## 4 Discussion  
Our results demonstrated that the resistance to migration of types 2, 3, and 4 was significantly greater than to that of type 1. Similar results were observed when comparing PSs with four flaps (types 3 and 4) with those with two flaps (type 2). No difference was found between PSs with the same number of flaps (types 3 and 4). These findings support that the AMF proportionally increased as the number of flaps increased, and the vertical and horizontal directions of the flaps did not affect the MAMF. However, the attachment position of the flaps can affect the AMF. Our histological results demonstrated that there were no mucosal injuries caused by attached flaps during stent removal. Stent migration can be sufficiently prevented when the number of flaps is increased, and using a PS with flaps angled 60\({}^{\circ}\) toward the papilla is an effective and safe strategy for preventing stent  
Figure 5: Continuous antimigration force (AMF) changes according to the times in the 3D-printed pancreatic phantom with the pancreatic duct and the extracted porcine pancreas. Figure 6: (**a**) Representative fluoroscopic images in type 3 obtained during the experiment in the 3D-printed pancreatic phantom and the extracted porcine pancreas and locations of tissue sampled for histologic examination where stent was present in proximal (P), middle (M), distal (D), and normal pancreatic duct without stent (N) portions. (**b**) Representative microscopic images (hematoxylin-eosin staining, magnification X2) in type 3 show no mucosal injuries in the stented p-duct (P, M, and D). migration. When developing an antimigration stent, the attachment location and number of flaps should be considered. The endoscopic placement of a PS in the p-duct can resolve or improve symptoms in patients with ductal stricture [12; 13]. Despite the several advantages of PS placement, periodic stent exchanges or re-interventional procedures are inevitable because of stent deterioration or obstruction and various other stent-related complications [14; 15; 16; 17; 18; 19]. Stent-related complications included proximal or distal migration, stent occlusion, and stent-induced p-duct changes. In addition, 5.2% of cases had proximal migration, and 7.5% had distal migration [9]. To overcome stent migration, PSs were developed in varying shapes and sizes and having varying numbers of flaps, barbs, or flanges. Depending on PS type, stent size usually ranges from 3 to 10 French, with variable numbers of internal and/or external flaps made of polyethylene [20]. The experimental results were properly acquired using the proposed device. Due to the compactness of the device that contains a small number of components with a small footprint, the 3 DP and EPP experiments were accomplished in a simple manner. From the acquired data in Table 2, the MAMF in the EPP shows a significantly small value compared with that in the 3 DP. This was derived from the frictional effect and elongation ratio of the test pieces. The silicone surrounding the p-duct may provide a small elongation ratio compared with that in the EPP. In contrast, the friction coefficient of the silicone can be higher than that of the p-duct tissues. These are considered major factors that made differences. Nevertheless, the overall tendency was similar in both experiments. This study has some limitations. First, the total number of PSs was relatively small for performing a robust statistical analysis, even though the Bonferroni-corrected Mann-Whitney U test was used in this study. Second, the EPP was too tenacious to faithfully reflect the characteristics (e.g., size, configuration, and quality) of the human pancreas. In vivo tests are required to precisely measure the AMF of the PS with flaps in the p-duct. Third, the push-pull gauge apparatus for measuring the AMF was operated at a fixed rate without a delicate control.