## 3 Results and Discussion
### Hemolytic Experiments and Cytotoxicity Assay by MTT  
Blood compatibility and cytotoxicity are extremely vital problems faced by biomaterial researchers, especially when considering implant biomaterials [46; 47]. Red blood cells may rupture when they contact blood incompatible implant materials, releasing hemoglobin and blood platelets to form blood clots caused by their destruction [48]. In order to evaluate the hemolysis of the materials, all the data obtained from the hemolysis test were collected and calculated in Table 3. All of the films exhibited slightly hemolysis (under 3%), which conformed to the American Society for Testing and Materials standards, so as to be considered as blood compatible materials. The hemolysis ratio was evaluated according to the equation below:  
\[HR(\%)=\left[(D_{sample}-D_{negative})/(D_{positive}-D_{negative})\right] \times 100\% \tag{3}\]  
where \(HR\) represents the hemolysis ratio, and \(D_{sample}\), \(D_{negative}\), and \(D_{positive}\) are the absorbance of the test samples, negative samples, and positive samples, respectively [49]. The hemolysis rates slightly decreased as the content of hard segment increased, which was in accordance with the analysis of the wettability of the membranes. Generally speaking, the lesser the \(HR\) value was, the better the blood compatibility of the material was. Therefore, the increment of the hard segment could lead to less damage of the red blood cells, showing desirable potential as a significant factor to obtain better blood compatibility for practical applications. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & **Tensile** & **Ultimate Tensile** & **Elongation at** & **Tensile Elongation** & **Tensile Elongation** \\  & **Modulus** & **Strength (MPa)** & **Break (\%)** & **(100\%) Tensile** & **(300\%) Tensile** \\  & **(MPa)** & **Strength (MPa)** & **Strength (MPa)** \\ \hline
20\%-PCU & \(9.4\pm 1.4\) & \(35.6\pm 3.7\) & \(490\pm 50\) & \(8.7\pm 1.2\) & \(15.6\pm 2.1\) \\
25\%-PCU & \(12.6\pm 1.1\) & \(38.5\pm 1.2\) & \(510\pm 70\) & \(10.2\pm 0.8\) & \(18.4\pm 1.6\) \\
30\%-PCU & \(15.4\pm 3.1\) & \(30.5\pm 5.4\) & \(530\pm 30\) & \(10.3\pm 1.4\) & \(20.5\pm 1.2\) \\
35\%-PCU & \(18.3\pm 2.7\) & \(43.3\pm 2.3\) & \(380\pm 50\) & \(11.6\pm 1.7\) & \(24.6\pm 1.5\) \\ Chronoflex® C & \(14.8\pm 1.8\) & \(45.5\pm 3.1\) & \(410\pm 30\) & - & - \\ Pellethane 2363-80A & \(13.0\pm 2.0\) & \(34.0\pm 2.0\) & \(430\pm 20\) & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison between the polyurethane films and other commercial polyurethane products. Figure 6: Tensile stress-strain curves of different contents of hard segments of the polyurethane. The method of the MTT assay, for its simplicity and high reproducibility, is a widely performed strategy which is specifically used for mitochondrial function and to evaluate cytocompatibility of biomaterials for cell viability and proliferation [50]. The percentage viability of cell lines with 7 days of contact of primary L929 cells with the leaching liquor exposed to some representative PCUs films is shown in Figure 7. The percentage of viable cells with the obtained values were collected and calculated according to the following equation [51]:  
\[\text{Cell viablitiy }(\%)=\frac{\text{Absorbance of samples and L929 cells }\times 100}{\text{Absorbance (control)}} \tag{4}\]  
Generally speaking, cytotoxicity can be rated based on the above equation relative to the controls; severe cytotoxicity (<30%), moderate cytotoxicity (30%-60%), slight cytotoxicity (60%-90%), and not cytotoxic (>90%) [52]. Clearly, these membranes showed a cell viability response of over 70%, indicating that the produced polyurethane could be classified into the level of slight cytotoxicity with relatively good biocompatibility. Obviously, all the groups with different contents of the hard segment exhibited significant differences and there were little changes in the cell viability with the increased hard segment.