This octopus-inspired smart skin can change shape and reveal hidden secrets on command.

Synthetic materials are used almost everywhere, from manufacturing to medicine, yet most are designed to perform only one or two specific functions. A team at Penn State is working to change that. Led by Hongtao Sun, assistant professor of industrial and manufacturing engineering (IME), the researchers developed a new fabrication method that produces multifunctional “smart synthetic skin.” These adaptable materials can be configured to encrypt or decrypt information, enable adaptive camouflage, support soft robotics, and more.

Using this approach, the team created a programmable smart skin made from hydrogel, a soft material rich in water. Unlike traditional materials with fixed properties, this hydrogel-based skin can be tuned to perform multiple functions. Its optical appearance, mechanical behavior, surface texture, and ability to morph into new shapes can all shift when exposed to heat, solvents, or mechanical stress.

The research was published in Nature Communications and was featured in Editors’ Highlights.

Octopus-Inspired Design and 4D Printing Technology

Sun, the project’s principal investigator, said the concept grew out of observing cephalopods such as octopuses. These animals can rapidly alter the color and texture of their skin to hide from predators or communicate.

“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 technique as 4D printing because the resulting 3D structures are designed to actively respond to environmental changes.

To achieve this, the researchers used halftone-encoded printing. This method converts image or texture information into binary ones and zeros and embeds that data directly into the material, similar to the dot patterns seen in printed photographs or newspapers. By integrating these patterns into the hydrogel, the team effectively programs how the smart skin will respond to different external triggers.

Different sections of the material are designed to react in specific ways. Some areas may swell less or soften more than others when temperatures shift, liquids are introduced, or physical forces are applied. By carefully arranging these digital patterns, the researchers control the overall behavior of the sheet.

“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.”

Hidden Images and Built In Encryption

One of the most striking demonstrations involved concealing and revealing images. Haoqing Yang, a doctoral candidate studying IME and first author of the paper, highlighted this feature as a key example of the material’s versatility.

To demonstrate the concept, the researchers encoded a photo of the Mona Lisa into the hydrogel film. After being washed with etha nol, the film appeared clear, with no visible image. The hidden portrait only became fully visible after immersion in ice water or during gradual heating.

Yang emphasized that the Mona Lisa was simply a demonstration. The same printing strategy can embed virtually any image 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 team also showed that concealed patterns can be detected by gently stretching the material and analyzing how it deforms using digital image correlation analysis. In other words, information can be revealed not only visually but also through mechanical interaction, providing an added layer of security.

Shape Morphing in a Single Material Layer

Beyond visual changes, the smart skin demonstrated impressive flexibility. According to Sun, the material can transform from a flat sheet into unconventional, bio-inspired shapes with detailed textures. Unlike many other shape-morphing materials, this transformation does not require stacking multiple layers or combining different substances.

Instead, the digitally printed halftone pattern within a single sheet governs both texture and form, allowing the surface to mimic the complex features seen in cephalopod skin.

Building on this capability, the researchers showed that the material can perform multiple functions at once. By co-designing the halftone patterns, they encoded the Mona Lisa image into flat films that later emerged as the material was reshaped into three-dimensional forms. As the sheets curved into dome-like structures, the hidden image gradually appeared, illustrating how shape change and visual display can be programmed together.

“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 Future of Smart Hydrogel Systems

Sun noted that this work builds on the team’s earlier research on 4D-printed smart hydrogels, which was also published in Nature Communications. In that previous study, the focus was on co-designing mechanical properties and programmable transitions from flat sheets to three-dimensional forms. The new research advances that foundation by employing halftone-encoded 4D printing to integrate additional functions into a single smart hydrogel film.

Looking ahead, the researchers plan to develop a scalable platform that enables precise digital encoding of multiple functions within one adaptive smart material system.

“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.

Reference: “Halftone-encoded 4D printing of stimulus-reconfigurable binary domains for cephalopod-inspired synthetic smart skins” by Haoqing Yang, Haotian Li, Juchen Zhang, Tengxiao Liu, H. Jerry Qi and Hongtao Sun, 12 November 2025, Nature Communications.
DOI: 10.1038/s41467-025-65378-8

Other contributors from Penn State include 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.

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