(464g) Microfluidic Formation of Ionically Crosslinked Polyamine Gels

Recent advances in microreaction engineering using microfluidic devices has led to the the generation of polymer structures with a variety of shapes and morphologies. Microfluidic devices allow for the confinement of reagents and precise control of the time of reaction. A current focus area in microfluidic research is on flow and formation of gels in microchannels. Primary applications of gels in microfluidic devices are in forming components for flow control, separations, detection and sensing, geared towards developing micro total analytical (μTAS) systems.¹

An important approach to incorporating gels within the microchannel is to form the gel structures in situ. This route enables gel deposition at precise locations in a microchannel. So far, in situ gel formation in microfluidic channels is based on polymerization and electrostatic cross-linking reactions. However, these routes often require additional fabrication steps of flow focusing, geometric confinement and on-line UV/photolithography. Bazargan and Stoeber recently reported on reversible gelation in microchannels, from phase-separation of thermoresponsive pluronics with salt solutions of Na₃PO₄.²

In this study, we demonstrate in situ polymer gelation from electrostatic cross-linking reactions of cationic poly(allylamine hydrochloride) and citrate anions. This is the first example of in situ and in-channel polymer gel formation, which occurs at room temperature and does not require an immiscible phase or a flow focusing geometry. Formation parameters such as charge ratio, flow shear stresses and pH along with existing laminar flow conditions are seen to influence gel morphology. We find that the polyamine exhibits shear-thickening behavior when reacting with the citrate ions to form a viscoelastic gel phase. This gel phase remains stable and intact after cessation of flow. We also show that the extent of polymer gelation and disintegration of gels can be controlled by changing individually, the pH of citric acid and PAH streams. Hence, reversibility of gelation can be built into the system using pH as a variable. A model is proposed based of a phase diagram in charge ratio and flow (shear stress) that describes the mechanism and kinetics of gel-formation.

References:

1. Peterson, D. S., Solid supports for micro analytical systems. Lab on a Chip 2005, 5, (2), 132-139.

2. Bazargan, V.; Stoeber, B., Moving temporary wall in microfluidic devices. Physical Review E 2008, 78, (6).