## 4 Discussion  
For the realization of the biocompatible resin system, a thiol-yne-methacrylate reactive system has been evaluated during the first step of the work. This reaction was based on a multiple-step radical mechanism leading to high monomer conversion and outstanding thermo-mechanical properties [31; 32]. In the second step, the investigated and optimized resin system was evaluated in a 3D printing process in collaboration with Lithoz. It turned out that the functional group conversion of the monomers in the 3D printed parts (methacrylate conversion ~76% using 1.6 wt% Irgacure 784 (Irg784) as photoinitiator) is significantly lower as the conversion, which could be obtained in thin model films (conversion methacrylate >95%; see first evaluation report). This behavior could be explained by the lower intensity of the utilized light in the DLP-based printing process. Since methacrylates provide a significant cytotoxic potential, the amount of remaining methacrylate building blocks has to be decreased in the printed parts to the lowest possible level. Moreover, also the influence of the choice and concentration of the photoinitiator (PhI) in the resin system on the methacrylate conversion was investigated. Besides Irg784, the also-biocompatible Germanium-based PhI Ivocerin was tested. Based on the conducted experiments, it was finally observed that a 2.4 wt% Irg784 content leads to 86% methacrylate conversion directly after printing, which increased after a post-illumination (at 80 \({}^{\circ}\)C) to 95% [33; 34; 35]. In the following experiment, the effect of the post-curing on the thermo-mechanical properties using a dynamic mechanical analysis was investigated. It turned out that the additional illumination at elevated temperatures leads to a significant increase in the glass transition temperature. This can be explained by the higher network density obtained with progressive polymerization of uncured monomers. It needs to be mentioned that this post-treatment must be performed above the glass transition temperature of the printed photopolymer to facilitate monomer mobility in the polymeric network, enabling a subsequent polymerization reaction. Aside from the methacrylate conversion, the storage modulus at body temperature (\(\sim\)37 \({}^{\circ}\)C) also increased significantly from 252 to 1073 in the post-curing step. The 3D printing technique used in present experiment needs to be improved in order to obtain a surface with greater smoothness, free of residual uncured polymer [36; 37; 38; 39]. Non-contact quantitative evaluation of surface topography was completed in the present study. Using confocal microscopy, surface roughness parameters were measured. The images were taken at 20\(\times\) and 50\(\times\) magnification, and then the results were analyzed in ZEN 3.6 software, equipped with the Topography module that allows for obtaining the surface roughness parameters. Measurements were conducted both for samples before and after indentation tests. For each sample and each magnification, five measurements were made so that statistical analysis could be performed. The results of the quantitative surface topography analysis are shown in Figure 7a,b. The Ra parameter is the arithmetic mean deviation of the profile from the mean line. The highest value of this parameter was found for sample 1 (according to Table 1), for which the average value was 1.6 \(\upmu\)m (Figure 7a). The other samples had at least twice the value of this parameter. It was noted that for the samples exposed at 70 \({}^{\circ}\)C, the value of the Ra parameter was twice less than the Ra values for the corresponding samples exposed at 100 \({}^{\circ}\)C. No effect of the applied layer on the surface roughness was noticed. The Rz parameter reports the height of roughness according to the ten points of the profile. The measure of this quantity is the average roughness. The highest value was obtained for samples 2 and 3 (according to Table 1) (Figure 7b).