What Is Invisalign Made Of? Materials and Safety

Invisalign aligners are made from a multilayered polyurethane-based plastic called SmartTrack, a proprietary material that Align Technology introduced to replace an older single-layer material. The safety profile of this plastic has been studied in terms of chemical leaching, hormone-mimicking potential, and cell toxicity, and the findings are broadly reassuring, though not without nuance worth understanding if you wear these trays for most of every day.

SmartTrack and What Came Before It

When Invisalign launched, the aligners were made from a material called EX30, a single-layer polyethylene terephthalate glycol (PETG) plastic.¹ PETG is widely used in consumer packaging and medical devices. It is rigid, transparent, and relatively easy to thermoform over a dental model. But rigidity was also a drawback: EX30 aligners delivered their force in a less sustained, less comfortable way, and clinicians noticed limitations in how precisely certain tooth movements could be achieved.

Align Technology eventually switched to SmartTrack, a polyurethane-based material with a layered internal structure. Under Raman microscopy, SmartTrack consists of a polyester layer sandwiched between two soft elastomeric polyurethane layers.² That sandwich structure matters because it gives the aligner both flexibility and controlled stiffness. The polyurethane layers provide a gentler, more sustained force on teeth, while the polyester core helps the tray maintain its programmed shape over a wear cycle. The company has since introduced a newer generation called ClearQuartz, which flips the arrangement: a soft polyurethane core between two polyester outer layers.²

Studies comparing outcomes between EX30 and SmartTrack have generally found improved clinical performance with the newer material, which is one reason you will not encounter EX30 in current Invisalign trays.³

The BPA Question

Bisphenol A (BPA) is probably the chemical people worry about most when they learn they will be wearing plastic in their mouths for 20-plus hours a day. BPA is an endocrine disruptor linked to developmental and reproductive concerns, and it shows up in certain polycarbonate plastics and epoxy resins. The good news is that Invisalign’s polyurethane material is not made from BPA-containing compounds, and laboratory testing consistently reflects that.

An in vitro study testing several clear aligner brands, including Invisalign, found that none of them leached detectable amounts of BPA.⁴ A systematic review pulling together data across multiple studies came to the same conclusion: the amount of BPA released from clear aligners was extremely low or zero.⁵ BPA can theoretically exist as a trace impurity in some plastics even when it is not an intentional ingredient, which is why researchers keep testing for it. But the amounts, when detected at all, fall far below any threshold of concern.

One wrinkle worth knowing about: if your orthodontist bonds composite resin attachments to your teeth (the small tooth-colored bumps that help aligners grip), those attachments introduce a different chemical profile into the picture. An in vivo study measuring saliva from Invisalign patients found that after attachments were removed at the end of treatment, several resin-related compounds became detectable in saliva, including breakdown products of Bis-GMA and UDMA, which are common dental resin monomers. Before removal, those compounds were below detection limits, suggesting the removal process itself releases trapped material.⁶ BPA levels in the same study actually dropped after attachment removal. The takeaway: the aligners themselves are not a meaningful BPA source, but the bonded attachments are a separate material with their own leaching characteristics.

Cytotoxicity and Hormone-Mimicking Effects

Beyond BPA specifically, researchers have tested whether the substances that leach from aligner plastic are toxic to cells or mimic estrogen. An early study exposed human gum cells and a breast cancer cell line (a standard test for estrogenic activity) to extracts from Invisalign aligners. There was no evidence of cell toxicity and no stimulation of the estrogen-sensitive cells at any concentration.⁷ That finding is particularly relevant because estrogen-mimicking chemicals are a common concern with plastics, and this test specifically ruled it out for Invisalign material.

More recent work paints a slightly more nuanced picture. A study comparing four clear aligner systems, including Invisalign, found that all of them showed some degree of toxicity to cells in culture, ranging from slight to moderate, with differences depending on how concentrated the extract solution was.⁸ In other words, at higher concentrations the cell viability dropped compared to a control group. This does not necessarily mean the aligners are dangerous in actual use. Laboratory cytotoxicity tests deliberately push concentrations higher than anything you would encounter in a real mouth, because the goal is to screen for potential problems. Still, it is honest to say that aligner plastics are not biologically inert in the way, say, a glass surface would be. They release trace amounts of material, and at sufficient concentrations those substances can affect cells in a dish.

