Fabrication Of Integrated Pneumatic Microvalves In Polymer Microfluidic Chips

Various fundamental operations of the microfluidic chip such as sampling, reaction, concentration, separation and detection are all finished during the fluid flow process. So the manipulation and control of microfluids is the key technique of the microfluidic chips. Microvalve is one of the main components of microfluidics, it has the ability to open or close microchannels to realize the handling of microfluids. Pneumatic microvalve is a kind of active valve actuated by external compressed gas. It has the advantages of simple structure, reliable performance, easy integration and wide application range, which make it to be the most commonly used valve in microfluidic chip. The topic of this dissertation is focused on the development of integrated pneumatic microvalve in thermoplastic polymer microfluidic chips.In chapter1,different types of the microvalves used in microfluidic chips were introduced, especially the pneumatic microvalves and its working principle. The pneumatic valves can be categorized into normally-open valves and normally-closed valves according to their structures. The comparison between these two types was studied. The recent progress in the pneumatic microvalve was then reviewed within the following three aspects:chip material and bonding technique, using fluorinated ethylene propylene as elastic membrane and fabrication of rounded fluid channel. Finally, the applications of the pneumatic microvalve in different fields were presented.In chapter2, a method for preparation normally-open pneumatic microvalves in PMMA chips with double-layer elastic PDMS membranes was developed. In this chapter, the hot-embossed control and fluid substrates made of PMMA sheets were irreversibly bonded to a PDMS membrane (each PDMS membrane for a substrate), respectively, after the PMMA substrates were silanized and treated with plasma. Thus, a control half assembly and a fluid half assembly were respectively formed. Relying on the native adhesive force of the PDMS membranes, the control half and fluid half were then reversibly bonded, forming the full microchip integrated with the PMMA-PDMS···PDMS-PMMA four-layer structure. This novel method for fabricating normally-open pneumatic microvalve has the advantages of①easy and reliable bonding process, low equipment requirement;②individually replaceable control assembly and fluid assembly, suitable for disposable use. The performances of the microvalve were systematically characterized, such as bonding strength, relation between flow rate and fluid pressure, maximum fluid pressure the closed valve can withstand and the valve durability. And experimental results showed that the microvalve had high bonding strength, favorable switching performance and excellent durability. Through this method, two kinds of PMMA microfluidic chip with integrated pneumatic microvalves were prepared. One is a microchip with a single pneumatic valve used for two-phase droplet generation. The size of generated droplets was dependent on the valve-opening time duration and the result showed a good linear relation. The other is a microfluidic chip with integrated multi-pneumatic valves array. With the coordinated open-closed switching of the four pneumatic valves to form a sample loading/injection system, micro flow injection chemiluminescence determination of hydrogen peroxide was successfully carried out. Meanwhile, the feasibility of disposable use of only the fluid half was also confirmed.In chapter3,a method for preparation normally-open pneumatic microvalves in PMMA chips with fluorinated ethylene propylene (FEP) as actuating membranes was developed. FEP membrane has the characteristics of air-tight, chemically-inert and non-adsorptive. When the FEP membrane was used instead of PDMS elastic membrane, the microchip will also has the features of non-adsorptive and corrosion-resistant. But the high Young’s modulus of FEP brought great difficulty to the fabrication of microvalve. For the normally-open pneumatic microvalve, the cross sectional of the fluid channel was required to be round, so the positive mold used for hot-embossing of thermoplastic materials was also required to have round relief. This positive mold with round relief was commonly made by photoresist-reflow process, but it no longer works when preparing round fluid channel with high width-depth ratio. In this chapter, the arched FEP top wall of the compressed-gas-fulfilled channel was used as the mother mold to fabricate the high temperature resin positive mold via PDMS soft-lithography. Then the round fluid channel was successfully hot-embossed onto PMMA substrate. The full chip was made by sandwiching another FEP membrane between the prepared PMMA substrate with round fluid channel and that with control channel and bonded with adhesive resin. Before the bonding process, the FEP film should be treated with plasma to make both sides cementable. The relation between flow rate and fluid pressure for the as-prepared microvalve under the open/closed status was characterized. A pneumatic peristaltic micropump was formed by connecting three microvalves in series, and was applied in flow driving. Linear relationship between flow rate and actuation frequency was observed within the range1-20Hz.