Superelastic leg design optimization for an endoscopic capsule with active locomotion

Nowadays Nitinol structures are accepted and highly exploited in the medical field. Therefore, studying the mechanical properties of this material—which is not trivial by considering the thermal behaviour of the alloy—is very important in order to get quantitative data for a reliable design of novel Nitinol components. In particular, this study focuses on the design optimization of superelastic Nitinol legs to be integrated into an endoscopic capsule for biomedical applications. The leg is provided with an elastic knee that adds a passive degree of freedom to the structure; this solution allows us to achieve a good locomotion adaptability in the unstructured environment of the gastrointestinal tract, characterized by different diameters. First, the mechanical behaviour of Nitinol was analysed. Tensile tests were carried on in order to extract the peculiar stress–strain curve of the material. The hysteretic behaviour was observed through 100 loading–unloading cycles. The acquired data were used to model the leg design by finite element methods (FEM) and to estimate the stress–strain internal state during the operative work. These data were used to optimize the leg, which was finally fabricated and tested, demonstrating an improvement of five times regarding the number of cycles before leg failure. (Some figures in this article are in colour only in the electronic version)