Microfluidic approaches to the synthesis of complex polymeric particles

Abstract

The synthesis of micron-sized polymeric particles with precise control over shape, monodispersity and chemistry is a technologically important objective. Varied applications including medical diagnostics. designer fabrics and optical devices could benefit from the availability of geometrically complex and chemically inhomogeneous particles. Microfluidics has recently emerged as an important alternative route to the synthesis of such complex particles. This thesis presents three new approaches to complex particle and structure synthesis in microfluidic devices. In the first approach. droplets formed by shearing a curable photopolymer. using a continuous water phase at a T-junction, were constrained to adopt non-spherical shapes by confining them using appropriate microchannel geometries. The non-spherical shapes formed were permanently preserved by photopolymerizing the constrained droplets in situ using focused ultraviolet (UV) light from an inverted microscope. The second and more general method called Continuous Flow Lithography (CFL) is a one-phase, projection photolithography based process to continuously synthesize polymeric microparticles in any 2-D extruded shape down to the colloidal length scale.

(cont.) Polymerization was also performed across laminar. co-flowing streams to generate Janus particles containing different chemistries, whose relative proportions could be tuned easily. CFL was also used to synthesize 'particle surfactants' that assembled at the interface of oil-water emulsions or formed micelle-like structures in water. While CFL was able to synthesize particles in non-spherical shapes with chemical anisotropy, particle throughput and resolution was a concern. To mitigate these problems, a new setup called Stop Flow Lithography (SFL) was devised. In SFL, a flowing stream of oligomner is stopped before polymerizing an array of particles into it, providing for much improved resolution over particles synthesized in flow. The formed particles are then flushed out at high flow rates before the cycle of stop-polymerize-flow is repeated. The high flow rates enable orders-of-magnitude improvements in particle throughput over CFL. However, the deformation of the PDMS elastomer due to the imposed pressure restricts how quickly the flow can be stopped before each polymerization event. We have developed a simple model that captures the dependence of the time required to stop the flow on geometric parameters such as the height, length and width of the microchannel, as well as on the externally imposed pressure.

(cont.) A third approach to synthesizing particles uses elastomeric phase masks to build all-PDMS devices. Coherent laser light passing through a phase mask generates a complex 3D distribution of intensity that selectively exposes certain regions while leaving out others. This results in the formation of 3-D structures whose features can be tuned at the micron scale and below. We have attempted the formation of 3-D structures in hydrogel polymers which could have important implications in the field of tissue engineering. Finally, we have developed a simple model of the oxygen inhibited polymerization that occurs in flow lithography. This model is able to qualitatively predict the presence of a thin, uncrosslinked layer of oligomer close to the walls of the PDMS device. This layer is critical to our ability to flow out particles in flow lithography. This thesis demonstrates that microfluidics is indeed a viable and promising route to the synthesis of complex polymeric particles and structures.