Introduction

The continued improvement of medical treatment demands easy to use, cheaper, and more durable products without compromising the treatment outcome itself. Due to inherent properties and availability, polymeric materials show high potential for medical applications. Polymeric materials are lightweight, easy to manufacture, cheap, and versatile. These properties allow them to be used in diverse medical applications such as implants, prostheses, and orthodontic appliances. As of any material used for medical applications (and intraoral applications in particular), the polymer must be biocompatible and must not induce adverse reactions.^1,2 The restrictions and the standard in choosing a polymer depend on the type of application.

In orthodontic applications, polymers are exposed to the intraoral environment, which comprises several different substances including water, electrolytes, enzymes, bacteria, among other components.³ Additionally, consuming different food and drink changes the acidity and ion concentration and may temporarily introduce organic solvent (e.g., alcohol) to the polymer environment. This means that the polymer must be resistant to chemical corrosion. Principally, corrosion causes a particle release, which—depending on the size and form of particles—might influence the mechanical properties of the polymer as well.

Another important aspect is the thermal properties of the polymer. Although the intraoral temperature remains relatively constant (near 37°C), a polymer could be subjected to varying temperature during intraoral application. This means that the intraoral temperature might change from a subzero range (e.g. while eating ice cream) to values as high as 60°C (e.g., while drinking tea). Such temperature variations lead to expansion or contraction of the material, which might have an influence on the interaction between the polymer and the teeth. Thus, a polymer must be able to tolerate temperature alteration without a pronounced volume and mechanical performance change.

The mechanical stability of the polymer also plays an important role in orthodontic applications. For instance, a polymer used for aligners must withstand high occlusal forces; otherwise, fractures or deformation might occur.⁴ A change in the mechanical properties of the polymer during the intraoral application period could also lead to unwanted changes of the mechanical loads applied to the teeth. Even for a chemically stable material (i.e., a material showing no corrosion), the mechanical properties of the polymer can still vary over time due to aging and creep.^5–7 There are two types of aging: physical and chemical aging.^5,6 Both chemical and physical aging render the polymer brittle and stiffer, thus a lower strain may be generated during the application.

This chapter will focus on the basic properties of polymers typically used for aligners. It will also include an explanation of the chemical structure and thermal properties of these polymers. As the effectiveness of a polymer for dental use depends on thermal, chemical, and mechanical stability, these issues also will be discussed briefly. Finally, future perspectives of polymers used for aligners are described.