Magnetic artificial cilia for microfluidic applications
Gowranga Hanasoge, Srinivas Kumar
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Biological organisms use hair-like cilia and flagella to perform fluid manipulations essential for their function and survival. This work aims at developing synthetic analogs of biological cilia. The fabrication, actuation, beating kinematics, and applications of bio-mimetic magnetic cilia that can be integrated into MEMS devices to enhance their functionality is detailed. Applications of the cilia to create metachronal waves, fluid mixing, and bacteria capture are demonstrated. In this work, a simple lithographic technique to realize metallic thin film cilia is developed. The magnetic cilia can be driven to oscillate with highly asymmetric strokes using a simply rotating magnetic field. The beating of the synthetic cilia resembles the beating of biological cilia. The fabrication method follows standardized steps of thin-film microfabrication which allows for a higher degree of accuracy and reproducibility. The asymmetric motion opens the possibility of harnessing such synthetic cilia for fluid pumping and other fluid manipulations. A computational model is employed for analyzing the fluid structure interactions and to validate and further examine cilium motion due to a rotating magnetic field. The key parameters governing cilium motion are established and the actuation regimes enhancing asymmetry of cilium beating are identified. The combined effect of multiple beating cilia generates rapid microfluidic pumping. The flow produced by the ciliary array depends on many factors like the direction of magnet rotation, the cilia properties, dimensions of ciliary array, and the microchannel dimensions. All these parameters are varied to characterize the fluid flow produced. It is found that the cilia arrays can generate transitional flow speeds of up to corresponding to a flow rate of in closed-loop micro channels. These flow rates are the highest reported for such ciliary systems. Furthermore, optimum operating conditions for maximum pumping are established. Inspired by biological cilia, it is shown that artificial cilia can be actuated in a sequential metachronal fashion. Multiple methods are explored to achieve sequential actuation of the cilia. The difference in magnetic cilium properties control the phase of the beating motion. This property is used to induce metachronal waves within a ciliary array and explore the effects of operation parameters on the wave motion. The metachronal motion in the artificial system is shown to depend on the magnetic and elastic properties of the filaments, unlike natural cilia, where metachronal motion arises due to fluid coupling. Microorganisms use biological cilia to create various kinds of transport, and most important among them is the transport of suspended particles. Biological cilia are shown to effectively capture and trap specific particles for feeding. Motivated by this, the use of artificial magnetic cilia to capture particles suspended in a fluid is proposed. Perhaps, such nature-inspired ciliary capture and isolation of particles can be incorporated in microfluidic lab-on-chip devices for pre-concentration of cells and analytes. The particle capture capabilities of magnetic cilia are demonstrated and characterized. The surface of cilia is functionalized by the target specific receptor and the target is captured on the ciliary surface upon contact. It is found that maximum particle capture is obtained for same operating conditions that produce the maximum pumping. Furthermore, Salmonella bacteria capture on the surface of the cilia is demonstrated as a proof of concept. Thus, the developed thin film magnetic cilia can find application in a wide variety of microfluidic devices. The ease of fabrication, actuation and reliability in operation make such cilia an attractive option for various applications. They can easily be incorporated into any microscale device where precise control and metering of fluid is necessary. Furthermore, they can be used to study and better understand the fluid transport produced by biological cilia.