Synthetic materials, ubiquitous across science, engineering, and industry, often excel at a limited set of functions. Researchers at Penn State University are challenging that limitation with a new fabrication technique yielding a programmable “smart synthetic skin” capable of performing a wide array of tasks, from information encryption to adaptive camouflage and supporting soft robotics.
The material, detailed in a paper in Nature Communications and highlighted in the journal’s Editors’ Highlights, is a programmable hydrogel – a soft, water-rich material – that dynamically responds to external stimuli like heat, solvents, or mechanical stress. Unlike traditional synthetics with fixed properties, this “smart skin” allows for the adjustment of optical appearance, mechanical response, surface texture, and even shape morphing.
The project, led by Hongtao Sun, assistant professor of industrial and manufacturing engineering (IME) at Penn State, draws inspiration from cephalopods, particularly octopuses, renowned for their ability to rapidly alter skin appearance and texture for camouflage and communication. “Cephalopods use a complex system of muscles and nerves to exhibit dynamic control over the appearance and texture of their skin,” Sun explained. “Inspired by these soft organisms, we developed a 4D-printing system to capture that idea in a synthetic, soft material.”
Sun describes the process as 4D printing because the resulting objects aren’t static; they actively change in response to their environment. This adaptability is achieved through a method called halftone-encoded printing, which translates image or texture data into binary code – ones and zeros – and embeds that information directly into the hydrogel. This is analogous to the dot patterns used in traditional newspaper printing to create images.
By encoding these digital patterns, the researchers program how the smart skin reacts to different stimuli. Different regions of the material respond uniquely, swelling, shrinking, or softening to varying degrees when exposed to temperature changes, liquids, or mechanical forces. Careful design of these patterns allows precise control over the material’s overall behavior. “In simple terms, we’re printing instructions into the material,” Sun said. “Those instructions tell the skin how to react when something changes around it.”
One striking demonstration of this capability involved concealing and revealing images. The team encoded a photo of the Mona Lisa into a hydrogel film. Initially, the image remained hidden. However, when the film was placed in ice water or gradually heated, the image became visible. Haoqing Yang, a doctoral candidate in IME and the paper’s first author, emphasized that the Mona Lisa was merely an example; the technique can encode virtually any image.
“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 noted. The team also discovered that concealed patterns could be detected by stretching the material and analyzing the resulting deformation using digital image correlation analysis, adding an additional layer of security by revealing information through mechanical interaction.
Beyond image manipulation, the smart skin exhibits remarkable shape-shifting abilities. According to Sun, the material can transition from a flat sheet into complex, bio-inspired shapes with detailed surface textures without requiring multiple layers or different substances. This contrasts with many other shape-changing materials that rely on complex construction.
The researchers demonstrated the coordinated control of multiple functions by encoding the Mona Lisa image into flat films that subsequently transformed into three-dimensional dome-like shapes. As the sheets curved, the hidden image gradually appeared, showcasing the simultaneous control of shape and visual appearance within a single 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.
This work builds upon the team’s previous research on 4D-printed smart hydrogels, also published in Nature Communications. That earlier study focused on combining mechanical properties with programmable transitions from flat to three-dimensional forms. The current research expands on this by utilizing halftone-encoded 4D printing to integrate even more functions into a single hydrogel film.
Looking forward, the researchers aim to develop a scalable and versatile platform for precise digital encoding of multiple functions within a single 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 concluded.
The research team 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 contributed to the project.
