Materials science of clear aligner therapy: composition, microstructure, and physical properties of thermoplastic and 3D-printed polymer systems: a structured literature review

Purpose: This review summarises current evidence on materials used for clear aligner fabrication, focusing on thermoformed PETG/TPU systems and directly 3D-printed photopolymers, and relates reported material properties to force decay, ageing, surface quality, optical behaviour, and biocompatibility. Methods: PubMed/MEDLINE, Scopus, Embase, and the Cochrane Library were reviewed, supplemented by manufacturer data sheets and technical reports when peer-reviewed data were limited. Studies reporting composition, microstructure, mechanical, optical, or thermal properties, surface behaviour, ageing, or biological safety of aligner materials were considered. Results: PETG is optically clear and stiff (1,800-2,200 MPa; Tg 75-81 °C) but shows marked early stress relaxation; one wet 37 °C study reported 62% force decay over 24 h, mainly in the first 8 h. TPU behaviour is formulation-dependent: single-layer TPU may relax similarly to PETG, whereas selected multilayer TPU/copolyester sheets generate lower initial forces with slower decay. Directly 3D-printed, photocurable urethane-acrylate-based aligners remove model-forming and thermoforming steps and may improve geometric control, but their properties depend on print orientation, washing, post-curing, and degree of conversion. Shape-memory photopolymers near oral temperature show partial recovery, but clinical force-decay data over routine wear intervals remain limited. Conclusions: Aligner material behaviour is formulation- and processing-dependent. Broad labels such as PETG, TPU, or printed resin are insufficient unless manufacturing route, thickness, post-processing, and ageing conditions are reported. Standardised force-decay and ageing protocols, followed by comparative clinical studies, are needed before material-specific claims can guide treatment planning.