Author

Chong Hu

Year of Award

1-31-2018

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Department of Chemistry.

Principal Supervisor

Ren, Kangning

Keywords

Microfabrication;Microfluidic devices.

Language

English

Abstract

The present thesis includes a series of studies on microfluidic technology from novel microfabrication methods in polymers to diverse microfluidic applications. Specifically, this study focuses on some key issues in microfluidics, regarding the development of microfluidic fabrication strategy, material selection for microfabrication, and applications, in particularly controllable reactions of novel polymer-based microfluidic devices. We have developed novel methods, which hold completely different idea/ concept with conventional approaches', for fabrication of microfluidic chips with polymer materials. While for the microfluidic applications, the thesis exhibits cell perfusion experiments with freestanding 3D microchannels made of alginate hydrogel, convenient and sensitive lead(Ⅱ) ions detection on a plastic membrane microfluidic chip which was fabricated by the proposed novel one-step strategy, as well as and microfluidic controllable synthesis of enzyme-embedded metal-organic frameworks in a laminar flow;In the first part, we proposed an inside-out fabrication strategy using a copper scaffold as the sacrificial template to create freestanding 3D microvascular structures containing branched tubular networks with alginate hydrogel. The microvascular structures produced with this method are strong enough to allow handling, biocompatible for cell culture, appropriately porous to allow diffusion of small molecules, while sufficiently dense to prevent blocking of channels when embedded in various types of gels. In addition, other materials and biomolecules could be pre-loaded in our hydrogel tubular networks by mixing them with alginate solution, and the thickness of tubule wall is tunable. Compared to other potential strategies of fabricating free-standing gel channel networks, our method is parallel processing using an industrially mass-producible template, making our method rapid, low-cost and scalable. We demonstrated cell culture in a nutrition gradient based on a microfluidic diffusion device made of agar, a hydrogel traditionally hard to microfabricate, by embedding the synthesized tubules into the agar gel. In this way, the freestanding hydrogel vascular network we produced is a universal functional unit that can be integrated with other gel-based devices to build up the supporting matrix for 3D cell culture outside the hydrogel vascular structure; allowing great convenience and flexibility 3D culture. The method is readily implementable to have broad applications in biomedicine and biology, such as vascular tissue regeneration, drug discovery, and delivery system in 3D culture.;The second part, we developed a one-step method to mass produce microfluidic chip with thermal plastic membranes. We used a perfluoropolymer perfluoroalkoxy (often called Teflon PFA) negative mold, which is very nonsticky and has ultrahigh melting point, as solid stamp to thermal-bond two pieces of plastic membranes, low density polyethylene (LDPE) and polyethylene terephthalate (PET) coated with ethylene-vinyl acetate copolymer (EVA), which have different coefficients of thermal expansion. During the short period of contact with the heated Teflon stamp, the pressed area of the membranes permanently bonded, while the LDPE membrane spontaneously rose up at the area not pressed, forming microchannels automatically. These two regions were clearly distinguishable even at micrometer scale so that we were able to fabricate microchannels with width down to 50 microns. By using thermal-bonding, the pattern of Teflon mold will be transferred to the plastic membrane forming channels while two membranes will be bonded at the same time. The method enables generation of microchannels and bonding process to accomplish in a single step without sophisticated instruments. One Teflon mold can be used to mass replicate many plastic membrane chips in a short time because each round needs only a few seconds. Our method can fabricate a plastic microfluidic chip rapidly (within 12 seconds per piece) at an extremely low price (less than 0.02{dollar} per piece). We also showed some identical microfluidic manipulations with the flexible plastic membrane chips including droplet formation, microfluidic capillary electrophoresis and squeezing-pump for quantitative injection. In addition, we demonstrated convenient on-chip detection of lead ion by a peristaltic-pumping design, as an example of the applications of the plastic membrane chips in resource-limited environment. Due to the fast production method and low-cost of plastic materials, this one-step method will hopefully lead to new opportunities for the commercial implementations of microfluidic technologies.;Finally, on the basis of preliminary study of microfluidic laminar flow synthesis of MOFs in aqueous system in Chapter 4, we successfully synthesized and investigated formation of enzyme-embedded metal-organic frameworks (MOFs) in a continuous laminar flow on a microfluidic chip. Resultant enzyme-MOF composites displayed higher enzymatic activity than enzyme-MOF composites from bulk solution synthesis. A possible reason was that the precisely controlled and yet changeable reaction conditions such as reaction time and diffusive mixing of reagents allowed the fast reaction to be isolated into controllable processes and studied with predesigned yet changing conditions. This, in return, led to distinct morphological characteristics and activities of the enzyme-MOF composites compared to those from bulk synthesis. The results indicated that the highest activity of enzyme-MOF composites was obtained when metal ions and organic ligands were first gradually mixed within a few seconds before enzyme molecules joined the gradual mixing process. We found that the crystallinity degree of as-produced enzyme-MOF composites was reduced via the microfluidic flow synthesis, containing more structural defects compared to those with high degree of crystallinity from bulk synthesis. The reduced crystallinity allowed more effective approaching of substrates with enzyme embedded in composites and therefore an increased enzyme activity compared to enzyme-MOF composites from bulk synthesis. We further demonstrated that enzyme-MOF composites showed enhanced stability against elevated temperature and protease digestion compared with free enzymes, allowing their wider utility in biotechnology.

Comments

Principal supervisor: Doctor Ren Kangning;Thesis submitted to the Department of Chemistry. ; Thesis (Ph.D.)--Hong Kong Baptist University, 2018.

Bibliography

Includes bibliographical references (pages 130-141).

Available for download on Thursday, August 01, 2019



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