Fig. 5: Halftone pattern-regulated strain mapping for multi-modal, multi-layered information encryption-decryption.

Halftone patterns oriented horizontally (a), vertically (e), or diagonally (i) across grayscale levels (G0 to G9), with corresponding optical halftone images of the Mona Lisa (b, f, j) and full-field strain mappings: y-strain (c, d, l) and x-strain (g, h, k) under uniaxial stretching (red arrows). Optical (m) and x-strain mapping images (n, o) of the Mona Lisa encoded with hybrid halftone patterns for enhanced contrast. The left side of the optical image in (m) and its x-strain mapping image (n) show the deswollen hydrogel in water at 35 °C, while the right side, where the image information is erased in (m) but preserved in the x-strain mapping (o), represents the film after ethanol immersion followed by water at 35 °C. p, q Second-layer encryption of the letter “M”, composed of diagonally oriented halftone patterns embedded within the horizontally oriented halftone patterns of the Mona Lisa at the same grayscale level, displayed in the optical image (p) and the y-strain mapping image under 10% y-axis strain \({\varepsilon }_{y}\) (q). Scale bars, 5 mm.