FROM thermoformed to directly printed and 4D clear aligners: a critical narrative review of materials, manufacturing variables and clinical translation

Clear aligner therapy has expanded rapidly, but the conventional thermoforming workflow remains limited by material thinning, dimensional change, viscoelastic force decay, and model-dependent manufacturing inefficiency. Directly 3D-printed aligners (DPAs) and shape-memory polymer (SMP)-based 4D aligners have emerged as promising alternatives that may improve geometric control, enable force customization, and introduce thermo-responsive recovery behavior. This review critically synthesizes current evidence on the transition from thermoformed aligners to directly printed and SMP-based systems, with specific emphasis on how material chemistry, additive-manufacturing variables, and post-processing conditions influence thermo-mechanical behavior, optical and surface stability, biocompatibility, and clinical translation. A structured narrative review was conducted using Web of Science Core Collection, PubMed/MEDLINE, and Scopus for studies published between 2016 and 2026. Following staged screening and domain-based eligibility assessment, 150 core records were retained for final synthesis. The evidence was interpreted through predefined analytical domains including material chemistry and resin selection, SMP/4D thermo-mechanical behavior, optical and aging-related performance, biocompatibility and leachable risk, printer-dependent variability, print orientation, aligner design parameters, post-processing protocol, and emerging SMP systems. The available literature suggests that directly printed and SMP-based aligners offer important conceptual advantages over conventional thermoformed systems, particularly in digital thickness control, geometric accuracy, and, for selected SMP resins, temperature-dependent shape recovery and more physiologic force profiles. However, these advantages are not uniformly validated across materials or workflows. Reported findings remain strongly dependent on resin formulation, printer type, build orientation, specimen geometry, cleaning method, curing atmosphere, and aging condition. Optical and surface stability are often reduced after intraoral aging, while biological safety remains closely linked to curing completeness and residual monomer release, especially for urethane dimethacrylate-containing systems. Generally, current evidence supports cautious optimism rather than routine clinical equivalence. The most defensible conclusion is that next-generation directly printed and SMP-based aligners are promising but remain protocol-dependent systems whose long-term reliability, safety, and clinical superiority require stronger standardization, broader independent validation, and well-designed clinical studies.