Are the big aligner companies missing out on the new material trends?
Growing aesthetic concerns in recent years have led to a substantial increase in the number of adult patients seeking dental care. In particular, orthodontic services have become surprisingly popular among adults and young adolescents. Removable clear aligners have triumphed over traditional metal braces owing to their increased comfort and discrete appearance. Studies have also identified that clear aligners are way more respectful of the patient’s periodontal health as would be the brackets and wires of fixed appliances.
As a clear aligner consumer, you’ll be able to maintain your daily oral hygiene properly due to its removability. Among the parameters that affect the clinical efficiency of clear aligners, the material employed for their manufacturing plays a key role. As we are aware, the material used to fabricate a clear aligner can directly affect its clinical performance, taking into consideration the biological, physiological, and physical features of the material. The type of material that is used to produce aligners will depend entirely on the manufacturing process.
The actual manufacturing process of clear aligners can be divided into two categories: (i) the conventional method of vacuum thermoforming with thermoplastic material moulding on physical models, (ii) direct 3D printing without intermediary physical models. The first technique is commonly used commercially and clinically to produce clear aligners. As for the direct 3D printing process, there is no photo polymerisable resin that is currently approved to be suited for direct printing. Therefore, only thermoformed aligners have been commercialized and clinically utilized to fabricate clear aligners.
Thermoplastic materials
Before we discuss the top materials in the market, we must learn some basics of the aligner material choices used today.
Thermoplastic polymers are classified based on their molecular structure into amorphous and semicrystalline polymers. Amorphous polymers possess irregularly arranged molecular structures that are characterized by a low degree of molecular packing. On the other hand, semicrystalline polymers contain both areas of uniformly and tightly packed chains (crystalline domains) and irregularly arranged areas (amorphous regions). These dual-configured structures make semicrystalline polymers hard, and opaque, with good chemical resistance, and a sharp melting point. In comparison, amorphous polymers are softer, transparent, have low shrinkage, and have better impact resistance.
For the production of transparent orthodontic aligners, the most used polymers (individually or blended) include polyester, polyurethane, and polypropylene.
PET/PETG
Among polyesters, polyethylene terephthalate (PET) and polyethylene terephthalate glycol (PETG) are widely used in the production of clear aligners, namely with brands such as Biolon and Duran. PET can be drawn into tapes, films, or fibres and demonstrates a more opaque and white appearance with good strength. PETG, another non-crystalline co-polyester, shows excellent transparency, adequate flow property, and resistance against various solvents. These materials are extremely durable, have high impact strength and are resistant to chemical changes, making them favourable choices for clear aligners to function inside the mouth.
Typical examples of PET/PETG aligners include Biolon from Dreve, Duran from Scheu Dental, ESSIX from Dentsply Sirona, and Track A from Forestadent. Amongst these four, Biolon appears to be the most flexible, ensuring patient comfort and preventing breakage.
PC
Polycarbonate (PC) is also used to fabricate clear aligners and is mainly praised for its durability, hardness and transparency.
TPU
Thermoplastic polyurethane (TPU) is another extremely versatile polymer that has many favourable properties such as excellent mechanical and elastomeric characteristics, chemical and abrasion resistance, adhesion properties, as well as simplicity of machining. [1] TPU is known for its elastic properties – when subjected to load, TPU changes its shape but can recover its original shape when the load is removed without damage. This high tear resistance allows it to change from a single layer of polyurethane to multiple layers.
The best-known example of TPU is Invisalign. Prior to 2013, Invisalign aligners were made with a single-layer polyurethane material known as Exceed-30 (EX30). In 2013, they transitioned to a new polymer, SmartTrack™ (LD30) – a multilayer aromatic thermoplastic polyurethane. According to Align, this new material would be able to provide the aligners with more elasticity and produce more constant forces, thereby improving their clinical efficacy.
Another example of this material is the Zendura FLX from Bay Materials (also known as “Clear Quartz” when used with ClearCorrect aligners.
Polymer blends
The primary requirement for materials to fabricate clear aligners is for them to exert continuous and controlled forces to produce correct tooth movements. Experts learned that the mechanical properties of the polymers could be improved by mixing various types of materials: polyester, polyurethane, and polypropylene to create a super-polymer. Various types of blending ratios have been employed to determine the essential features of the blend. For example, blending PETG/PC/TPU at the 70/10/20 ratio showed the best mechanical properties compared to other blending ratios.
These blends are able to enhance the clinical performances of aligners, with the best combinations of tensile strength, impact strength, and elongation at break. It seems that these blends may be the best formula to obtain the best clear aligners, however, many of the big aligner brands prefer using single or multi-layered TPU or PET/PETG to manufacture their aligners.
Today, Invisalign makes use of SmartTrack™ – a multilayer aromatic TPU, Spark uses TruGen™ – a multi-layer polyurethane, and ClearCorrect uses ClearQuartz™ – a polyurethane resin. Perhaps these brands will integrate more complicated and new materials into their products in the future.