For the 3-D analysis of the structures, we use CLSM. The channel is filled with index matching oil (Nikon immersion oil, ) to limit image deterioration by diffraction. We use a home-built CLSM based on a Nikon TE300 inverted microscope, with a 25 mW, 488-nm laser (iFlex-Mustang 488, QiOptiq), spinning disk-based confocal scanner (UltraView ERS, Perkin Elmer), and camera (C4742-95-12ERG, Hamamatsu). The axial -position of the objective (S Plan Fluor ELWD 60X NA 0.7, Nikon) was scanned using an objective piezo scanner (PIFOC P-720, Physik Instrumente) with a range of . Only the emitted autofluorescence from the structure was transmitted to the camera using a low-pass emission filter with a cut-off wavelength of 525 nm. For the overview image in Fig. 3, a regular fluorescence microscope (TE2000, Nikon) was used with a lower magnification objective (S Plan Fluor ELWD 45X NA 0.45, Nikon). An autofluorescence excitation-emission map of the cured polymer can be found in the 1.