Comparative microscopic analysis of plastic dispersion from 3D-printed and thermoformed orthodontic aligners

To compare directly printed aligners (DPA) and thermoformed aligners (TFA), evaluating the potential release and dispersion of microplastic (MP) and nanoplastic (NP) particles under simulated oral conditions.

DPA samples (Graphy Tera Harz TC-85-DAC resin) and TFA samples (Invisalign® SmartTrack) were subjected to rubs in an ultrapure water bath. The liquid was collected post-friction and analyzed for MPs and NPs using various techniques: optical microscopy (OM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). Also, plastic residues were quantified by weighing after drying within a laminar flow hood. Microscopic image analyses comprised the quantification of the average size of MPs and NPs, their concentration by TEM, and the roughness analysis by AFM.

The masses of MPs and NPs separated after rubbing were 0.001 g/200 µl and 0.004 g/200 µl for TFA and DPA samples, respectively. TEM analysis confirmed that DPA samples had larger (203.08 ± 2651.65 μm²) and more numerous particles compared to TFA (0.23 ± 27.53 μm²), even though it was not possible to distinguish the MPs and NPs due to clustering of the plastic residuals. AFM analysis indicated a bigger root mean square grain size for TFA than DPA; similarly, the mean roughness was lesser in the DPA sample than TFA one.

DPA generated larger and more numerous plastic particles compared to TFA, though grain-size characterization was challenging due to particle aggregation. This suggests that the manufacturing process and materials used in DPA could impact the creation of MPs and NPs during simulated mastication, highlighting a potential area for process optimization.