3.2. UV Curing and 2PP Structuring of 3D Scaffolds

Eleven different precursor mixtures were prepared as samples for mechanical analysis as well as for the 2PP structuring, namely, five different formulations each based on the two different PEtOx-DAs (DP of 10 or 20) and one PEG-DA for reference purposes (M_n = 700 g mol⁻¹, Sigma Aldrich). Four out of five formulations based on the PEtOx-DA precursors were mixtures with PEtOx-As (DP10 or DP50) in weight percent ratios of 80:20 and 20:80 for each PEtOx-A macromonomer. The fifth formulation represented the pure DA macromonomer. For example, the precursor consisting of PEtOx-DA DP10 and MA DP10 in a weight percent ratio of 80:20 was prepared by 1.6 g (1.35 mmol) of PEtOx-DA DP10 and 0.4 g of PEtOx-A DP10 (0.37 mmol). In the case of PEtOx-A DP10, the macromonomer was dried in vacuo before usage because of its high hygroscopy. All other PEtOx mixtures were prepared accordingly. Then, 2 g ultrapure water was added to obtain processable solutions. Even if the PEG-DA was already in a liquid processable form, water was added to get a proper comparison. Finally, 10 mg of the photoinitiator IRGACURE^® 2959 (0.004 mmol, Sigma Aldrich) for UV curing or BA740 (0.02 mmol) for 2PP structuring were added to the mixtures. All mixtures were stirred overnight at room temperature to ensure complete dissolution.

The UV curing was performed with a Vacuum UV Exposure Unit 2 from proMa systro, which works at a wavelength of 365 nm and power of 120 W. Silicone masks for cylindrical samples with height of 4 mm and diameter of 6 mm were placed in the UV chamber, filled with the precursor, and covered with glass slides to get a flat sample surface. Subsequently, curing was performed for 30 min.

The 2PP system M3DL from Laser NanoFab GmbH (Hannover, Germany) was equipped with (i) a Ti:sapphire femtosecond laser working at 800 nm wavelength, delivering 140 fs pulses at 80 MHz repetition rate (VISION II, Coherent, Scotland); (ii) a beam splitter combination consisting of a cube polarizer and a λ/2-plate for attenuation of the laser power in the 2PP experiment; (iii) a CCD (charge-coupled device) camera for monitoring the process; and (iv) an acousto-optic modulator for rapid on/off switching of irradiation. The beam was focused by an ×50 objective lens with NA 0.75 (EC EPIPLAN, Zeiss, Germany). Before the 2PP experiments were performed, the precursor/PI mixtures were placed in a home-made 1 mm thick silicone frame on a glass slide equipped with 3-(trimethoxysilyl) propyl methacrylated surface functionalization (Sigma Aldrich, Germany) to improve adhesive strength between the glass slide and the 2PP structure. A second glass slide on top formed the upper cover plate. The 3D structure was produced by moving the focal volume of the laser beam with an ultraprecise linear motor-driven three-axis positioning stage according to the computer model. The stage movement was coupled to a Galvano scanner (ALB 10100 Air Bearing Stage, Aerotech Inc., Pittsburgh, PA, USA) to increase the writing speed and shorten the necessary processing time. Arrays of spider web structures were processed with increasing laser power from 100 to 400 mW in 100 mW steps in one dimension (X) and increasing writing speed from 1 to 7 mm/s in 2 mm/s steps in another dimension (Y). Furthermore, arrays differing in line distances in the X- and Y-directions of 1, 2, 4, and 6 µm were produced, while the line distance in the Z-direction was kept constant at 5 µm. For imaging purposes, ground plates of 1.5 × 1.5 × 0.1 mm (line distances of 10 µm in the X- and Y-directions and 5 µm in the Z-direction) were processed for arrays with line distances of 1 µm before the process started because the arrays delaminate from the glass slide when the material is soaked in water during development. To remove the nonilluminated precursor after irradiation, the samples were eluted in ultrapure water for 24 h as developer (acetone was used for PEG-DA). The developer was replaced periodically according to common laboratory practice.