(168k) Microfluidic Formation of Polyamine Gels and Capsules by Crosslinking and Flow Focusing Approaches

Recent advances in microreaction engineering using microfluidic devices have resulted in polymer structures with a variety of morphologies and shapes. The microfluidic environment allows for confinement of reagents and precise control of reaction times. A current focus area in microfluidic research is on formation of gels and capsules. Gels in microchannels have been utilized for flow control, separations, detection, and sensing.¹ Capsule synthesis in microchannels has focused on utilizing laminar flow and flow-focusing geometries to achieve size and compositional control on a continuous mode basis.²

In this study, we demonstrate microfluidic approaches to tune interactions between a polyamine (poly (allylamine hydrochloride) or PAH) and a multivalent anion (citric acid) to form gels and capsules. We recently reported electrostatic mediated crosslinking of polyamines by citrate ions in a microfluidic channel that resulted in formation of viscoelastic gel phases. Gelation occurred in situ and at room temperature, without requirements of flow focusing or an immiscible fluid phase, where gel morphology was found to depend on reactant charge ratios, shear stresses and the pH of reactant streams.³

Here, we first investigate applications of these gels as flow regulatory devices of aqueous systems in microchannels. Gel thickness and consequently flow regulation could be controlled by varying individually, the pH of PAH and citric acid streams. In varying pH to span the pK_a value(s) of the reactants, gel thickness was found to be reversible, although it exhibited a hysteresis profile. Gel hysteresis could be rationalized by diffusion-limited transport of anions across different gel thickness barriers. To regulate flow of organic liquids in microchannels, covalently crosslinked polyamine gels were synthesized. These gels were formed in situ using an aldol condensation reaction scheme wherein, pre-formed ionically crosslinked polyamine gels were covalently crosslinking with glutaraldehyde. The resulting gels were permanent and demonstrated successful flow regulation of organic liquids without gel collapse. To form capsules, a strategy using flow focusing microfluidic geometry with immiscible fluids (CO₂ /paraffin oil) was employed.

References:

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

2. Steinbacher, J. L.;McQuade, D. T., Polymer chemistry in flow: New polymers, beads, capsules, and fibers. Journal of Polymer Science Part a-Polymer Chemistry 2006, 44, (22), 6505-6533.

3. Kini, G.;Lai, J.;Wong, M.;Biswal, S. L., Microfluidic Formation of Ionically Crosslinked Polyamine Gels. Langmuir 2010, DOI: 10.1021/la903983y.