Scientists Create Smart Material Mimicking Octopus Skin
University Park, PA — While synthetic materials are prevalent across industries and scientific fields, they often serve limited functions. To address this limitation, researchers at Penn State, led by Assistant Professor Hongtao Sun, have developed a groundbreaking fabrication method. This method enables the printing of multifunctional 'smart synthetic skin' — materials that can perform various tasks such as encrypting or decrypting information, enabling adaptive camouflage, and powering soft robotics. The research team, including doctoral candidates Haoqing Yang, Haotian Li, and Juchen Zhang, along with lecturer Tengxiao Liu and Professor H. Jerry Qi, has made significant progress in this field.
Their innovative approach involves creating a programmable smart skin from hydrogel, a water-rich gel-like material. Unlike traditional synthetic materials with fixed properties, the smart skin offers enhanced multifunctionality. Researchers can adjust the gel's dynamic control over optical appearance, mechanical response, surface texture, and shape morphing when exposed to external stimuli like heat, solvents, or mechanical stress. This breakthrough was published in Nature Communications and featured in Editors' Highlights.
The inspiration for this material stems from the natural biology of cephalopods, such as octopuses, which can control their skin's appearance for camouflage or communication. Sun explains, 'Cephalopods use a complex system of muscles and nerves to dynamically control their skin's appearance and texture. Inspired by these soft organisms, we developed a 4D-printing system to replicate this concept in a synthetic, soft material.'
The 4D-printing method involves translating image or texture data onto a surface using binary ones and zeros, similar to the dot patterns in newspapers or photographs. This technique encodes digital information directly into the material, allowing the smart skin to change its appearance or texture in response to stimuli. The team's careful design of patterns determines how the material behaves overall, enabling them to 'print instructions' into the material that dictate its reactions to environmental changes.
One of the most remarkable demonstrations of the smart skin is its ability to hide and reveal information. The team encoded a photo of the Mona Lisa onto the hydrogel. When washed with ethanol, the image disappeared, but it reappeared when the film was immersed in ice water or gradually heated. This behavior could be utilized for camouflage or information encryption, where messages are hidden and revealed under specific conditions. Additionally, the material can be stretched gently to uncover hidden patterns, adding another layer of security through mechanical deformation.
The smart skin is highly malleable, transforming easily from a flat sheet into non-traditional, bio-inspired shapes with complex textures. Unlike other shape-morphing materials, this effect doesn't require multiple layers or different materials. Instead, the shapes and textures can be controlled by the digitally printed halftone pattern within a single sheet, mimicking the intricate patterns on cephalopod skin.
Building on these shape-morphing capabilities, the researchers demonstrated that the smart skin can combine multiple functions simultaneously. By co-designing the halftone patterns, they encoded the Mona Lisa image directly into flat films that later emerged as 3D shapes. As the flat sheets curved into dome-like structures, the hidden image information gradually became visible, showcasing how changes in shape and appearance can be programmed together.
Sun highlights the significance of this work, stating, 'Similar to how cephalopods coordinate body shape and skin patterning, the synthetic smart skin can simultaneously control its appearance and deformation within a single, soft material.' This research builds upon previous efforts to 4D-print smart hydrogels, focusing on the co-design of mechanical properties and programmable 2D-to-3D shape morphing. The current study introduces a halftone-encoded 4D printing method to co-design multiple functions within a single smart hydrogel film.
Looking ahead, the team aims to develop a general and scalable platform for precise digital encoding of multiple functions into a single adaptive smart material system. Sun emphasizes the broad implications of this interdisciplinary research, which includes stimulus-responsive systems, biomimetic engineering, advanced encryption technologies, and biomedical devices.
The co-authors affiliated with Penn State include doctoral candidates Haotian Li and Juchen Zhang, and lecturer Tengxiao Liu. Additionally, Professor H. Jerry Qi from the Georgia Institute of Technology collaborated on this groundbreaking work.
