Biopolymerization-driven self-assembly of nanofiber air-bridges

Abstract

Several proteins, including actin and fibrin, polymerize in vivo to form nanometre diameter fibers. These processes can be duplicated in vitro using only the essential enzyme and protein precursors. These same protein solutions are directed to self-assemble into oriented arrays of air-bridges using only the crude operation of hand brushing them over textured micron-scale surfaces. The creation of these suspended structures could be used as nanomechanical elements in various sensors and actuators, and their fabrication by this rapid directed self-assembly method would be useful, especially during the early phases of prototype device development. The fabrication method extends earlier studies (Harfenist et al., Nano Lett., 2004, 4, 1931) in which an organic polymer dissolved in a volatile solvent forms nanofiber air-bridges through a combination of capillary force driven thinning of liquid bridges and evaporation driven solidification of the polymer solution. However, in the current study polymerization is initiated when a monomeric solution of soluble protein is brushed over the micro-textured surface. When fibrinogen solutions are brushed over a thrombin primed surface, or monomeric actin over a KCl primed array, fiber air-bridges are formed, sometimes reaching diameters as small as 16 nm. The uniformity in diameter of one hand-brushed array of 358 parallel fibrin air-bridges was 36.4 nm (6.8 nm standard deviation), with no more than 3 broken fibers. The fibrin bridges are shown to be both highly elastomeric and adhesive through demonstration of the construction and stretching of a three point bridge using a micromanipulator. Also the brush-on method produced ordered arrays of suspended fibrin membranes, which sometimes were anchored perpendicular to the vertical sidewalls of the textured surface and other times were anchored parallel to the sidewalls. The demonstration of air-bridge formation by biopolymerization suggested that air-bridges might also be formed during initiated polymerization of organic monomers. Even though the monomer of norbornylene has a much smaller molecular weight than fibrinogen, hand brushing of norbornylene in toluene with Grubbs' catalyst resulted in the self-assembly of fiber air-bridges as small as 4 nm diameter over nearly 6 microns length.