In this blog post, we explore how shape-memory materials—once the stuff of sci-fi movies—are being utilized in the real world. Let’s examine the innovations these transformative materials will bring!
In the movie TERMINATOR 2, the Terminator T-1000 freely changes its body shape to hunt down the protagonist, John Connor. The T-1000, which adapts its form to the situation and quickly recovers from damage, is a formidable threat to the hero. This was made possible by the liquid metal that composed the T-1000’s body.
This film shocked audiences at the time, and the concept of liquid metal, straddling the line between science and science fiction, sparked viewers’ imaginations. However, materials with similar properties are now being developed in reality, serving as examples of the potential for future technological advancement.
While not capable of such free-form transformation, materials that similarly change shape can be found around us today. These are shape memory materials. Shape memory materials exhibit the shape memory effect. The shape memory effect refers to the phenomenon where an alloy remembers a specific shape and, even after being deformed by force, restores that shape when heated. The most widely used shape memory material today is shape memory alloy. Shape memory alloys remember their shape when maintained above the transformation temperature for a certain period. Once the shape is memorized, no matter how many times it is deformed, heating it above the transition temperature causes a phase transformation, restoring it to the remembered shape. This occurs because the material undergoes thermoelastic martensitic transformation, which restores it to its pre-deformed state during the phase transformation, even after plastic deformation. All alloys exhibiting thermoelastic martensitic transformation are shape memory alloys. They are widely known to the public for their ability to restore a dented bumper to its original shape simply by pouring hot water over it. An alloy similar to shape memory alloys is the superelastic alloy. Superelasticity refers to the phenomenon where an alloy deformed under applied stress restores its shape when the stress is removed. Unlike shape memory alloys, where phase transformation due to temperature change is the cause, superelastic alloys undergo phase transformation due to stress. When stress is applied, they transform into a martensite phase; upon stress removal, they revert to their original phase, restoring their shape.
These shape memory alloys and superelastic alloys are currently establishing themselves as innovative materials across various industrial fields. They are not only lightweight and robust but also significantly aid in effectively solving problems that existing materials could not address. Particularly, these materials are becoming increasingly important in terms of sustainability and efficiency, and are expected to make a major contribution to future environmentally friendly technological development.
Since their shape changes with temperature, shape memory alloys are utilized wherever temperature-driven actuators are needed. They are already commonly found in everyday items like coffee pots, electric rice cookers, and boiler hot water valves. In aerospace, hinges and mirrors previously used motorized deployable actuators, but these suffered from complex structures, failure risks, and heavy weight. In contrast, shape memory alloys are lightweight, structurally simple, and can be utilized in confined spaces, making them a highly sought-after material to replace conventional methods. Shape memory alloys also receive high marks for biocompatibility. They are used in stents that expand within the body to widen blood vessels, artificial muscles for artificial hearts, and fracture treatment devices. Furthermore, superelastic alloys, which offer significantly superior shock absorption compared to conventional alloys, are being applied in eyeglass frames, mobile phone wires, and functional underwear.
Particularly, the biocompatibility of shape memory alloys is infinitely expanding their potential applications in the medical field. For instance, various medical devices developed to shorten patient recovery times after surgery can now operate more efficiently and safely by leveraging the characteristics of shape memory alloys. This enables advances in medical technology and significantly improves patients’ quality of life.
Despite these advantages, shape memory alloys face challenges such as difficult processing, welding, and forming, the difficulty of controlling the transition temperature, and high unit costs. Research on shape memory polymers is being conducted to address these issues. The mechanism of action for shape memory polymers differs from that of shape memory alloys and also varies depending on the type of polymer used. However, the overall process enabling shape memory polymers to function can be summarized as follows. Shape memory polymers are broadly composed of a fixed phase (crosslinking points) that determines the shape and a reversible phase that connects them. When pulled and deformed above the transition temperature, the specimen elongates, aligning the polymer chains. This leads to a decrease in structural entropy. Upon cooling, the specimen maintains its deformed shape while forming secondary bonds. However, upon reheating, the secondary bonds break, and the polymer chains move toward a disordered state, causing the material to return to its original shape. Unlike shape memory alloys, shape memory polymers only require the formation of bonds between the crosslinking points and the polymer chains to memorize a shape. Therefore, they can be programmed to remember shapes using various stimuli such as heat, light, electric fields, or magnetic fields.
Shape-memory polymers have lower strength compared to shape-memory alloys. However, they offer high elastic deformation capability, low cost, light weight, biocompatibility, and biodegradability. These advantages enable their use in applications where shape-memory alloys were previously unsuitable. Furthermore, the methods to induce the shape memory effect are diverse—including heat, electricity, magnetic fields, light, and changes in acidity—and the transition temperature is easily controllable. Above all, the greatest advantage is the simpler manufacturing process. Leveraging these benefits, medical suture threads that form knots when heated are made from shape memory polymers. Shape-memory polymer fibers that flatten and remove wrinkles when washed in hot water can also be made from shape-memory polymers. Shape-memory polymers are being applied in wearable displays and solar panels. Research is also underway to replace various aerospace components using shape-memory alloys and biomedical stents.
Future technology focuses not merely on solving existing problems but on opening new possibilities. Shape-memory materials are central to this future technology, playing a crucial role in transforming our lifestyles and creating a more efficient and sustainable society. For example, smart clothing or self-healing building materials will further expand the application range of shape-memory materials, significantly enhancing the quality of human life.
In this way, shape memory materials enable what was previously impossible with conventional materials, and their applications become limitless as technology advances. Though once considered solely a technology of the future, a look around reveals shape-memory materials are already deeply embedded in our lives—from household appliances to surgical sutures and spacecraft. Research continues to reduce the cost and improve the properties of shape-memory materials, accelerating their commercialization. At this rate, the day will come when we can easily see shape-memory materials in our daily lives.
