关键词： Biology and life sciencesEngineering and technologyMaterials scienceMedicine and health sciencesPhysical sciencesResearch Article  

摘要： The link between mechanics and biology in the generation and the adaptation of bone has been well studied in context of skeletal development and fracture healing. Yet, the prediction of tissue genesis within - and the spatiotemporal healing of postnatal defects, necessitates a quantitative evaluation of mechano-biological interactions using experimental and clinical parameters. To address this current gap in knowledge, this study aims to develop a mechanistic mathematical model of tissue genesis using bone morphogenetic protein (BMP) to represent of a class of factors that may coordinate bone healing. Specifically, we developed a mechanistic, mathematical model to predict the dynamics of tissue genesis by periosteal progenitor cells within a long bone defect surrounded by periosteum and stabilized via an intramedullary nail. The emergent material properties and mechanical environment associated with nascent tissue genesis influence the strain stimulus sensed by progenitor cells within the periosteum. Using a mechanical finite element model, periosteal surface strains are predicted as a function of emergent, nascent tissue properties. Strains are then input to a mechanistic mathematical model, where mechanical regulation of BMP-2 production mediates rates of cellular proliferation, differentiation and tissue production, to predict healing outcomes. A parametric approach enables the spatial and temporal prediction of endochondral tissue regeneration, assessed as areas of cartilage and mineralized bone, as functions of radial distance from the periosteum and time. Comparing model results to histological outcomes from two previous studies of periosteum-mediated bone regeneration in a common ovine model, it was shown that mechanistic models incorporating mechanical feedback successfully predict patterns (spatial) and trends (temporal) of bone tissue regeneration. The novel model framework presented here integrates a mechanistic feedback system based on the mechanosensiti

关键词： Molecular dynamics   mechanical properties   nanowires   beta-Zn4Sb3  

摘要： Recent experimental studies reveal that beta-Zn4Sb3 nanowires have greater potential commercial application compared with conventional bulk materials. Mechanical stability of beta-Zn4Sb3 nanowires is an important foundation for their commercial application. In the present work, mechanical properties of beta-Zn4Sb3 nanowires are studied by using the molecular dynamics method, which theoretically reveals the influence mechanism of various effects (temperature, strain rate, and size effect) on the mechanical behavior of beta-Zn4Sb3 nanowires. The simulation results show that mechanical properties of beta-Zn4Sb3 nanowires decrease remarkably compared with bulk beta-Zn4Sb3. Due to the disordered Zn atoms in beta-Zn4Sb3, mechanical properties of beta-Zn4Sb3 nanowires show a weak temperature softening effect, weak strain rate hardening effect, and slight size dependence. The present simulations of beta-Zn4Sb3 nanowires represent a fundamental study and shed light on the understanding of the essential mechanical behavior of beta-Zn4Sb3 nanowires.

关键词： Copper   Lamellae   Aluminium   Dislocations   Molecular dynamics  

摘要： This paper attempts to gain an understanding of the effect of lamellar length scale on the mechanical properties of two-phase metal-intermetallic eutectic structure. We first develop a molecular dynamics model for the in-situ grown eutectic interface followed by a model of deformation of Al-Al2Cu lamellar eutectic. Leveraging the insights obtained from the simulation on the behaviour of dislocations at different length scales of the eutectic, we present and explain the experimental results on Al-Al2Cu eutectic with various different lamellar spacing. The physics behind the mechanism is further quantified with help of atomic level energy model for different length scale as well as different strain. An atomic level energy partitioning of the lamellae and the interface regions reveals that the energy of the lamellae core are accumulated more due to dislocations irrespective of the length-scale. Whereas the energy of the interface is accumulated more due to dislocations when the length-scale is smaller, but the trend is reversed when the length-scale is large beyond a critical size of about 80 nm. (C) 2014 Author(s).

关键词： METAL-organic frameworks   PHASE transformations (Physics)   PYRIDINE   MACROCYCLIC compounds   PHASE change materials  

摘要： Metal-organic framework (MOF) materials have been prepared that contain a mechanically interlocked molecule (MIM) as the pillaring strut between two periodic Zn-carboxylate layers. The MIM linker is a [2]rotaxane with a [24]crown-6 (24C6) macrocycle and an aniline-based axle with terminal pyridine donor groups. The single-crystal X-ray structures of MOFs UWDM-2 (1,4-diazophenyl-dicarboxy-late) and UWDM-3 (1,4-biphenyl-dicarboxylate) show that both frameworks are large enough to contain the free volume required for rotation of the interlocked 24C6 macrocycle, but the frameworks are interpenetrated (UWDM-2, three-fold, and UWDM-3, two-fold). In particular, for UWDM-3 the 24C6 rings of the pillaring MIM are positioned directly inside the square openings of neighboring zinc dicarboxylate layers. Variable-temperature (VT) H-2 SSNMR demonstrated that the 24C6 macrocycles in UWDM-2 and UWDM-3 can only undergo restricted motions related to ring flexibility or partial rotation but are incapable of undergoing free rotation. VT-powder X-ray diffraction studies showed that upon activation of UWDM-3, by removing solvent, a phase change occurs. The new beta-phase of UWDM-3 retained crystallinity, and 2H SSNMR demonstrated that the 24C6 macro cyclic ring of the pillared MIM strut is now free enough to undergo full rotation. Most importantly, the phase change is reversible;the beta version of the MOF can be reverted to the original a state by resolvation, thus demonstrating, for the first time, that the dynamics of a MIM inside a solid material can be controlled by a reversible phase change.

