Controlled Transport of Micrometre-Sized Cargoes by Hydrogel Actuation

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An important issue in nanoscience and engineering is the controlled, directed motion of of small objects or volumes of liquid. A typical example are diagnostic arrays (e.g. DNA or protein chips) that are able to probe a large number of chemical reactions on a small surface area. While typically manufactured by robotic devices that place different reagents at specific lattice positions, it is desirable to develop strategies that provide a more simple and precise control to position chemical substances at microscopically defined positions on a surface. Two strategies to achieve this are micro-electro-mechanical systems (MEMS) or microfluidic devices. Both require a highly localised actuation that either controls the local motion of a mechanical device or the flow of an externally driven liquid. The most common way to design such micro actuators makes use of established lithographic methods to pattern silicon, silicon oxide, or related materials. There is, however, an increasing realisation that the use of soft materials may be beneficial for the design of such systems. Polymeric rubbers and gels have a much lower elastic modulus compared to silicon or ceramic materials, requiring much lower actuation pressures to achieve a large displacement. The potentials that drive these pressures are therefore much smaller compared to conventional actuators.Good candidates for soft actuation are swollen polymer networks (gels). Polymer gels are known to undergo a reversible discontinuous volume phase transition (they swell or deswell by a factor of more then 100) in response to infinitesimal changes in its environment. It is well established that a relatively small change in temperature, pH, or electric field can give rise to a dramatic change in its volume and the related mechanical properties. But despite the early realisation that this phase transition is potentially important for a number of technological applications, the technological implementation of gel actuation has yet to emerge. In this project we plan to make use of swelling-deswelling cycles of thin gel layers to transport micrometre-sized cargoes. The main idea is to induce waves on the surface of the gel, by which cargo particles are moved along. We have identified three physical mechanisms that can be used to propel weakly adsorbed particles across a gel surface. The purpose of this project is to explore the swelling/deswelling kinetics and morphology of thin supported hydrogel layers and their use to transport micrometre-sized cargoes in a well controlled fashion.