Multi-wavelength centrifugal processing enables 3D printing of functionally graded medical devices: Construction and validation of mechanically tunable orthodontic aligners

Functionally Graded Materials (FGMs) have gained substantial attention in biomedical device development, particularly for creating functionally adaptive solutions. In recent years, grayscale vat photopolymerization 3D printing has emerged as a promising technology for FGMs fabrication owing to its advantages of high efficiency and precision. However, the residual unreacted monomers in grayscale printing components have brought a large amount of toxicity, becoming a bottleneck restricting their application in biomedical fields. This study proposes a multi-wavelength stepwise curing strategy that integrates wavelength-selective photoabsorber (PA) into the resin, using clear orthodontic aligners as a platform, to achieve a highly polymerized surface state while enabling gradient mechanical properties. Based on the integration of light field simulation and photopolymerization kinetics, a mathematical model was developed to predict the degree of conversion (DoC) distribution in multi-layer printing. The printed aligners demonstrated validated biocompatibility, with in vitro experiments showing that grayscale modulation effectively reduced orthodontic forces on non-targeted teeth while resisting stress relaxation during 7-day continuous monitoring. Furthermore, a centrifugation-based post processing method was developed to effectively eliminate surface layer steps and reduce bacterial adhesion. This process is compatible with the majority of current photopolymer resin systems and provides a technical framework for developing advanced functional medical devices.