Enhanced Cytocompatibility Properties of Hydroxyapatite Doped with Trivalent Ions
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Enhanced Cytocompatibility Properties of Hydroxyapatite Doped with Trivalent Ions Elizabeth A. Massa, Elliott B. Slamovich*, and Thomas J. Webster Department of Biomedical Engineering and * School of Materials Science Engineering, Purdue University, West Lafayette, IN 47907, U.S.A. ABSTRACT Hydroxyapatite (HA) is a bone-like ceramic used as a coating for dental and orthopedic implants. It is well known for its good cytocompatibility properties, but is limited in use due to its high solubility within the body and mechanical properties that differ from surrounding tissue and bone. The present in vitro study investigated a variety of dopants as a way to further enhance the good cytocompatibility properties of HA as well as address these problematic properties. The dopants investigated were divalent (magnesium and zinc) and trivalent (yttrium, lanthanum, indium, and bismuth) ions. Of the trivalent ions, yttrium and indium have smaller ionic radii than calcium. The present study showed osteoblast adhesion to be statistically greater (p < 0.1) on yttrium-doped HA and indium-doped HA than on undoped HA. Since yttrium and indium are both trivalent ions of a smaller ionic radius than calcium, and other ions of different size and charge did not show significant changes in osteoblast adhesion, these results suggest that substituting divalent calcium with a trivalent ion of smaller size encourages osteoblast adhesion. INTRODUCTION Hydroxyapatite (HA; Ca10(PO4)6(OH)2) is a naturally occurring mineral in the body which composes a large part of bone. Synthetic HA, of the same chemical formula, is relatively simple to make and has been shown to bond well with bone. This has led to its use as a coating for orthopedic and dental implants. Coating an implant with HA utilizes HA’s excellent biocompatibility properties in conjunction with the good mechanical properties of a lessbiocompatible material, such as titanium. Using HA in this way promotes osseointegration of an implant, stabilizing it and preventing motion-induced damage to surrounding tissue. However, due to HA’s high solubility rate in vivo and poor mechanical properties, the use of HA as a coating sometimes clinically fails to induce the desired osseointegration [1]. One approach to improving the properties of HA for bone prostheses is to dope it with small amounts of impurities. Doping HA with magnesium and zinc has been shown to alter the solubility and mechanical properties of HA crystals [2]. The present in vitro study investigated a variety of dopants in order to determine a way to further enhance the cytocompatibility properties of HA. The dopants investigated were divalent (magnesium and zinc) and trivalent (yttrium, lanthanum, indium, and bismuth) ions. Magnesium, zinc, and yttrium have been shown to substitute for calcium in the HA lattice [3].
HH3.10.1
EXPERIMENTAL DETAILS Substrate Synthesis HA was synthesized by dripping 1 M calcium nitrate and 0.6 M ammonium phosphate into a solution of distilled water and ammonium hydroxide [3]. Dopants (specifically, magne
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