Stretchable Shape Memory Alloy Thin Films for Bioelectronics

Sensors, actuators, and power microsystems intended for bioelectronics require reliable long-term operation under severe mechanical deformation scenarios. Such systems must be fabricated directly onto a substrate material that is flexible, stretchable, and conformal to the underlying biological surface. Bioelectronics systems must also be fabricated with a high area density. This work presents freestanding sputtered shape memory alloy thin films in novel large area geometries for both stretchable electrodes and stretchable substrates for bioelectronics.<br/>Sputtered freestanding thin film shape memory alloys (SMAs) are metallic smart materials with exceptional mechanical strength, large actuation densities, and electrical conductivities similar to that of liquid metals. Freestanding SMA films can achieve significantly higher elastic strains (~6% – 8%) than achieved by traditional metals (~1% - 3%). The shape memory effect allows elastic recovery of large intrinsic strains in SMAs by heating the material above its austenite transformation temperature to induce a phase transformation between a low-temperature martensite phase and high-temperature austenite phase. Superelasticity is a stress-induced phase transformation that results in the recovery of large strains when loading/ unloading in the SMA at a temperature above its austenite finish temperature. Previously the SMA materials TiNiCu and TiNiCuCo were both demonstrated to show ultra-low fatigue, reversibly deforming through 10+ million cycles with no change to their functional properties [1]. The reversible phase transformation known in SMAs is especially advantageous for electronics that require reliable operation for multiple stretching situations.<br/>Here, we present the mechanical and electrical stretchable performance of novel geometries, fabricated from sputtered freestanding SMAs Ti₅₀Ni₃₅Cu₁₅ and Ti_53.3Ni_30.9Cu_12.9Co_2.9 with film thickness ranging between 15 – 80 μm [2]. We characterize their cyclic thermal-induced and stress-induced phase transformations. For example, a TiNiCuCo serpentine with a width of 25 μm and thickness of 20 μm was able to superelastically recover macroscopic strains of 50% for 100,000 cycles. The stress-induced phase transformation results in a reversible electrical resistance change of 0.72% when loading/ loading the serpentine (0% strain- 22.31 ohms, 50% strain- 22.15 ohms) . Due to functional fatigue, after 1,000 loading/ unloading cycles the electrical resistance of the serpentine slightly decreased (0% strain- 22.27 ohms, 50% strain 22.13 ohms). Superelastic TiNiCuCo serpentine structures with triangle islands had an area density of 83% and were shown to elastically stretch &gt;150%. TiNiCu serpentine structures with rectangle islands had an area density of 33% and could stretch to macroscopic strains of 75%. Upon removal of the applied force, the structure partially recovered its original shape to a 35% macroscopic strain. Thanks to the shape memory effect, after heating the sample above its austenite finish temperature, the SMA fully recovered all deformation returning to its original shape (0% macroscopic strain). Additionally, we show stretchable SMA films can serve as functional high-temperature substrates to enable other MEMS thin-film systems (i.e piezoelectric AlN [3]) to become flexible/ stretchable. Overall, our results show SMA thin films are extremely promising materials for integration into bioelectronic and wearable electronic systems.<br/>[1] Chluba, et al. “Ultralow-fatigue shape memory alloy films". Science, 348 (6238), 1004-1007. (2015)<br/>[2] Lima de Miranda, et al. Advanced Engineering Materials 15.1-2 (2013): 66-69<br/>[3] Curtis, et al. "Integration of AlN piezoelectric thin films on ultralow fatigue TiNiCu shape memory alloys." Journal of Materials Research 35.10 (2020): 1298-1306.<br/>This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE 1840340.