Evidence of chemical bonding at biomaterial-hard tissue interfaces

For many years, glass-polyalkenoate cements have been described as possessing the unique properties of self-adherence to human hard tissues, such as bones or teeth. However, direct experimental evidence to prove the existence of chemical bonding has not been advanced. X-ray Photoelectron Spectroscopy (XPS) was used to analyze the chemical interaction of a synthesized polyalkenoic acid with enamel and synthetic hydroxyapatite. For both enamel and hydroxyapatite, the peak representing the carboxyl groups of the polyalkenoic acid was detected to have significantly shifted to a lower binding energy. De-convolution of this shifted peak disclosed two components with a peak representing unreacted carboxyl groups and a peak suggesting chemical bonding to hydroxyapatite. On average, 67.5% of the carboxyl groups of the polyalkenoic acid were measured to have bonded to hydroxyapatite. XPS of hydroxyapatite also disclosed its surface to be enriched in calcium and decreased in phosphorus, indicating that phosphorus was extracted at a relatively higher rate than calcium. Analysis of these data supports the mechanism in which carboxylic groups replace phosphate ions (PO4(3-)) of the substrate and make ionic bonds with calcium ions of hydroxyapatite. It is concluded that an ultrathin layer of a polyalkenoic acid can be prepared on a hydroxyapatite-based substrate by careful removal of non-bonded molecules. With this specimen-processing method, XPS not only provided direct evidence of chemical bonding, but also enabled us to quantify the percentages of functional groups of the polyalkenoic acids that bonded to calcium of hydroxyapatite.