Micro-nano Scale Channels:Its Fabrication,Characterization And Applications

Abstract:Microfluidics and nanofluidics are two booming research fields that have been started at the end of the20th century. It is based on the special characteristics of structures at the microscale. Taking the advantages of the specialty, a serial of delicate controls and operations over the fluids can be realized. These controls and operations bear in them powerful functions that can be widely applicable in areas like bio-mimicking systems, drug screening&controlled release, in vivo&on line life monitoring systems and even microelectronics. However, in the present state, the two areas don’t have close connections because they neither share research goals nor fabrication techniques. Thus there are only a few researches that explore the smaller microscales (100nm～10μm) can bridge the two scales. Many important characters under the scale have not been fully explored or applied. Considering this, we are motivated to explore the smaller microscale. Researchers reported in this thesis follow the route of structure-function and application. So we first developed the chip fabrication technique that can help realize the structure of smaller microchannels. Then we integrated the channels to the chip for special functions that cover interfaces of liquid-gas. By applying their specialties, we are able to spread the structure to application areas covering ion preconcentration, cell culture&condition screenings, nanomaterial preparation and molecular analysis on the chip.1. Glass over-etching to bridge micro and nano channelsIn this study, a simple and economical fabrication technique bridging micro-and nanostructures is proposed. Glass molds with micro-nanostructures are fabricated by glass micro-lithography. The micro-lithography provides flexibility for structure design, and the glass etching contributes to transform the micro glass ridge to the nanoscale. Glass ridge structures with triangular cross sections are generated by undercutting, which coupled the isotropic character of glass and the shield effect of the top Cr layer upon HF etching. Further etching induced the height of the glass ridges to shrink from micro-to nanometers due to the edge effects. At the late etching stage, the geometrical change of the glass greatly slows down, which gives better control over the size of the glass ridge. By glass structure mold-copy, well repeatable, mechanically stable and tunable polydimethylsiloxane (PDMS) channels and cones are fabricated. Scanning electron microscopy (SEM) and laser interferometry (LI) are carried out to characterize the micro-nanostructures. Lastly, ion preconcentration was taken to confirm that the structures fabricated are really applicable.2. On chip steady liquid-gas interface generation based on smaller microchannels array structuresIn this work, the smaller microchannels are applied to bridge the liquid-gas phases in the two neighboring channels for a steady liquid-gas phase separation. In this basic structure, the steady phase is preserved by the strong capillary forces generated by the smaller microchannels. The basic physical principles behind the phenomenon is discussed. The relationship between the geometrical structure of the smaller microchannels arrays and the degree of stability is quantified by experiment. This work has made a solid foundation for the later applications that liquid-gas interfaces are based.3. On chip static dissolved gas concentration gradient generation for dynamic flowsIn this work, it is found that by coupling the one directional fluidic flow field and that of a steady liquid-gas interface, it is available to generate a dynamically steady dissolved gas concentration gradient (DgCG). By controlling the design of the density, distribution and the width of the smaller microchannels array, we have established theoretical models for four types of liquid-gas interfaces. It is proved that linear and even logarithmic distribution for the dissolved gas gradient can be achieved merely by adjusting the above mentioned three factors.4. On chip static dissolved gas and gas-liquid dual concentration gradient for quiescent liquid based on the combination of two typical gas diffusion interfaces with different diffusion speedThis paper introduced a novel dissolved gas concentration gradient (DgCG) generation mechanism for quiescent liquid systems. The chip adopted the typical3layered sandwich structures. On the top layer of the chip is the liquid channel network, in the middle layer is the array of through-hole reservoirs with gas channel networks on the bottom face and the bottom layer is a smooth surface. We added special structures to the gas channel layer for gas gradient generation. The mechanism combined the basic structure of smaller microchannels array for fast gas diffusion and the PDMS membranes for slower gas diffusion on the gas channel layer. And by tuning the thickness of the PDMS membranes the static DgCG can be effectively adjusted, which is proved both theoretically and experimentally. In addition to this, we are also succeeded in realizing the liquid-gas dual gradient by the combination of the liquid gradient as a second dimension to the statvic DgCG for the first time.5. On chip quantitative sample loading for microfluidic cell culture and condition screening systemsIn this work a novel microfluidic platform for cell culture and assay is developed. On the chip a static cell culture region is coupled with dynamic fluidic nutrition supply structures. The cell culture unit has a sandwich structure with liquid channels on the top, the cell culture reservoir in the middle and gas channels on the bottom. Samples can be easily loaded into the reservoir and exchange constantly with the external liquid environment by diffusion. Since the flow direction is perpendicular to the liquid channel on the top of the reservoir, the cells in the reservoir are shielded from shear-force. Cell culture models both for continuous perfusion and one-off perfusion were established on the chip. Both adherent and suspended cells were successfully cultured on the chip in2D and3D culture modes. After culturing, the trapped cells were recovered for use in a later assay. By assembling the basic units into an array, a steady concentration gradient can be generated. As a competitive candidate for a standard cell culture and assay platform, this chip is also successfully applied for cytotoxicity assays for nanomaterials.6. Liquid-gas dual phase microfluidic system for biocompatible CaCO3hollow nanoparticles generation and simultaneous molecule dopingIn this work, by combining the carbon dioxide gas-liquid interface with the chemical process of CaCO3crystal growth, we are succeeded in preparing CaCO3micro-nano structures that gradually changed within the DgCG. The structural transition of CaCO3are in the same trend with the change of the DgCG. Based on the results a specially designed microfluidic platform for doped CaCO3hollow nanoparticles generation is established, which utilized the smaller microchannels array to bridge the liquid and gas channels. It is available to generate CaCO3hollow nanoparticles continuously on this platform. By adjusting the flow velocity in the liquid channel, we can easily modulate the distribution of the DgCG and thus control the size of the CaCO3hollow nanoparticles. All the preparation procedures can be taken at room temperature and all in one step. Thus it is also easy to realize in-situ molecular dopping just by adding the molecular into the reactants in the liquid. Experiment proved that adding the molecular would have no observable influence to the structure of the hollow particles. Also, if fluorescence molecular is added, the special structure would have some resistance for fluorescence bleaching. Lastly, it is proved that the hollow particles have good biocompatibility. 7. Sample preconcentration for signal amplification based on controllable liquid evaporation in the microfluidic system and its application for multiplex DNA detectionIn this paper, sensitive and reliable multiplex DNA detection is realized in a specially designed microfluidic chip, which is based on microscale evaporation preconcentration. The chip is composed of eight evaporation reservoirs (each with0.1μL) in an array linked to the liquid channel in one end and a smaller microchannels array (SMA) in the other. The liquid channel enables sample loading to the evaporation reservoirs and the SMA bridges the evaporation reservoirs to the branched gas channel. After purging the solution in the liquid channel out by air, samples in the reservoirs are physically separated. Then the sample is subject to evaporation preconcentration, which can be accelerated by flowing gas. The evaporation would reach a plateau due to capillary condensation effect. After evaporation, the limit of detection (LOD) for DNA can be reduced from10nM to100pM. Moreover, the presence of evaporation plateau makes the preconcentration process highly repeatable and reliable. Multiplex DNA analysis can be realized simply by dip-coating different molecular beacons (MBs) in different evaporation reservoirs before chip bonding. This on chip evaporation approach is expected to have wide applications in bio-analysis due to the simple chip fabrication, easy operation, small sample consumption and efficient signal amplification.