Thayer School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, NH 03755, USA.
J Mech Behav Biomed Mater. 2013 Mar;19:87-93. doi: 10.1016/j.jmbbm.2012.10.013. Epub 2012 Nov 3.
Nacre possesses a remarkable combination of mechanical properties. Its high stiffness, strength and toughness are attributed to a highly aligned structure of aragonite platelets "glued" together by a small fraction (∼5vol%) of polymer; theoretically it can be described by a shear-lag model of staggered tensile elements between which loads are transferred via shear. Despite extensive research, it has not been possible yet to manufacture this aligned structure as a bulk material of considerable volume with a fast and easy production process. Particularly porous materials would benefit from enhanced wall material properties to compensate for performance loss due to their high porosity. An important application for such porous materials are tissue scaffolds for bone substitution. Bone, like nacre, exhibits excellent mechanical properties, particularly an exceptionally high toughness, because of its composite structure of hydroxyapatite platelets aligned in a ∼35vol% polymer matrix. Through the freeze casting process, which results in a fast and straightforward self-assembly of platelet-shaped particles during directional solidification, highly porous bulk materials with nacre-like cell walls can now be created. This porous nacre outperforms by a factor of 1.5-4 in terms of stiffness, strength and toughness materials that have the same amount of porosity but do not exhibit the nacre-like microarchitecture. The self-assembly process presented in this study thus has tremendous potential for the creation of highly porous, yet mechanically strong tissue scaffolds for low or medium load bearing bone substitute materials. Due to the versatility of the freeze casting process, materials with a self-assembled cell wall structure can be created from high-aspect ratio particles of all material classes. This enables material optimization for a great variety of applications such as impact protection, filtration, catalysis, energy generation and storage, in addition to those with excellent mechanical properties at high porosity.
珍珠母具有显著的机械性能组合。其高刚度、高强度和高韧性归因于文石薄片的高度取向结构，这些薄片通过少量（约 5vol%）聚合物“粘合”在一起；理论上，它可以通过交错拉伸元件的剪切滞后模型来描述，其中负载通过剪切传递。尽管进行了广泛的研究，但尚未能够制造这种具有相当体积的整体材料，并且具有快速且简单的生产工艺。特别是多孔材料将受益于增强壁材料性能，以补偿由于其高孔隙率而导致的性能损失。这种多孔材料的一个重要应用是用于骨替代的组织支架。与珍珠母一样，骨由于其羟磷灰石薄片在约 35vol%聚合物基质中取向的复合结构，表现出优异的机械性能，特别是异常高的韧性。通过冷冻铸造工艺，在定向凝固过程中快速且直接地组装板状颗粒，可以制造出具有珍珠母样细胞壁的高度多孔整体材料。这种多孔珍珠母在刚度、强度和韧性方面的表现优于具有相同孔隙率但不具有珍珠母样微观结构的材料，其性能提高了 1.5-4 倍。因此，本研究中提出的自组装过程在制造用于低或中等承重骨替代材料的高度多孔、机械强度高的组织支架方面具有巨大潜力。由于冷冻铸造工艺的多功能性，可以从所有材料类别中具有高纵横比的颗粒创建具有自组装细胞壁结构的材料。除了具有高孔隙率的优异机械性能之外，这还可以实现各种应用的材料优化，例如冲击保护、过滤、催化、能量产生和存储。
