Superelastic leg design optimization for an endoscopic capsule with active locomotion

Abstract

Nowadays Nitinol structures are accepted and highly exploited in the medical field.Therefore, studying the mechanical properties of this material—which is not trivialby considering the thermal behaviour of the alloy—is very important in orderto get quantitative data for a reliable design of novel Nitinol components. Inparticular, this study focuses on the design optimization of superelastic Nitinol legs tobe integrated into an endoscopic capsule for biomedical applications. The legis provided with an elastic knee that adds a passive degree of freedom to thestructure; this solution allows us to achieve a good locomotion adaptability in theunstructured environment of the gastrointestinal tract, characterized by differentdiameters. First, the mechanical behaviour of Nitinol was analysed. Tensile tests werecarried on in order to extract the peculiar stress–strain curve of the material. Thehysteretic behaviour was observed through 100 loading–unloading cycles. The acquireddata were used to model the leg design by finite element methods (FEM) and toestimate the stress–strain internal state during the operative work. These data wereused to optimize the leg, which was finally fabricated and tested, demonstratingan improvement of five times regarding the number of cycles before leg failure.