Jacques Lemaître

Can bioactivity be tested in vitro with SBF solution

A large part of the scientific community has accepted the paradigm that a simulated body solution (SBF) can be used to test the bioactivity of a material. This is exemplified by the rapidly increasing number of publications using this test. The aim of this document is to demonstrate that (i) there is presently not enough scientific data to support this assumption, and (ii) even though the assumption was valid, the way the test is generally conducted leaves room for improvement. Theoretical arguments and facts supporting these statements are provided, together with possible improvements of the proposed bioactivity test.

Mechanical characterization of brushite and hydroxyapatite cements

Compression, tension and torsion tests were designed and completed successfully on a brushite and a precipitated hydroxyapatite cement in moist condition. Elastic and strength properties were measured for these three loading cases. For each cement, the full set of strength data was fitted to an isotropic Tsai-Wu criterion and the associated coefficients identified. Since the compressive Youngs moduli were about 10% larger than the tensile moduli, the full set of elastic data of each cement was fitted to a conewise linear elastic model. Hysteresis of the stress-strain curves was also observed, indicating dissipation mechanisms within these cement microstructures. A comparison of the measured mechanical properties with human cancellous bone confirmed the indication of brushite as a bone filling material and the potential of the hydroxyapatite cement as a structural biomaterial.

Calculation of the Solubility Diagrams in the System Ca(oh)2-h3po4-koh-hno3-co2-h2o

A computer program has been developed for calculating the solubility isotherms of sparingly soluble calcium phosphates (including octacalcium phosphate and β-tricalcium phosphate) and calcite in the system Ca(OH)2-H3PO4-KOH-HNO3-CO2-H2O. It allows the influence of such parameters as temperature, pH, partial CO2 pressure and ionic strength to be investigated. The calculation process takes into account the effects of ion-pair formation and ionic strength. Selected solubility isotherms are presented and compared to literature data. The influence of temperature, Ca/P ratio, ionic strength and CO2 pressure on the stability isotherms of hydroxyapatite and dicalcium phosphate are discussed in detail.

Biomechanical characterization of a biodegradable calcium phosphate hydraulic cement: A comparison with porous biphasic calcium phosphate ceramics

Biomechanical properties of a biodegradable calcium phosphate hydraulic cement (CPHC) were tested with rabbits. The cement was composed of beta-tricalcium phosphate (beta-TCP), monocalcium phosphate monohydrate (MCPM), and calcium sulfate hemihydrate (CSH), beta-TCP-MCPM-CSH cement. Cylinders of 4.7 mm in diameter and 10 mm in length were put into bone cavities created in the distal epiphysis of femurs in rabbits. Cylinders of the same size of porous biphasic calcium phosphate ceramics (BCPC, 75% hydroxyapatite and 25% beta-TCP) were implanted as references. Two, 4, 12, and 16 weeks after the operation, the rabbits were sacrificed. Histomorphometry showed that the cement was resorbed, leaving only 7.67 +/- 1.81% of bone cavity after 12 weeks. Newly formed bone occupied 34.59 +/- 4.00% of the cavity. Cylindrical bone-material composites were cut out with a small dental burr. Compressive force was applied to the specimens and compressive strength, elastic modulus, and toughness were calculated. The same tests were performed on cylinders of normal bone from the same site, which served as controls. The compressive strength and the toughness of the cement-bone composite were higher than those of normal bone and porous ceramics 12 weeks after the operation (p < 0.05). At 16 weeks the compressive strength and the toughness returned to the normal bone values. The elastic modulus of the porous ceramic-bone composite was higher than the normal bone at 4, 12, and 16 weeks after surgery (p < 0.05). We found that the beta-TCP-MCPM-CSH cement is replaced by new bone and that the cement-new bone composite has similar or better mechanical properties than normal bone within 16 weeks. This study suggests the usefulness of a particular cement for filling bone defects or for temporary fixation of orthopedic implants.

Hydroxyapatite cement scaffolds with controlled macroporosity: fabrication protocol and mechanical properties

Precipitated hydroxyapatite cement scaffolds with macroporosity of controlled size and shape were successfully built using a solid freeform fabrication process. The negative macroporosity was designed using CAD software and built with an inkjet machine. A slip casting process using a plaster mold was used to precipitate the cement and manufacture samples for mechanical testing. The negative macroporosity was then extracted from the samples using a thermal process. Compression, tension and torsion tests were completed successfully on the precipitated hydroxyapatite cement samples in moist condition. Elastic and strength properties were measured for these three loading cases and compared to the plain hydroxyapatite cement properties reported in a previous study. Homogenization theory was applied to estimate the elastic properties of the manufactured scaffolds. A good correlation was obtained between the experimental data and the theory.

