# Characterizing the Mechanical Performance of a Bare-Metal Stent with an Auxetic Cell Geometry
## 4 Discussion  
The primary aim of this study was to show the potential of auxetic structure as a bare-metal stent. Mechanical evaluation of polymer/plastic auxetic stents is available in the literature [32; 33; 61; 62]; however, the auxetic bare-metal stent produced by femtosecond micromachining has not been studied for in vitro mechanical and FEA simulations under mechanical loading. In this study, there was a good agreement observed between simulations and mechanical experiments under the different loading conditions. In vitro mechanical testing for three-point bending significantly delayed loss of stent function because the passage of the stent never contracts, and the auxetic stent never develops sharp corners under bending, which may reduce concerns regarding artery damage. The stent did not break for a very prolonged deformation (Figure 6a), which stems from the re-entrant geometrical structure of the auxetic stent. Furthermore, the loading indenter interacts with its upper side, and it warps the loading indenter first while the bottom part of the stent starts bulging (Figure 6a). Due to nonlinear behavior, there was no stiffness value generated from the load-displacement curve. Thus, the auxetic stent becomes folded with a synclastic deformation and was never fully obstructed under three-point bending. For torsion, both torque and stress values in simulations show a steep increase, up to approximately \(14^{\circ}\), no warping up to \(45^{\circ}\), and this linear behavior, followed by a curve with much lower stiffness. Under torque-twist angle behavior, the auxetic stent also shows radial expansion, which presents an advantage of keeping the stent channel opened. There was a notable expansion in the cross-section in the auxetic stent, which is an indication of flexibility without yielding. For a high level of twisting angles up to 180, the auxetic stent brings an exceptional advantage by ensuring an opening preserved for the blood flow without obstruction (Figure 6b). Simulations match very well with the behavior of auxetic stents observed in direct mechanical tests for torsional loading. For longitudinal compression, the auxetic stent starts buckling under 0.6 mm displacement where buckling may occur either at the edges or around the middle of the stent, and the auxetic stent folds towards the inside, which can avoid artery damage. Simulations and direct mechanical results match well in predicting the point of buckling in the auxetic stent. It occurs almost at 5% compression, in the form of folding with a narrowed cross-section instead of in the form of bulging that can damage arteries (Figure 6c). Under lateral compression, the auxetic stent becomes almost completely folded towards the inside, and the diameter becomes smaller and avoids artery damage (Figure 6d). The auxetic stent can expand stably approximately 10% from its crimped form while showing longitudinal elongation. The current design of the stent can be a good alternative for arteries with a diameter around 5 mm, and it brings the advantage of keeping the blood vessel open with a stent implanted even for large deformations. The auxetic design brings the potentials of adjusting required stiffness by varying unit-cell dimensions without wrecking the deformation mechanism. However, the auxetic stent produced for this study has laser welding points, and they are prone to fail under radial expansion. One of the specimens tested under balloon expansion showed more augmentation at one edge because of the failure of one of the welding points. Thus, laser welding points require specific attention because they may be prone to extreme pressure conditions. In this study, the fabricated auxetic stent specimens were only utilized for in vitro analysis under mechanical loadings, but not for in vivo animal models or clinically. Though the diameter of the fabricated auxetic stent is approximate, 5 mm (4.77 mm) is not fit for the coronary artery. Nonetheless, this auxetic stent design (re-entrant geometry) can be potentially extended for coronary artery stents.