| Working in Nanofabrication and Biophysics Labs at Caltech, I have developed several building blocks for rapid DNA sizing, cell sorting, molecular fingerprinting, and hybridization assays. First, a microfabricated flow-cell device was developed using 'soft lithography'. It offers a small, cheap and robust alternative to the complicated glass-capillary structure used in conventional flow cytometers. Based on this device, a highly sensitive single-molecule DNA sizing system was demonstrated. It is 100 times faster and requires a million times less sample than pulsed-field gel electrophoresis. Moreover, DNA and cell sorting has also been demonstrated under this system, where simple enclosed actuation schemes are implemented. System downtime for capillary cleaning and the cross-contamination issue are eliminated, because the device costs only pennies to make and becomes disposable. Using this system, prototype work for rapid DNA fingerprinting was also devised as an alternative to the Southern blot protocol. Molecular evolution, VNTR fingerprinting of forensic samples, disease diagnosis based on RFLP, and DNA genomic mapping can all be accomplished with this system. In addition, a multilayer soft lithography technique was invented, allowing monolithic microvalves and micropumps to be built into these flow-channel devices. Active microfluidic systems containing on-off valves, switching valves and pumps were made, entirely out of elastomer. The softness of these materials allows the device area to be reduced by more than two orders of magnitude compared with silicon-based devices. An actuation volume as small as one picoliter was demonstrated. The other advantages of soft lithography, such as rapid prototyping, ease of fabrication, and biocompatibility, are retained. Based on these components, an active integrated diagnostic chip was built. More than two orders of magnitude improvement in terms of binding speed and efficiency over passive devices was shown. Selective surface patterning of various diagnostic probes within the chips by elastomeric flow channels was also shown. With active pumping, we are able to make a rotary motion in these microfluidic devices and show fast inline mixing which overcomes the limitation of laminar flow in this low-Reynolds number regime. Moreover, problems of buffer depletion due to electrolysis in electroosmotic flow control do not exist in these devices. |