关键词： DNA folding   RESEARCH   MACROMOLECULES   RING formation (Chemistry)   BIOMECHANICS   NANOTECHNOLOGY  

摘要： DNA origami represents a class of highly programmable macromolecules that can go through conformational changes in response to external signals. Here we show that a two-dimensional origami rectangle can be effectively folded into a short, cylindrical tube by connecting the two opposite edges through the hybridization of linker strands and that this process can be efficiently reversed via toehold-mediated strand displacement. The reconfiguration kinetics was experimentally studied as a function of incubation temperature, initial origami concentration, missing staples, and origami geometry. A kinetic model was developed by introducing the j factor to describe the reaction rates in the cyclization process. We found that the cyclization efficiency (j factor) increases sharply with temperature and depends strongly on the structural flexibility and geometry. A simple mechanical model was used to correlate the observed cyclization efficiency with origami structure details. The mechanical analysis suggests two sources of the energy barrier for DNA origami folding: overcoming global twisting and bending the structure into a circular conformation. It also provides the first semiquantitative estimation of the rigidity of DNA interhelix crossovers, an essential element in structural DNA nanotechnology. This work demonstrates efficient DNA origami reconfiguration, advances our understanding of the dynamics and mechanical properties of self-assembled DNA structures, and should be valuable to the field of DNA nanotechnology.

关键词： POLYMORPHIC transformations   CHEMICAL kinetics   ISOTACTIC polymers   POLYBUTENES   DEFORMATIONS (Mechanics)   X-ray diffraction  

摘要： Isotactic poly(1-butene) (i-PBu) crystallizes upon cooling form the melt in a metastable tetragonal structure (form II),which slowly evolves toward the state of ultimate stability, i.e., the trigonal form I. It is well-known that this polymorphic transformation, which typically requires few weeks at room temperature, can be greatly accelerated by the application of mechanical stresses and/or deformation. However, the exact mechanism of this kinetics enhancement is not completely understood. In this work, the polymorphic transformation of i-PBu under tensile deformation is investigated in details. Thanks to properly designed mechanical histories-including experiments at different true strain and true stress rates-and to in situ wide-angle X-ray diffraction experiments, the role of the various deformation parameters is elucidated. The use of different time scales during the experiments enabled us to gain kinetics data on the transition, information which is disregarded in current literature. The set of experiments performed permit to highlight a stress-driven mechanism, active up to a fraction of transformed form I of about 0.4-0.5. After this value is reached, the stress-transformation time superposition principle does not hold anymore and the transition kinetics slows down, since a major part of the total applied stress is carried by the mechanically stronger form I lamellae.

标准号: GB/T 30575-2014

摘要： 本标准规定了用于生物动力学测量,相关标准起草参考以及人体暴露于机械振动与冲击精确描述的解剖学坐标系和基本中心坐标系。本标准中规定的局部解剖学坐标系适用于头部、颈根(头和颈系统的驱动点)、骨盆和手。对于其他骨胳局部的相应解剖学坐标系的建立,本标准规定了一般原则。本标准规定的生物动力学坐标系适用于影响人体的直线和旋转振动与冲击的描述及测量参考系统。

关键词： 机械   液压   系统动力   控制技术  

摘要： 在国外,很早就有人研究液压系统的功率匹配控制技术,随着人们认识的深入,这项技术发展很快,新的技术与成果不断涌现。这种技术很快传到我国,但是,我国目前主要还停留在介绍、模仿等层次,鲜有独立的技术,创新与改进就更谈不上了。

关键词： 漂浮基   柔性空间机械臂   捕获卫星   碰撞动力学   镇定运动控制   柔性振动抑制  

摘要： 讨论漂浮基柔性空间机械臂在轨捕获卫星过程的接触、碰撞动力学建模和碰撞后混合体系统镇定运动控制及柔性振动主动抑制问题。在机械臂末端手爪与被捕获卫星接触、碰撞前,结合假设模态法和拉格朗日法建立空间机械臂系统动力学模型。在接触、碰撞持续过程,考虑空间机械臂与卫星间的运动几何关系、力传递关系,并基于动量守恒关系,分析接触、碰撞对空间机械臂系统刚性运动和柔性振动状态的影响效应。在接触、碰撞后,根据空间机械臂与目标卫星的相互作用内力关系,建立两者混合体系统的综合动力学模型,并针对上述接触、碰撞的影响效应,基于奇异摄动理论将混合体系统综合动力学模型分解为快、慢变子系统,其中,快变子系统表征系统柔性振动,慢变子系统表征系统刚性运动,并对慢变子系统设计镇定运动的径向基函数(Radial basis function,RBF)神经网络控制算法,而对快变子系统设计振动主动抑制的线性二次最优控制算法。数值仿真证实上述控制算法的有效性。

关键词： 柔性机器人连杆   动力学模型   模态截断   振动   弹性变形  

摘要： 本文采用假设模态法和Lagrange方程建立了柔性机器人连杆的动力学模型,利用数值仿真,对比分析了截取模态的阶数对连杆振动的影响,结果表明,三阶模态截断可以得到较良好的振动响应分析结果。仿真结果还表明,柔性机器人连杆的结构参数对系统振动的影响,适当增加连杆截面高度可以有效抑制其振动和弹性变形。末端集中质量会对连杆振动产生较大影响。仿真结果表明,集中质量越大,连杆振动也越剧烈。