In Situ Investigation Of Micro-Nano Mechanical Behaviors Of Bone

Bone is a natural biological composite with a complex hierarchical structure,showing excellent mechanical properties,such as high strength and toughness.The structure-mechanical function relations of bone have long been an important topic,which of related researches provide important reference for the treatment of bone diseases and the development of biomimetic materials.However,it is still lack of a systematic and in-depth research on the deformation mechanism of bone hierarchical structure at different spatial and time scales.In this work,based on the novel in-situ mechanical testing platform,the relationship between bone structure and mechanical function has been explored in situ.This work provides an intuitive perspective for the deformation mechanisms of bone structures at different scales.(1)According to the requirements of Asylum Research MFP-3D atomic force microscope(AFM),three generations of in-situ mechanical testing devices have been designed and manufactured.The miniaturized and light-weight third-generation device was successfully developed through optimizing parameter conditions and structural design as many times as required.The in-situ AFM-mechanical test platform could carry out a variety of in-situ mechanical test modes with high precision for materials and provide the nano-level resolution of AFM,which unified the two independent processes of the characterization of micro-/nanomorphology and the testing of mechanical property of materials.The stability and feasibility of the in-situ AFM-mechanical test platform was verified by in-situ bending experiments of pearl shell samples.The in-situ AFM-mechanics test platform designed in this paper is universal to most materials,which will provide new ideas and perspectives for material research.(2)The in-situ AFM-mechanical test platform was applied for in situ tracking the crack propagation in bone microstructures and the microscopic toughening mechanisms in bone microstructure were explored at multiscale levels.The natural nanostructure on bone canal surfaces was scanned by AFM and verified the nanomorphology of mineral and collagen phases at the surface of polished samples.The development of microscopic toughening mechanisms,such as microcrack,crack bridging and crack deflection,was observed in situ under load condition,and the effects of microstructure and internal stress on these microscopic toughening mechanisms were analyzed.The influence of nanoscale deformation behaviors on these toughening mechanisms was explored,which cannot be observed in the non-stress state,and it was found that the internal stress forced the extrusion or tension between mineralized collagen fibrils,which resulted in the interface separation between mineralized fibrils.The results showed that the nanoscale deformation behaviors provided the structural preparation for the growth of crack tip,and the catastrophic failure of nano-interface determined the initiation path of the microcrack and crack tip.(3)The mechanical failure behaviors of bone with different orientations were observed in situ to record the effects of osteon and its internal ordered structure on crack propagation under load.The mechanical properties of osteon structure and lamellar structure were evaluated by indentation method.The crack propagation in bone transverse samples during in-situ bending was tracked.The zigzag path of crack with periodic deflection among the lamellae and interlamellae in the longitudinal samples was analyzed through in-situ bending test from the level of mineralized collagen fibril array.The results showed that the crack propagation along the external structure of osteon(especially cement line)was easy in the transverse orientation,and the failure nano-interfaces between mineralized collagen fibril arrays arranged orderly within the lamellae showed dendritic nanocracks with multidirectional nano-deflection,which was synergistic with the interlamellae lead to these microscopic zigzag paths.Combined with a series of observations on the resistance of crack propagation behaviors from osteon structure,lamellar structure to mineralized collagen fibril array through optical microscope and AFM,a multi-level composite material model was proposed,i.e.,the hollow fiber reinforced model(Haversian system)-the ring-shaped layered composite model(lamellar structure)-the ordered fibril reinforced model(mineralized collagen fibril array).The results showed that the hierarchical structure offers the bone a multi-level crack resistance through a multi-level composite material model,and especially the longitudinal orientation of bone can better play the role of the multi-level composite material model.(4)This study further specifically explored the nano-failure behavior of mineralized collagen fibrils.The medullary surface of the bovine femur as the research object was scanned using high resolution AFM to observe the nanostructure such as mineralized collagen fibrils,and combining the nanoindentation mapping,the relationship between the structure and mechanical distribution of mineralized collagen fibrils on the medullary surface was analyzed.Mineralized collagen fibrils on the medullary surface retained the characteristics structure of collagen,which were interconnected by the mineral aggregate grains and the distribution of the nano-mechanical properties was inhomogeneity and oriented.These results showed that the medullary surface can be ideal to represent the internal nanostructures of bone.The mechanical behaviors of mineralized collagen fibrils observed during in situ tensile test were compared with the failure nano-behaviors of the crack tip in polished bone samples through in situ bending experiment.The results showed that the nanoscale plastic failure of bone was local and nonsimultaneous,and the failure of the interface between nano-components provided the nanoscale structural conditions for the initiation and development of microscopic failure.The toughening mechanism of bridging plays an important role in the process of bone failure in the multilevel forms of nanoscale fibrillar bridging,sub-microscale fibril array bridging and microscale crack bridging.(5)To reveal the nano-level origin of irreversible plastic deformation in bone,in-situ mechanical testing was conducted on antler bone and bovine femur bone to further explore the plastic mechanical behaviors at the level of mineralized collagen fibril.The macroscopic tensile test and cyclic tensile test of antler bone showed that the irreversible plastic deformation was accompanied by the whole process of tensile deformation.The in-situ tensile and in-situ bending experiments of antler bone showed that the deformation at the level of mineralized collagen fibrils was one of the nano-level origins of this irreversible plastic deformation and the nanoscale deformation behaviors were irreversible.The in-situ bending of bovine femur samples with the medullary surface provided a visual perspective for the study of the deformation mechanisms of nano-components under load.The results showed that the mineralized collagen fibril array,formed by several microns of mineralized collagen fibrils array,initiated to deformation in the form of "anchor" at the beginning of the nanometer plastic deformation and the inter-array slippage was the main way of the plastic behavior.This is the first stage of the nanometer plastic deformation,which of the main deformation structure is mineralized collagen fibril arrays.As the internal stress increased within the mineralized fibril arrays,the interfaces between mineral phase and collagen phase were debonding,and then rubbing and slipping in the form of "mechanical interlock",which further leaded to the collapse of nano-components and the exposure of mineralized collagen fibrils.This is the second stage of the nanoscale plastic deformation,in which the deformation behavior of the mineralized collagen fibrils played a major role.The fracture of nano-interface further aggregated and finally led micro-failure in local area.Therefore,the inter-array and interfibrillar slippage were the nanoscale origins of the macroscopic plastic deformation.