Functionalization of polycaprolactone/hydroxyapatite scaffolds with Usnea lethariiformis extract by using supercritical CO2.

Investigation of an integrated supercritical fluid extraction and supercritical solvent impregnation process for fabrication of microporous polycaprolactone-hydroxyapatite (PCL-HA) scaffolds with antibacterial activity is presented. The HA content and particle size as well as the operating conditions of the integrated process is optimized regarding the amount of impregnated antibacterial agent (Usnea lethariiformis extract) in the PCL-HA matrix, scaffold morphology and antibacterial activity against methicillin resistant Staphylococcus aureus (MRSA) strains. High pressure differential scanning calorimetry (HP-DSC) assay reveals that an increasing amount of HA results in decreasing melting temperature as well as crystallinity at an operating pressure of 17 MPa. The PCL-HA composites with micrometric sizes of the HA particles are convenient for being processed by the integrated process due to the simple preparation, a good interaction between the PCL matrix and filler and the advantageous impact on sorption. The scaffold obtained from PCL-HA with 20% of the HA shows the highest impregnation yield at 17 MPa and 35 °C (5.9%) and subsequently also the best bactericidal effect on the tested MRSA strains at an initial bacterial inoculum of 2 × 10(-4)CFU/mL.

Application of impedance spectroscopy to evaluate the effect of different setting accelerators on the developed microstructures of calcium phosphate cements.

The main goal of the present study was to evaluate the effect of different setting accelerator agents on the developed microstructures of calcium phosphate cements (CPCs) by employing the impedance spectroscopy (IS) technique. Six compositions of CPCs were prepared from mixtures of commercial dicalcium phosphate anhydrous (DCPA) and synthesized tetracalcium phosphate (TTCP) as the solid phases. Two TTCP/DCPA molar ratios (1/1 and 1/2) and three liquid phases (aqueous solutions of Na(2)HPO(4), tartaric acid (TA) and oxalic acid (OA), 5% volume fraction) were employed. Initial (I) and final (F) setting times of the cement pastes were determined with Gillmore needles (ASTM standard C266-99). The hardened samples were characterized by X-ray powder diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and apparent density measurements. The IS technique was employed as a non-destructive tool to obtain information related to porosity, tortuosity and homogeneity of the cement microstructures. The formulation prepared from a TTCP/DCPA equimolar mixture and OA as the liquid phase presented the shortest I and F (12 and 20 min, respectively) in comparison to the other studied systems. XRD analyses revealed the formation of low-crystallinity hydroxyapatite (HA) (as the main phase) as well as the presence of little amounts of unreacted DCPA and TTCP after 24 h hardening in 100% relative humidity. This was related to the proposed mechanisms of dissolution of the reactants. The bands observed by FTIR allowed identifying the presence of calcium tartrate and calcium oxalate in the samples prepared from TA and OA, in addition to the characteristic bands of HA. High degree of entanglement of the formed crystals was observed by SEM in samples containing OA. SEM images were also correlated to the apparent densities of the hardened cements. Changes in porosity, tortuosity and microstructural homogeneity were determined in all samples, from IS results, when the TTCP/DCPA ratio was changed from 1/1 to 1/2. The cement formulated from an equimolar mixture of TTCP/DCPA and OA as the liquid phase presented setting times, degree of conversion to low-crystallinity HA and microstructural features suitable to be used as potential bone cement in clinical applications. The IS technique was shown to be a very sensitive and non-destructive tool to relate the paste composition to the developed microstructures. This approach could be very useful to develop calcium phosphate bone cements for specific clinical demands.

Synthesis of tetracalcium phosphate from mechanochemically activated reactants and assessment as a component of bone cements.

The aim of this work was to gain a better understanding about the synthesis of tetracalcium phosphate (TTCP, Ca(4)(PO(4))(2)O) through a solid-state reaction from mechanochemically activated CaCO(3)-(NH(4))(2)HPO(4) mixtures. The evolution of the reaction was followed by DTA, XRD, FTIR and SEM techniques. An enhanced reactivity of the mixtures was detected as the mechanochemical treatment times increased. This effect was related to both the loss of crystallinity of the reactants and the production of defects on their surfaces. 6 h of mechanochemical processing at 1190 rpm, followed by 3 h of thermal treatment at 1500 degrees C, were enough to obtain pure TTCP. The crystallinity and purity of the obtained TTCP were checked by XRD and FTIR. The morphologic characteristics were analyzed by SEM and BET analysis. The behavior of synthesized TTCP powder in combination with commercial dicalcium phosphate anhydrous (DCPA, CaHPO(4)), as the solid phase of bone cements, was tested. Both the combination of different particle sizes of TTCP and DCPA and the effect of different kinds of accelerator agents (disodium hydrogen phosphate, tartaric acid, citric acid and oxalic acid) on setting time and degree of conversion to hydroxyapatite (HA, Ca(10)(PO(4))(6)(OH)(2)) were evaluated. The combination of TTCP (0.32 m(2)/g) with DCPA (1.52 m(2)/g), in a 1/1 molar ratio, showed the shortest setting times and high conversions to HA when an oxalic acid solution (5% volume fraction) was used as the liquid phase of the formulation. Results obtained from this work demonstrated that synthesized TTCP shows promising behavior as a component of bone cements, exhibiting not only a smaller particle size than that usually reported but also a low degree of crystallinity, all of which increases the reactivity of the obtained TTCP. This study provided a very efficient method for synthesizing pure TTCP through a modified solid-state reaction from mechanochemically activated reactants, employing very short times of thermal treatment in comparison with the conventional processes.

