Post-washing using a Soxhlet apparatus improves the mechanical properties and biocompatibility of 3D printing resin in contaminated solvent.

Photopolymerization-based 3D printing is widely used in dental manufacturing due to its precision and customizability. However, printed resin specimens require effective post-washing to remove unpolymerized resin, which is a critical step that affects both their mechanical properties and biocompatibility. A conventional ultrasonic bath may fail to achieve effective post-washing due to solvent contamination by the residual resin, resulting in incomplete cleaning and possible redeposition. This study investigated the use of a Soxhlet apparatus as an innovative post-washing method. By continuously refreshing the solvent, Soxhlet post-washing ensures the effective removal of residual resin. The results showed that Soxhlet post-washing restored the turbidity of the contaminated solvent to that of fresh ethanol, indicating a significant reduction in surface contamination. Mechanical tests showed that the Vickers hardness and tensile strength were significantly higher for specimens treated by a Soxhlet apparatus than when using a contaminated ultrasonic bath. Fourier-transform infrared spectroscopy, Raman spectroscopy and photo-DSC showed that the degree of conversion was higher in the contaminated Soxhlet apparatus group than in the contaminated ultrasonic bath group. Cell viability assays confirmed that cytotoxic leachates were reduced after Soxhlet post-washing, resulting in higher biocompatibility. These findings suggest that Soxhlet-based post-washing overcomes the limitations of solvent contamination and provides a promising approach to improve the quality and clinical performance of 3D printing resin.