Memory effect in aligners – where are we?

The vast expanse of the orthodontic sector has only been growing in the last two decades or so and we are fortunate to have sustained a heterogenous pool of resources, all dedicated to addressing malocclusion in all its malicious forms. In recent years, there have been tremendous advancements in orthodontic technology including improvements in biomaterials, radiography, metallurgy, and CAD/CAM technology.

What continues to intrigue researchers today are the materials that are employed for aligner fabrication. And so, we’ve been witness to experts constantly attempting to improve treatment efficacy by altering the materials and their properties, giving way to many novel materials that have played a significant role in enhancing aligners’ clinical performances in the treatment process.

Commonly, several dental materials (such as cement, ceramics, or composites) were designed to survive long periods in the oral cavities without interacting with the oral environment. You can call them inert or passive in terms of their minimal tissue responses. This is how things have been to date. However, experts have now shifted their focus to creating materials that are able to undergo purposeful change, playing an active role in the way the device works.

These so-called “smart” materials have definitely brought on a paradigm shift to the orthodontic industry and are the newest and most promising materials to have established a place in aligner treatment.

What is the shape memory effect?

Before we delve into the world of “smart” materials, we must first acquaint ourselves with the theory of the shape memory effect.

Certain metallic materials possess a very interesting property. Even after apparent plastic deformation, these materials are able to return to their original shape when heated. Additionally, in a certain temperature range, these materials can be strained to approximately 10% and still return to their original shape when unloaded. These peculiar effects are known as thermal shape memory and superelasticity respectively.

Both of these effects will depend on the occurrence of a specific type of phase change known as thermoelastic martensitic transformation. Experts have long tried to harness these effects into practical applications in dentistry and were successful with the recent use of the shape memory effect in Ni-Ti alloys. Nickel-titanium archwires were so successful in correcting dental crowding that some research articles even claimed that the crowding Ni-Ti archwires could fix would require multiple aligners to make the same difference.

What are shape memory polymers? (references [1] [2])

When we talk about clear aligners like Invisalign® and the mystery material it is made out of, we are mostly bombarded with the term ‘thermoplastic material’. Shape memory polymers are similar but with additional features.

Smart materials (SMs) or stimuli-responsive materials (SRMs) are the new best thing for aligner fabrication. Any material that can respond appropriately to various types of environmental changes or external stimuli such as electrical, thermal, or magnetic impulses, and generate a predictable and repeatable response is known as a stimuli-responsive material. The key feature of these smart materials is their ability to return to their original state after a stimulus has been removed.

The “smartness” feature of these materials is determined by two different mechanisms:

1. Property change: These result from the change in conditions of the environment and cause the material’s properties (chemical, mechanical, electrical, magnetic, or thermal) to change. The change is typically direct and reversible.
2. Energy exchange: This results from the change in the condition of the environment surrounding the material and causes the energy state of the material to change without affecting its properties. This change is also direct and reversible.

Since we already established that Ni-Ti alloy is a smart material, in this context, we learn that this material in archwires is good at storing energy (loading) and transferring this energy (unloading) to teeth over a period of time. The energy state is changed but the properties of the archwire do not change.

Shape memory polymers display low density, considerable elastic deformation, and high chemical stability. Additionally, not only do they have the potential for significant self-shape recovery, but they are also relatively transparent and thus are well-suited to function as a clear aligner material.