New water-soluble photo-initiators for two-photon polymerization based on benzylidene cyclopentanones

Hydrogels, mostly known for their use in biomedical applications, are of particular interest in recent research fields. Beside their usage in biomedical applications, 3D-structured hydrogels possess properties which are interesting for other fields of research like the fabrication of lenses, sensors or electronics. Two-photon polymerization (2PP) emerged as a suitable technique to process hydrogels, due to its high spatio-temporal resolution. In order to obtain precise 3D structues via 2PP from aqueous solutions, efficient water-soluble two-photon photo-initiators (2PIs) are required. In this contribution, several 2PIs based on benzylidene cyclopentanone structures bearing carboxyl or amino groups were synthesized via aldol condensation. These 2PIs were converted into sodium carboxylate or ammonium chloride salts to increase water solubility. Also, 2PIs equipped with oligo(2-ethyl-2-oxazoline) chains to enhance water solubility were investigated concerning their suitability for two-photon polymerization. Z-scan measurements of the synthesized 2PIs revealed two-photon cross-sections of 80 to 253 GM at a wavelength of 800 nm in water. Cytotoxicity tests showed that six of the initiators featured low or no cytotoxicity up to a concentration of 1 mg mL−1. Five out of the nine tested 2PIs revealed good initiation efficiencies at a concentration of 0.5 wt% in aqueous formulations containing 50 wt% of poly(2-ethyl-2-oxazoline) diacrylate (PEtOx-DA, DP = 10), showing broad fabrication windows at a writing speed of up to 10 mm s−1.