Adhesion of carboxylate cements to hydroxyapatite. I. The effect of the structure of aliphatic carboxylates on their uptake by hydroxyapatite.
An investigation was made into the effect of the structure and functionality of aliphatic carboxylates on their sorption onto hydroxyapatite with a view to ascertaining factors affecting the adhesion of polyacrylate cements to tooth materials. In general, the amount of carboxylate sorbed was found to increase with the number of groups contained in the molecule. Thus, polyacrylate was found to be much more strongly sorbed than low molecular weight species. Sorption of the low molecular weight species appeared to be related to stereochemical factors rather than to the stability constants of their calcium chelates.
Mechanism of Adhesion of Polyelectrolyte Cements to Hydroxyapatite
The interaction between polyacrylate ions and hydroxyapatite was examined using extraction techniques and infra-red spectroscopy. These studies have shown that polyacrylate ions become irreversibly attached to the surface of hydroxyapatite by displacing existing phosphate ions. The mechanism is not one of simple ion exchange, since calcium ions are displaced by the phosphate ions.
Adsorption and Ionic Crosslinking of Polyelectrolytes
Materials formed by polymers containing ionic or ionizable groups in their chemical repeating units (polyelectrolytes) have been of interest in the area of biomaterials over the past two decades. Presently, two largely polyelectrolytic compositions are utilized as dental cements or as restorative materials for hard oral tissues. These two types of dental biomaterials are the zinc polycarboxylate cements (1) and the glass ionomer cements and restoratives (2). Dental impression materials, known as alginates (3), represent an additional example, as they are partly based on an ionizable polysaccharide, alginic acid. Polyelectrolyte-based materials have been tested for other biomedical applications, such as the formulation of cardiovascular elastomers (4), dialysis membranes (5), drug delivery systems (6), or surgical adhesives (7). As a consequence of past and current research activity, it is possible that polyelectrolytes will find application in some of these areas in the near future. It is interesting to consider that biological tissues themselves contain considerable amounts of polyelectrolytic material. This fact alone does not, of course, guarantee that synthetic polyelectrolytes should lead to the successful development of nearly ideal biomaterials from a mechanical, chemical or biocompatibility standpoint. Nonetheless, a strong interest in these materials, and their potential advantages can be justified on the basis of some of their characteristic features, depending on the specific application involved. Some of these characteristic features, and their relevance in the formulation of biomaterials, have been described below.