Evidence of chemical bonding at biomaterial-hard tissue interfaces

For many years, glass-polyalkenoate cements have been described as possessi ng the unique properties of self-adherence to human hard tissues, such as b ones or teeth. However, direct experimental evidence to prove the existence of chemical bonding has not been advanced. X-ray Photoelectron Spectroscop y (XPS) was used to analyze the chemical interaction of a synthesized polya lkenoic acid with enamel and synthetic hydroxyapatite. For both enamel and hydroxyapatite, the peak representing the carboxyl groups of the polyalkeno ic acid was detected to have significantly shifted to a lower binding energ y. De-convolution of this shifted peak disclosed two components with a peak representing unreacted carboxyl groups and a peak suggesting chemical bond ing to hydroxyapatite. On average, 67.5% of the carboxyl groups of the poly alkenoic acid were measured to have bonded to hydroxyapatite. XPS of hydrox yapatite also disclosed its surface to be enriched in calcium and decreased in phosphorus, indicating that phosphorus was extracted at a relatively hi gher rate than calcium. Analysis of these data supports the mechanism in wh ich carboxylic groups replace phosphate ions (PO43-) Of the substrate and m ake ionic bonds with calcium ions of hydroxyapatite. It is concluded that a n ultrathin layer of a polyalkenoic acid can be prepared on a hydroxyapatit e-based substrate by careful removal of non-bonded molecules. With this spe cimen-processing method, XPS not only provided direct evidence of chemical bonding, but also enabled us to quantify the percentages of functional grou ps of the polyalkenoic acids that bonded to calcium of hydroxyapatite.