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控制小物体或大量液体的定向运动。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上探测大量的化学反应。虽然通常由机器人设备制造，将不同的试剂放置在特定的晶格位置，但人们希望开发一种策略，提供更简单和精确的控制，将化学物质定位在表面上微观定义的位置。实现这一目标的两种策略是微机电系统（MEMS）或微流体装置。两者都需要高度局部的驱动，要么控制机械装置的局部运动，要么控制外部驱动液体的流动。设计此类微致动器的最常见方法是利用已建立的光刻方法对硅、氧化硅或相关材料进行定影。然而，越来越多的人认识到，使用软材料可能有利于这种系统的设计。与硅或陶瓷材料相比，聚合物橡胶和凝胶具有更低的弹性模量，需要更低的驱动压力来实现大的位移。因此，与传统的执行器相比，驱动这些压力的电位要小得多。软驱动的良好候选者是膨胀的聚合物网络（凝胶）。已知聚合物凝胶经历可逆的不连续体积相变（它们膨胀或膨胀超过100倍），以响应其环境的微小变化。众所周知，温度、pH值或电场的一个相对较小的变化可以引起其体积和相关机械性能的巨大变化。但是，尽管早期认识到这种相变对于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-膨胀循环来运输微米大小的货物。其主要思想是在凝胶表面产生波，从而使货物颗粒移动。我们已经确定了三种物理机制，可以用来推动弱吸附粒子通过凝胶表面。该项目的目的是探索薄支撑水凝胶层的膨胀/溶胀动力学和形态，以及它们以良好控制的方式运输微米尺寸货物的用途。

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.