The objective of this study was to develop biologically compatible resins with optimum safety profiles and physical properties that can be used for long periods inside the mouth, and to attempt to use these resins in a 3D printing process to produce direct aligners for use in mouthpiece orthodontics. Low toxicity water-soluble monomers (1M) were used to develop 3D printer-compatible biocompatible resins (polymers) that have no skin reactivity, carcinogenicity, or reproductive toxicity. Cytotoxicity testing (LDH test), proliferation testing (WST1 test), and mechanical testing were also performed. We attempted to use these resins to produce direct aligners. We successfully produced acrylic-epoxy hybrid light-curing resins that have optimal safety profiles composed of water-soluble monomers alone. The results of cytotoxicity testing and proliferation testing showed that all the water-soluble monomers used for 3D printing (3D-1M) exhibited low cytotoxicity, but the cell survival rates suggested that the composition ratio of the raw materials may be an issue. The results of mechanical testing demonstrated that the 3D-1M met the mechanical strength requirements for base polymers used in orthodontics. We successfully produced direct aligners using these biocompatible resins. However, their middle sections were fragile, suggesting that their physical properties must be improved if they are to be used in aligners. We intend to improve the mechanical properties of these monomers, and perform safety testing for approval as Class-II controlled medical devices.