Squid Skin Inspires New Smart Material

Researchers from Nanjing University, China, and the Nanyang Technological University, Singapore, have developed a new soft material that can block heat, light and microwaves. Inspired by the natural properties of squid skin, the material can be switched between transparent and opaque to block a wide range of electromagnetic wavelengths from passing through it. It was presented in a research paper published in ACS Nano.  

Electromagnetic spectrum modulation

Data encryption and decryption, energy conservation, thermal management and tactical camouflage applications have all necessitated the development of new materials that can block visible light, as well as other wavelengths of electromagnetic (EM) radiation. 

This has trickled down to the commercial level, with the development of smart frosted glass and privacy films that can be quickly toggled between opaque and transparent. These work by applying an electric field to the glass or film, which causes liquid crystals present in the material to either align or scatter, determining whether visible light will pass through the glass or be blocked. 

But for industrial applications, these materials still have significant challenges to overcome. Crucially, these materials are good at blocking very narrow bands of EM radiation but do not perform so well when placed in complicated multiband EM environments. With the growing prevalence of 5G and other wireless technologies, more adaptive materials that can act across wider or multiple EM wavelength ranges are an important area of focus for researchers. 

In search of an alternative, some researchers are turning to nature for inspiration. 

Squid, octopus and other cephalopods have a unique and specialized skin layer that is not found in other animals. This layer contains two types of cells – iridocytes and chromatophores – that control the light-scattering properties and pigmentation of the animal’s skin, respectively. By using muscles to control the surface structure of this skin layer, whether it is wrinkled or flat, cephalopods can dynamically and reversibly change their coloring in response to environmental threats. 

In a similar vein, researchers are now investigating the use of artificial materials that can transition between reflecting and transmitting visible light and infrared wavelengths by shifting their surface structures from wrinkled to cracked. 

Squid skin inspires new switchable material

Artificial materials that can block visible light based on surface nano- and microstructures have been developed before. But these materials were also only effective against materials with short wavelengths; the wavelength of microwaves is around five orders of magnitude greater than visible light, and is significantly larger than the surface structures produced by these materials.

In the new paper, researchers present a novel adaptive, multispectral and mechano-optical system that uses bilayer acrylic dielectric elastomer (ADE) and silver nanowire (AgNW) films. The use of electrically conductive materials, such as these silver nanowires, appears to block microwaves. By adding a conductive network of silver nanowires to the tunable elastomer that is capable of altering its surface structure, the new material can quickly transition between shielding and transmitting EM radiation across the visible-to-microwave ranges.   

“Inspired by the variable appearance of cephalopods skins, we have designed a class of adaptive multispectral mechano-optical systems with both on-demand tunability and wide spectral range from the visible to the microwave, which operate by mechanically reconfiguring the surface morphology of bilayer ADE/AgNW film,” the researchers write in their new paper. 

To make the material, the researchers created a two-layer film by spraying a thin coating of silver nanowires onto a stretched elastomer. Stretching and contracting the material produced cracks and bumpy wrinkles in the metal surface. 

The researchers found that when the material was contracted to a -30% strain, it blocked light, trapped infrared heat and shielded up to 99.9% of microwave radiation directed at its surface. Conversely, the more the material was stretched, the more visibly transparent it became while also allowing more heat and microwaves to penetrate through it. 

Applications for tunable radiation-blocking films

The researchers conducted several proof-of-concept experiments to demonstrate the potential applications of their material. 

In one experiment, where they wrapped a volunteer’s forearm in stretched and unstretched material, they found that it can help to trap body heat in cold environments and disguise warm regions from the view of IR heat-seeking cameras. In another, they found that the material was able to selectively transmit or block wireless electrocardiography (ECG) signals being sent from a commercial Bluetooth heart monitor. 

“Given these excellent and versatile properties, we demonstrated multiple potential applications of these films for smart windows, smart EMI shields, personal thermal management, dynamic infrared stealth, and human motion detection,” the researchers write. “This work demonstrates a promising approach for designing adaptive multispectral mechano-optical systems and provides great opportunity for many emerging fields, including intelligent multispectral stealth technology, energy-conversion buildings and individual protection and healthcare.”

Reference: Liang L, Yu R, Ong SJH et al. An adaptive multispectral mechano-optical system for multipurpose applications. ACS Nano. 2023. doi: 10.1021/acsnano.3c01836

This article is a rework of a press release issued by the American Chemical Society. Material has been edited for length and content.