Light-Responsive 3D Printed Smart Gels Replicate Octopus Camouflage

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Rutgers University engineers have created a 3D printed smart gel that changes shape when exposed to light, becoming an “artificial muscle.” The material may lead to some exciting research for various engineering applications, including new military camouflage, soft robotics, and flexible displays. Inspired by the highly sophisticated camouflage mechanisms found in cephalopods – like octopuses, cuttlefishes, and squids – that allow them to change skin color, the research team presented artificial color-changing cells called chromatophores that can alter their color pattern in response to light.

Evidence from a paper published in the journal ACS Applied Materials & Interfaces shows that a multi-material 3D printed light-responsive artificial chromatophore (LAC) can sense light and alter its color pattern at the individual unit level. The study co-authors intended to replicate the cephalopods’ incredible ability to use the thousands of color-changing chromatophore cells distributed on their soft skin to alter color and texture. With the chromatophores acting individually, cephalopods can create extremely complex skin color patterns, which they use for exquisite functions like camouflage and communication, and even alter their skin’s texture to match rocks or corals. This sophisticated biological evolution has given cephalopods the tools to survive millions of years of predation from eels, sharks, and countless fishes.

Octopus camouflages. Image courtesy of Special Area of Conservation Spain via Wikimedia Commons.

Although there are many camouflage mechanisms in nature, cephalopods are studied extensively because of their dramatic and elaborate camouflage mechanism. The chromatophores distributed throughout cephalopods’ skin play a critical role and have inspired scientists for its unparalleled potential in novel engineering systems with distributed intelligence. The innovative cephalopod-inspired LAC developed at Rutgers consists of three components: light-responsive muscle, stretchable sac, and rigid frame.

In the study, the researchers described the development of two 3D printable hydrogels, or smart gels, used to create the LAC. They developed a light-responsive muscle made from a 3D printable hydrogel that senses light and changes shape. To do this, the engineers incorporated a light-sensing nanomaterial in the hydrogel, turning it into an “artificial muscle” that contracts in response to light alterations.

They also developed a 3D printable, stretchable acrylic acid hydrogel material that can reveal colors when the light changes. When combined with the light-sensing smart gel, the 3D printed stretchy material changes color, resulting in the highly desired camouflage effect. In addition to creating the photoactive hydrogel as a light-responsive muscle and the acrylic acid hydrogel as a stretchable sac, the team also used the synthetic prepolymer cross-linker solution PEGDA 250 as a rigid frame material because of its relatively stable swelling behavior over the temperature change.

Cephalopod-inspired 3D printed chromatophore using smart gels. Image courtesy of Rutgers University School of Engineering via ACS Applied Materials & Interfaces.

To fabricate the LACs, the engineers employed a custom-built multi-material projection micro stereolithography 3D printing technique, a high-resolution additive manufacturing technology capable of fabricating complex centimeter-scale 3D architectures with extreme feature resolution approaching those of single mammalian cells. After printing the rigid frame using PEGDA 250, they projected a patterned UV light to photopolymerize the precursor solution. Next, they removed the residual material in the printing area with a vacuum pump, followed by an automatic sequence for cleaning with ethanol three times. Then, two separated light-responsive muscles were created, integrating the muscles with the rigid frame. Finally, they fabricated the stretchable sac, and repeated this process until printing all three materials to manufacture the LAC, reported the authors in the paper.

Schematic illustration of the multi-material projection micro stereolithography 3D printing process. Image courtesy of Rutgers University School of Engineering via ACS Applied Materials & Interfaces.

“Electronic displays are everywhere, and despite remarkable advances, such as becoming thinner, larger and brighter, they’re based on rigid materials, limiting the shapes they can take and how they interface with 3D surfaces,” said senior author Howon Lee, an Assistant Professor at Rutgers’ Department of Mechanical and Aerospace Engineering. “Our research supports a new engineering approach featuring camouflage that can be added to soft materials and create flexible, colorful displays.”

The printed light-responsive artificial chromatophore. Image courtesy of Rutgers University School of Engineering via ACS Applied Materials & Interfaces.

Along with co-authors Daehoon Han, Post-Doctoral Associate at the University of Minnesota’s Department of Mechanical Engineering; and Rutgers mechanical engineering doctoral students Yueping Wang and Chen Yang, Lee presented an artificial materials system that can turn light input into color patterns.

This National Science Foundation (NSF)-supported study successfully proved that the LAC color tone could shift from black to white within two minutes of light from a digital projector. Additionally, it revealed the creation of various binary color patterns from an array of three LACs, mimicking the unique ability of the cephalopods’ chromatophores to sense and act as an independent unit. In the future, by adding different dyes to the stretchy hydrogel, the researchers can change the current black-and-white binary color pattern to a more vibrant color expression. The next steps include improving the technology’s sensitivity, response time, scalability, packaging, and durability.

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