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Setting Kinetics of Calcium Phosphate Cement by Dielectric Spectroscopy



H J. Mueller, R W. Hirthe


Calcium phosphate cements (CPC) are useful because of their excellent biocompatibility and self-hardening behavior. The setting reaction involving equimolar quantities of tetracalcium phosphate (TTCP, Ca4(PO4)2O) and dicaalcium phosphate anhydrous (DCPA, CaHPO4) with water proceeds by hydrolysis, dissolution, and precipitation of hydroxyapatite (OHAp;Ca5(PO4)3OH) from supersaturated solution. Acidic (H3PO4) and basic (Ca(OH)2) byproducts are neutralized so that the setting reaction occurs close to pH = 7, and is complete at 37 C in 4 h. Since the reaction occurs at a near-constant rate, as determined by titration, inference is that zero order reaction kinetics prevail. Since all aspects of the reaction kinetics and mechanisms for the setting of CPC have not been completely clarified, the purpose of this project was to investigate the setting kinetics of CPC with dielectric spectroscopy (DS).Methods: DS and equivalent electrical circuit (EEC) modeling were used. CPC powder (0.4 g) was mixed with water at a powder/H2O ratio of 3.5. A 12.5 mm diameter by a 2 mm thick disk of the cement was formed at 25 C by initially pressing with light force the mixed, unset paste between the faces of two gold-plated copper disk electrodes positioned inside a poly methylmethacrylate tube of the same diameter. Both electrodes were connected to the input of a Hewlett Packard model 4192 impedance analyzer. Lumped complex impedances (Z*) with a 1 V sinusoidal signal at frequencies between 5 Hz and 13 MHz were obtained by computer every 10 min between 10 min and 8 h from the start of mixing. The measured real (Z') vs. imaginary (Z) components of Z* were plotted and modeled with an EEC. A computer program with complex nonlinear least squares iterations tested the simulation of the chosen EEC to the measured data. The goodness of the simulation was revealed with a chi squared (c2) value and values within the low 10-5 range gave good fits. The EEC with the lowest c2 was chosen.(Extended abstract provides research results and continues this discussion.)
International Symposium on Advanced Materials With Biomedical Applications


calcium phosphate cements, complex plane impedance plots, dicalcium phosphate anhydrous, dielectric spectroscopy, equivalent electrical circuit, hydroxyapatite, reaction order, setting kinetics, tetracalcium phosphate


Mueller, H. and Hirthe, R. (2008), Setting Kinetics of Calcium Phosphate Cement by Dielectric Spectroscopy, International Symposium on Advanced Materials With Biomedical Applications (Accessed April 15, 2024)
Created October 16, 2008