Schedule Jun 08, 2012
Modeling the Interaction of Active Cilia with Species in Solution: From Chemical Reagents to Microscopic Particles
Anna Balazs (Univ. Pittsburgh)

Biological cilia can sense the presence of particulates or minute chemical variations in their environment, transmit this information to their neighbors, and thereby produce a global response to the local change. Using computational modeling, we demonstrate two distinct examples of analogous sensing and communicating behavior performed by artificial cilia. The first example involves cilia formed from chemo-responsive gels undergoing the oscillatory Belousov-Zhabotinsky (BZ) reaction. The activator for the reaction, u , is generated within these BZ cilia and diffuses between the neighboring gels. We find that the spatial arrangement of the BZ cilia affects the local distribution of u, which in turn affects the dynamic behavior of the system. For two closely spaced cilia, the chemo- mechanical traveling waves within the gels are observed to propagate from top to down. When we increase the inter-cilia spacing, we not only alter the directionality of these traveling waves, but also uncover a distinctive form of chemotaxis: the tethered gels bend towards higher concentrations of u and hence, towards each other. This chemotaxis is particularly pronounced in an array of five cilia, where we observe a "bunching" of the cilia towards the highest concentration in u, accompanied by the synchronization of the chemo-mechanical waves. We also show that the cilial oscillations can be controlled remotely and non-invasively by light. By selectively illuminating certain cilia, we could "play" the array like a keyboard, causing a rhythmic variation in the heights of the gels. These attributes could be exploited in a range of microfluidic applications, where the controllable communication among the BZ cilia and self-oscillating surface topology can be harnessed to transport microscopic objects within the devices. In our second example, 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. Through this adhesive interaction, the cilia can both sense and regulate the motion of the particle along the ciliated surface. By varying the cilia-particle adhesion strength and the cilia stiffness, we can pinpoint the parameters where the particle can be "released", "propelled" or "trapped" by the cilial layer. In the propelled state, the particle is effectively flung from neighbor to neighbor, leading to significant increases in the particle velocity. 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 anti-fouling applications, while surfaces that can trap and thus, remove particulates from the solution are useful for efficient filtration systems.

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