The development of practical two-photon absorption photoinitiators (TPA PIs) has been slow due to their complicated syntheses often reliant on expensive catalysts. These shortcomings have been a critical obstruction for further advances in the promising field of two-photon-induced photopolymerization (TPIP) technology. This paper describes a series of linear and cyclic benzylidene ketone-based two-photon initiators containing double bonds and dialkylamino groups synthesized in one step via classical aldol condensation reactions. Systematic investigations of structure–activity relationships were conducted via quantum-chemical calculations and experimental tests. These results showed that the size of the central ring significantly affected the excited state energetics and emission quantum yields as well as the two-photon initiation efficiency. In the TPIP tests the 4-methylcyclohexanone-based initiator displayed much broader ideal processing windows than its counterparts with a central five-membered ring and previously described highly active TPA PIs. Surprisingly, a writing speed as high as 80 mm/s was obtained for the microfabrication of complex 3D structures employing acrylate-based formulations. These highly active TPA PIs also exhibit excellent thermal stability and remain inert to one-photon excitation. Straightforward synthesis combined with high TPA initiation efficiency makes these novel initiators promising candidates for commercialization

Hydrogels are polymeric materials with water contents similar to that of soft tissues. Due to their biomimetic properties, they have been extensively used in various biomedical applications including cell encapsulation for tissue engineering. The utilization of photopolymers provides a possibility for the temporal and spatial controlling of hydrogel cross-links. We produced three-dimensional (3-D) hydrogel scaffolds by means of the two-photon polymerization (2PP) technique. Using a highly efficient water-soluble initiator, photopolymers with up to 80 wt.% water were processed with high precision and reproducibility at a writing speed of 10  mm/s. The biocompatibility of the applied materials was verified using Caenorhabditis elegans as living test organisms. Furthermore, these living organisms were successfully embedded within a 200×200×35 μm³ hydrogel scaffold. As most biologic tissues exhibit a window of transparency at the wavelength of the applied femtosecond laser, it is suggested that 2PP is promising for an in situ approach. Our results demonstrate the feasibility of and potential for bio-fabricating 3-D tissue constructs in the micrometre-range via near-infrared lasers in direct contact with a living organism.

Several novel aromatic ketone-based two-photon initiators containing triple bonds and dialkylamino groups were synthesized and the structure-activity relationships were evaluated. Branched alkyl chains were used at the terminal donor groups to improve the solubility in the multifunctional monomers. Because of the long conjugation length and good coplanarity, the evaluated initiators showed large two-photon cross section values, while their fluorescence lifetimes and quantum yields strongly depend on the solvent polarity. All novel initiators exhibited high activity in terms of two-photon-induced microfabrication. This is especially true for fluorenone-based derivatives, which displayed much broader processing windows than well-known highly active initiators from the literature and commercially available initiators. While the new photoinitiators gave high reactivity in two-photoninduced photopolymerization at concentration as low as 0.1% wt, these compounds are surprisingly stable under one photon condition and nearly no photo initiation activity was found in classical photo DSC experiment.

Two-photon-induced photopolymerization (2PP) has gained increased interest due to the capability of manufacturing three-dimensional structures with very high feature resolution. To assess the suitability of photopolymer systems for 2PP, methods have to be developed that allow a screening of the efficiency of monomer-initiator combinations in the context of high throughput, large processing window and geometric quality of the final parts. In this paper, a method for evaluating 2PP structures is described. For this purpose, the double-bond conversion of fabricated 2PP structures was measured giving quantifiable results about the efficiency of the photoinitiator. The method is based on local measurement of the double-bond conversion of the photopolymer using a microscope in combination with infrared spectroscopy. The obtained double-bond conversion is a measure for the efficiency of the photopolymer system (initiator in combination with monomer), and thus allows to compare different photopolymers in a quantitative way. Beside this evaluation of 2PP structures, fabrication of complex 3D structures was done to determine the limits of the 2PP technology for miscellaneous components.

Due to its high feature resolution and the capability to produce 3D-structures without the need of support-structures, two-photon polymerization (2PP) is one of the fastest growing technologies in the field of additive manufacturing. To ensure good quality of the fabricated parts, proper methods for evaluating the suitability of different material systems for 2PP are necessary. In this paper, we present a fast screening method based on measuring the Young's modulus of micro-cantilevers giving a quantifiable measure and representing the material- and fabrication-properties in one experiment.