Hydrolytic stability of experimental hydroxyapatite-filled dental composite materials.

OBJECTIVES: The purpose of this study was to analyze the behavior in water, related to mechanical properties, of experimental composites for dental restoration. METHODS: The studied materials were composed of a visible-light-curing monomer mixture (Bis-GMA and TEGDMA or HEMA) and micrometric, nanometric or a mixture of both sizes hydroxyapatite particles as a reinforcing filler. Filler particles were modified with a coupling agent (citric, hydrosuccinic, acrylic or methacrylic acid or silane). The hydrolytic stability of the evaluated materials was studied through total elution and water-uptake tests. Percent net-mass variation was daily monitored and analyzed as a function of time. Mechanical performance was examined through flexural properties and Vickers hardness. Morphological surface changes were observed with scanning electron microscopy. ANOVA statistical analysis was performed (P<0.05). RESULTS: In general, the use of HEMA instead of TEGDMA did not substantially worsen the composite quality. Dental composites containing only nanometric particles of hydroxyapatite as a filler are unsuitable for clinical performance. Midway-filled composite resins loaded with micro-HAP particles, coated with citric, acrylic or methacrylic acid displayed low percent elution and water-uptake values. Mechanical properties were similar or even superior to those measured for silane treated particles. SIGNIFICANCE: More research is needed to further improve the interaction of nano-HAP particles with the polymeric matrix, either as a single filler or, preferentially, mixed with micro-HAP, that will allow to increase the total loading of reinforcing filler and, hence, to improve the mechanical properties.

Dental composites reinforced with hydroxyapatite: mechanical behavior and absorption/elution characteristics.

The purpose of this study was to analyze the behavior in water as well as the mechanical and surface properties of experimental composites designed for dental restoration. Studied materials were composed of a visible-light-cured monomer mixture as a matrix (bisphenol-alpha-glycidyl methacrylate with triethyleneglycol dimethacrylate or hydroxyethyl methacrylate) and either micrometric or nanometric hydroxyapatite (HA) particles as a reinforcing filler. The surface of the filler particles was modified by using different coupling agents (citric, hydroxysuccinic, acrylic, or methacrylic acid). The hydrolytic stability of the evaluated materials was studied through elution-in-water and water-uptake tests. Mechanical and surface properties were examined through the results of flexural, hardness, and surface roughness tests. Means and standard deviations were calculated for each variable. Analysis of variance and multiple comparison tests were performed. Materials containing bisphenol-alpha-glycidyl methacrylate:triethyleneglycol dimethacrylate and micrometric-HA coated with citrate, acrylate, or methacrylate displayed the most favorable results. Improvements should be obtained by increasing the total filler amount, and by the introduction of nanometric-HA filler into a micrometric-HA reinforced composite resin system.

Polymethylmethacrylate-based bone cement modified with hydroxyapatite.

A commercial acrylic bone cement was modified by the incorporation of different weight fractions of polycrystalline hydroxyapatite (HA), and the modified formulation was investigated. The influence of the filler proportion on the flow characteristics and the mechanical behavior of the resultant composite was evaluated. The residual monomer present in the cured materials was measured by gas chromatography. The comparison of the residual monomer present in the cements with and without reinforcement demonstrated that the degree of polymerization was not affected by the addition of HA. Porosity morphology was analyzed by optical and scanning electron microscopy. Image examination revealed that the porosity and the pore size of the hardened cement increased with an increasing amount of particulate filler. Flexural, compressive, and fracture properties of the cement with varying amounts of HA reinforcement were measured. It was found that up to 15 wt% HA could be added for increases in flexural modulus and fracture toughness. HA acts as a rigid filler that enhances fracture resistance and flexural modulus. Our results show that the workability of the modified formulation limited the incorporation of the ceramic filler to a maximum value of 15 wt%.

Influence of temperature and additives on the microstructure and sintering behaviour of hydroxyapatites with different Ca/P ratios.

Sintering of two hydroxyapatite (HA) samples with different Ca/P ratios was studied as a function of thermal pretreatments, sintering temperature and additives (0-0.6 ion % Li+ or 0-5 ion % Mg2+). The samples were sintered in air and characterized by density measurements, scanning electron microscopy, differential thermal analysis, X-ray diffraction and dilatometry. Upon sintering, samples with Ca/P ratio of 1.51 (HA C) transformed to beta-Ca3(PO4)2 and Ca10(PO4)6(OH)2, resulting in materials with low densities and containing agglomerated beta-Ca3(PO4)2 when sintered above 1200 degrees C. Samples with a Ca/P ratio of 1.77 (HA S), without beta-Ca3(PO4)2, showed better sinterability and homogeneous microstructures. Li+ additions favoured liquied-phase sintering and reduced the beta-Ca3(PO4)2 content in sintered materials. Mg2+ additions did not result in higher densities, but inhibited the hydroxyapatite grain growth rate. A significant percentage of the added Mg2+ was incorporated into the beta-Ca3(PO4)2 structure.