Shape memory materials, like the T-1000 in the movie TERMINATOR 2, are replacing conventional materials in aerospace, medicine, and everyday life, showing the promise of future technologies.
In the movie “TERMINATOR 2,” the Terminator “T-1000” tries to kill the protagonist “John Connor” by freely changing its body shape. The T-1000 is a threat to the protagonist because it can change its form to suit the situation, and it can quickly recover from damage. What makes this possible is the liquid metal that makes up the T-1000’s body.
The movie shocked audiences at the time, and the concept of liquid metal captured their imagination, blurring the lines between science and science fiction. However, materials with similar properties are being developed in the real world, and it’s a testament to the promise of future technology.
Although they can’t change as freely, we can see similar shape-shifting materials around us today. These are shape memory materials. Shape memory materials are materials that exhibit the phenomenon of shape memory. Shape memory is a phenomenon in which an alloy memorizes a shape and restores its shape when heated, even if it is deformed by applying force. Currently, the most commonly used shape memory materials are shape memory alloys. Shape memory alloys remember the shape by maintaining a certain time above the deformation temperature, and once the shape is remembered, no matter how many times it is deformed, it is restored to the remembered shape with phase transformation when heated above the transition temperature. This is because it undergoes thermoelastic martensitic transformation, which is restored to the shape before plastic deformation upon phase transformation. All alloys that undergo thermoelastic martensitic transformation are shape memory alloys. The public is familiar with the image of a bumper being dented and then being restored by pouring hot water over it. Similar to shape memory alloys are superelastic alloys. Superelasticity is a phenomenon in which an alloy is deformed by applying stress to it and then restores its shape when the stress is removed. Unlike shape memory alloys, which undergo phase transformation due to temperature changes, superelastic alloys undergo phase transformation due to stress, changing to the martensitic phase when stress is applied and returning to the original phase when the stress is removed.
These shape memory alloys and superelastic alloys are currently being used as innovative materials in many industries. They are not just lightweight and strong, they are helping to effectively solve problems that traditional materials have been unable to solve. In particular, these materials are becoming increasingly important in terms of sustainability and efficiency, and are expected to make a significant contribution to the development of future environmentally friendly technologies.
Shape memory alloys are used wherever temperature-driven devices are needed because they change shape with temperature. They are used in everyday objects such as coffee pots, rice cookers, and hot water valves for boilers. In the aerospace field, hinges, reflectors, etc. used to be motorized devices that unfolded using motors, but they had problems such as complex structures, possible failures, and heavy weight. Shape memory alloys, on the other hand, are lightweight, simple in structure, and can be utilized in small spaces, making them a promising alternative to conventional methods. Shape memory alloys have been recognized for their biocompatibility and are used in stents that expand to widen blood vessels, artificial muscles for artificial hearts, and fracture repair devices. In addition, super-elastic alloys, which have excellent shock absorption compared to conventional alloys, are applied to eyeglass frames, cell phone wires, and functional underwear.
In particular, the biocompatibility of shape memory alloys opens up endless possibilities for applications in the medical field. For example, various medical devices developed to shorten the recovery time of patients after surgery are able to operate more efficiently and safely by utilizing the properties of shape memory alloys. This is enabling advances in medical technology and significantly improving the quality of life for patients.
Despite these advantages, shape memory alloys are difficult to machine, weld, and form, their transition temperature is difficult to control, and their unit cost is high, which is why shape memory polymers are being researched. The mechanism of action of shape memory polymers is different from shape memory alloys and varies depending on the type of polymer they are composed of. However, the overall process that makes shape memory polymers work can be summarized as follows. Shape memory polymers are composed of a stationary phase (crosslinks) that largely determine their shape and a reversible phase that connects them. When you deform a shape by pulling it above its transition temperature, the polymer chains align as the specimen stretches, which leads to a decrease in structural entropy. When the specimen is cooled, it retains its deformed shape and forms secondary bonds. However, upon reheating, the secondary bonds are broken and the polymer chains revert back to their original shape as they try to return to their disordered state. Unlike shape-memory alloys, shape-memory polymers only need to form crosslinks and bonds between polymer chains to remember their shape. This is why they can be shape-memorized with a variety of stimuli, including heat, light, electric and magnetic fields.
Shape memory polymers are not as strong as shape memory alloys. However, they are highly elastically deformable, have a low unit cost, are lightweight, and are biocompatible and biodegradable. Because of these advantages, shape memory polymers are being used in applications where shape memory alloys are not. In addition, the shape memory effect can be triggered by heat, electricity, magnetic fields, light, or changes in acidity, and the transition temperature can be easily controlled. The biggest advantage is that the process is simpler. Shape memory polymers are used to make medical suture threads that knot when heated. Shape memory polymer fibers can also be made from shape memory polymers, which are wrinkle-free and flatten when washed in hot water. Shape memory polymers are being used in wearable displays and solar panels. Shape memory alloys are also being explored to replace various aerospace components and stents in biomedical applications.
The technologies of the future are not just about solving existing problems, but about opening up new possibilities. Shape memory materials are at the heart of these future technologies, playing a key role in transforming the way we live and creating a more efficient and sustainable society. For example, smart clothing and self-healing building materials will further expand the range of applications for shape memory materials, further enhancing the quality of human life.
Shape memory materials are enabling things that traditional materials cannot, and their uses are only getting better as the technology evolves. Once thought of as a futuristic technology, shape memory materials are already in our everyday lives, from home appliances to surgical instruments to spacecraft. Even now, research is continuing to lower the unit cost of shape memory materials and improve their properties, accelerating their commercialization. At this rate, we will soon see shape memory materials in our daily lives.
