Design and optimization of functionally graded superelastic niti stents

Endoscopic stents are being used by surgeons in off-label uses to manage leaks and perforations in the gastrointestinal tract. Commercially available stents are primarily designed to open strictures in the esophagus through tissue compression. The stents incorporate a woven NiTi wire to produce a stiff and linear tubular shape that conforms to the esophagus. In off-label uses, where the stents are placed in non-esophageal locations the stents must bend, the stents show a high propensity to migrate from their initial location causing unwanted complications. In this paper, a new stent design incorporating functionally graded NiTi is presented and explored. First, a functionally graded NiTi stent design is proposed. Next, a mechanical model using finite element analysis is developed to predict the bending moment and stiffness of the functionally graded stent designs. Finally, the mechanical model is coupled with a genetic algorithm in MATLAB to identify optimal designs. For a 90° bending angle, the best design parameters of the newly proposed flexible stents are found for three different stent design families. The results of the functionally graded stents show how tailoring the material properties locally in a structure can lead to highly compliant behavior. The tailoring of the geometric and material design developed may be applied to design of highly flexible and optimized medical devices.