PubMed · 8794032
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A P Yoganathan. 1996. An engineer's perspective.. https://pubmed.ncbi.nlm.nih.gov/8794032/
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The calcium phosphate cement (CPC) used in this study was formed by combining equimolar amounts of tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA). This powder, when mixed with water, sets to a hard cement in about 30 min. However, the water-based CPC paste is not highly cohesive and is vulnerable to washout until hardening occurs. The objectives of this study were to investigate the effects on handling properties, washout resistance, cement hardening behavior, and mechanical properties of adding several gelling agents to CPC paste. Aqueous solutions that contained a mass fraction of 2-4% of hydroxypropyl methylcellulose (HPMC), carboxyl methylcellulose (CMC), chitosan acetate, and chitosan lactate were used as cement liquids. Hardening time was measured by the Gilmore needle test; resistance to washout was evaluated by the disintegration of the cement specimen in water with agitation; and mechanical strength was evaluated by the measurement of diametral tensile strength and compressive strength. Handling properties were greatly improved by the addition of HPMC, CMC, chitosan acetate, and chitosan lactate. Hardening time was retarded by the additions of HPMC and CMC, and mechanical strength was weakened by the addition of either the chitosan lactate or the chitosan acetate.
The kinetics of stoichiometric (Ca/P = 1.67) and calcium-deficient (Ca/P = 1.5) hydroxyapatite formed in aqueous solution by acid-base reactions involving CaHPO4 and Ca4(PO4)2O were determined. Complete reaction occurs within 6 h at 37.4 degrees C regardless of composition with stoichiometric hydroxyapatite forming more rapidly. Stoichiometric hydroxyapatite formed more rapidly because the particle sizes of its precursors were smaller. Hydroxyapatite formation is characterized by an initial period of surface hydration of the precursors, an induction period, and a period during which the bulk of the conversion to hydroxyapatite occurs. During the first 3 h of reaction at 37.4 degrees C, the pH is about 8.25 and 7.6, respectively, as the stoichiometric and calcium-deficient hydroxyapatite are being formed. Subsequently the pH values move toward those of the related invariant points: Ca10(PO4)6(OH)2-Ca(OH)2 solution, and CaHPO4-Ca9HPO4(PO4)5OH solution. The concentrations of calcium and phosphate in solution never exceed those in serum. Seeding with 5 wt% hydroxyapatite eliminates induction regardless of composition. The kinetics are first-order and follow the Arrhenius relationship regardless of composition. The total heats of reaction (delta Hr) were determined at constant temperatures between 25 degrees C and 70 degrees C. delta Hr values of 261.3 and 320 kJ/mol were determined for the formation of calcium-deficient hydroxyapatite and stoichiometric hydroxyapatite, respectively. Activation energies of 84.5 and 87.4 kJ/mol were calculated for the formation of calcium-deficient and stoichiometric hydroxyapatite, respectively. Heats of formation for Ca4(PO4)2O and Ca9HPO4(PO4)5OH were calculated to be -4764.1 and -12707.7 kJ/mol, respectively.
The high corrosion resistance and good biocompatibility of titanium and its alloys are due to a thin passive film that consists essentially of titanium dioxide. There is increasing evidence, however, that under certain conditions extensive titanium release may occur in vivo. An ion-beam-assisted sputtering deposition technique has been used to deposit thick and dense TiO2 films on titanium and stainless steel surfaces. In this study, using the following measurements these TiO2 films have been investigated in a phosphate-buffered saline solution: (1) open-circuit potential versus time of exposure, (2) electrochemical impedance spectroscopy, (3) potentiodynamic polarization, and (4) Mott-Schottky plot. A higher electrical film resistance, lower passive current density, and lower donor density (in the order of 10(15) cm-3) have been measured for the sputter-deposited oxide film on titanium in contrast to the naturally formed passive oxide film on titanium (donor density in the order of 10(20) cm-3). The improved corrosion protection of the sputter-deposited oxide film can be explained by a low defect concentration and, consequently, by a slow mass transport process across the film. As opposed to TiO2 on titanium, a deviation from normal n-type semiconducting Mott-Schottky behavior was observed for TiO2 on stainless steel.