Microfluidics with Nanoscale Additive Manufacturing: Devices and Applications

Over the past years, additive manufacturing technologies have emerged as a powerful tool in the field of microfluidic engineering, owing to their powerful capabilities in fabrication of 3D microstructures for diverse functions. However, most of the current applications in microfluidics only adopt these technologies to create simple microchannels rather than functional components, since the resolution of conventional additive manufacturing techniques usually fall within the range of tens to hundreds of micrometers. Herein, this dissertation has centered on the use of a specific additive manufacturing technique that possesses extremely high resolution, the two-photon polymerization technique, for different applications in microfluidics. Given the fact that two-photon polymerization builds objects through a voxel-by-voxel manner, the time of fabrication could be significantly long, therefore it is more suitable to create only the essential parts at regions of interest (ROI) in a microfluidic device, instead of creating the whole device. More specifically, based on this criterium, we have contributed to several applications of microfluidics, including novel fabrication method for soft lithography, superhydrophobic foil with hierarchical structures, acoustofluidic micromixer and micropump, as well as the cell traps. First of all, a novel hybrid fabrication method based on photolithography and two-photon polymerization has been developed to create master moulds in soft lithography. As a result, this proposed method prevents the huge time expenses from printing the entire parts using two-photon polymerization, along with taking advantages of conventional photolithography. Afterwards, we have investigated the capabilities of two-photon polymerization in the control of surface wetting by creating fractal Sierpinski tetrahedron and hierarchical pyramid microstructures on glass slides and flexible plastic films. Since different microstructures can be built at different regions, the proposed method could be applied for a controllable management of surface wetting. Based on the experience of creating microstructures on flexible foils, we have developed a novel acoustofluidic micromixer by means of embedding the created microstructures in flexible devices made of off-the-shelf materials such as plastic films and double-sided tapes. The mixing effect is generated from 3D printed microstructures, through which the acoustically oscillated air-liquid interfaces are connected to the ambient air, thus preventing the oscillation of these interfaces from dissolution and compression. Extending the capabilities of the embedded microstructures proposed, we have also developed an acoustofluidic micropump on the foils. A maximal flow rate of 420 nL/min can be obtained when the driving voltage was set to 4 Vpp. To further investigate the pumping abilities of the proposed micropump, it has been utilized to drive flows for single-cell trapping. Despite the successful development and implementation of devices in several applications, the work has been not comprehensive enough. This is because the field is still at its beginning stage, and encompasses numerous uncovered matters and possibilities. Therefore, fundamental studies on topics such as photopolymerization, hydrophobicity, post treatments, acoustofluidics, and others, have to be done in the next stage to further improve the performance of two-photon polymerization in these applications and expand its capabilities in other fields. On the other hand, two-photon polymerization is still not a widely accessible technique, since it requires expensive fabrication systems and long fabrication time. Therefore, future works should empathize on further improvements in critical aspects such as the speed and cost of fabrication. Nevertheless, we truly believe the proposed devices and applications in this dissertation have manifested the capabilities of two-photon polymerization in microfluidics and other fields, and will serve as an important example and the foundation for future developments in microfluidics and nanoscale additive manufacturing technologies.