## 2 Materials and Methods  
### Preparation of SA-GEL Composites Material  
Homogeneous sodium alginate solution (concentration from 1.5-4.5%) was prepared with distilled water, and then each solution was mixed with a gelatin solution of various concentrations, respectively. The homogeneous solution of SA-GEL was obtained by using a constant temperature magnetic stirrer (37 \({}^{\circ}\)C, 8 h), and then its composite material properties were tested. Both viscosity concentration value was obtained for convenient observation and logarithmic value was obtained, the viscosity test results of the solutions with different proportions are shown in Table 1.  
According to Table 1, the viscosity of the mixed solution increases exponentially with the increase in sodium alginate concentration at different gelatin concentrations. Hydrogen bonding and electrostatic interaction also occur, which increases the viscosity of the mixed solution. When gelatin is added too much (gelatin content is 10% or above), it hinders the formation of network structure of sodium alginate and calcium chloride to a certain extent. As result, the network structure cross-linking becomes insufficient, the gel hardness decrease, and the molding time is too fast for three-dimensional forming. Therefore, at the same temperature, 0.08 g/mL gelatin was mixed with sodium alginate of different concentrations to prepare composite gel, then cross-linked with 3% calcium chloride solution. The specific content in the composite solution (considering the effect of viscosity) is shown in Table 2.  
The cross-linking with 3% concentration of calcium chloride solution was performed and the effects of different proportions on the hardness and elasticity of gel were tested. The results are shown in Figure 1. It can be seen from Figure 1 that the strength and elasticity of SA-GEL are also improved with the increase in sodium alginate content. When the concentration is relatively low, the hardness of the SA-GEL is only about 330 g, and the elasticity is poor. This is because there is not enough alginate combined with calcium ions. When the concentration reaches 3%, the SA-GEL structure is compact, the  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Algin** & **Gelatin** & **1.5\%** & **2\%** & **2.5\%** & **3\%** & **3.5\%** & **4\%** & **4.5\%** \\ \hline \multicolumn{1}{c}{} & \multicolumn{6}{c}{**Viscosity (cp)**} \\ \hline
2\% & 5.15 & 7.42 & 8.72 & 8.93 & 9.44 & 10.34 & 10.96 \\
4\% & 5.96 & 8.08 & 9.01 & 10.3 & 11.5 & 12.65 & 14.01 \\
6\% & 6.44 & 8.45 & 9.40 & 11.3 & 12.9 & 15.16 & 18.16 \\
8\% & 7.36 & 8.98 & 9.63 & 13.5 & 15.01 & 18.27 & 23.24 \\
10\% & 7.68 & 9.88 & 12.58 & 15.88 & 18.23 & 24.23 & 29.1 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Viscosity table of gelatin–sodium alginate mixed solution.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Number** & **1** & **2** & **3** & **4** \\ \hline Algin (g/mL) & 0.015 & 0.02 & 0.025 & 0.03 \\ Gelatin (g/mL) & 0.08 & 0.08 & 0.08 & 0.08 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Sample number of different concentration ratios.  
hardness of the gel can reach 568.6 g, the elasticity also increases to 1.79 mm, the toughness is positive, and the mechanical properties increase significantly. This is because calcium ions and enough sodium alginate molecules undergo a cross-linking reaction, and enhance the intermolecular interaction force.