A team of researchers has developed an innovative printing method that allows them to encode an image of the Mona Lisa onto a material they call "smart skin." This image can be hidden within the material and only becomes visible under certain conditions—such as stretching, heating, contact with liquids, or when the material changes shape from flat (2D) to three-dimensional (3D).

Multifunctional Synthetic Materials

Synthetic materials are widely used in science, engineering, and industry, but most are designed for a limited range of applications. A group of researchers from Penn State University, led by Associate Professor Hongtao Sun, has developed a new technology for producing multifunctional "smart synthetic skin." These adaptive materials can be programmed to perform various functions: from concealing and revealing information to creating adaptive camouflage and supporting soft robotic systems.

Programmable Smart Skin Made from Hydrogel

Using a new approach, the scientists created programmable smart skin based on hydrogel—a soft, water-rich material. Unlike traditional synthetic materials with fixed properties, this smart skin can be tuned to respond to a wide range of stimuli. Its appearance, mechanical properties, surface texture, and ability to change shape are controlled by external factors such as heat, solvents, or physical manipulation. The research results were published in the journal Nature Communications and featured in the Editors' Highlights section.

Inspired by Nature

Hongtao Sun explained that the idea for the material came from observing octopuses and other cephalopods, which can rapidly change the appearance and texture of their skin for camouflage or communication. "Cephalopods use a complex system of muscles and nerves to dynamically control their skin's appearance and texture. Inspired by these soft-bodied creatures, we developed a 4D printing system to achieve similar capabilities in a synthetic material," Sun said.

Halftone Encoded Printing Technology

To achieve adaptability, the team used a halftone encoded printing method. This technique converts image or texture data into a binary code and embeds it directly into the material, similar to how dot patterns are used in newspapers or photographs to create images.

By encoding digital patterns within the hydrogel, researchers can program the smart skin's response to different stimuli. The printed patterns determine how specific areas of the material will react: some may swell, contract, or become softer when exposed to changes in temperature, liquids, or mechanical forces. By carefully designing these patterns, the team controls the overall behavior of the material. "Simply put, we print instructions inside the material that determine how it responds to changes in the environment," Sun noted.

Demonstrating Concealment and Revelation of Information

One of the most striking demonstrations was the material's ability to hide and reveal visual information. Graduate student and lead author Haoqin Yang noted that this feature highlights the potential of smart skin. For the demonstration, the team encoded an image of the Mona Lisa into a hydrogel film. When the material was rinsed with ethanol, it became transparent and the image disappeared. The hidden image reappeared only after the film was placed in ice water or gradually heated.

Yang emphasized that the Mona Lisa was used merely as an example—the technology allows virtually any image to be encoded into the hydrogel. "Such behavior can be used for camouflage, where the surface blends into its surroundings, or for information encryption, where messages become visible only under certain conditions," he explained.

The researchers also showed that hidden patterns can be detected by carefully stretching the material and analyzing its deformation using digital image correlation. This means information can be revealed not only visually but also through mechanical manipulation, adding an extra layer of security.

Flexibility and Coordination of Functions

The smart skin demonstrated remarkable flexibility: the material easily transitions from a flat sheet to complex, nature-inspired shapes with detailed surfaces. Unlike many other shape-shifting materials, this transformation does not require multiple layers or different substances. Changes in shape and texture are entirely controlled by the printed halftone patterns within a single sheet, enabling effects similar to cephalopod skin.

Building on this capability, the team showed that multiple functions can be programmed to work simultaneously. By carefully designing the halftone patterns, they encoded the Mona Lisa image into flat films, which then transformed into three-dimensional shapes. As the sheets bent into dome-like structures, the hidden image gradually appeared, demonstrating that changes in shape and appearance can be coordinated within a single material. "Just as cephalopods coordinate body shape and skin patterns, synthetic smart skin can simultaneously control its appearance and deformation—all within one soft material," Sun noted.

Outlook and Future Development

Sun added that the new work builds on the team's previous research on 4D printing of smart hydrogels, also published in Nature Communications. That study focused on combining mechanical properties with programmable transitions from flat to three-dimensional forms. In the current work, the team expanded their approach, using halftone 4D printing to integrate even more functions into a single hydrogel film.

In the future, the researchers plan to create a scalable and universal platform that enables precise digital encoding of multiple functions within a single adaptive material. "Interdisciplinary research at the intersection of advanced manufacturing technologies, intelligent materials, and mechanics opens new possibilities for creating systems that respond to stimuli, biomimetic engineering, encryption technologies, biomedical devices, and much more," Sun concluded.