Design Considerations for Magnetically Actuated Biomimetic Cilia

Abstract

Biology has produced extraordinary solutions for functions necessary for life. Sensing and movement appear to be two such necessary functions. For both of these, biology has developed extensions from the cell body called cilia (Sleigh, 1962). For movement – either of the entire organism or for moving fluid and hence nutrients toward the stationary organism – nature has evolved motile cilia (Satir, Mitchell, & Jekely, 2008). These remarkable structures, consisting of a highly organized collection of several hundred thousand proteins packed in a single 250 nanometer diameter cylinder 7 microns long, beat in regular bend shapes autonomously, fuelled by chemical energy (Nicastro, 2009; Satir & Christensen, 2007). Cilia exist throughout biology, from single cell organisms to humans, and are essentially identical down to the protein level across this vast range of life forms. Within the human body, the literature on their significance to human health has exploded over the past decade (Cardenas-Rodriguez & Badano, 2009). Cilia have long been understood to be the critical mechanism for fighting lung infections through the propulsion of mucus (Antunes & Cohen, 2007; Boucher, 2007). More recently, cilia have been found to guide neurons within the brain and to be responsible for the left-right asymmetry of the human body (Okada & Hirokawa, 2009; Sutherland & Ware, 2009). Whether for organism propulsion or for its more complex physiological functions, the cilium is the primary manner in which the cell interacts with surrounding fluids (Cartwright, Piro, Piro, & Tuval, 2008; Smith, Gaffney, & Blake, 2009). Each of these issues: the structure, operation and function of cilia remains a major challenge for biological physics. Difficult unanswered questions remain in the protein-level organization of the cilium, the coordination of 4000 molecular motors to produce bend shapes and the structure-fluid interaction of beating, slender bodies with Newtonian and viscoelastic fluids. From an engineering perspective, cilia present a challenge for replicating their function at the micrometer length scale. Applications of such structures may include microfluidics for pumping and mixing, sensing surfaces for measuring local fluid flows and active surfaces for energy applications and the inhibition of biofouling. We are far from having the ability to replicate the extraordinary, nanoscale architecture of the cilium. However, we have been successful recently at replicating the functional capability of cilia at the sub-micron length scale through a variety of advances in materials and fabrication strategies. We begin our discussion with magnetic materials as magnetic actuation offers flexibility in design and