Propulsion and Trapping of Microparticles by Active Cilia Arrays Amitabh Bhattacharya, † Gavin A. Buxton, ‡ O. Berk Usta, § and Anna C. Balazs* ,† † Department of Chemical and Petroleum Engineering, University of Pittsburgh, 1249 Benedum Hall, Pittsburgh, Pennsylvania 15261, United States ‡ Robert Morris University, 6001 University Boulevard, Moon Township, Pennsylvania 15108-1189, United States § The Center for Engineering in Medicine, 51 Blossom Street, Boston, Massachusetts 02114, United States ABSTRACT: We model the transport of a microscopic particle via a regular array of beating elastic cilia, whose tips experience an adhesive interaction with the particle’s surface. At optimal adhesion strength, the average particle velocity is maximized. Using simulations spanning a range of cilia stiffness and cilia−particle adhesion strength, we explore the parameter space over which the particle can be “released”, “propelled”, or “trapped” by the cilia. We use a lower-order model to predict parameters for which the cilia are able to “propel” the particle. This is the first study that shows how both stiffness and adhesion strength are crucial for manipulation of particles by active cilia arrays. These results can facilitate the design of synthetic cilia that integrate adhesive and hydrodynamic interactions to selectively repel or trap particulates. Surfaces that are effective at repelling particulates are valuable for antifouling applications, while surfaces that can trap and, thus, remove particulates from the solution are useful for efficient filtration systems. 1. INTRODUCTION In various living organisms, the coordinated motion of hairlike filaments, called “cilia”, is vital for performing such activities as movement, feeding, or pumping of fluids within the animal. For example, paramecia use cilia to undergo directed movement and marine suspension feeders use cilia to propel food into their bodies. In humans, the synchronized undulations of cilia in the respiratory tract help to pump viscous fluids away from the lungs, as well as propel dust and other particles out of the body. 1 Inspired by the utility and efficiency of such biological cilia, researchers have begun to design synthetic analogues that could be harnessed to regulate flow in microfluidic devices. 2−11 The synthetic cilia are fashioned from flexible filaments that are anchored to the floor of a microchannel and actuated by an applied field 2−4,6,12−14 or forces. 15 These oscillating filaments not only provide effective pumping of fluids, 2b,3 but can also promote the mixing of dissolved components 2b,3 and controllably switch the directionality of the net flow within microchannels. 15 To date, however, the possibility of using active, synthetic cilia to direct the movement of microscopic particles, such as biological cells and microcapsules, within microchannels has not been extensively explored. 16−18 Developing approaches for conveying cells or microcarriers to specified locations within microfluidic devices 19,20 is vital for performing accurate micro- scale analysis or chemical synthesis. While the coordinated motion of the cilia is effective at propelling the surrounding fluid, 1,3,15,21,22 it is also plausible that an adhesive interaction between the cilia and particulates is necessary for controlling the particle movement. In mammals, for example, beating cilia located at the entrance of the oviduct cannot transport egg cells unless there is a critical level of adhesion between the cilia tips and cells. 23,24 The combined effects of such cilial adhesion and generated flows might be responsible for transport in other biological environments. 24 Notably, to the best of our knowledge, there have been no prior computational or theoretical studies aimed at probing the role of adhesion between motile cilia and particles in solution. Here, we use simulations and a lower-order analytical theory to model the interactions between microscopic particles and actuated cilia (see Figure 1) that encompass a “sticky” tip. As we show below, for a critical adhesion strength, active cilia can propel particles from one neighbor to the next, significantly increasing the particle’s velocity. If the adhesive interaction between a particle and this sticky tip is sufficiently high, the particle can become trapped within the cilia layer. These results can facilitate the design of synthetic cilia that integrate adhesive and hydrodynamic interactions to selectively repel or trap particulates. Surfaces that are effective at repelling particulates are valuable for antifouling applications, while surfaces that can trap and, thus, remove particulates from the solution are useful for efficient filtration systems. Furthermore, the findings can provide insight into physical factors that influence adhesive interactions between biological cilia and particulates. The paper is organized in the following manner. In section 2, we first describe the hybrid computational approach we devel- oped to simulate the cilia−particle system, and then in section Received: December 8, 2011 Published: January 10, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 3217 dx.doi.org/10.1021/la204845v | Langmuir 2012, 28, 3217−3226