Solubility Properties of Hydroxyapatite Doped with Divalent and Trivalent Ions
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Solubility Properties of Hydroxyapatite Doped with Divalent and Trivalent Ions Jennifer L. Smith, Elizabeth A. Massa-Schlueter*, and Thomas J. Webster* School of Chemical Engineering and *Department of Biomedical Engineering, Purdue University, West Lafayette, IN 47907, U.S.A. ABSTRACT Hydroxyapatite (HA) is a bone-like ceramic with good cytocompatibility properties and, consequently, has been used as coatings for dental and orthopedic implants. However, the main limitation of using HA as a coating material is its high calcium dissolution rate. The present study investigated an assortment of dopants in order to lower the calcium dissolution rate of HA. The HA dopants used in the present study were divalent (magnesium and zinc) and trivalent (yttrium, lanthanum, indium, and bismuth) cations. In a 21 day study, undoped HA and HA doped with magnesium (Mg), lanthanum (La), and bismuth (Bi) were found to increase in mass while formulations doped with zinc (Zn), indium (In), and yttrium (Y) were found to decrease in mass. Decreased mass may be due to loss of a number of chemical groups (such as calcium, phosphorous, and hydroxyl groups). Studies were also performed to specifically evaluate calcium dissolution. The divalent and trivalent-doped HA averaged samples had calcium concentrations in the supernatant consistently below the undoped HA samples suggesting lower calcium dissolution rate for all doped formulations. The results of the present study demonstrated that of those dopants tested that increased in mass, HA doped with Bi had the lowest amount of calcium released into the supernatant media. Since calcium has been shown to increase adsorption of proteins important for new bone growth, the present study provided evidence that to decrease the calcium dissolution rate of HA, dopants should be used, particularly Bi. INTRODUCTION Hydroxyapatite (HA; Ca10(PO4)6(OH)2) is a ceramic that has been studied and used for dental and orthopedic implants. It is naturally found in the body, composing a large part of bone and has excellent biocompatibility properties. For orthopedic implants, HA is used to coat titanium which has good mechanical properties. However, due to high calcium solubility in vivo, sometimes the HA coating has been found to partially or completely vanish before osseointegration has occurred thus causing the implant to loosen [1]. Reports have established that HA coatings are degraded by dissolution of many elements (including calcium, phosphorous, hydroxyl groups) in extracellular fluids [2]. Moreover, studies have implicated increased desorption of calcium (an element which has been shown to increase adsorption of vitronectin, a protein that enhances bone cell function) in HA is a major problem [2-4]. A possible approach to improving the properties of HA for use in the body is to dope it with small amounts of impurities. Doping HA with magnesium or zinc has been shown to alter calcium solubility [3]. The HA dopants investigated in the present study were divalent (magnesium and zinc) and trivalent (
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