Monosized Spherical Granules of Bio-Active Ceramics

2000 Fiscal Year Final Research Report Summary

• Project/Area Number: 10045035
• Research Category: Grant-in-Aid for Scientific Research (B).
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Material processing/treatments
• Research Institution: Tohoku University
• Principal Investigator: KAWASAKI Akira Graduate School of Engineering, Tohoku University, Professor, 大学院・工学研究科, 教授 (50177664)
• Co-Investigator(Kenkyū-buntansha): LI Jing-feng Graduate School of Engineering, Tohoku University, Asoc.Prof., 大学院・工学研究科, 助教授 (50241542) ; WATANABE Ryuzo Graduate School of Engineering, Tohoku University, Professor, 大学院・工学研究科, 教授 (20005341)
• Project Period (FY): 1998 – 2000
• Keywords: bioceramics / bio-active / bone repair / spherical particles / spherical particles / spray drying / sintering / slurry

Research Abstract

Through discussions on the preparation of β-TCP granules to be used for the reinforcement of bioresorbable calcium-phosphate self-setting materials (Brushitic cements), which are contemplated as injectable bone grafting materials. In order to ensure good final mechanical strength, while keeping acceptable rethological properties of the fresh cement, the desired granules should be spherical, exhibit a narrow size distribution in the 250-500 micrometer range, and should be sintered up to over 95% theoretical density.
The restults obtained on the preparation of monosized β-TCP spherical granules by the POEM/Freeze-Drying process developed by Prof. Kawasaki's group have been thoroughly discussed. Even though the particles are globally perfectly spherical in shape and exhibit good size uniformity, they present severe cracking after drying. Some twinned granules are also formed, though to a very limited extent.
The observed formation of cracks on the generated granules may have diverse origins , non mutually exclusive : i) Excessive cooling rate, which results in sharp temperature gradients in the grantules, and may cause severe mechanical stresses due to faster thermal contraction at the granule surgace where the material solidifies first ; ii) Recrystallization of 3water into ice during the freeze-drying process, which takes place at a temperature close to the triple point of water (0.01℃) ; and iii) Excessive brittleness of the dried granules, due to their relatively low green density (appr. 40% vol) and their low content in binding polymer (less than 2% wtPAA). Moreover, the high glass transition temperature of the said polymer makes it very brittle at the freeze-drying temperature, so that no plastic deformation of the drying granules can release the mechanical stresses which develop during the drying process.
Two directions of research will be undertaken in order to overcome the cracking problems : i) Adaptation of the solidification/cooling step, including the choice of the coolant, and ii) Modifications of the ceramic suspension composition, so as to improve the mechanical strength of the β-TCP granules, and maybe prevent ice-crystal formation during the freeze-drying step.