| Cortical bone, as a poroelastic material and composite system, often bears cyclic loads that might come from walking, running or other daily activities. These activities can make bone "feel" and "adapt", but what is the mechanism? In order to explore it, researches should be conducted in the level of osteon, because the mechanism may be related with strain-derived fluid stimuli such as fluid shear stress, fluid pressure gradient, chemotransport and stress/strain-generalized potentials (SGP), etc. These effects may be the activated signals of bone growth and bone remodeling. Therefore, it is significant to propose a biomechanical osteon model to examine its intraosseous pressure characteristics and fluid flowing behavior.Bone's Stress-Generated Potentials (SGP) mainly includes piezoelectric potential and streaming potential. The piezoelectric potential generally arouses in the loaded dry bone, while the streaming potential is in the wet or living bone. Therefore, we chose our task to model the streaming potential produced in bone canaliculi to explore the mechanisms of mechanotransduction and electromechanotransduction.Besides, the macroscopical bone material specimens were modeled by using the poroelasticity theory and its poroelastic responses were studied under laboratory conditions. Most previous experimental studies were conducted on the level of bone material. However, physiological bones bear the external dynamic loading as a whole structure, such as walking, running and jumping. Therefore, it has a more realistic reference value to test the SGP produced in the large whole bone structure. Thus, under simulated physiological loading states (walking and running), we examined the cattle femur (large bone structure)'s SGP by INSTRON (8847) testing machine.From the above, the more detailed task list and the main conclusion are as follows:(1) We respectively proposed a hollow and Haversian fluid (pressure) contained osteon model (～150μm) to examine its intraosseous pressure characteristics and fluid flowing behavior under the external loading environments. The relationship among the external loads (axial and cyclic), intraosseous pressure, and fluid velocity was established. Some conclusions are obtained below:(1) The fluid pressure amplitude in the Haversian fluid (pressure) considered osteon model is much higher than that in hollow osteon model.(ii) Generally, the increase of axial strain amplitude and frequency can result in the increase of fluid pressure and velocity amplitudes.(iii) Both the pore pressure and fluid velocity amplitudes are proportionate to the amplitude of strain rate.(iv) With an external cyclic loading, the induced fluid pressure gradient and flow behavior are also cyclic.(v) At the hollow osteon scale, the pressure is strongly affected by permeability variations whereas fluid velocity is not.(2) Base on (1), a streaming potential model produced in bone canaliculi (-500nm) was established and examined its influence of loading amplitude, frequency, and permeability, etc.(i) The streaming potential amplitude (SPA) is proportional to the pressure amplitude difference, strain amplitude, frequency and strain rate amplitude. However, the key loading factor governing the SP is the strain rate and it can be seen as a representative loading parameter under the physiological state.(ii) The SPA produced in the canaliculi of Haversian fluid contained osteon model is larger than that of hollow osteon (not considering Haversian fluid) model.(iii) With an external cyclic loading, the induced fluid pressure gradient and flow behavior are also cyclic.(iv) The larger canaliculi radius, the larger SPA produced, but the SPA is independent of the canaliculi length.(3) The macroscopical bone material specimens (-mm) were modeled by using the poroelasticity theory and its poroelastic responses were studied under laboratory conditions. Its poroelastic behaviors also have been compared with the osteon scale model.(i) The poroelastic behaviors of bone material specimens are similar to that of osteon scale:Both the pore fluid's pressure and velocity amplitudes are proportionate to the strain amplitude, frequency and strain rate amplitude. However, the key loading factor governing its poroelastic behavior is the strain rate.(ii) With an external cyclic loading, the induced fluid pressure gradient and flow behavior are also cyclic.(iii) At the bone material scale, the pressure is strongly affected by permeability variations whereas fluid velocity is not.(iv) In the one dimensional flow model, the streaming potential amplitude (SPA) is proportional to the pressure amplitude difference, strain amplitude, frequency and strain rate amplitude. Besides, the increase of the permeability results in the decrease of SPA.(4) Under simulated human physiological loading states (walking and running), we obtained the cattle femur's (large bone structure:-mm) SGP by INSTRON (8847) testing machine and made a comparison with theoretical analysis.(ⅰ) The cattle femur, being given the human's normal walking and running state of load and frequency, will generate electric potential. Voltage amplitude generated by running is larger than that of walk. The electric potential amplitude will become larger with the load and frequency increases.(ⅱ) There will have two electric potential peaks under the conditions of walking and running. One of them is a negative peak which is generated by heel landing (loading), the other is a positive peak generated when the front sole off the ground (unloading). The second potential peak is larger than that of the first one.(ⅲ) Bones have different electric properties in the condition of tension and compression. Under the same strain rate or stress gradient, the electric potential value generated by tensile is greater than that of compression.(ⅳ) The electric potential amplitude is approximately proportional to the loading amplitude, frequency, strain rate, which agrees with the conclusion of theoretical model. |
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