HAp Samples Research Articles

Role of surface charge and wettability on early stage mineralization and bone cell–materials interactions of polarized hydroxyapatite

Our objective was to determine the role of surface charge and wettability on early stage mineralization as well as bone cell adhesion and proliferation on polarized HAp surface. To estimate the surface wettability, contact angles were measured in water, simulated body fluid (SBF) and Dulbecco’s modified Eagle’s medium/nutrient mixture F-12 Ham (DMEM). Experimental results show that HAp surface wettability and surface energy can be tailored by inducing surface charge without introducing any volumetric effects in the material. Increasing the surface charge increased the wettability and also the energy of HAp surfaces in all tested media. A maximum surface energy of 49.47 ± 3.76 mJ/m 2 was estimated for positively charged HAp surfaces polarized at 400 oC. The in vitro bioactivity of polarized HAp samples was evaluated by soaking in SBF and DMEM (cell media). Cell–materials interaction was studied by culturing with human fetal osteoblast cells (hFOB). In vitro results show that tailoring the combined effect of wettability and charge polarity on the HAp surface enable differential binding of inorganic ions (e.g., Ca 2+, Cl −, Na +, HCO 3 − etc) and organic cell adhesive proteins (e.g., fibronectin, vitronectin etc) with different surface properties, which results in accelerated or decelerated mineralization as well as cell adhesion and proliferation on polarized HAp surface.

Nanocrystalline hydroxyapatite powders by a chitosan–polymer complex solution route: Synthesis and characterization

Nanocrystalline hydroxyapatite (HAp) powders were successfully synthesized by a simple method using chitosan–polymer complex solution. To obtain HAp nanopowders, the prepared precursor was calcined in air at 400–800 °C for 2 h. The phase composition of the calcined samples was studied by X-ray diffraction (XRD) technique. The XRD results confirmed the formation of HAp phase with a small trace of monotite phase. With increasing calcination temperature, the crystallinity of the HAp increased, showing the hexagonal structure of HAp with the lattice parameter a in a range of 0.94030–0.94308 nm and c of 0.68817–0.68948 nm. The particle sizes of the powder were found to be 55.02–73.36 nm as evaluated by the XRD line broadening method. The chemical composition of the calcined powders was characterized by FTIR spectroscopy. The peaks of the phosphate carbonate and hydroxyl vibration modes were observed in the FTIR spectra for all the calcined powders. TEM investigation revealed that the prepared HAP samples consisted of rod-like nanoparticles having the particle size in the range of 100–300 nm. The corresponding selected-area electron diffraction (SAED) analysis further confirmed the formation of hexagonal structure of HAp.

Mechanical Characterization of Hydroxyapatite-Based, Organic-Inorganic Composites

Hydroxyapatite-based materials have been used for dental and biomedical applications. Newly developed synthesis techniques give cause to a broad field in the study of these materials and industry demands products with better properties day by day. The purpose of the present work was to evaluate the mechanical properties of hydroxyapatite-based (HAp-based), organic-inorganic composites. HAp-based, organic-inorganic composites were obtained by modified gel casting process and organic molecules in a gelatin solution. HAp samples of different sizes and shapes were obtained with controlled micro and macro porosity and then were immersed into several gelatin solutions with different concentrations. X-ray powder Diffraction (XRD), Infra Red (IR) Spectroscopy and Scanning Electron Microscopy (SEM) techniques were used to analyze samples before and after gel casting process in order to assure that chemical and physical properties remains the same after this process. IR Spectroscopy and SEM techniques were used to characterize samples after the introduction of organic phase in order to analyze the final morphology of samples. Mechanical characterization was made in compression mode to samples without and with different concentrations of organic phase in order to establish the optimum conditions in which the highest compressive strength and Young’s modulus is reached.

Colloidal processing and sintering of hydroxyapatite

Abstract This paper describes the preparation of hydroxyapatite ceramics through slip casting and pressureless sintering. Based on the surface properties of HAp powder, a combination of two components, polyacrylic acid sodium salt (PAA-Na) and l -glutamic acid (GA), was used as the dispersing agents. HAp slurries with the solid content as high as 63 vol% could be prepared in the presence of the dispersants. After slip casting, green samples exhibited homogeneous microstructure. The samples were further densified at different temperature and soaking time. The microstructure, dehydroxylation behavior and mechanical properties were evaluated. In consideration of dehydroxylation, grain growth and blowhole development, it was found that 1250 °C with 2 h soaking is appropriate for sintering of HAp. Results showed that it was feasible to introduce l -glutamic acid as the second dispersant for improving the slurry stability and making more reliable HAp samples.

