Influence of Thermoformation and Clinical Use on Surface Free Energy and Biocompatibility of Polyethylene Clear Aligners

Clear aligners are widely used in orthodontic therapy; however, thermoformation and intraoral exposure may alter their surface properties and biological performance. This study evaluated the effects of aligner fabrication and two-week clinical use on surface free energy and cytocompatibility of polyethylene clear aligners.

Commercial polyethylene intended for clear aligner fabrication was investigated. Three groups were analyzed: (1) unprocessed polyethylene, (2) thermoformed aligners prior to clinical use, and (3) aligners retrieved after two weeks of intraoral wear. Cytocompatibility was assessed using L929 cells cultured under standard conditions. Cells were seeded onto sterilized specimens and incubated for 72h. Cell viability was determined using the MTT assay and expressed relative to untreated, control cells (100% viability). Surface free energy (γ), including polar and dispersive components, was measured by contact angle analysis using deionized water and ethylene glycol.

Polyethylene showed slightly reduced baseline cell viability compared with the control (95%), while thermoformation induced a mild further decrease in the number of viable cells. A more pronounced reduction was observed after clinical use, where viability decreased by approximately 25% relative to control. Surface free energy increased markedly and progressively across all groups, rising from 29.3 in unprocessed polyethylene to 39.0 mJ/m² after thermoformation and 67.4 mJ/m² after intraoral exposure. This increase was driven by a strong rise in the polar component, which increased from 14.3 to 27.0 mJ/m² and 67.1 mJ/m², respectively. In contrast, the dispersive component decreased from 15.0 to 12.1 mJ/m² and 0.27 mJ/m², respectively, indicating a pronounced shift in surface chemistry after clinical use.

Thermoformation and intraoral exposure induce significant physicochemical remodeling of polyethylene clear aligners, characterized by increased surface polarity and surface free energy. These changes are accompanied by a meaningful reduction in cytocompatibility after clinical use, indicating that intraoral aging is the dominant factor governing biological response of aligner materials.