Controlled Transport of Micrometre-Sized Cargoes by Hydrogel Actuation

纳米科学和工程中的一个重要问题是微小物体或体积液体的受控定向运动。一个典型的例子是诊断阵列(例如DNA或蛋白质芯片)，它能够在很小的表面积上探测大量的化学反应。虽然通常由将不同试剂放置在特定晶格位置的机器人设备制造，但希望开发出提供更简单和更精确的控制以将化学物质放置在表面上微观定义的位置的策略。实现这一目标的两种策略是微电子机械系统(MEMS)或微流控设备。两者都需要高度局部化的驱动，或者控制机械设备的局部运动，或者控制外部驱动液体的流动。设计这种微致动器最常见的方法是利用已有的光刻方法对硅、氧化硅或相关材料进行图案化。然而，人们越来越意识到，使用软材料可能对此类系统的设计有利。与硅或陶瓷材料相比，聚合物橡胶和凝胶的弹性模数要低得多，需要低得多的驱动压力才能实现大位移。因此，与传统的致动器相比，驱动这些压力的势能要小得多。适合软致动的候选材料是膨胀的聚合物网络(凝胶)。众所周知，聚合物凝胶会经历可逆的不连续体积相变(膨胀或膨胀超过100倍)，以响应其环境的无限微小变化。众所周知，温度、pH或电场的相对较小的变化可以引起其体积和相关机械性能的剧烈变化。但是，尽管人们很早就意识到这种相变对许多技术应用具有潜在的重要意义，但凝胶驱动的技术实现尚未出现。在这个项目中，我们计划利用薄凝胶层的膨胀-消胀循环来运输微米级的货物。主要的想法是在凝胶表面诱导波，从而使货物颗粒沿其移动。我们已经确定了三种可以用来推动弱吸附颗粒穿过凝胶表面的物理机制。本项目的目的是探索薄层支撑水凝胶层的膨胀/消胀动力学和形态，以及它们在以良好控制的方式运输微米级货物方面的应用。

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.