## 4 Discussion  
I-SEMSs were developed and designed to increase flexibility and fracture resistance [12; 13; 14]. They have a uniform and small cell size resulting from fabrication by braiding machines using multiple nitinol wires [13]. I-SEMSs have a significantly smaller cell size than conventional K-SEMSs. Stent-induced tissue hyperplasia commonly and definitely grows through the wire mesh within 4 weeks after stent placement. According to the results of previous studies, stent-induced tissue hyperplasia occurred after stent placement in the rat within 4 weeks, so this study also processed a follow-up for 4 weeks [17; 18; 19; 20]. Our results demonstrated that I-SEMS placement successfully inhibited stent-induced tissue hyperplasia caused by mechanical injury after stent placement in a rat esophageal model. Tissue penetration through the wire meshes may have been relatively preventable due to the small cell size. The high flexibility also relatively reduced the mechanical damages to the esophageal wall. After stent placement, the luminal diameter of the stented esophagus was significantly larger in the I-SEMS group than in the K-SEMS group. Consistently, the histologic results also demonstrated that the tissue hyperplasia-related variables in the K-SEMS group were significantly higher than those in the I-SEMS group. These findings support that the smaller cell size and high flexibility of I-SEMSs are key factors for inhibiting stent-related tissue hyperplasia formation.  
The physical properties of I-SEMSs yielded high flexibility to maintain a round shape when they are bent. The flexibility of these stents is associated with stent-related complications, such as stent collapse, fracture, abutment, and kinking [20; 21; 22; 23; 24]. In our study, the small cell size of the I-SEMSs increased the stent flexibility. In the bending test, the K-SEMSs showed severe kinking at the middle portion owing to their low flexibility. Conversely, the I-SEMSs successfully maintained an inner lumen without any stent deposition. With their superior physical advantages, I-SEMSs may prevent various stent-related complications in non-vascular luminal organs, including the esophagus, biliopancreatic duct, colon, and airway. Furthermore, they seem to be particularly useful in tortuous anatomical structures of non-vascular luminal organs.  
I-SEMSs have been extensively used in managing vascular stenosis [11; 12; 25]. Clinical investigators reported excellent outcomes of I-SEMS placement for primary superficial femoral artery lesions to prolong stent patency and reduce stent fracture [26; 27; 28; 29; 30]. However, I-SEMSs have high variations of stent length caused by shortening and elongation during stent placement [12]. In our study, wherein I-SEMSs were applied for the first time in the rat esophagus, stent shortening and elongation during stent placement did not occur because the esophageal stent delivery system has a better profile than the vascular stent delivery system.  
Our study has several limitations. First, tissue responses were observed in the normal rat esophagus after stent placement. Stent-induced hyperplasia formation after stent placement may differ with esophageal strictures. Second, only a few tests of the stent samples were conducted in this study. Although the bending test was successfully performed to compare the degree of stent deformation, the bending stiffness could not be investigated because of the lack of a measuring device. Additional mechanical properties of the newly developed I-SEMS for non-vascular luminal organs, including radial force, axial tension, and torsion test findings, should be evaluated. Although the tortuous anatomical structures of non-vascular luminal organs have not been investigated to prove the benefits of the high flexibility of I-SEMSs, our results demonstrated the basic concept of the effectiveness and safety of these stents in the rat esophagus.