Fabrication of size- and shape- controllable mixed-scale PDMS channel networks including 3D microfunnels

Abstract

In this thesis, a novel batch fabrication technology for a mixed-scale PDMS (Polydimethylsilane) channel networks using carbon- micro electromechanical systems (MEMS) and single molding process is introduced. Nanofabrication has been used in broad research fields including single electron memories, multiple tunnel junction (MTJ) devices, micro electromechanical systems (MEMS) sensors, and nanofluidic devices. Nanofluidic channel networks are crucial parts in various applications such as DNA electrophoresis, bio sensors, molecular preconcentration, ionic transport nanofluidic diodes, desalination, nanofludic transistors because of their unique phenomena including ion concentration polarization, ion rectification effect, nanocapillarity and electrical double layer overlap. However, the research on the nanofluidics has been limited because of the lack of simple and cost-effective nanofabrication technologies. Microfabrication technologies that are compatible with nanofabrication processes also needs to be developed so as to integrate microfluidic channels with nanochannels, because the microchannels guide sample fluid into and out of the nanochannels and thus mixed-scale channel networks are the basic architecture of the nanofluidic devices. However, the fabrication of mixed-scale channel networks is limited by difficult alignment processes between nanostructures and microstructures, high fabrication cost, and time consuming and complex processes. These limitations can be overcame by utilizing carbon-MEMS technology and single polymer molding process. A mixed-scale carbon structure as the mold of a Poly (dimethylsiloxane) (PDMS) channel network was fabricated using conventional UV lithography and pyrolysis. A single molding process using multi-layers of hard PDMS and soft PDMS completes the fabrication of mixed-scale PDMS channel networks including 55-nm-high and 441-nm-wide nanochannels. The quality of the PDMS molding process was evaluated using a scanning electron microscope (SEM) and an atomic force microscope (AFM). The surface energy of the pyrolyzed carbon mold could be modulated by controlling pyrolysis temperature resulting in high surface energy. As a result, a single carbon mold could replicate PDMS channel networks more than 40 times without anti-adhesion coating. The hermetic sealing and uniformity of the PDMS nanochannel were evaluated by filling the mixed-scale PDMS channel networks with fluorescein isothiocyanate (FITC). The properties of PDMS nanochannels were characterized by measuring I-V relationship in KCl solution. Except for the simple fabrication of the complex carbon mold, the pyrolysis process also enabled the formation of smoothly tapered mold side wall because of good adhesion between the photoresist and substrate, and volume reduction in pyrolysis. As a result, 3-D funnels could be integrated at the entrance and exit of the nanochannel. By this novel 3-D funnel structure, efficient entrapment of a single micro particle at the entrance of a nanochannel was enabled. It is expected that the mixed-scale PDMS channel networks with 3D-funnels can be applied to nanoelectroporation for efficient cell transfection