Stop-Flow Lithography for Complex Particle Synthesis and Application in Directed Assembly

The synthesis of complex microparticles is an important objective. These particles can find use in a number of applications ranging from tissue engineering to ceramics and assembly. Tuned assembly of anisotropic particles can give rise to macrostructures with complex morphologies. Externally applied fields like electric or magnetic fields are useful ways to tune the assembly of anisotropic particles. This thesis begins with an understanding of the flow conditions in stop-flow lithography (SFL), the technique used for anisotropic microparticle synthesis, followed by the demonstration of the versatility of SFL by synthesizing soft cell-laden microgels, hard ceramic microcomponents and 3D curved microparticles. The thesis ends with the study of the assembly of anisotropic magnetic hydrogels synthesized using SFL. In the first section of the thesis, we introduce SFL and identify optimal conditions for particle synthesis using SFL. We do so by analyzing the dynamic response of a retracting PDMS wall after the removal of an external stress. We realized that for small deformations the problem lends itself to a regular perturbation analysis that is analytically solvable at zeroth-order. We compared the zeroth-order solution to the numerically solved full solution and to trends seen in experiments. In the second section we demonstrate the ability to synthesize complex particles using SFL. We generated anisotropic cell-laden microgels with reasonable cell viability. This work required the use of careful, benign conditions to ensure good cell viability and precise stop of the flow to ensure good resolution of cell-laden hydrogels. We determined an optimal cell density in a mixture of the cell suspension with the oligomer and photoinitiator. Then, we varied the concentrations of the oligomer and photoinitiator in the mixture to achieve reasonable polymerization times while simultaneously ensuring the desired cell viability. In a different work, we demonstrated the ability to make colloidal glass and silicon microcomponents using SFL. We flew a shear thinning colloidal silica suspension mixed with oligomer and photoinitiator through a microchannel and flashed UV light through a photomask to synthesize polymeric microcomponents of desired shape. In order to enhance their structural integrity, these colloidal microgears were transformed into fully dense, glassy silica microparticles by sintering at 1150 ◦C for 3 10 hours. SFL has traditionally been used to synthesize 2D extruded particles. We demonstrated the ability to synthesize 3D curved particles using SFL by introducing curvature in the direction orthogonal to the projection of UV light. We achieved this by co-flowing two streams which we called the polymerization and tuning fluid respectively, through a microchannel. On stopping the fluids, curvature developed at the