementless joint prostheses or as a load-bearing bone substitute.     

In vivo response of porous hydroxyapatite and beta-tricalcium phosphate prepared by aqueous solution combustion method and comparison with bioglass scaffolds

Pure hydroxyapatite (HAp) and a biphasic calcium phosphate [containing 90% of beta-tri-calcium phosphate (beta-TCP) and 10% HAp] were tailored through an aqueous solution combustion synthesis. Porous struts were prepared using all the powders along with bioglass, a known bioactive material, and subsequently characterized. Sterilized struts were implanted to the lateral side of radius bone of 24 black Bengal goats of either sex, in which a blank hole was left unfilled in a group of six specimens to act as control. The bone formation response of the three implanting materials in vivo has been studied using scanning electron microscope and histological analysis in contrast with positive controls. Push-out tests were used to assess the mechanical strength at the bone-biomaterial interface. It was observed that interfacial response was strongly dependent on combinations of different physical and chemical parameters. The surface of beta-TCP exhibited similar characteristics of bone and was distinct from those of intervening apatite layer of bioglass. Lower bone ingrowth and reduced strength was observed with HAp compared to beta-TCP/bioglass-based implants. Bone formation response of the Ca-P material varied according to the composition of the implanting material, which could be tailored through this novel synthesis.