The objective of this propOsed program is to optimize the properties which can be developed in hydroxyapatite (HAp)-based dental restorative materials when they are formed near physiological temperature. HAp-based restorative materials potentially offer significant advantages in that HAp is compositionally close to tooth material and should exhibit very similar thermal conduction and thermal expansion. The opportunity exists to develop a """"""""chemical"""""""" bond between an HAp-based restorative material and enamel or dentin, and physical properties can be tailored depending on the specific application. The basis for the approach is that basic and acidic calcium phosphates react in aqueous solution at low temperature to form HAp. A variety of studies carried out in our laboratory have demonstrated the viability of the approach. Depending on the specific conditions, HAp formation has been shown to reach a substantial degree of completion within 15 minutes by acid-base reaction. Based on the conditions in solution during reaction, HAp formation occurs under conditions compatible with both hard and soft tissue. In spite of these attributes, the reactions have not been well characterized with respect to kinetics, microstructural development, mechanical property development, and to the bond developed with tooth surfaces. Such development is related to both the characteristics of the reactants and to the conditions of reaction. The principal objective of this study is to establish these relationships. Once these have been established, property development can be optimized. Five task areas are envisaged. In the first task, factors which influence the kinetics of HAp formation will be investigated. These studies will determine of the rate-limiting factors in the reactions. The second task is to use high temperature processing of reactants to enhance both reactivity and property development. Compositions limited only to calcium phosphates will be investigated. A third task will study conditions in solution which influence the reactions forming HAp. These findings will be used to optimize property development in a fourth task. Included is an assessment of the effects of the inclusion of reactive and inert filler materials on property development. Materials will be tested mechanically and, based on failure analyses, microstructures will be controlled accordingly. In a final task, the quality of the bond developed with tooth surfaces will examined. Bonding will be examined depending on treatment of tooth surfaces in manner to identify opportunities likely to result in tooth- HAp restorative material interfacial regions of high strength and low porosity.
