Encoding and Encryption via Dual-Wavelength 3D Digital Light Processing.

Additive manufacturing of multi-responsive materials is opening new opportunities in a wide range of applications, including information storage, security, and sensing. Herein, complex structures enabling both the encoding and encryption of information in three dimensions (3D) are achieved by combining photo- and pH-responsive chromophores, spiropyran and phenolphthalein derivatives, respectively, within a single ink formulation. To this end, a multi-wavelength digital light processing (DLP) system is employed to fabricate initially colorless structures using a 405 nm wavelength LED to trigger the photopolymerization process. In parallel, a 365 nm wavelength LED integrated into the same system can induce locally controlled isomerization of spiropyran to the colored merocyanine, enabling information encoding in 3D in a layer-by-layer fashion. The stored information can be reversibly encrypted on demand via pH variation. The pH-responsive chromophore, phenolphthalein, enables modulation of optical contrast, rendering the encoded patterns temporarily unreadable. This work opens new pathways toward engineering multi-responsive functionality in complex 3D architectures.