## 3 Results  
In this paper, we propose a novel nitinol \"spherical occlusion device\" for future cancer treatment. This is the first of its kind in the world and also represents the first \"spherical\" variation from traditional cylindrical stents. Such a shape transformation was achieved through a series of expansions and heat treatments. Figure 7 shows the FEA contour plots of strain distribution during a series of expansions. It was found that the maximum strain of the device usually occurred at the inner surface of the most-curved region. At the end of each expansion, the subsequent heat treatment relieved all stresses  
Figure 6: Hepatic artery model (**left**) and occlusion experiment setup (**right**). within the device, which became stress-free. The deformed geometry of the device was then extracted for the next expansion until the final diameter of 5.2 mm was reached. The spherical occlusion device has to withstand large deformation without fracturing during the dramatic shape transformations from crimping to expansion and from cylinder to sphere. FEA simulation results showed that the maximum strain encountered during the entire process was 10.0% when the device was crimped inside a catheter of 1.6 mm diameter for delivery. Since the nitinol failure strain is approximately 12%, this shows that our device is able to withstand large deformation safely. Table 2 lists the maximum strains encountered during the manufacturing and deployment procedures. A hepatic artery bifurcation model was used to evaluate the device's occlusion performance. The device was placed at the upstream of outlet 2, as shown in Figures 3 and 6. CFD simulation results showed that devices with metal densities of 27% and 14% were able to reduce the blood flow by 50% and 30%, respectively (Figure 8). The reduction of the blood flow in the target branch implied that a portion of the blood flow was redirected towards the other branch, according to the conservation of mass. The spherical occlusion device can be custom-made by tailoring its metal density to reduce blood flow by specific amounts. During heat treatment, the two most important parameters are temperature and time. Higher temperature and longer operating time negatively impact the device's mechanical properties. After laser cutting, the spherical occlusion device was expanded gradually to the desired shape and diameter and heat-treated multiple times in a salt bath at 500 \({}^{\circ}\)C for 200 s. The device surface was then sand-blasted and electro-polished to achieve a mirror-like surface finish. Sand-blasting was first applied with aluminum oxide particles (37-44 \(\upmu\)m) sprayed at a pressure of 2 kg/cm\({}^{2}\) for one minute to remove large debris, followed by electro-polishing to further improve the surface conditions. The electro-polishing solution was a mixture of 79% (volume) of acetic acid and 21% (volume) of perchloric acid. The anode and cathode were made of a stainless steel flake and a titanium wire, respectively. The electro-polishing process was conducted at room temperature in two stages, with the first stage at 7.7 volts for 60 s and the second stage at 11.3 volts for another 60 s. Figure 9 presents the world's first prototype of the spherical occlusion device for demonstration of our novel design concept. Prior to the occlusion experiments, the spherical occlusion device was deployed at the upstream of outlet 2 using a home-made assembly of the delivery system including a guide wire, a catheter, and a propelling tube. The spherical occlusion device was crimped inside the catheter cylindrically, while the guide wire ran through its center as shown in Figure 10a. The guide wire was delivered to the far distal end of outlet 2 in the artery model to provide support, while the rest of the components followed the guide wire to the target site. The propelling tube, sitting between the catheter and the guide wire, moved forward to push the occlusion device out of the catheter for deployment.