How Saliva Reshapes Dental Appliances at the Microscopic Level
That seemingly smooth plastic in your mouth is constantly changing at a microscopic level, and these changes might surprise you.
Have you ever wondered what happens to your clear aligners or retainers while they sit in your mouth? These transparent dental appliances lead a secret life at the microscopic level, constantly interacting with your saliva in ways that reshape their very structure. The same environment that makes life possible in our mouths—with its varying pH levels, enzymes, and temperature fluctuations—slowly transforms the plastic materials we depend on for straight teeth.
Oral temperature varies between 5.6°C and 58.5°C, creating stress on materials .
Saliva with varying pH levels creates a challenging chemical environment for materials .
Surface transformations occur at nanometer scale, invisible to the naked eye 1 .
Polyethylene terephthalate glycol (PETG) has become the go-to material for clear dental appliances—aligners, retainers, and mouthguards—thanks to an ideal combination of strength, flexibility, and transparency 1 . Unlike their metal counterparts, these clear devices offer discreet orthodontic treatment, but they exist in one of the most challenging environments in the human body.
Our saliva itself is far from simple—it's a complex biological fluid containing water, electrolytes, mucus, enzymes, and antimicrobial compounds that can interact with foreign materials in unexpected ways 7 .
What makes thermoplastics like PETG so useful is their amorphous molecular structure—meaning their polymer chains are arranged randomly rather than in orderly crystalline patterns. This irregular structure gives them transparency and flexibility but also creates microscopic pathways for water molecules to penetrate 1 .
Random polymer arrangement allows light to pass through
Irregular structure creates microscopic channels for moisture
To understand exactly how saliva transforms dental appliances, researchers designed a sophisticated experiment that simulated the oral environment with remarkable precision 1 .
The study focused on four commercially available PETG materials used in dental practices worldwide:
The samples were immersed in artificial saliva with different pH levels to represent varying chemical conditions in our mouths 1 :
Acidic (pH 4)
Neutral (pH 7)
Basic (pH 9)
The experiment simulated 14 days of oral exposure through cycles of hydration in artificial saliva followed by desiccation (drying) 1 .
Using a pressure molding unit, the researchers thermoformed 1 mm thick sheets of each PETG material into standardized specimens, replicating how dental appliances are made in actual practice 1 .
The samples were immersed in artificial saliva with different pH levels—neutral (pH 7), acidic (pH 4), and basic (pH 9)—to represent the varying chemical conditions in our mouths 1 .
The experiment simulated 14 days of oral exposure through cycles of hydration in artificial saliva followed by desiccation (drying), with careful measurements taken at each stage 1 .
The researchers employed two powerful measurement techniques: contact profilometry for general surface roughness and atomic force microscopy (AFM) for high-resolution three-dimensional images of the surface at nanometer scale 1 .
The findings revealed a fascinating story of transformation happening at the microscopic level.
The researchers discovered that all materials absorbed moisture from the artificial saliva, with their weights increasing after immersion. The highest sorption occurred in basic environments (pH 9), suggesting that materials are particularly vulnerable to chemical changes from certain foods or conditions. Leone samples showed the highest water uptake, while Erkodur demonstrated the lowest 1 .
| Material | Highest Sorption Environment | Relative Sorption Level | Solubility |
|---|---|---|---|
| Leone (L) | Basic (pH 9) | Highest | Significant |
| Crystal (C) | Basic (pH 9) | High | Significant |
| Duran (D) | Basic (pH 9) | Moderate | Not Significant |
| Erkodur (E) | Basic (pH 9) | Lowest | Not Significant |
Perhaps most strikingly, the surface of the materials changed in unexpected ways:
This counterintuitive finding—that materials could become both smoother and more irregular depending on the scale of observation—reveals the complexity of how saliva interacts with dental plastics.
| Material | Microscale Change | Nanoscale Change |
|---|---|---|
| Leone (L) | Smoother | More Irregular |
| Crystal (C) | Smoother | More Irregular |
| Duran (D) | Smoother | More Irregular |
| Erkodur (E) | Smoother | More Irregular |
Complementary research found that these physical changes affected the appearance of the materials as well. After simulated oral environment exposure, the materials showed increased translucency and opalescence, with colors migrating toward red and yellow zones . In some cases, these color changes reached levels classified as "extremely marked" .
These scientific findings have tangible implications for dental patients and professionals.
The changes in surface topography and increased nanoscale roughness create more attachment points for oral bacteria, potentially leading to increased biofilm formation 1 . This explains why older aligners often feel less clean and develop a cloudy appearance.
Different materials showed significantly different behaviors in the same environments. For instance, Erkodur demonstrated lower water sorption than other materials 1 . This knowledge empowers dentists to select the most appropriate materials for specific clinical situations and patient needs.
The optical changes observed—increased translucency and color shifts—explain why clear aligners gradually become more visible over time, potentially affecting aesthetics during treatment .
Perhaps most importantly, these findings provide scientific justification for the standard recommendation to replace aligners every 1-2 weeks during orthodontic treatment 1 . The material degradation documented in these studies would eventually compromise the precise force delivery required for effective tooth movement.
The cloudiness results from microscopic changes in the material's surface topography. As saliva interacts with the plastic, it creates nanoscale irregularities that scatter light differently and provide more attachment points for bacteria and proteins, leading to a cloudy appearance 1 .
Different pH levels in your mouth (from foods and drinks) affect how much moisture the aligner material absorbs. Basic environments (pH 9) cause the highest water sorption, potentially accelerating material changes 1 . Acidic environments can also affect the material but to a lesser extent.
As aligners absorb moisture and their surface changes at a microscopic level, their mechanical properties change. This can affect the precise forces they apply to teeth, potentially compromising treatment effectiveness. Regular replacement ensures consistent force application 1 .
Research into how dental materials behave in the oral environment continues to evolve.
Scientists are now working to develop next-generation thermoplastics with enhanced resistance to moisture absorption and surface changes 1 .
The growing understanding of material-oral environment interactions promises a future where dental appliances last longer, perform more predictably, and maintain their aesthetic qualities throughout their intended lifespan.
The hidden life of your clear aligners reveals a fascinating dance between human biology and material science—a microscopic drama that plays out every day in millions of mouths around the world. The next time you pop in your aligner or retainer, remember that there's more to that simple piece of plastic than meets the eye—and thanks to scientific inquiry, we're beginning to understand its secret world.