Post-Functionalization of Photocured Microstructures by Living Polymerization

Lithographic technologies have enabled fabrication of 3D microstructures and robotics with<br/>sub-micron precision. Despite the rapid expansion of available printable materials, there is<br/>still a fundamental constraint in the state-of-the-art technology, which is a versatile method to<br/>functionalize the structures after the fabrication process. Post-functionalization has the<br/>potential to change the properties of a structure, for example by introducing hydrophobic or<br/>hydrophilic groups to the structure’s surface. Furthermore, it can allow for introduction of<br/>materials that are incompatible with lithographic techniques.<br/><br/>We approached this challenge by combining lithographic curing with ‘living’ polymerization,<br/>specifically ‘reversible addition−fragmentation chain-transfer’ (RAFT). In contrast with regular<br/>free radical polymerization used for curing, which produces ‘dead’ polymer chains, RAFT-<br/>polymerization still has ‘living’ end groups that can reactivate and continue polymerization<br/>with different monomers in a later stage. For the introduction of living polymerization, recent<br/>development of photo-activated initiation and oxygen tolerant systems are crucial for facile<br/>implementation. These have mainly been developed for RAFT (reversible addition-<br/>fragmentation chain-transfer polymerization), which is based on the reversible breaking of a<br/>covalent bond in di- or trithio species. [1] Recently, the group of Boyer have shown 3D<br/>structures on a macro-scale and showed the possibility to post-functionalize them. [2] Their<br/>system to build the 3D structures however uses layer-by-layer green-light 3D printing and<br/>requires 3 to 30 minutes of light irradiation for every layer of the structure. The inherent slow<br/>speed compared to free radical polymerization, makes RAFT probably unsuitable for<br/>micrometer scale lithographic techniques like direct laser writing.<br/><br/>For this reason, in the approach proposed here, we cure structures using regular UV, but<br/>crucially introduce a RAFT-group to the photoresist either through covalent attachment or as<br/>a macro-initiator. In a second step, we use photo-RAFT for chain extension and thereby<br/>modify the surface properties of the 3D microstructure. As simple proof of the livingness of<br/>the process, we perform the chain-extension with a fluorescent monomer, which can be<br/>easily identified post-synthesis. We are furthermore looking into the use of different<br/>monomers and photoresist compositions that are not limited to the surface, but suitable for<br/>chain extension in the matrix of the structure. Ultimately, we aim to fabricate 3D<br/>microstructures by direct laser writing, using photoresists containing agents suitable for post-<br/>functionalization by RAFT.<br/><br/>[1] <i>J. Chiefari, Y. K. (Bill) Chong, F. Ercole, J. Krstina, J. Jeffery, T. P. T. Le, Roshan T. A.<br/>Mayadunne, G. F. Meijs, C. L. Moad, G. Moad, E. Rizzardo, and S. H. Thang,<br/>Macromolecules 1998 31 (16), 5559-5562</i><br/><br/>[2] <i>A. Bagheri, K. E. Engel, C. W. A. Bainbridge, J. Xu, C. Boyer, and J. Jin, Polym. Chem.,<br/>2020,11, 641-647</i><br/><br/>This research received funding from the European Horizon 2020 Research and Innovation Programme (No. 899349 - 5D NanoPrinting).