Synthetic materials are widely used across science, engineering, and industry, but most are designed to perform only a narrow range of tasks. A research team at Penn State set out to change that. Led by Hongtao Sun, assistant professor of industrial and manufacturing engineering (IME), the group developed a new fabrication technique that can produce multifunctional “smart synthetic skin.” These adaptable materials can be programmed to perform a wide variety of tasks, including hiding or revealing information, enabling adaptive camouflage, and supporting soft robotic systems.
Using this new approach, the researchers created a programmable smart skin made from hydrogel, a soft, water-rich material. Unlike conventional synthetic materials with fixed behaviors, this smart skin can be tuned to respond in multiple ways. Its appearance, mechanical behavior, surface texture, and ability to change shape can all be adjusted when the material is exposed to external triggers such as heat, solvents, or physical stress.
The findings were published in Nature Communications, where the study was also selected for Editors’ Highlights.
Inspired by Octopus Skin and Living Systems
Sun, the project’s principal investigator, said the concept was inspired by cephalopods such as octopuses, which can rapidly alter the look and texture of their skin. These animals use such changes to blend into their surroundings or communicate with one another.
“Cephalopods use a complex system of muscles and nerves to exhibit dynamic control over the appearance and texture of their skin,” Sun said. “Inspired by these soft organisms, we developed a 4D-printing system to capture that idea in a synthetic, soft material.”
Sun also holds affiliations in biomedical engineering, material science and engineering, and the Materials Research Institute at Penn State. He described the process as 4D printing because the printed objects are not static. Instead, they can actively change in response to environmental conditions.
Printing Digital Instructions Into Material
To achieve this adaptability, the team used a method called halftone-encoded printing. This technique converts image or texture data into binary ones and zeros and embeds that information directly into the material. The approach is similar to how dot patterns are used in newspapers or photographs to create images.
By encoding these digital patterns within the hydrogel, the researchers can program how the smart skin reacts to different stimuli. The printed patterns determine how various regions of the material respond. Some areas may swell, shrink, or soften more than others when exposed to temperature changes, liquids, or mechanical forces. By carefully designing these patterns, the team can control the material’s overall behavior.
“In simple terms, we’re printing instructions into the material,” Sun explained. “Those instructions tell the skin how to react when something changes around it.”
Hiding and Revealing Images on Demand
One of the most eye-catching demonstrations involved the material’s ability to conceal and reveal visual information. Haoqing Yang, a doctoral candidate in IME and the paper’s first author, said this capability highlights the potential of the smart skin.
To demonstrate the effect, the team encoded an image of the Mona Lisa into the hydrogel film. When the material was washed with ethanol, it appeared transparent and showed no visible image. The hidden image became clear only after the film was placed in ice water or gradually heated.
Yang noted that the Mona Lisa was used only as an example. The printing technique allows virtually any image to be encoded into the hydrogel.
“This behavior could be used for camouflage, where a surface blends into its environment, or for information encryption, where messages are hidden and only revealed under specific conditions,” Yang said.
The researchers also showed that concealed patterns could be detected by gently stretching the material and analyzing how it deforms using digital image correlation analysis. This means information can be revealed not only visually, but also through mechanical interaction, adding an extra level of security.
Shape Shifting Without Multiple Layers
The smart skin also demonstrated remarkable flexibility. According to Sun, the material can easily shift from a flat sheet into complex, bio-inspired shapes with detailed surface textures. Unlike many other shape-changing materials, this transformation does not require multiple layers or different substances.
Instead, the changes in shape and texture are controlled entirely by the digitally printed halftone patterns within a single sheet. This allows the material to replicate effects similar to those seen in cephalopod skin.
Building on this capability, the team showed that multiple functions can be programmed to work together. By carefully designing the halftone patterns, they encoded the Mona Lisa image into flat films that later transformed into three-dimensional forms. As the sheets curved into dome-like shapes, the hidden image slowly appeared, showing that changes in shape and visual appearance can be coordinated within one material.
“Similar to how cephalopods coordinate body shape and skin patterning, the synthetic smart skin can simultaneously control what it looks like and how it deforms, all within a single, soft material,” Sun said.
Expanding the Potential of 4D-Printed Hydrogels
Sun said the new work builds on earlier research by the team on 4D-printed smart hydrogels, which was also published in Nature Communications. That earlier study focused on combining mechanical properties with programmable transitions from flat to three-dimensional forms. In the current research, the team expanded the approach by using halftone-encoded 4D printing to integrate even more functions into a single hydrogel film.
Looking ahead, the researchers aim to create a scalable and versatile platform that allows precise digital encoding of multiple functions within one adaptive material.
“This interdisciplinary research at the intersection of advanced manufacturing, intelligent materials and mechanics opens new opportunities with broad implications for stimulus-responsive systems, biomimetic engineering, advanced encryption technologies, biomedical devices and more,” Sun said.
The study also included Penn State co-authors Haotian Li and Juchen Zhang, both doctoral candidates in IME, and Tengxiao Liu, a lecturer in biomedical engineering. H. Jerry Qi, professor of mechanical engineering at Georgia Institute of Technology, also collaborated on the project.