A shape memory polymer that returns to its original shape after deformation like a rubber band! We will learn about the principles and applications of this amazing material that uses its elasticity and resilience to drive innovation in various industries, including medicine and robotics.
Two young children are playing with a yellow rubber band. They are making stars with their small fingers and trying to make a starfish, competing to see who can make more shapes. How can you make so many different shapes with just one rubber band? It’s because rubber bands have a property of returning to their original shape. If you give it some force to make a star shape and then release the force, it will return to its original shape, allowing you to make a spear shape again. In this simple game, we can easily observe the unique elasticity and resilience of rubber bands.
Elasticity and resilience of rubber bands have many useful applications. For example, they are used in sports equipment, clothing, and even building materials, and their properties make them useful in various ways in our daily lives. Thanks to their elasticity and resilience, rubber bands are useful in various situations that require strong tensile strength and flexibility. These properties are expanding their uses in our daily lives.
Shape memory polymers are elastic and have this kind of property. Shape memory polymers are polymers that have the property of returning to their original shape when the environment is created in the same conditions as the environment in which they were originally formed, even if the shape of the object changes. This technology is opening up innovative possibilities in various fields such as medicine, aerospace, and robotics.
The principle of shape memory polymers can be explained through cross-links, which are points that chemically connect polymer chains. When the positional relationship of the cross-links changes due to deformation, it remembers it internally and returns to its original form. This property has great potential especially in the medical field. For example, a stent made of shape-memory polymer is inserted into a narrowed blood vessel and then returns to its original shape by body temperature, thereby widening the blood vessel.
Let’s take a closer look at the principle. A polymer with an initial shape is transformed by an increase or decrease in heat and temporarily fixed. When heat is applied above a critical temperature, the polymer recovers from the temporary shape and returns to its original shape. This is the shape memory effect. The force that restores the deformation comes from a change in entropy that comes from the elasticity of the polymer. Entropy, simply put, indicates the degree of disorder. To make an analogy, students in class are in an orderly state, so their entropy is small, while students during recess are in a very disordered state, so their entropy is large. According to the second law of thermodynamics, the entropy of the entire universe increases in the direction of reaction. Since the initial polymer has a large entropy due to the disordered molecular arrangement, it is an unstable reaction in the direction of decreasing entropy because the transformation of this polymer is to align the molecular arrangement. Therefore, when heat is applied in this temporary fixed state, conditions are provided for increasing entropy, so it returns to its initial shape. This is the principle of shape memory polymers.
The structure of shape memory polymers is similar to that of a jungle gym or a net. This structure generally originates from the coexistence of a fixed (hard) and a reversible (soft) part. The reversible phase is the main part of the shape memory polymer and plays an elastic role in deformation and recovery. Above the critical temperature, the reversible phase takes the form of a fluid and has the property of being able to move freely. At this point, if a shape-memory polymer is deformed, the polymer chains align and the entropy decreases. This unstable state can be maintained by rapidly cooling the deformed state. The structural rearrangement of the reversible phase by pulling deformation is strictly limited at temperatures below the critical temperature, and the recovery of the polymer chain does not occur.
Shape memory polymers have low density and high elasticity, making them flexible, and depending on the characteristics of the polymer, they may also have properties such as biocompatibility and biodegradability. Because of these properties, it is used as a composite material combined with other materials in many fields, including toys and medical tools. Currently, recovery to its original state through temperature is widely used, but its use under other conditions, such as light and pH, is not yet widespread, so it is a promising material for the future.
Shape memory polymers also have the potential to provide innovative solutions in a variety of industries. For example, in robotics, shape memory polymers can be used to create flexible and adaptive artificial muscles. These artificial muscles can enable more natural movements than traditional robotic components, thereby expanding the scope of robot applications.
