Shape Memory Alloy Thin Film Auxetic Structures

Auxetic structures provide an interesting approach to solving engineering problems due to their negative Poisson's ratio, which allows for elongation perpendicular to applied stresses, opposite to a conventional structure's necking behavior. Thus, they can function well in applications requiring compacting the device into a small volume during the deployment (e.g., implants inserted with catheters) or stretchability with area coverage (e.g., stretchable electronics). Fabricating them with shape memory alloys (SMAs) expands the possibilities. The high strains experienced by auxetic structures may become reversible compared to ordinary metals due to superelastic or shape memory effect. This work studies four different auxetic microstructures using thin film SMAs that are capable of surviving strains up to 57.4%. Since these structures are fabricated by layer deposition and lithography, other components, such as microelectronics, can be seamlessly integrated into the fabrication process. These auxetic thin films are investigated for their mechanical behavior under tension for their stretchability and stability. Under tension, thin films are known to show wrinkling instabilities. In two of four designs, the large auxetic behavior leads to wrinkling, while the other two display stable, non-wrinkling behavior. These designs can be candidates for stretchable electronics, wearable medical devices (e.g., biosensors), or implants (e.g., stents).

Automated summary

Auxetic structures with negative Poisson’s ratio show promising potential in solving engineering problems by allowing elongation perpendicular to applied stresses. This study investigates four different auxetic microstructures using shape memory alloys, capable of surviving high strains. The mechanical behavior of these thin film structures under tension is evaluated, with two designs showing wrinkling instabilities and the other two displaying stable, non-wrinkling behavior, making them suitable for stretchable electronics, wearable medical devices, or implants.