Microscopic Acid-Induced Degradation and Elemental Release From Thermoformed and 3D-Printed Orthodontic Aligners in a Simulated Gastric Environment

Abstract

Clear aligners have revolutionized orthodontic treatment, yet concerns are rising about microplastics (MPs) and nanoplastics (NPs) released from these devices through mechanical wear and chemical degradation. Once ingested, these particles may undergo structural and chemical transformations in the gastrointestinal tract, particularly under acidic gastric conditions. Despite growing environmental and toxicological awareness, the degradation patterns of aligner materials remain largely unexplored. This study evaluated the acid-induced degradation and elemental release of thermoformed (TFA) and direct-printed (DPA) aligners in a simulated gastric environment. TFA (Invisalign SmartTrack) and DPA (Graphy TC-85DAC) samples were exposed to hydrochloric acid (pH 2). Surface acid-induced degradation was monitored using atomic force microscopy (AFM) over 60 min, while elemental release was quantified using inductively coupled plasma mass spectrometry (ICP-MS) following acid digestion on 0.5 M HCl leachates after 7 days. TFA rapidly disintegrated into an amorphous gel, preventing AFM imaging at pH 2. DPA maintained integrity and showed progressive roughening: RMS roughness rose from 10.06 to 10.97 nm (+ 9%; p < 0.001), mean roughness from 7.85 to 8.49 nm (+ 8%; p = 0.002), and maximum height from 68.31 to 76.51 nm (+ 12%; p = 0.038). ICP-MS of digested matrices revealed distinct elemental fingerprints: TFA was dominated by Sn (33.42 mg/kg), K (21.35 mg/kg), and Na (13.34 mg/kg); DPA by Ca (36.63 mg/kg), Na (11.87 mg/kg), and Fe (3.2 mg/kg). In 7-day 0.5 M HCl leachates, TFA released Sb 0.13 and Sn 0.09 mg/kg, whereas DPA showed Sb 0.03 and Sn 0.11 mg/kg; DPA leachates were richer in Ca (7.57 mg/kg) and Fe (1.57 mg/kg). DPA exhibited quantifiably slower acid erosion than TFA and distinct elemental release profiles at longer extraction, supporting greater acid-phase stability of DPA and providing elemental markers to trace aligner-derived particles. The results pertain to Invisalign SmartTrack and Graphy TC-85DAC and should not be generalized to all thermoformed or 3D-printed aligners. These findings emphasize the need for biostable, environmentally safer materials in orthodontics, especially considering the ingestion and systemic distribution of MPs.