Fabrication of porous calcium polyphosphate implants by solid freeform fabrication: A study of processing parameters and in vitro degradation characteristics

Solid freeform fabrication (SFF) involves the creation of a solid 3-D objec t of desired shape by successively adding raw materials in particles or lay ers. Its use in fabricating surgical implants is being explored. The object ive of this study was to determine the feasibility of using SFF to build po rous parts of calcium polyphosphate (CPP), a linear condensed phosphate tha t has been suggested as a material for forming bioresorbable skeletal repla cement implants. CPP powders (< 25 mum in particle size) were added to an U V curable monomer (SOMOS 6110) at a solids loading of 25 vol %, with the ad dition of a commercial dispersant to prevent particle agglomeration and set tling. Viscosity and cure depth measurements were performed to insure that CPP suspension met the requirements deemed necessary for use in SFF. The CP P suspension was bulk cured and sintered in molds in order to assess binder removal and sintering parameters. Using a three-point bend test, the ultim ate bending strength and energy-to-fracture of sintered CPP samples simulat ing parts to be formed by this strategy were characterized. In vitro degrad ation studies using 0.1M of tris-buffered solution were performed to assess the effect of aging on mechanical properties of the samples as a function of the processing route and resulting structures. The polymer binder succes sfully was removed from the cured ceramic suspension by developing a proced ure that combined slow heating rates with low temperature dwells. Sintering CPP at 585 degreesC for 1 h produced amorphous material samples with avera ge porosity of 27.7 +/- 2.0%. Sintering CPP at 600 degreesC for 1 h produce d a crystalline material with samples having an average porosity of 22.9 +/ - 1.3%. Crystalline CPP was found to exhibit superior bend strength and tou ghness compared with amorphous CPP. Both samples experienced a decline in m echanical properties during in vitro degradation; however, the effects were more pronounced with the amorphous CPP samples. Amorphous CPP was found to degrade four times faster than crystalline CPP, as shown by high levels of phosphate present in the degradation solution and a noticeable increase in the porosity of the samples. Crystalline CPP was more resistant to attack as dissolution was limited to surface features of the sintered particles. ( C) 2001 John Wiley & Sons, Inc.