MEMBRANE-INTEGRATED AND MEMBRANE-FREE MICRO AND NANOFLUIDICS FOR ACCURATE MOLECULAR TRANSPORT IN BIOLOGICAL ASSAYS

Abstract

Nanofluidics has a comparable characteristic length to the size of ions and biomolecules, so that it can be used as an efficient platform to conduct accurate biochemical assays/analyses. In particular, the nanofluidic elements are often embedded into microfluidics, forming integrated micro/nanofluidic networks for even more complex and systematic applications such as electrokinetic pumps, transistors, and energy convertors. Such innovative applications using unique mass-transport phenomena in micro/nanofluidic devices become available with the development of novel and precise nanofabrication techniques. For example, conventional nanolithography techniques such as electron or focused-ion beam lithography are used widely because of high degree of precision and accuracy of patterning. Another general nanofabrication method is micro-electromechanical system (MEMS)-based techniques that include both top-down (e.g., silicon etching) and bottom-up (e.g., thin-film deposition) approaches. However, it has been a challenge to fabricate the mixed-scale micro/nanofluidic devices by using either the conventional nanolithography utilizing the high-energy beams or the conventional MEMS-based nanofabrication because of the cost, time, throughput, and incompatibility issues of the methods. In particular, the limitations become more critical when both the microfabrication and the nanofabrication techniques need to be used in series to make micro/nano multi-scale structures. Therefore, an innovative alternate technique is specifically required to address the current weaknesses of both the conventional nanolithography and the MEMS-based nanofabrications. This dissertation describes novel and unconventional methods to fabricate mixed-scale micro/nanofluidic devices by integrating nanoporous hydrogels and ion selective membranes (ISMs) into microfluidic devices (membrane-integrated micro/nanofluidics). On the other hand, the micro/nanofluidic devices can be also fabricated by employing microfabricated ratchet structures that perform the same functions of a membrane, and by intentionally creating nanoscale cracks to produce nanochannels (membrane-free micro/nanofluidics). The dissertation’s early chapters deal with the development of novel nanomaterial-integrating methods to accurately control mass-transport phenomena at the micro/nanofluidic interfaces. A variety of hydrogel membranes are employed to enable pure diffusive or pure electrophoretic transport for accurate and active controls of chemical environments. In addition, ISMs are used to perform permselective ion transport for electrokinetic applications. The late chapters of this dissertation introduce membrane-free mixed-scale micro/nanofluidic devices that possess enhanced capabilities compared to the membrane-based devices, including higher precision and robustness in mass-transport controls, and higher compatibility with existing microfluidic components. First, an arrowhead-shaped ratchet microstructure in a microfluidic device physically compartmentalizes micron-sized bacterial cells but allows diffusion-controlled chemical environments without convective drag to the cells, which is commonly performed by a nanoporous membrane or a nanochannel. That is, the microfabricated ratchet structure acts the same function of a nanofluidic element without nanofabrication. Second, nanochannels and microchannels are fabricated simultaneously by an unprecedented cracking-assisted nanofabrication technique (called crack-photolithography) that relies only on a standard photolithography process. The crack-photolithography produces well-controlled micro/nanochannels in any desired shapes and in a variety of geometric dimensions, over large areas and with a high-throughput. Hence, mixed-scale micro/nanofluidic devices can be fabricated by the same technique that is used to fabricate a microchannel without additional nanofabrication processes and expensive equipment. Basically, the membrane-integrated and membrane-free micro/nanofluidic devices in this dissertation have the same mission, the transport control of biomolecules and chemical species to conduct biological assays in an accurate and high-throughput manner. As practical applications, the mixed-scale micro/nanofluidic devices are used for performing electrokinetic biosample pretreatments such as concentration and separation for ultra-sensitive and ultra-selective detection of target analytes such as proteins, particles and bacterial cells. In addition to the electrokinetic biomolecular/bacterial handling, the devices are also used for accurate characterizations of bacterial behavior such as chemotaxis and gene expression under convection-free and diffusion-controlled chemical stimulations. The role of a nanofluidic element such as a nanoporous membrane and a nanochannel array in microfluidics is essential to enable accurate and permselective transports of ions and molecules in various bioassays. In this context, the proposed membrane-based or membrane-free micro/nanofluidic devices play both microfluidic and nanofluidic functions without complicated nanofabrications, resulting in time-/cost-efficient and high-throughput fabrication. Thus, the research achievements in this dissertation substantially contribute to popularize and revolutionize the micro/nanofluidic systems and technologies, which have been hindered due to expensive and time-consuming conventional nanofabrications.