Superelastic Auxetic Structures For Deployable Stretchable Implants

Non-invasive, deployable, medical implants will be important for next-generation bioelectronics intended for brain machine interfaces, diagnostics, and disease management. Deployable medical implants face several challenges such as biocompatibility, complex shape adaptation of vascular system, high crimpability that requires large volume reduction for the deployment via catheter and self-deployment. Furthermore, the device must use design concepts from stretchable electronics to withstand the constant stretching and compressing forces associated with typical movement of the body. These challenges can be overcome by using thin-film shape memory alloys (SMAs) patterned into auxetic structures. Auxetic structures are special type of designs that show negative Poisson’s ratio and synclastic bending. Negative Poisson’s ratio leads to expansion under uniaxial strain which leads to larger area coverage. Synclastic bending allows for easy adaptation from 2D thin films to 3D complex concave and convex shapes in the body, creating seamless contact between SMA device and biological tissue.<br/><br/>Binary TiNi SMAs are biocompatible materials already used for medical stent applications, because SMA can be crimped to fit into a micro-catheter and then expanded upon heating to body temperature at 37 °C. Any complex 3D shapes can be set by crystallizing SMA with high temperature annealing. SMAs can remember those shapes by transitioning between two phases through a stress-induced or temperature-induced phase transformation. These effects lead to the recovery of large strains up to 6% intrinsically, allowing the device to become self-deployed at body temperature. Freestanding, self-supporting, biocompatible TiNi alloys gives the advantage that they do not require a polymer substrate or polymer encapsulation. These features allow TiNi to come into direct contact with the biological materials [1].<br/><br/>In this study, six different auxetic structures TiNi free standing thin films (25 µm – 45 µm) are fabricated by using photolithography, magnetron sputtering, etching and rapid thermal annealing [2]. Photolithography provides us with the advantage of fabricating 2D micro structures with complex shapes. Furthermore, the fabrication process used is compatible for the integration of other MEMS devices onto SMA substrates. Each fabricated 2D structure is checked under tension and compression to imitate deployment and crimped state respectively. Superelastic tensile tests on auxetic structures are performed under isothermal loading conditions at 37 °C. Under tension their stretchability, area coverage and wrinkling behavior are analyzed. The TiNi auxetic designs show between 25% and 105% macroscopic recoverable strains. Combining superelastic material with auxetic structures results in high stretchability and high area coverage. With high area coverage, less material can be used to achieve same area coverage of conventional designs. Therefore, it enhances the crimpability. Our results suggest SMA auxetics structures as promising substrates with superior mechanical properties for bioelectronics and deployable medical implants.<br/>This study is founded by GRK 2154, German Research Foundation.<br/><br/><b>REFERENCES</b><br/>[1Loger, Klaas & Engel, A. & Haupt, J. & Li, Qian & Miranda, Rodrigo & Quandt, Eckhard & Lutter, Georg & Selhuber-Unkel, Christine. (2015). Cell adhesion on NiTi thin film sputter-deposited meshes. Materials Science and Engineering C 59, S. 611–616.<br/>[2] Miranda, Rodrigo & Zamponi, Christiane & Quandt, Eckhard. (2013). Micropatterned Freestanding Superelastic TiNi Films. Advanced Engineering Materials Volume 15, Issue 1-2 p. 66-69.