# Electrospun Nanofibrous Membranes for Tissue Engineering and Cell Growth
## 8 Polyurethane  
Polyurethane (PU) belongs to the materials that were only briefly mentioned before as a possible partner for one of the other polymers; however, PU is biocompatible, non-toxic, and has good elastic and mechanical properties, which makes it suitable for soft tissue engineering, e.g., for the soft tissues of the cardiovascular system [115]. [PERSON] et al. prepared PU nanofiber mats by electrospinningning and tested attachment, proliferation, and viability of VERO fibroblast cells (African green monkey kidney fibroblasts) in vitro [116]. They found no cytotoxic effect after 24 h but did after longer times. Nevertheless, the live/dead assay showed mainly living (stained green) cells and only a few dead (stained red) cells after 24-72 h (Figure 6). Generally, the nanofiber mats were found to show an ideal porosity, with suitable pore size and pore interconnectivity, to support cell penetration into the scaffolds, as well as the exchange of nutrients and metabolic products. One of the possible blending partners for PU is hydroxyapatite (HA), as already mentioned in combination with chitosan and PLA. [PERSON] et al. investigated PU nanofiber mats with HA particles in different ratios, electrospun from various combinations of DMF and tetrahydrofuran (THF), and for use in bone tissue engineering and found that beads vanished when THF was part of the solvent, while fibers became more uniform and thicker [117]. The inclusion of HA particles significantly increased the tensile properties of the nanofiber mats. Tests with osteoblastic mouse cells and human embryonic mesenchymal  
Figure 5: Fluorescent images of ST-2 cells on as-spun nanofiber mats of different PCL/PGS ratios S1-S7 after 7 days of incubation. Reprinted from [114], originally published under a CC-BY license. Figure 6: Live/dead assays showing fibroblasts incorporated in control and PU membranes, after 24, 48, and 72 h. Reprinted from [116], with permission from Elsevier. progenitor cells showed no difference of viability on all tested PU scaffolds, while nano-HA was shown to increase the cell proliferation rate. This finding was attributed to a stronger interaction between the polymer and ceramic phase for a large HA surface area, which allowed for improved protein attachment. Significantly lower cell viability was additionally found on nanofiber mats with significantly larger average fiber diameters; underlining the importance of the scaffold morphology for cell growth and viability. Similar results were found in other studies [118]. Similarly addressing bone tissue engineering, [PERSON] et al. added corn and neon oil to polyurethane scaffolds and found slightly reduced fiber diameters, as compared to pure PU nanofiber mats, increased tensile strength, and hydrophobic or hydrophilic behavior for PU/corn oil or PU/corn oil/neem oil, respectively [119]. The hydrophilicity of PU/corn oil/neem oil was found to be in an ideal range of contact angles between 40\({}^{\circ}\) and 70\({}^{\circ}\), which was mentioned as a possible reason for the increase of the adhesion and proliferation of the fibroblast cells seeded on these substrates. Furthermore, blood compatibility assessments showed anticoagulant properties, as opposed to pure PU nanofiber mats, and non-toxicity against red blood cells and human fibrost cells. Combining PU with PCL and graphene oxide (GO), the resulting nanofiber mats showed an increasing fiber diameter with increasing GO concentration [120]. Seeding human skin fibroblast cells on them, [PERSON] et al. found good biocompatibility and showed that GO could significantly increase the hydrophilicity and biocompatibility, as compared to pure PU/PCL nanofiber mats, in this way leading to stretched cell growth on the more \"cell-friendly\" surface. Additionally aiming at skin tissue engineering, [PERSON] et al. investigated PU/starch core-shell nanofibers, partly with additional hyaluronic acid in the shell [121].