The skeleton is able to adapt to mechanical loading through bone remodeling, i.e. bone resorption followed by bone formation. The osteoclasts close to microdamages are believed to initiate bone resorption, but whether local mechanical loading such as fluid flow regulates recruitment and differentiation of osteoclast precursors at the site of bone resorption has yet to be investigated. In the present study, finite element analysis first revealed that there exists low fluid shear stress (FSS) field inside microdamage. Basing on a custom-made device of cone-and-plate fluid chamber, finite element analysis and particle image velocimetry measurement were performed to verify the formation of gradient FSS flow field. Furthermore, the effects of gradient FSS on the migration, aggregation, and fusion of osteoclast precursors were observed.Results showed that osteoclast precursor RAW264.7 cells migrate along radial direction toward the region with lower FSS during exposure to gradient FSS stimulation for 40 min, obviously deviating from the direction of actual fluid flow indicated by fluorescent particles. When inhibiting calcium signaling pathway with gadolinium and thapsigargin, cell migration toward low-FSS region was significantly reduced. For other cell lines, MC3T3-E1, PDLF, rMSC and MDCK, gradient FSS stimulation did not lead to the low-FSS-inclined migration. After being cultured under gradient FSS stimulation for 6 days, the density of RAW264.7 cells and the ratio of TRAP-positive multinucleated osteoclasts in low-FSS region were significantly higher than those in high-FSS region. Therefore, osteoclast precursor cells may have special ability to sense FSS gradient and tend to actively migrate toward low-FSS region, which is regulated by calcium signaling pathway.

Mechanical microenvironments, such as characteristics defining mechanical environments and fluid flow play an important role in steering the fate of mesenchymal stem cells (MSCs). However, the synergistic effect of adhesion morphology and fluid flow on the biological behavior of MSCs is seldom investigated. In this article, 0.5 or 0.8 Pa fluid shear stress (FSS) was applied to the MSCs on micropatterned substrates, and the apoptosis and osteogenic differentiation of MSCs were measured by double fluorescent staining. Results showed that the cellular adhesion patterns with low circularity and large area are beneficial to the osteogenic differentiation of individual MSCs. Meanwhile, FSS facilitated osteogenic differentiation of MSCs, as shown by the expression of alkaline phosphatase, osteocalcin, and collagen I. In addition, nuclear transfer of Yes‐associated protein, a transcriptional regulator in MSCs, was enhanced after being exposed to FSS. These results demonstrated the synergistic effects of FSS and adhesion morphology in directing the fate of MSCs, and these effects may be adopted to design bio‐functional substrates for cell transplantation in tissue engineering.

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The replaceable energy-dissipating beam is convenient for post-earthquake repair and reinforcement or direct replacement. A quasi-static test study of an eccentrically supported steel frame with replaceable energy dissipating beam sections was performed, and the hysteretic performance and failure mode of the eccentrically supported steel frame with replaceable energy dissipative beam sections were obtained. Based on the test results, the structure of the energy dissipating beam segment was improved, and a number of K-shaped eccentric support frame finite element models for the web opening ratios of different energy dissipating beam segments were established. Nonlinear numerical analysis was performed on its hysteretic performance. The research shows that the finite element analysis is in good alignment with the test results. The opening of the web in the energy-consuming beam segment can effectively reduce the stress level of the frame beam and ensure that large plastic deformation occurs within the energy-consuming beam segment. With the increase of the opening ratio, the ultimate bearing capacity, initial stiffness and energy consumption of the frame have all shown a downward trend. The research provides a basis for the seismic design of similar structures and can guide the design of related projects.