Effect of Electrochemical Parameters on the Morphology and Ca/P Ratios of Deposited Apatite Coatings on Metal and Alloy Substrates
Vallerie DeLeon* and Teresa D. Golden
University of North Texas
1155 Union Circle #305070, Denton, Texas 76203
In this research the optimal conditions of electrochemically depositing uniform apatite coatings on stainless steel was investigated. Apatite is a ceramic with many different phases and compositions that have beneficial characteristics for biomedical applications. Of those phases Hydroxyapatite (HA) is the most biocompatible and is the primary constituent of the inorganic material in bones. HA coatings on metal and alloys have the ability to bridge the growth between human tissues and implant technology. The metal provides the strength and the HA provides the needed bioactivity.
One of the major concerns with current implants is after a certain number of years the implant begins to corrode and degrade in the body causing dangerous toxic effects. HA coatings have been applied on recently new implants, however the HA is deposited via plasma spray. Plasma spray is the current method most studied and employed in industry, but is costly, provides non-uniform coatings, and has some uncontrollable variables. Electrochemically deposited coatings lower production costs, provides uniform coatings, and allows for better experimental control. Another major concern with the HA coated implants is the need to determine the appropriate Ca/P ratio, which determines the apatite phase, and morphology needed in order for the implant to be successful. The correct composition and morphology is still being debated and not yet known completely (exception being HA is known to be the major phase). The composition of apatite phases and morphology must be completely compatible for the tissue to adhere and the bone material to grow properly. If it fails to match the coating will degrade and lose its purpose. Electrochemical deposition allows for control and manipulation of the Ca/P ratio and morphology.
The calcium phosphates were electrochemically deposited using a modified simulated body fluid adjusted to different pHs (from 4 to 10) for 1-3 hours at 65°C while maintaining cathodic potentials of -1.3V, -1.5V, and -2.0V (vs. SCE). It was observed that the composition and morphology of HA coatings can be changed during deposition by the concentration of counter ions in solution, pH, temperature, and applied potential. Also post deposition treatment such as, sintering has an effect on the composition of the apatite. Optimum electrochemical conditions give uniform coatings with pure crystalline HA coating and powders after post treatment. The coatings were characterized by powder x-ray diffraction, Fourier transform infrared spectroscopy, energy dispersive x-ray microanalysis, and scanning electron microscopy.
HA has a Ca/P ratio of 1.67 and any ratio greater or less gives other phases of apatite such as brushite, octacalcium phosphate, tricalcium phosphate or others that are not as biocompatible. However if these impurities are in the HA coating their unharmful degradation can be beneficial by assisting the HA to adhere to the surrounding tissues quicker. Therefore, improving the implant acceptance rate, increasing the healing time, reduce corrosion occurrence, improve the longevity of the implant and reduce tissue damage and inflammation. The percent of impurity necessary to improve the application of the implant is a controllable through electrochemical depositionunlike other methods. Therefore, results indicate electrochemical deposition is a better alternative to current methods.
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