Exploration of post-print modification of 3D photo-printed materials for microfabrication by means of RAFT polymerization

A rather underappreciated aspect in the field of lithographic microfabrication is the post-print modification of 3D printed structures, which could be relevant to introduce or modify properties of both the bulk and the surface of materials. This study focusses on a strategy revolving around controlled polymerization, specifically reversible addition–fragmentation chain-transfer (RAFT) polymerization, to create a “living” resin material. We explored three different strategies which all focus on optimizing a different virtue of introducing a RAFT agent in microfabrication by two-photon lithography (TPP). The first method introduces a trithiocarbonate in a densely cross-linked material and changes the hydrophobicity of the surface in the chain extension process. To activate the chain extension, a zinc-based catalyst in the chain extension solution was irradiated by green light. The second approach instead altered the properties of the bulk of the structure. A different RAFT agent, a xanthate, was applied to increase the reaction rate and the cross-linking density was reduced. The property changes as a result of the chain extension were monitored by the change in the FT-IR spectrum and a lowering of the Young's Modulus of TPP printed microstructures. Finally, a tetra-functional self-polymerizing xanthate was synthesized, which allows for initiator-free polymerization and subsequent chain extension. These results show the successful modification of both bulk and surface properties of microstructures using RAFT control. Since all of the chain extensions are all light induced, this study opens avenues for localized property modification with focused light.