Initiation efficiency and cytotoxicity of novel water-soluble two-photon photoinitiators for direct 3D microfabrication of hydrogels

The lack of efficient water-soluble two-photon absorption (TPA) photoinitiators has been a critical obstruction for three dimensional hydrogel microfabrications with high water load by two-photon induced polymerization (TPIP). In this paper, a series of cyclic benzylidene ketone-based two-photon initiators, containing carboxylic acid sodium salts to improve water solubility, were synthesized via classical aldol condensation reactions. The cytotoxicity of cyclopentanone-based photoinitiators is as low as that of the well-known biocompatible photoinitiator Irgacure 2959 as assessed in the dark with MG63 cell line. In z-scan measurement, the TPA cross sections of the investigated initiators are only moderate in water, while the TPA values for hydrophobic analogues measured in chloroform were much higher. All novel initiators exhibited broad processing windows in TPIP tests using hydrophilic photopolymers with up to 50 wt% of water. Impressively, microfabrication of hydrogels with excellent precision was possible at a writing speed as high as 100 mm s ^-1.