Invisalign aligners are made from a medical-grade thermoplastic called SmartTrack, a multilayer material combining polyurethane and co-polyester. This proprietary plastic was designed specifically for orthodontic use and replaced Invisalign’s original material in 2013. It’s what gives the aligners their flexibility, clarity, and ability to apply steady pressure to your teeth over the course of each wear cycle.
SmartTrack’s Layered Structure
SmartTrack isn’t a single sheet of plastic. It has a layered architecture: a polyester core sandwiched between polyurethane outer skins. This combination is deliberate. Polyester provides rigidity and shape retention, while polyurethane adds elasticity and comfort. The result is an aligner that’s stiff enough to push teeth into new positions but flexible enough to snap on and off without cracking.
Before SmartTrack, Invisalign used a single-layer material called EX30, made from PETG (polyethylene terephthalate glycol), a common clear plastic also used by other aligner brands. PETG is transparent and resistant to solvents, but it has a significant drawback for orthodontics: it loses force quickly. Studies comparing aligner materials found that single-layer PETG showed the highest rate of stress relaxation over 24 hours, meaning the force it applied to teeth dropped off dramatically in just the first few hours of wear.
Multilayered materials like SmartTrack perform better on this front. They deliver lower initial force but maintain it more consistently over time, which matters because steady, gentle pressure moves teeth more predictably than a strong force that fades fast. Align Technology (Invisalign’s manufacturer) describes SmartTrack as having greater elasticity, better chemical stability, and a more precise fit than EX30.
How Aligners Are Manufactured
Every Invisalign treatment starts with a 3D scan of your teeth. That scan gets imported into orthodontic design software, where your provider plans the sequence of tooth movements, places virtual attachments, and adjusts aligner thickness. Each stage of your treatment gets its own unique aligner shape.
Traditionally, these designs are used to 3D-print a physical model of each tooth position, and the SmartTrack material is then thermoformed (heated and vacuum-pressed) over those models. The industry is also moving toward directly 3D-printing aligners from photopolymer resins, skipping the model step entirely. In that process, the printed aligner goes through centrifugation to remove uncured resin, UV curing in a nitrogen environment to fully harden the plastic, polishing where support structures were attached, and a brief hot water rinse to clear any residue.
Printing resolution sits around 100 microns on the vertical axis, which is fine enough that the aligner surface feels smooth against your cheeks and tongue. The resin temperature during printing is kept near 30°C to prevent defects.
Safety and Chemical Concerns
SmartTrack does not contain latex, gluten, or phthalates (based on available testing), and it’s classified as hypoallergenic. The BPA question is more nuanced. Laboratory testing using liquid chromatography found that SmartTrack released detectable amounts of both BPA and BPS, and it released more of both than other clear aligner brands tested. However, the amounts measured were far below the acceptable daily intake level established by regulatory agencies. In practical terms, the exposure from wearing Invisalign is a tiny fraction of what you’d encounter from other everyday sources like food containers and receipts.
Allergic reactions to Invisalign are rare but not unheard of. Even though the aligners don’t contain latex or acrylic, some people are sensitive to synthetic polymers in general. Symptoms can include gum inflammation, mild swelling, or irritation where the aligner contacts soft tissue. People with known sensitivities to plastics or dental adhesives are most likely to notice this.
Staining and Durability During Wear
SmartTrack starts out nearly invisible, but it’s not immune to discoloration. In laboratory staining tests, turmeric and saffron caused the most noticeable color changes, followed by Kashmiri red chili, coffee, and tea. Even green tea produced mild discoloration after 12 to 24 hours of exposure. Black tea specifically caused significant color change in Invisalign aligners after seven days of contact.
This is why you’re told to remove your aligners before eating or drinking anything other than water. Since each set of aligners is only worn for one to two weeks before switching to the next tray, minor staining rarely becomes a serious cosmetic issue. But drinking coffee or tea with aligners in will accelerate the yellowing noticeably, especially toward the end of a tray’s lifespan.
Heat is the other practical concern. Thermoplastics soften when they get hot, so leaving aligners in a car on a summer day, rinsing them with hot water, or drinking very hot beverages while wearing them can warp the shape. Once warped, an aligner won’t fit properly and can’t be reshaped.
What Happens After You’re Done With a Tray
Used Invisalign trays are currently not part of any widespread recycling program. Most end up in general plastic waste and get incinerated. The polyurethane and polyester blend isn’t easily recyclable through standard municipal systems, and the small size of each tray makes sorting impractical. Some orthodontic offices have started setting up collection points for used aligners to explore more responsible disposal, but this is still the exception. Given that a typical Invisalign treatment involves anywhere from 20 to 50 individual trays, the cumulative plastic waste from millions of patients worldwide is a growing environmental consideration.