## 5 Conclusions  
Different characteristics emerged based on the various studies analyzed.  
The Smart Track (PU) material was hard, wear-resistant, elastic but more brittle, and with lower sliding resistance than PETG-based devices, while compared to PU devices, it was smoother and less bumpy.  
The PETG materials were then more transparent, flexible, and therefore thinner than the PU materials, even the roughness was low.  
Stress decay was also tested, which was rapid for both PETG and PU devices in the first 8 h of application, subsequently tending to decrease to a plateau.  
After thermoforming, both PETG and PU materials decreased the mechanical properties (Table 3).  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Property** & **PETG (e.g., Duran, Essix Ace, Essix Plus, Biolon)** & **PU (e.g., Zendura, SmartTrack, Eon, SureSmile, Clarity)** \\ \hline Elastic Modulus & Generally higher, decreases significantly after thermoforming & Higher hardness and modulus values \\ \hline Tensile Strength & High, but decreases after heat treatment and immersion in artificial saliva & Maintains strength, but may show slight decrease after thermoforming \\ \hline Flexibility & Less flexible compared to PU & More elastic, allowing for predictable orthodontic movements \\ \hline Transparency & High, can be affected by crystallinity (e.g., Essix C+ is less transparent) & High transparency, practical invisibility \\ \hline Stress Decay & Rapid stress decay, especially in the first few hours & Rapid initial stress decay but reaches a plateau over time \\ \hline Chemical Resistance & Good, but varies (e.g., Essix C+ has higher chemical resistance) & Generally good, with specific formulations possibly affecting resistance \\ \hline Surface Characteristics & Smooth but can exhibit impurities after thermoforming & Smooth but may show irregularities and impurities \\ \hline Yield Stress & Generally higher but decreases significantly after thermoforming & Slightly lower, decreases about 30\% after thermoforming \\ \hline Applications & Widely used in orthodontics for clear aligners and retainers & Used for aligners reporting more elasticity and flexibility \\ \hline \hline \end{tabular}
\end{table}
Table 3: Summary of the properties of PETG and PU.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14156533/s1](https://www.mdpi.com/article/10.3390/app14156533/s1)]([https://www.mdpi.com/article/10.3390/app14156533/s1](https://www.mdpi.com/article/10.3390/app14156533/s1)), Excel spreadsheet.
* [1]**Author Contributions:** Conceptualization, A.C. and M.D.F.; methodology, N.C.; software, C.M.C.-P.; validation, G.D.I., N.C. and M.D.F.; formal analysis, A.C.; investigation, C.M.C.-P.; resources, N.C.; data curation, A.C.; writing--original draft preparation, M.D.F.; writing--review and editing, G.D.I.; visualization, N.C.; supervision, G.D.I.; project administration, A.C.; funding acquisition, C.M.C.-P. All authors have read and agreed to the published version of the manuscript.
* [2]**Funding:** This research received no external funding.
* [3]**Institutional Review Board Statement:** Not applicable.
* [4]**Informed Consent Statement:** Not applicable.
* [5]**Data Availability Statement:** Not applicable.
* [6]**Conflicts of Interest:** The authors declare no conflicts of interest.