Fig. 3: Halftone-enabled 4D printing for dynamic information encryption-decryption.

Halftone patterns generated using the FM method (a) and AM method (b) within a 6 × 6-pixel unit matrix, displaying defined grayscale levels from G0 to G9 (top panels) and corresponding printed hydrogel films in the deswollen state (bottom panels). A 7000 × 7000-pixel RGB image of the Mona Lisa (c) converted to a 120 × 120-pixel grayscale image (d) and further encoded as 720 × 720-pixel halftone images using the FM method (e) or AM method (f). c “Mona Lisa” by Leonardo da Vinci, image courtesy of Rawpixel.com (https://www.rawpixel.com/image/7726890/image-mona-lisa-art-public-domain), licensed under the Creative Commons Attribution 4.0 International License (CC BY 4.0). d Adapted from the same source as (c), converted to grayscale image, licensed under the Creative Commons Attribution 4.0 International License (CC BY 4.0). e Adapted from the same source as (c), converted to halftone image, licensed under the Creative Commons Attribution 4.0 International License (CC BY 4.0). f Adapted from the same source as (c), converted to halftone image, licensed under the Creative Commons Attribution 4.0 International License (CC BY 4.0). g 4D printed halftone-encoded hydrogel films for image concealment and revelation. The encrypted image information remained concealed in ethanol (dark red dot-labeled image) but gradually emerged within 60–100 s upon immersion in ice water, revealing contrast against a black background (yellow dot-labeled images) (Supplementary Movie 1); in the swollen state (25 °C water), the films appeared uniformly translucent against a black background (green dot-labeled images); image information was gradually revealed upon heating above the LCST through two protocols: during initial phase separation (orange dot-labeled images) and in the deswollen state (blue dot-labeled images for Route-1, red dot-labeled images for Route-2). Scale bars, 500 μm (a, b); 10 mm (g).