多层级是承力生物材料普遍采用的设计策略，解决了人工复合材料设计难以克服的诸多挑战性难题。然而，对于纳米尺度的力学性能和缺陷是如何在层级间传递的，人们仍然知之甚少。本项目以一种典型的承力生物材料——矿化胶原纤维为对象，研究1）纳米尺度的优异力学性能是如何在层级间保持和传递的，2）不同层级的结构间是如何关联、耦合与协作的，3）微裂纹和病变等缺陷的影响是如何通过多层级结构分别被抑制和传播的，4)如何建立反映缺陷影响的骨材料本构关系。为此，我们将1）发展一套跨尺度、多尺度耦合的粗粒化模型开展计算研究，2）对健康和不同程度骨质疏松症的大鼠骨组织开展微纳结构表征和宏、微、纳观力学测试，3）结合计算和实验结果构建反映缺陷影响的骨材料本构模型。本项目的研究将不仅加深和完善人们对多层级生物材料“结构—功能”关系和相关疾病病理的理解，对于多层级仿生复合材料的发展也有借鉴和指导意义。

Hierarchical design is a strategy widely adopted by nature to produce high performance composites with relatively weak constituent materials, and successfully conquer many challenges facing the traditional design of synthetic composites. However, there is still a major gap in our knowledge as to how the superior mechanical properties and the defect’s effect at nanoscale pass through several structural hierarchies to macroscale. Taking the mineralized collagen fiber as a model hierarchical material, which consists of 3 levels of structural hierarchies, i.e. fiber, fibril and tropocollagen molecules, the project will attempt to understand 1) how the superior mechanical properties of nanomaterials are scaled up via hierarchical design, 2) how the different levels of structural hierarchy are coupled and synergized, 3) how the random defects such as micro/nano-cracks are tolerated while the systematic defects due to some diseases like osteoporosis, osteogenesis imperfecta etc. lead to fragile bone, and establish 4) a constitutive relationship to reflect the defect’s effect on bone quality. To this end, 1) a cross-scale “Beam-Bead-Spring” model will be developed to perform computational studies, 2) a series of experiments will be conducted to characterize the micro/nano-structures and mechanical properties of bone tissues from rats of different osteoporosis severity, 3) a constitutive model considering the defect’s effect will be constructed based on the combined numerical and experimental results. The expected outcomes here will not only add to our understanding of the “structure--function” relationship of bone tissue and the pathology of some relevant bone diseases, but also provide guidelines for the design of bioinspired hierarchical composites.