Cephalopods like squids, octopuses, and cuttlefish can change colors quickly because of tiny organs in their skin called chromatophores.

The choromatophores contain sacs of pigment. When light strikes the pigment granules, the chromatophores absorb most of the wavelengths and reflect back only a thin band of color.

By using muscles to expand and contracting the chromatophores, squid change the way light bounces off their bodies, leading to a variety of presented colors.

Inspired by the chromatophore, Rutgers engineers set out to create an artificial one.

The researchers developed a 3D-printable hydrogel, or smart gel, that changes shape once the material senses light. The light-responsive achievement enables new features for camouflaging and displays, say the hydrogel's creators.

The team's findings appeared in a recent study in the journal ACS Applied Materials & Interfaces .

The artificial chromatophore unit consists of three components: a photoactive hydrogel "muscle," a stretchy hydrogel, and a rigid frame to maintain the structure.

Rutgers, The printed light-responsive artificial chromatophore's three components: light-responsive muscle, stretchable sac, and rigid frame. The sac expands upon exposure to light.
The printed light-responsive artificial chromatophore's three components: light-responsive muscle, stretchable sac, and rigid frame. The sac expands upon exposure to light. (Reprinted with permission from ACS Appl. Mater. Interfaces, Jan 3. Copyright 2021 American Chemical Society.)

To make a light-responsive, squid-like material, the engineers also had to embed nanomaterials into the photoactive hydrogel. The tiny components convert light to heat.

Upon light irradiation, the nanoparticles generate heat to de-swell the hydrogel matrix, which in turn stretches the middle hydrogel sac containing the desired pigment.

According to the study, the color tone of the light-responsive artificial chromatophore, or LAC, shifted from black to white within two minutes of light projection from a digital projector.

The stretchy hydrogel carries the desired color to express upon light exposure.

Rutgers, Active volume change of the photoactive hydrogel in response to light irradiation. The photoactive hydrogel consists of a temperature-responsive hydrogel and polydopamine nanoparticles (PDA-NPs) as a photothermal agent.
Active volume change of the photoactive hydrogel in response to light irradiation. The photoactive hydrogel consists of a temperature-responsive hydrogel and polydopamine nanoparticles (PDA-NPs) as a photothermal agent. (Reprinted with permission from ACS Appl. Mater. Interfaces, Jan 3. Copyright 2021 American Chemical Society.)

"In our case, we just used the native white color of the hydrogel," lead researcher Howon Lee told Tech Briefs.

Howon Lee is senior author of the research  and an assistant professor in the Department of Mechanical and Aerospace Engineering in the School of Engineering at Rutgers University–New Brunswick.

Next steps for Lee's team include improving the technology’s sensitivity, response time, scalability, and durability.

In a short Q&A with Tech Briefs below, Lee brings us into specific applications for a light-responsive, squid-like material.

Tech Briefs: Where will this kind of light-sensing nanomaterial be most valuable?

Prof. Howon Lee: Current camouflage strategy relies on predetermined color patterns, which lacks the ability to adapt to a possibly changing environment. Some reported works utilized separate sensors to detect the surroundings, which may require auxiliary devices and power consumption. Our work mimics exactly the way cephalapods create color expression, namely using artificial muscle and chromatophores with soft materials to sense and change color simultaneously.

This [nanomaterial] may lead to new military camouflage surface, new types of flexible/soft display devices, or perhaps new types of windows or glass that may vary their transparency/color depending on the need.

Tech Briefs: How sensitive is this technology to light? In potential applications, where do you envision the source of this kind of light and how will it be delivered?

Prof. Howon Lee: What we see — color, brightness — is always light, so the source is everywhere. Currently [the technology] requires quite intense light, but we hope that we will be able to improve the sensitivity by scaling down the size of the artificial chromatophores.

Tech Briefs: How finely can the material change its shape based on its exposure to light? Does the material go from one shape to another, or are there a range of shapes that are possible, depending on the light?

Prof. Howon Lee: It can change gradually from one shape to another — for example, from fully stretched to fully contracted, just like our muscle.

Tech Briefs: What are you working on now?

Prof. Howon Lee: We are working on various responsive and active materials as well as new 3D-printing technologies. We are living in a dynamic and three-dimensional world, so we believe that 3D printing with active materials will create fascinating engineering applications.

What do you think? How do you envision this material being used? Share your comments and questions below.