Auxetic shape memory alloy (SMA) materials are candidates for ballistics, deployable antennas, actuators, stretchable electronics, and biomedical devices. Auxetic materials are periodic structures characterized by a negative Poisson's ratio, meaning they expand laterally when stretched longitudinally. The work presented here shows the mechanical properties of auxetic structures are significantly improved by using sputtered superelastic shape memory alloy materials compared to traditional materials (e.g., Cu, Si). In particular, sputtered freestanding TiNiCuCo SMAs offer advantages as substrates for stretchable electronics. Two novel auxetic structures with enhanced expandability and compressibility are presented, fabricated from sputtered TiNiCuCo. The novel geometries presented in this work are based on the combination of the auxetic rotating rectangle structure where the rotating hinges are replaced by two common stretchable interconnects (e.g., serpentines and Archimedean spirals). The influence of functional fatigue on the electrical properties, thermal-induced and stress-induced phase transformations of the novel stretchable auxetic TiNiCuCo thin-films are presented.

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