1) US4413978A – Orthodontic retainer
https://patents.google.com/patent/US4413978A/en
2) Retainer wear and compliance in the first 2 years after active orthodontic treatment
Kacer, Valiathan, Narendran, & Hans
https://www.sciencedirect.com/science/article/abs/pii/S0889540610005391
3) The quality of information on the Internet on orthodontic retainer wear: a cross-sectional study
https://www.tandfonline.com/doi/abs/10.1080/14653125.2015.1114711
4) Mechanical and geometric properties of thermoformed and 3D printed clear dental aligners
Jindal, Juneja, Siena, Bajaj, & Breedon
https://www.sciencedirect.com/science/article/abs/pii/S0889540619306262
5) Evaluation of fit for 3D-printed retainers compared with thermoform retainers
Cole, Bencharit, Carrico, Arias, & Tüfekçi
https://www.sciencedirect.com/science/article/abs/pii/S088954061831117X
6) Clear retainer
Chaimongkol et al.