Raman Microspectrometry Studies of Brushite Cement: In Vivo Evolution in a Sheep Model

Calcium phosphate hydraulic cements are promising synthetic bone grafting materials. Brushite-based cements were implanted for 6 and 12 months in the distal condyle of sheep femur, and their in vivo evolution was investigated by Raman microspectrometry. This new technique can probe small volumes in the cubic micrometer range. Its resolution allows a very fine analysis of crystalline changes in calcium phosphate mixtures at the microscopic level. First, Raman spectra of pure brushite, monetite, and beta-tricalcium phosphate (beta-TCP) were recorded, in order to set a data base for the basic components of brushite cements. These spectra show significant differences in the vibration mode v1 for the phosphate ion (988 and 878 cm(-1) for brushite, 988 and 900 cm(-1) for monetite, 968 and 948 cm(-1) for beta-TCP). These differences are strong enough as to allow the qualitative and quantitative analysis of these crystalline phases in the cement. Implanted sheep femur samples were harvested after 24 and 52 weeks post-op, and prepared for Raman analysis in the form of 1-mm-thick sections. Implants at 24 weeks show a core of residual cement isolated from the surrounding bone by fibroconnective tissue. No trace of brushite was detected by micro-Raman analysis in this area, but instead, a mixture of beta-TCP and Type-B carbonated apatite, the latter being very close in composition and structure to the mineral fraction of normal bone in the vicinity of the implant. Implants recovered after 52 weeks show a decrease of the bone/residual cement perimeter, whereas new trabeculations are formed in the implanted zone; the small amounts of residual cement still present are substantially transformed into Type-B carbonated apatite containing small amounts of proteins. In the same area, some beta-TCP particles are also detected showing that, contrary to brushite, the excess beta-TCP originally present in the cement is not completely metabolized. In the implanted zone already converted into trabecular bone, Raman microspectrometry shows the characteristic spectrum of normal bone.

Composition effects on the pH of a hydraulic calcium phosphate cement.

The pH of a hydraulic calcium phosphate cement (HCPC) made of monocalcium phosphate monohydrate (Ca(H2PO4)2·H2O; MCPM), β-tricalcium phosphate (β-(Ca3(PO4)2; β-TCP) and water was measured as a function of reaction time and composition at room temperature. During setting, the cement pH varies from very acidic pH values, i.e., 2.5, to almost neutral pH values, i.e., 6. The cement pH profile significantly depends on the initial cement composition. However, all profiles are characterized by a sharp initial decrease of the pH due to the dissolution of MCPM crystals and the precipitation of dicalcium phosphate dihydrate (CaHPO4· 2H2O; DCPD) crystals. With an excess of MCPM, the final pH stays low, and its value can be predicted from the initial composition of the cement and solubility data. With an excess of β-TCP, the end pH is close to 5, which is much lower than 5.9, the value predicted by calculation. Results suggest that the difference may be due to the presence of impurities in the cement. Replacing MCPM by phosphoric acid renders the cement paste very acidic for the initial 30 s, but then the pH profile follows that obtained with MCPM. Adding pyrophosphate ions into the cement paste postpones the position of the pH minimum. The delay, which is proportional to the concentration of pyrophosphate ions, is thought to be due to the inhibiting action of pyrophosphate ions on the precipitation of DCPD crystals.

Mechanical characterization of brushite cements : A Mohr circles' approach

Dry and wet brushite cements with various solid/liquid ratios were tested in compression and tension. Two different testing techniques were used to determine tensile strength: Direct Tensile test (DT) and Diametral Compression test (DC) (Brazilian test), which is an indirect way of measuring tensile strength on brittle materials. Statistical analysis of the results obtained on dry cements points out a constant ratio between the values measured by DT and Brazilian tests (DC/DT = 85%). The Mohrs circles representation allows us to understand that, for a material like our cement, ultimate stress measured with the Brazilian test can only underestimate tensile strength, because the compressive/tensile strength ratio is lower than 8. The second consequence of this low ratio is that, in the Brazilian test, the plane along which fracture initiates undergoes not only a normal tensile stress, but also a tangential stress component. Thus, the state of stress on the fracture plane differs from the one taking place in the direct tensile test. Consequently, with such a material (sigma(c)/sigma(t) < 8), the Brazilian test does not estimate the true tensile strength.

Effect of several additives and their admixtures on the physico-chemical properties of a calcium phosphate cement

Combinations of citrate (C6H5O73-−), pyrophosphate (P2O74−) and sulfate (SO42−) ions were used to modify the physico-chemical properties of a calcium phosphate cement (CPC) composed of β-tricalcium phosphate (β-TCP) and phosphoric acid (PA) solution. The results obtained with only one additive at a time are similar to those previously published. New facts are: the positive effect of C6H5O73− ions on cement failure strain and their negative effect on cement pH. The position of the setting time maximum measured at an SO42− concentration of 0.09 M was not displaced by the addition of C6H5O73− and P2O74− ions. However, the effect of SO42− ions on the setting time was depressed by C6H5O73− ions. Moreover, no increase in tensile strength was observed when increasing amounts of SO42− were added into a C6H5O73−-containing cement. The latter results suggest a competitive effect of C6H5O73− and SO42− on setting time and tensile strength. Anhydrous dicalcium phosphate (DCP; CaHPO4) appeared in cement samples dried just after setting, but not in cement samples incubated for 24 h in deionized water before the drying step. It is believed that the setting reaction is stopped by the drying step, leaving a low internal pH in the sample, hence providing favorable conditions for the transformation of dicalcium phosphate dihydrate (DCPD) into DCP. Interestingly, even though C6H5O73− ions dramatically lowered the equilibrium pH of the cement with 5 ml of deionized water, they still prevented the occurrence of the transformation of DCPD into DCP.

Synthesis of nickel hydroxide powders by urea decomposition

Abstract Synthesis of nickel hydroxide from aqueous solutions by decomposition of urea was investigated. Spherical agglomerates of nanocrystalline particles were obtained. Precipitated powders show turbostratic α-phase with significant carbonate intercalated into the structure. Addition of dispersant HPMC increases the specific surface area and reduces the median agglomerate size, whereas increase in aging time increases the precipitation yield considerably.