Shape Memory Alloy Thin Film Auxetic Structures

Abstract

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).

Introductory Chapter: Introduction to Shape Memory Alloys

“…The use of shape memory alloys, often known as SMAs, has been noted to be increasing in the medical industry, particularly in the realm of minimally invasive therapies and implanted devices. In this context, SMAs are being utilized more and more in the design of orthodontic appliances, stents, and surgical equipment [19,20]. These materials, which are renowned for their exceptional properties, facilitate the development of cutting-edge solutions such as self-expanding stents, which can autonomously adapt to vessel constraints, and smart materials that are highly responsive to bodily cues, such as fluctuations in temperature.…”

Development, fabrication and mechanical characterisation of auxetic bicycle handlebar grip

“…Three different basic unit cell shapes were chosen: + (non-auxetic), X (non-auxetic) and S (auxetic based on tetra chiral geometry, named also S-shape 34 ), as shown in Fig. 1 a.…”

Thermal Stability of Cu-Al-Ni Shape Memory Alloy Thin Films Obtained by Nanometer Multilayer Deposition

“…This working capability is due to a reversible martensitic transformation between the high-temperature phase (austenite) and the low-temperature phase (martensite) through a structural phase transition involving an atomic lattice shearing, by shuffling and distortion of the austenite lattice; see the textbooks [ 3 , 4 , 5 ] for a review of the fundamental aspects of SMA. In recent years, emerging flexible technologies have incorporated thin films of SMA into the design of auxetic materials [ 6 ] and mechanical metamaterials [ 7 , 8 ], as well as stretchable electronics [ 9 ], origami-inspired or programmable surfaces [ 10 , 11 ], and in general, all technologies of thin-film flexible actuators [ 12 ]. In addition, because of the advent of miniaturization, many research efforts have focused on the characterization of SMA at the micro- and nanometer scale in order to develop active micro-/nanodevices; devices such as microgrippers [ 13 ], microswitches [ 14 ], microvalves [ 2 ], microwrappers [ 15 ], and bimorph actuators [ 16 ] have already been developed for MEMS applications; see [ 17 , 18 ] for a review in this field.…”