Molecular Shuttles Operating Undercover:  A New Photolithographic Approach for the Fabrication of Structured Surfaces Supporting Directed Motility

The integration of active transport into nanodevices greatly expands the scope of their applications. Molecular shuttles represent a nanoscale transport system driven by biomolecular motors that permits the transport of molecular cargo under user-control and along predefined paths. Specifically, we utilize functionalized microtubules as shuttles, which may be transported by kinesin motor proteins along photolithographically defined tracks on a surface. While it was thought that efficient guiding along these tracks requires a combination of surface chemistry and topography, we show here that channel-like tracks with a particular wall geometry can be created to efficiently guide microtubules in the absence of selectively adsorbed motor proteins. This new wall geometry consists of an undercut 200 nm high at the bottom of the channel wall fabricated by image reversal photolithography using AZ5214 photoresist. Microtubules move unencumbered in the undercut, suggesting applications for nanofluidic systems and for in vitro motility assays mimicking the restricted environment characteristic of intracellular transport. Because adsorbed kinesin supports motility on top and bottom surfaces of the guiding channels, this guiding mechanism may serve as a first step toward the development of three-dimensional architectures.