Design of Polymers With Orthogonal Gelation Schemes for Water-Soluble Photo-Patterned Release Layers

Photolithographic microfabrication, pervasive in semiconductor device fabrication, micro-electromechanical systems, and microfluidic channels, often involves sacrificial release layers that must be washed away with harsh chemical processing. Fabricated polymer-based microstructures, including stimuli-responsive surfaces, shape-morphing particles, and biomedical devices, may often not survive these steps, motivating the development of easily-removable release materials. Through controlled radical polymerization, we have synthesized water-soluble polymers with two orthogonal gelation mechanisms, one reversible and chemically driven, the other irreversible and photochemically driven. In solution, we first form Schiff bases between pendent benzaldehyde groups and diamine crosslinkers, yielding branched polymers that remain below the gel point. Next, in the solid-state, pendent benzophenone groups are used to photochemically crosslink the material, selectively forming gelled structures in irradiated regions that remain after unexposed regions are removed. After microfabrication of desired polymer structures, washing with a slightly acidic aqueous solution causes the photo-patterned release film to dissolve due to reversible dissociation of the Schiff base. We have studied how the content of the two crosslinking monomers and stoichiometry of diamine to benzaldehyde control the ability to reversibly cross the gel point, enabling effective use of this approach.