Monosized Spherical Granules of Bio-Active Ceramics

通过讨论制备用于增强生物可吸收磷酸钙自固化材料（磷灰石水泥）的β-TCP颗粒，预期将其作为可注射骨移植材料。为了确保良好的最终机械强度，同时保持新鲜水泥的可接受的流变性能，所需的颗粒应该是球形的，在250-500微米范围内表现出窄的尺寸分布，采用POEM/Freeze法制备了单粒径β-TCP球形颗粒，并对其进行了表征。对川崎教授小组开发的干燥过程进行了深入的讨论。尽管颗粒在形状上总体上是完美的球形并且表现出良好的尺寸均匀性，但它们在干燥后存在严重的开裂。一些孪晶颗粒也会形成，但程度非常有限。在生成的颗粒上观察到的裂纹的形成可能有不同的起源 ...更多信息 不互相排斥：i）过度冷却速率，这导致颗粒中的急剧温度梯度，并且由于在材料首先凝固的颗粒间隙处的更快热收缩而可能导致严重的机械应力; ii）在冷冻干燥过程中，3水再结晶成冰，这发生在接近水的三相点的温度下（0.01℃）;和iii）由于干燥颗粒相对较低的绿色密度（约0.01 ℃），40%vol）和它们在粘合聚合物中的低含量（小于2%wtPAA）。此外，所述聚合物的高玻璃化转变温度使其在冷冻干燥温度下非常脆，因此干燥颗粒的塑性变形不能释放在干燥过程中产生的机械应力。i）固化/冷却步骤的调整，包括冷却剂的选择，和ii）陶瓷悬浮液组合物的改性，以提高β-TCP颗粒的机械强度，并且可以防止在冷冻干燥步骤期间形成冰晶。少

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 … More , 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. Less