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Combustion Synthesis of Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> Net-Shape Surgical Implants

More-biocompatible materials are produced in fewer processing steps.

ASelf-propagating high-temperature combustion synthesis (SHS) is the basis of a method of making components of porous tricalcium phosphate [Ca3(PO4)2] and related compounds in net sizes and shapes for use as surgical implants that are compatible with bone. Ca3(PO4)2-based materials are among those preferred for use in orthopedic, restorative, and reconstructive surgery. As explained below, the SHS method offers advantages over prior methods of manufacturing Ca3(PO4)2-based surgical implants.

Ca3(PO4)2 occurs in at least two crystalline forms: a monoclinic form denoted the α phase and an orthorhombic form denoted the β phase. The β phase is the preferred form for bone replacements because it can be resorbed by the body, facilitating bone remodeling. At an appropriate porosity, β Ca3(PO4)2 resembles natural bone and serves as a scaffold into which osteogenic cells can migrate. Thus, bone becomes directly attached to and grows into a β Ca3(PO4)2 implant. The body generally resorbs β Ca3(PO4)2 within about two years, replacing it with natural bone.

Prior methods of making surgical implants of Ca3(PO4)2 and related materials have not yet been perfected. The prior methods involve, variously, synthesis of Ca3(PO4)2-containing bioceramics from aqueous solutions, sintering, sol-gel processing, and/or casting of polymeric foams mixed with slurries of Ca3(PO4)2- containing bioceramic particles. All of these prior methods are energy- and labor-intensive, and each requires several time-consuming steps. Of particular interest is the sintering method, which includes molding by compacting a powder into a die having the size and shape of the desired part, then heating to temperature just high enough that the powder particles undergo solid-state bonding to each other but do not melt. The great disadvantage of this method is that at the high sintering temperature, β Ca3(PO4)2 becomes converted to α Ca3(PO4)2, which is not preferable as a bone replacement material.

Relative to any of the prior methods, the present SHS-based method requires fewer steps, takes less time, enables better tailoring of porosity, and yields a greater ratio between the desired β phase and the undesired α phase. Processing according to this method begins with preparation of a mixture of CaO and P2O5 powders and possibly other ingredients described below. Processing must be done in a protective dry, nonreactive atmosphere (e.g., argon) because P2O5 is hygroscopic and strongly reactive. The mixture is compacted into a combustible or noncombustible die having the size and shape of the desired part. If the die is noncombustible, the preform of compacted powder is then removed from the die carefully so as not to deform or break it.

Next, the compacted powder preform is heated to initiate the main combustion synthesis reaction, 3CaO + P2O5 → Ca3(PO4)2, which is accompanied by some other reactions that yield a variety of calcium-, oxygen-, and phosphorus-containing byproducts. The main combustion synthesis reaction is exothermic and selfsustaining: once it has been initiated, a wavefront comprising a reaction zone moves through the mixture. In the reaction zone and its vicinity, the reactant having the lowest melting temperature momentarily spreads by means of capillary action, leading to a large dispersion of the reaction products.

In general, Ca3(PO4)2 is formed if the mixture contains between about 60 and 90 mole percent of CaO and between about 10 and 40 mole percent of P2O5. The proportions of these ingredients can be adjusted to tailor the proportions of the α and β phases of Ca3(PO4)2 in the combustion-synthesis product. Optionally, the reaction mixture can include one or more dopants and/or a gasifying agent. Also optionally, the combustionsynthesis product can be subjected to a further process of controlled heating and cooling to increase the ratio between the β and α phases of Ca3(PO4)2.

Process parameters can also be varied to tailor the degree of porosity, the proportion of interconnected pores, and the shapes of the pores in the finished product, and to impart functionally graded porosity as might be required for a particular application. Examples of such parameters include, but are not limited to, the pressure used to compact the reactant mixture, the amount of the gasifying agent, the proportions of CaO and P2O5 in the reactant mixture, the sizes of the reactant powder particles, and the pressure of the atmosphere in which the reaction takes place.

This work was done by Reed A. Ayers, Martin Castillo, Guglielmo Gottoli, John J. Moore, and Steven J. Simske of the Colorado School of Mines for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Materials category.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn:

Steve Fedor
Mail Stop 4-8
21000 Brookpark Road
Cleveland, Ohio 44135

Refer to LEW-17951-1.

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