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E. Zerobin, M. Tromayer, A. Dobos, W. Steiger, M. Markovic, S. Baudis, A. Ovsianikov, R. Liska:
"Promoting Initiation-Efficiency and Biocompatibility of Two-Photon Initiators by the Introduction of Photolabile Moieties";
Poster: Symposium 'BSTE 2018: Next Generation Tissue Engineering', Ghent, Belgien; 2018-11-21 - 2018-11-23; in: "BSTE 2018: Next Generation Tissue Engineering", (2018), PO43.

INTRODUCTION
Two-photon polymerization (2PP) is a versatile technique to produce complex microstructures with very high spatial resolution.1 Various applications in the biomedical sector are accessible via the precise engineering of hydrogel scaffolds. 2PP enables photopolymerization right in the focal point of a pulsed laser beam. In order to create crosslinked networks efficient two photon initiators (2PIs) are necessary. Reactive radical species induce polymerization after excitation via the applied NIR laser. In order to reduce the formation of cytotoxic species a new initiation mechanism have been developed.2 Cleavage of photo-labile groups reduces the chance of energy transfer to molecular oxygen and therefore the formation of cytotoxic reactive oxygen species. Two different types of 2PIs based on cleavable functional groups, diazosulfonates (DAS) and oxime esters3 (OE), have been developed and studied towards 2PP.
MATERIALS AND METHODS
DAS was synthesized by diazotization of amsonic acid and subsequent coupling of the correspronding tetrazonium salt with alkali sulfite. OE functionalities were introduced onto two photon active chromophores by aldol condensation of terephthalaldehyde with methyl cycloketone. Subsequent oximation and esterification with various acid chlorides yielded in OE-based 2PIs. Cell encapsulation experiments were performed with a Ti:sapphire laser (λ = 800 nm). Confocal 3D image stacks of the fabricated scaffolds were taken with a Zeiss Axio Observer Z1 and a LSM 700 unit.
RESULTS AND DISCUSSION
Two photon activity of DAS has been tested as well as its performance as low toxic 2PI in the presence of living cells. Promising results compared to state-of-the-art 2PIs have been shown concerning metabolic cell activity and long term cell survival. Furthermore, novel OE-2PIs with comparable photo-reactivity have been synthesized and tested towards their performance during 2PP.
CONCLUSIONS
A new concept was developed to employ novel 2PIs based on intramolecular cleaving mechanisms. DAS showed promising results regarding cell viability. Organo soluble OE-2PIs have already been synthesized and showed comparable reactivities compared to state of the-art 2PIs.
REFERENCES
[1] X.-H. Qin, A. Ovsianikov, J. Stampfl, R. Liska, BioNanoMaterials, 2014. 15(3-4), 49-70
[2] Tromayer, M., Dobos, A., Gruber, P., Ajami, A., Dedic, R., Ovsianikov, A. and Liska, R., Polymer Chemistry, 2018. 9(22): p. 3108-3117.
[3] Dworak, C. and Liska, R., Wiley Online Library, 2010.
ACKNOWLEDGEMENT
The authors acknowledge the financial support by the Austrian Science Fund (FWF,
Proj. Number I2444 N28).

Two-photon polymerization, complex microstructures with very high spatial resolution, applications in the biomedical sector, novel 2PIs based on intramolecular cleaving mechanisms