# Production of Synthetic Models for Neuro-Oncology Training by Additive Manufacturing
## 2 Materials and Methods
### Materials
#### 2.1.2 Brain  
To simulate the brain, we identified the optimal material in a preliminary phase, comparing the elasticity modulus of three different samples, and taking into account the fact that the reference elastic modulus of the brain is 21.16 kPa [17; 18; 19; 20; 21]. The three different materials analyzed in the preliminary tests were: \begin{table}
\begin{tabular}{c c c} \hline \hline
**Property** & **Smartfil® EP** & **PLA Without Additives** \\ \hline Flexural elastic modulus & \(\times\) & \(\checkmark\) \\ Flexural strength & \(\checkmark\) & \(\times\) \\ Maximum cutting force (drilling) & \(\checkmark\) & \(\times\) \\ Maximum cutting force (milling) & \(\times\) & \(\checkmark\) \\ Chip behavior & \(\checkmark\) & \(\times\) \\ Surface hardness (Shore D) & \(\checkmark\) & \(\times\) \\ Validation by a neurosurgeon & \(\checkmark\) & \(\times\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Criteria for the choice of material for the skull model [16]. AljaSafe(r): alginate used to produce molds. This material was processed in different proportions, but it was discarded due to its dimensional instability and higher stiffness compared to a real brain. These samples lose their elastic properties over time, thus resulting in a hard and stiff material that is not feasible for this application. * Contor 5005: three-component silicone with tin catalyst (A:B:C). Six samples of five different proportions were manufacture with the Contor 5005. The A:B ratio was maintained according to the recommendation of the manufacturer (100:2.5), while the C content compared to A+B was modified in different percentages because it is important to check the behavior of the material when increasing or decreasing the catalyst. The codes used for these five groups were C30, C50, C70, C90, and C100. * Ecoflex(r) 00-10 (Smooth-On, Inc., Macungie, PA, USA): two-component silicone with platinum catalyst (A,B). Six samples of three different proportions were manufactured with this material. The A:B proportions were 1.0:0.5 (E0.5), 1:1 (E1), and 1.0:1.5 (E1.5). These samples were subjected to mechanical characterization under compression load. The mechanical testing was carried out using a LYI (LI-1065, Dongguan Liyi Environmental Technology Co., Ltd., Dongguan, China) testing machine in displacement control mode. The crosshead speed was set to 10 mm/min, and the compression modulus was calculated as the slope of the initial segment in the stress-strain graph. The results of the compression tests, in terms of average compressive elastic modulus, are compared in Figure 1.  
According to the acquired data, E1.5 sample achieved the most similar compressive elastic modulus (37.83 kPa) to the reference value (21.16 kPa), even if still far from it. To reduce the stiffness, we incorporated a new component (Slacker, Smooth-On, Inc., Macungie, PA, USA). Four different groups with different proportions of silicone component and Slacker were produced (A:B:Slacker) and subjected to compression tests to obtain the modulus of elasticity: Table 2 summarizes these results and the standard deviation of all of them. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Mix Groups** & **Mean Modulus of** & **Standard Deviation** \\ **(Ecoflex Comp. A: Ecoflex Comp. B: Slacker)** & **Elasticity (kPa)** & **SD (kPa)** \\ \hline Group A (1:1:1) & 40.41 & 3.45 \\ Group B (1:1:2) & 37.90 & 1.01 \\ Group C (1:1:3) & 31.29 & 6.10 \\ Group D (1:1:4) & 27.95 & 2.62 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mean modulus of elasticity and standard deviation of the tested samples (Ecoflex A:B:Slacker). Figure 1: Compressive elastic modulus of the different groups tested with the preliminary selection to simulate the brain. Finally, among the produced samples and in accordance with the neurosurgeons' feelings, the best material to mimic the brain was group C (1:1:3), with an elastic modulus of 31.29 kPa. On the other hand, a mold was needed for the silicone casting. This mold was produced by material extrusion AM with Smartfli(r) PLA (20% filling).