Theoretical Soft Matter and Biophysics, Institute of Complex Systems and Institute for Advanced Simulation, Forschungszentrum Jülich, 52425 Jülich, Germany.
Proc Natl Acad Sci U S A. 2013 Mar 19;110(12):4470-5. doi: 10.1073/pnas.1218869110. Epub 2013 Mar 4.
Propulsion by cilia is a fascinating and universal mechanism in biological organisms to generate fluid motion on the cellular level. Cilia are hair-like organelles, which are found in many different tissues and many uni- and multicellular organisms. Assembled in large fields, cilia beat neither randomly nor completely synchronously--instead they display a striking self-organization in the form of metachronal waves (MCWs). It was speculated early on that hydrodynamic interactions provide the physical mechanism for the synchronization of cilia motion. Theory and simulations of physical model systems, ranging from arrays of highly simplified actuated particles to a few cilia or cilia chains, support this hypothesis. The main questions are how the individual cilia interact with the flow field generated by their neighbors and synchronize their beats for the metachronal wave to emerge and how the properties of the metachronal wave are determined by the geometrical arrangement of the cilia, like cilia spacing and beat direction. Here, we address these issues by large-scale computer simulations of a mesoscopic model of 2D cilia arrays in a 3D fluid medium. We show that hydrodynamic interactions are indeed sufficient to explain the self-organization of MCWs and study beat patterns, stability, energy expenditure, and transport properties. We find that the MCW can increase propulsion velocity more than 3-fold and efficiency almost 10-fold--compared with cilia all beating in phase. This can be a vital advantage for ciliated organisms and may be interesting to guide biological experiments as well as the design of efficient microfluidic devices and artificial microswimmers.
纤毛推动是生物体内一种迷人且普遍的机制，可在细胞水平上产生流体运动。纤毛是一种毛发状的细胞器，存在于许多不同的组织和许多单细胞和多细胞生物中。大量的纤毛组装在一起，其摆动既不是随机的，也不是完全同步的——相反，它们以律动波（MCWs）的形式表现出惊人的自组织。早期有人推测，流体动力相互作用为纤毛运动的同步提供了物理机制。从高度简化的受驱动粒子阵列到少数几个纤毛或纤毛链的物理模型系统的理论和模拟都支持这一假设。主要问题是单个纤毛如何与相邻纤毛产生的流场相互作用，并同步它们的摆动以产生律动波，以及律动波的特性如何由纤毛的几何排列决定，例如纤毛间距和摆动方向。在这里，我们通过在 3D 流体介质中对 2D 纤毛阵列的介观模型进行大规模计算机模拟来解决这些问题。我们表明，流体动力相互作用确实足以解释 MCWs 的自组织，并研究了拍打模式、稳定性、能量消耗和输运性质。我们发现，与所有纤毛同步拍打相比，MCW 可以将推进速度提高 3 倍以上，效率提高近 10 倍。对于有纤毛的生物来说，这可能是一个至关重要的优势，并且可能对指导生物实验以及设计高效的微流控装置和人工微型游泳者都很有趣。