Aqueous processing of hydroxyapatite

This paper describes a study of the dispersion of hydroxyapatite (OHAp) powder in aqueous media. The slurry stability is achieved by utilizing a combination of a commercially available dispersant, polyacrylic acid sodium salt (PAA-Na) and ethylenediaminetetraacetic acid, tetrasodium salt, dihydrate (EDTA·4Na·2H 2O). Adsorption, sedimentation and rheology measurements have been performed to investigate the suspensions properties. It was shown that HAp slurries could be stabilized at pH 10.50 with the PAA-Na concentration around 0.45 wt%. Rheological measurement confirmed that well dispersed slurries could be obtained with the solid content as high as 63 vol%. After sintering, the relative density of HAp sheets reached 98.8%. The as sintered HAp samples exhibited excellent properties for cell attachment and proliferation. Results showed that the aqueous processing technique is effective for preparation HAp samples for biological applications.

Surface properties of various powdered hydroxyapatites

Electrophoretic mobilities of various synthetic and semisynthetic hydroxyapatites (Ca 10(PO 4) 6(OH) 2, HAP) suspended in aqueous solutions have been measured as a function of pH and calcium concentration. The studied powders differ in particle size, crystallinity degree and surface contamination (carbonate). When equilibrated in mineral acids or bases, a large plateau of negative mobility is observed in the pH range 5–8, with increasing negative values at higher pH. Only in the case of the sample composed of nanoparticles, positive mobility obtains at pH < 8.9. When Ca 2+ is added, positive mobility values are observed for all samples, and a bell-shaped profile results as a function of pH. Two possible models are explored to describe the results: the Nernstian approach, which assumes solubility equilibrium and surface potentials determined by the three potential-determining ions (Ca 2+, PO 3− 4, and OH −), and the surface complexation approach, based on the idea of negligible phase transfer of structural phosphate. The Nernstian model is inadequate, whereas a very simple surface complexation model based on the equations Ca 5(PO 4) + 3 = Ca 4(PO 4) − 3 + Ca 2+, Ca 4(PO 4) − 3 + H + = Ca 4(PO 4) 2(PO 4H), Ca 5(PO 4) + 3 + OH − = Ca 5(PO 4) 3(OH), coupled with a very simple electrical double layer, model suffices to reproduce the bell-shaped profile of the mobility as a function of pH in the presence of added calcium salts. The results also show that the sample composed of nanoparticles exchanges ions more easily with the solution, without reaching the solubility equilibrium in the explored timespans. In the presence of soluble phosphate salts, it is postulated that the same surface ensembles define the surface charge, with participation of phosphate as described by the equation 4 5 Ca 5(PO 4) + 3 + 3 5 PO 3− 4 = Ca 4(PO 4) − 3. HAP is just one member of a family of calcium phosphates with different (Ca)/(P) ratios. Electrophoretic mobilities of another member, tricalcium diphosphate, Ca 3(PO 4) 2, were also measured and shown to be described by the same basic model. Comparison with previous literature data shows that the negative plateau in the mobility is a general feature of many HAP samples at low Ca 2+, again in agreement with the surface complexation model. FTIR data demonstrates that surface phosphate indeed undergoes protonation, as postulated in the model.

Preparation of Translucent Hydroxyapatite Ceramics by HIP and Their Physical Properties

The translucent and colorless calcium and strontium hydroxyapatite (Ca, Sr)-HAp ceramics with compositions of (Ca10-10xSr10x)(PO4)6(OH)2, where x=0-0.8, were successfully prepared by hot isostatic pressing (HIP). The physical properties of obtained samples with x=0-0.8 were investigated. The lattice constants and hexagonality, c/a, of (Ca, Sr)-HAp samples gradually increased with increasing Sr content and were in good agreement with the data listed in JCPDS cards. The relative density for all samples HIP-treated was in the range of 96.4 to 99.7%. The values of Vickers hardness, HV, for all samples were almost constant in the range 5.5-5.9GPa regardless of the change in x, exhibiting the good approximation to the HV values of natural teeth. From the FE-SEM observations, the obtained samples had almost pore free microstructures consisting of about 1.6-1.9μm HAp grains. Judging from FT-IR measurements, there is the possibility that the (Ca, Sr)-HAp ceramics HIP-treated are applicable to special uses such as optical matched filters.

Vapor Phase Adsorption of Water on Hydroxyapatite

Water vapor adsorption isotherms were determined gravimetrically on three hydroxyapatite sampel differing in preparation and with surface areas of 70.4, 22.5, and 3.0 M2/gm, respectively. Heats of adsorption for the first layer of water were found to be 13.3, 13.2, and 13.9 kcal/mole on these hydroxypatites. From repeated thermal desorption and adsorption cycles of water, stepwise adsorption was observed which diminished with each outgassing cycle until it disappeared after the fourth cycle. Cross-sectional area of adsorbed water molecule on hydroxyapatite surface was estimated at 11.5 A2. Standard free energies, isosteric heats, changes in enthalpy, and entropy of adsorption of water on HAP samples outgassed at 300 C were determined.