Design and fabrication of microfluidic valves using poly(N-isopropylacrylamide)

Abstract

A compact printable microfluidic valve composed of poly(N-isopropylacrylamide) has been designed, fabricated, and tested. The design of the valve consists of filling microwells with poly(NIPAAm) and bonding PDMS channels above them. This filling is achieved using thermal inkjet printing of a prepolymer solution and subsequent polymerization using UV irradiation. When the gel is swollen, it blocks flow from passing through the channel. Upon heating, the gel shrinks and allows flow in the channel. Poly(NIPAAm) is a thermosensitive hydrogel that exhibits an inverse temperature expansion behavior. When the temperature of the swollen gel is raised above a lower critical solution temperature (LCST) of approximately 32°C, the gel becomes hydrophobic. This change in hydrophobicity results in expulsion of the water molecules from within the hydrogel network, thus resulting in shrinking of the gel. By adding magnetic nanoparticles to the hydrogel and exposing it to an external magnetic field, volumetric change of the hydrogel can be locally and externally induced. External heating of the magnetic nanoparticles, however, is not included in this thesis. In order to ensure shrinkage that is predictable in favor of flow control, microanchor structures have been designed, modeled, and fabricated at the bottom of the microwells. These microanchors hold the poly(NIPAAm) at the bottom of the plug such that the shrinkage of the gel always acts to open the flow channel at the top yielding a minimum pressure drop. Design decisions were made using the principles of Axiomatic Design in order to minimize the response time and pressure drops in the valve. Modeling of the underlying mechanisms is described along with the application of these models to the final device. Results of fabrication suggest the feasibility while also eliciting possible improvements to the fabrication process.

(cont.) Profilometry measurements of the swollen and shrunken valves reveal flow control operation as intended. Additionally, design and modeling of magnetic heating using mixed-in nanoparticles is presented. A fabrication plan designed to include this mechanism is proposed.