NSF Materials World Network: Microscopic models of cross-linked active gels

这项提议旨在支持美国、英国和南非的参与者之间开展联合研究和教育的国际合作。软生物物质的研究正在成为材料研究的一个重要新方向。其高度的国际形象从世界各地越来越多的马乔尔倡议地点可见一斑。这项建议侧重于研究细胞组织和动力学的物理方面，特别是分子马达和其他相关蛋白质对细胞骨架细丝的集体动力学和力学性质的影响。细胞骨架主要由半柔性的极性蛋白质细丝组成。此外，还有许多辅助蛋白与这些细丝结合，调节它们的组装，将它们连接到细胞器，并提供发动机，要么沿着细丝移动细胞器，要么移动细丝本身。马达蛋白利用重复的三磷酸腺苷水解循环所产生的能量，沿着极丝在一个方向上产生运动。我们将对(I)活性液体--由细胞骨架细丝和相关的马达蛋白悬浮液组成的纯化(体外)细胞提取物--和(Ii)活性固体--细胞骨架(体内)和由活性(马达蛋白)和被动交联剂连接的细胞骨架体外凝胶--的分析和数值性质进行理论研究。我们的研究将加深对细丝的微观性质(例如它们的硬度)和活性元素(例如运动过程能力和集体运动学)在控制活性流体和凝胶的宏观行为中的作用的理解。这对于即使是最简单的线性粘弹性响应的定量公式以及与实验的比较也是至关重要的。同时，我们将了解发动机动力的非平衡涨落的作用。对于活性流体，我们将(I)扩展现有的分析，以研究细丝柔性对主动悬架宏观性能的影响，(Ii)结合电机特性，以及(Iii)用数值研究来研究约束和边界的作用。我们提出的关于活性固体的工作包括(Iv)将交联型活性凝胶的玩具模型扩展到三维，以及(V)使用详细的微观描述来研究这种凝胶的弹性性质，这将导致(Vi)连续介质模型和线弹性。这样，我们将建立一个研究(Vii)宏观涨落和()活性凝胶中液晶相的微观理论框架。建议的理论研究将通过与一些致力于活性纤维凝胶物理性质的实验小组的合作来进行。

This proposal seeks to support an International collaboration for joint research and education between participants in the US, the UK and South Africa. The study of soft biological matter is emerging as an important new direction in materials re-search. Its highly international profile is evident from the increasing number of ma jor initiativestaking place across the world. This proposal focuses on the study of the physical aspects ofcell organization and dynamics, in particular the effect of molecular motors and other associatedproteins on the collective dynamical and mechanical properties of cytoskeletal filaments. Thecytoskeleton is predominantly composed of a network of semiflexible polar protein filaments. Inaddition, there are many accessory proteins that bind to these filaments, regulate their assembly,link them to organelles and provide the motors that either move the organelles along the filamentsor move the filaments themselves. Motor proteins use the energy derived from repeated cycles ofATP hydrolysis to generate motion in one direction along the polar filaments. We shall undertaketheoretical studies both of an analytical as well as of a numerical nature of (i) active liquids --purified (in vitro) cell extracts consisting of suspensions of cytoskeletal filaments and associatedmotor proteins -- and (ii) active solids -- the cell cytoskeleton (in vivo) and in vitro gels ofcytoskeletal filaments linked by both active (motor proteins) and passive crosslinkers.Our study will enhance the understanding the role of microscopic properties of the filaments(e.g., their stiffness) and of the active elements (e.g., motor processivity and collective motordynamics) in controlling the macroscopic behavior of both active fluids and gels. This is crucialfor a quantitative formulation of even the simplest linear viscoelastic response of these systemand a comparison with experiments. At the same time we shall gain understanding of the role ofnon-equilibrium fluctuations of the motor dynamics.For active fluids we shall (i) be extending existing analyses to study the effect of filament flexibilityon macroscopic properties of active suspensions, (ii) incorporate motor properties, and (iii) studythe role of confinement and boundaries with numerical studies. Our proposed work on activesolids includes (iv) extending to three dimensions a toy model of a cross-linked active gel, and(v) using a detailed microscopic description to study elastic properties of such gels, that willlead to (vi) continuum models and linear elasticity. In this way we shall establish a microscopictheoretical framework for the study of (vii) macroscpic fluctuations and the (viii) liquid crystallinephase in active gels. The theoretical investigations proposed will be informed by collaborationswith a number of experimental groups working on physical properties of active filament gels.