What Happens to the Plastic in Your Mouth

A polyurethane aligner is not the same object after two weeks of wear as it was when you unwrapped it. Your mouth is a harsh environment: warm, wet, acidic, full of enzymes, and subject to chewing forces. Research has tracked how Invisalign material changes under those conditions.

Scanning electron microscopy of aligners worn for two weeks showed voids and early signs of surface delamination. Chemical analysis indicated the possible release of trace elements, including aluminum, during clinical use. At the same time, the deeper molecular structure of the material remained relatively stable, meaning the polymer backbone was not breaking apart wholesale.⁹ So the surface takes a beating while the bulk of the material holds up.

Retrieval studies, in which researchers analyze aligners returned by patients rather than artificially aged in a lab, confirm surface-level wear patterns. Used aligners show abrasion at the cusp tips (where teeth press hardest), buildup of biological material on the surface, and localized mineral deposits where saliva stagnates. Interestingly, the buccal (cheek-side) segments of used aligners were harder than new ones, likely from cold-working effects of repeated chewing pressure.¹⁰ Infrared spectroscopy comparing new and used Invisalign trays has confirmed that the core polyurethane chemistry does not change dramatically during a normal wear cycle, though some mechanical properties do shift: the material becomes slightly more brittle and slightly less resistant to creep, meaning it deforms a bit more easily under sustained load.¹¹

This gradual mechanical decay is one reason aligners are designed to be replaced every one to two weeks. By the time you swap in a new tray, the old one has lost some of its ability to deliver consistent force.

Force Delivery and Stress Relaxation

The therapeutic value of an aligner depends on its ability to push teeth with a steady, gentle force over the full wear period. Polyurethane materials like SmartTrack are specifically chosen because they resist “stress relaxation,” the tendency of a stretched polymer to gradually lose tension. If a material relaxes too quickly, the aligner becomes a passive retainer within hours rather than an active tooth-mover for days.

A study comparing five commercial aligner materials under simulated oral conditions found that most polyurethane-based materials maintained their stress relaxation properties after two weeks of immersion in artificial saliva or water. One exception was Essix ACE, a copolyester material, which showed a significant decrease in stress relaxation rate after the same period.¹² This kind of difference explains why material choice is not just a marketing decision. A polymer that softens too quickly in a wet, warm environment will underperform clinically.

Why Aligners Turn Yellow

If you have worn aligners, you probably noticed them discolor over a wear cycle, especially if you drink coffee or tea. This is not just surface staining; the chemistry of polyurethane makes it inherently susceptible to yellowing. The carbamate group in polyurethane’s molecular structure is highly polar, which makes it form hydrogen bonds with water and hydrophilic pigments more readily. The urethane group also oxidizes over time into a yellowish chromophore. On top of that, water absorption causes the material to swell slightly, creating microcracks that let pigments from food and drink penetrate deeper into the plastic.¹³

This is essentially a tradeoff built into the material. Polyurethane’s flexibility and force delivery are excellent, but the same chemical features that make it a good aligner material also make it prone to absorbing color. Removing your aligners before drinking anything besides water is standard advice, and the science of how polyurethane absorbs pigment explains why that advice is worth following.

How Cleaning Affects the Material

Because aligners accumulate biofilm and staining during wear, people try various cleaning methods. The choice matters more than you might expect, because some cleaning agents change the surface properties of the plastic itself. A randomized controlled trial comparing several cleaning methods on Invisalign trays found that a colourant-free clear soap caused the least color change. Invisalign’s own Cleaning Crystals and whitening toothpaste caused the most.¹⁴ The Cleaning Crystals group also had significantly higher surface roughness than most other methods. A rougher surface is a problem: it creates more places for bacteria to anchor and pigments to accumulate, which defeats the purpose of cleaning in the first place.

