# In Situ Transformation of Electrospun Nanofibers into Nanofiber-Reinforced Hydrogels
## 4 Discussion
While water was absorbed instantaneously by the uniaxially electrospun scaffolds (G, GP, GPP and TGPP), the coaxial scaffolds (zeinPEO-GPP and zeinPEOT-GPP) did not fully absorb water droplets within 50 s. However, the contact angle decreased steadily (Figure 3a), which is in agreement with a study on uniaxial PDA-PEI-crosslinked gelatin fibers [38]. This slight difference in the wettability behavior may be due to the different electrospinning conditions. During coaxial electrospinning, a more pronounced stretching of the shell solution in comparison to uniaxial nanofibers is likely to have occurred due to a twice larger distance from the nozzle tip to the collector (Table 1). A higher jet stretching leads to a more constrained environment for the polymer in the shell solution and a slightly different interplay with the solvent, potentially changing the surface properties of the fibers as compared to the uniaxially electrospun scaffolds. In this case the constrained environment may have resulted in more non-polar groups orienting themselves towards to fiber surface, leading to higher contact angles as has been described earlier [58]. A biomaterial should show a high elasticity as well as a high tensile strength to allow for easy handling as well as resistance against wear and tear [3]. Both the dry uniaxial (G, GPP) and core-shell scaffolds (zeinPEO-GPP, zeinPEOT-GPP) demonstrated comparable low Young's moduli (high elasticities) and high tensile strengths, which is beneficial for their use as biomaterials (Table 2). In fact, elasticities and tensile strengths are similar to those obtained for zein-polycaprolactone core-shell nanofibers developed by our group in an earlier study [52]. Importantly, these correspond well with those of forearm skin [59], indicating that the materials may potentially be suited for tissue regeneration purposes after further optimization. However, analysis of the compression behavior was only possible for the core-shell fiber mats (zeinPEO-GPP and zeinPEOT-GPP), as the uniaxial samples (G, GPP) showed a too high fragility and loss of integrity during compression. Indeed, texture profile analysis of the samples confirmed good compressibility and hardness of zeinPEO-GPP and zeinPEOT-GPP, and the results obtained are comparable with previous findings for carboxymethyl chitosan-silk fibroin nanofiber-reinforced hydrogels [27]. Knowing the solid-state properties of nanofiber scaffolds in their dry state is important to estimate and understand their physical stability upon storage. The TGA data (Figure 4b) revealed three degradation steps, where the first corresponds to loss of absorbed and bound water [60], the second is related polymer degradation [61], and the third step corresponds to the final thermal decomposition of the sample. High stability of all samples > 90% of the original weight at the onset of sample degradation of \(\sim\)270 \({}^{\circ}\)C was confirmed by TGA, with the zein-containing samples showing higher onsets of degradation in accordance with previous studies [62; 63], while the onset of degradation for the uniaxial samples G and GPP correspond to reported degradation temperatures of gelatin powder [61]. Gelatin is an amorphous polymer, and the broad XRD patterns (Figure 4a) of the uniaxially electrospun samples (G, GPP) are in line with results obtained for gelatin reported earlier [60; 64]. Antimicrobial biomaterials should release the loaded antibiotic in a controlled manner and thus inhibit bacterial growth. Approximately 76% of T was released from the zeinPEOT-GPP scaffolds within an hour (Figure 5). The release kinetics of T best followed the Korsmeyer-Peppas model, which describes drug release from hydrogel systems as a combined result of diffusion, swelling and subsequent erosion of the hydrogel [5; 64]. With respect to an inhibition of bacterial growth, it was found that both zeinPEOT-GPP in dried and wetted states as well as T control samples effectively inhibited both investigated bacterial strains (Figure 6, Figure A2).