Clear aligners: Material structures and properties

3: Clear aligners: Material structures and properties

Masoud Amirkhani, Fayez Elkholy, Bernd G. Lapatki


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.57 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.

Polymer molecular structure and thermal properties

Polymers are very long and entangled molecules with nonconventional thermal and mechanical behavior. In this section, the structure of a polymer and its thermal behavior will be described. This comprises the specification of glass transition, aspects of aging, and the stability of the polymer in the intraoral milieu.

What is a polymer?

The word polymer is derived originally from the Greek words poly (“many”) and méros (“units”). This indicates that polymers consist of many repeating units connected to each another through chemical bonding. Normally, if a substance contains just a few molecules, an addition or removal of only a few atoms would change the material properties significantly. For example, if one would add just CH2 to heptane (C7H16), then the boiling point of the resulting molecule (C8H18) increases by 27°C. With polymers, in contrast, the number of repeating units could be changed by one or more units without any noticeable change in the polymer properties.

Typically, a polymer chain is made of several thousand repeating units with a length of several micrometers and a diameter just around 1 nm. The polymer chain is usually flexible, twisted, and intertwined. The molecular weight and chemical structure of the polymer determine most of its properties. In contrast to small molecules having a specific size and molecular weight (expressed in g/mol or kg/mol), a polymer bulk contains polymer chains with many different sizes and molecular weights. Hence the molecular weight of the polymer reflects an average of many different polymer chains.

Based on their thermal behavior, the three different classes of polymers are thermoplastic, elastomer, and thermoset.8 Clear aligners belong to the thermoplastic group. Thermoplastic polymers melt and flow upon heating above a certain temperature. Two widely used polymers for aligners are polyethylene terephthalate glycol (PET-G) and thermoplastic polyurethane (TPU).911 The latter is a special thermoplastic form of polyurethane which melts by heating, facilitating the thermoforming process. Both of these thermoplastic materials are transparent in the visible light spectrum, are impact-resistant, and highly ductile. Just these properties in particular make them very suitable for use as aligner material.

PET-G is a copolymer that constitutes two repeating units (Fig. 3.1): polyethylene terephthalate and glycol. The addition of glycol prevents the crystallization of the PET upon heating. This makes PET-G less brittle and more resistant to mechanical stress. PET-G is a versatile polymer used in many other applications such as protective cover (e.g., smart card), electronic devices, food containers, and medical instruments. One can thermoform, print, drill, bend, polish, and cut PET-G easily without noticeable impact on its stability and physical properties. As PET-G can be easily thermoformed and also recycled, it is also the material of choice for three-dimensional printing.

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Jan 16, 2022 | Posted by in General Dentistry | Comments Off on Clear aligners: Material structures and properties
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