Separate testing across multiple aligner brands found that simple water rinsing and brushing resulted in less optical change than cleaning tablet solutions or mild liquid soap, though the magnitude depended on the specific brand.¹⁵ Invisalign trays were among the more resilient brands in that comparison. The practical takeaway is that gentle, non-abrasive cleaning tends to preserve both the clarity and the smoothness of the plastic better than aggressive chemical soaks.

Bacterial Buildup on Aligner Plastic

One concern with any appliance that covers teeth is whether its surface attracts more bacteria than bare enamel. Lab testing of several aligner materials for initial bacterial adhesion and biofilm formation found no significant differences between the aligner plastics and either natural enamel or metal orthodontic brackets.¹⁶ In other words, the plastic itself does not appear to be uniquely hospitable to oral bacteria compared to what is already in your mouth. The hygiene challenge with aligners is less about the surface chemistry of the plastic and more about the trapping effect: a tray held snugly against teeth creates a low-oxygen, low-saliva-flow environment between the plastic and enamel, which can encourage bacterial growth if you are not cleaning regularly.

Microplastics and Nanoplastics

A newer area of investigation is whether aligners shed microplastics or nanoplastics during normal wear. Mechanical forces from chewing, temperature changes from hot and cold drinks, and chemical exposure from saliva and food all create conditions that could potentially fragment tiny particles from the polymer surface. A recent narrative review flagged this as an area that warrants more research, given the long-term nature of aligner therapy (treatments often last a year or more) and the total hours of plastic-to-tissue contact involved.¹⁷

At this point, the evidence is more of a question mark than an alarm. Microplastic release from dental appliances has not been quantified in the same way it has been for, say, plastic food containers or synthetic textiles. The surface degradation patterns seen in retrieval studies, including the voids and delamination mentioned earlier, are consistent with the idea that small particles could be shedding, but dedicated studies measuring particle counts and sizes from worn aligners are still limited. This is a space worth watching, especially as broader microplastics research continues to evolve.

The Rise of 3D-Printed Aligners

Most aligners today, including Invisalign, are still manufactured by thermoforming: heating a flat sheet of plastic and vacuum-pressing it over a 3D-printed model of the patient’s teeth.¹⁸ This process works, but it has inherent limitations. The stretching thins the plastic unevenly, so the walls of the aligner are not the same thickness everywhere, which affects force distribution.

Direct 3D printing of aligners, where the tray itself is printed rather than the model it is formed over, is an emerging alternative that uses entirely different materials. One notable example is TC-85, a photocurable resin with shape-memory properties. Shape-memory means the material can be deformed and then return to its original programmed shape when warmed, which in theory allows for more controlled and sustained force delivery throughout a wear cycle.¹⁹

Comparative testing of thermoformed polyurethane aligners against directly 3D-printed TC-85 aligners found that the printed material had lower chemical extractables, better resistance to stress relaxation, and superior shape-recovery behavior under short-term aging conditions.²⁰ Lower extractables is a direct safety advantage, since it means less chemical leaching into saliva. Researchers and manufacturers see directly printed aligners, particularly those made from biocompatible photocurable resins, as the likely next generation of the technology.²¹

The Environmental Side of Aligner Plastic

A full course of Invisalign treatment can involve anywhere from a dozen to over fifty individual trays, each worn for a week or two and then discarded. That is a considerable volume of single-use medical-grade plastic, and unlike a water bottle, it cannot go into a standard recycling bin because it is contaminated with biological material and often made from a polymer blend that municipal recyclers are not set up to process.

Research into the environmental footprint of clear aligner therapy has identified several strategies for reducing waste, including minimizing material consumption per tray, incorporating recycled content into aligner plastic, and developing take-back or recycling programs specifically for used aligners.²² Direct 3D printing could help on this front as well, because it uses only the material needed for each tray rather than cutting a large sheet and discarding the excess. Some newer resin-based printing processes also claim lower carbon emissions during manufacturing. For now, though, the sustainability challenge remains largely unsolved, and most used aligners end up in general waste.