# Characterizing the Mechanical Performance of a Bare-Metal Stent with an Auxetic Cell Geometry
## 1 Introduction
The inefficiency of stent designs is causing the high failure rate of arterial stenting [27; 32; 33; 34; 35; 36]. Thus, stents still have significant research potential for optimization despite being used clinically for decades. Stent optimization can be accomplished based on the design, choice of materials, fabrication method, surface functionalization, and implantation procedure [37]. Since the mechanical behaviors of the stent are of critical importance for efficient functional recovery of blood vessels, auxetic designs have recently been proposed for mechanically tunable stents [38; 39; 40; 41; 42] to enhance the radial expansion ability while reducing the axial expansion [43; 44; 45; 46]. Briefly, compared to most conventional materials, auxetics are mechanical metamaterials artificially designed to exhibit negative Poisson's ratios that contract in transverse directions upon being compressed uniaxially. Auxetics presents enhanced mechanical properties such as indentation resistance, fracture toughness, and shear stiffness, which greatly facilitate a range of applications, including biomedical applications [47; 48; 49; 50]. Different auxetic designs, including chiral, rigid node rotational, re-entrant, and elastic instability for stent designs, have been presented a potential for optimization between strut thickness, mechanical capacity, and deformation ability [38; 39; 40; 51; 52; 53; 54; 55; 56; 57; 58; 59]. Studies also consider that arterial endothelium subjected to both wall shear stresses and a cyclic circumferential strain due to pulsatile blood flow exhibits concurrent axial and transverse expansion (contraction) similarly to an auxetic structure [59]. Thus, an auxetic stent could likely better integrate with native tissues. In this regard, plastic and polymer auxetic stents were evaluated mechanically, numerically, and analytically [32; 33; 60; 61]. To the authors' knowledge, bare metal auxetic design stents for mechanical performance have not been studied in the literature. Thus, in our paper, we have presented a 316 L stainless steel stent with an auxetic cell geometry fabricated with high precision femtosecond micromachining and laser welding. Stents are a permanent implants and according, they experience various mechanical loadings such as bending, longitudinal compression, buckling, and torsion on the implantation site either in delivery or implantation (Figure 1) from the pulsation of the heart or body movement or pressures of the surrounding vessels and blood flow [8; 62; 63; 64; 65; 66; 67]. Thus, here we performed analysis of the mechanical capacity of the stent under mechanical loadings (Figure 1) combined with in vitro mechanical/finite element analysis (FEA) to understand the performance of this design. The novelty of the paper not just lies in studying extensive mechanical loadings representing realistic conditions, but also in the production methodology of the stent produced from stainless steel sheets by femtosecond laser cutting. This combination brings different mechanical aspects compared to the auxetic structures studied in the literature. Some of the mechanical issues such as stability of struts were already discussed in the author's previous work for auxetic plates [51], but this work extends these discussions for the tube form. Mechanical evaluations are conducted for both experimental and FE simulations to verify the assumptions in mechanical modeling in simulations. This study focused on evaluating the mechanical potential of cylindrical auxetic stent design, which was modeled and analyzed in the cylindrical state as the final shape produced by laser cutting and welding, and was not crimped nor expanded, which should be noted.