This research effort explores a short time densification process - termed Field Assisted Sintering Technique (FAST) - to manufacture fully dense nanocrystalline hydroxypaptite (HA) ceramic parts. First, the fundamental factors governing the FAST processing of nanocrystalline HA ceramics will be established. The relationship between processing parameters (electrical, pressure, sintering environment, and kinetic) and the resulting structure and properties of dense HA parts will be investigated. Next, mechanical properties and biocompatibility of fully dense HA ceramics as a function of sintering conditions will be determined. Finally, the FAST process will be used to manufacture near net shape nanocrystalline HA orthopedic implants. The mechanical and in vivo properties of these dense parts will be validated. Our industrial partner, Angstrom Medica (AM) in Woburn (MA), will contribute HA nanopowders, will design the real orthopedic implants for net shape manufacturing, and test the dense calcium phosphate ceramics for in-vitro biocompatibility.
If successful, the results of this research will be used to develop a technology to efficiently manufacture orthopedic implants with improved mechanical and biological properties. The proposed work will create the engineering foundations on which a new FAST manufacturing practice may be based. FAST sintering process enables densification of HA powders in minutes, as compared to hours in the currently used hot pressing process, thus enhancing the processing productivity. The net shape formability of this process will be adapted to manufacture orthopedic implants in shorter time and with a reduced number of processing steps. The retention of nanocrystallline structure in the final HA material has a high potential to produce implants with superior mechanical and biological properties that expedite healing, and reduce pain and inflammation upon surgery. The evaluation of process dependent microstructure and its relationship to these final properties will be an integral component of manufacturing reliable HA orthopedic implants.
The project aimed at understanding the physical, chemical and biological phenomena to fabricate sound implants with improved durability and biocompatibility. Hydroxyapatite (HA) is a synthetic material similar to natural bone. However, the use of HA in orthopaedic applications has been limited by its inadequate mechanical behavior. Enhanced mechanical properties can be obtained starting with very fine HA particles which are consolidated using a process called sintering. This is not a trivial task for very fine particles. Conventional sintering techniques require long times at elevated temperatures which compromise mechanical behavior. For example processing at 900ºC under pressure requires 6 hours. Clearly, there was a need to dramatically reduce sintering time in order to enable efficient manufacturing of very fine HA for orthopaedic applications. A new electrical field assisted sintering technique (FAST) was demonstrated as a flexible and viable technique to manufacture transparent biomedical HA implants which combine required mechanical behavior with biological compatibility. FAST dramatically reduces processing time which enables efficient manufacturing of nanocrystalline HA parts for orthopaedic applications. The process development was supported by numerical simulation to identify and resolve issues associated with the fabrication of large parts ("scale-up") and assist in part optimization by guiding experimental geometry design. Combined experimental and numerical scale-up studies demonstrated FAST capability to fabricate near net shape biomedical components. Finally, a pulsed electro-chemical deposition process was investigated to incorporate Ag nanoparticles in HA coatings for improved biocompatibility and bacterial infection prevention.
