Invisibility Cloak Thanks to Liquid Metal Ink?

Special Ink Developed for Camouflage Technologies and Wearable Electronics

Printable Metamaterial: A liquid metal-based ink could help develop flexible invisibility cloaks in the future. It can be printed onto various materials, remains soft, and can be stretched to 120 times its original dimensions—while still maintaining its electrical conductivity and stability. Surfaces printed with this ink can redirect electromagnetic waves. This could, for example, make robots invisible in radar images or allow for the design of wearable electronics.

Illustration of how to use invisibility technology. © KAIST (AI-generated)

When Harry Potter puts on his invisibility cloak, he becomes invisible to other people. For decades, scientists have been trying to develop so-called metamaterials with similar properties: Through specific surface structures, they are intended to redirect electromagnetic waves, especially visible light, in such a way that they make objects invisible. However, existing versions can only conceal very small, stationary objects and are also limited to a small portion of the visible light spectrum. Others can only fool thermal imaging cameras or block magnetic fields.

Printable Liquid Metal Ink

A team led by Jeongsu Pyeon from the Korea Institute of Science and Technology in Daejeon has developed a technology that could enable new types of stealth devices—and simultaneously open up numerous applications for wearable electronics. "We have developed a liquid metal-based ink suitable for creating metamaterial absorbers, structures that can effectively absorb or deflect electromagnetic waves," the research team reports.

The researchers used gallistan, an alloy of gallium, indium, and tin that is liquid at room temperature, as the base material for their ink. Because this alloy has a very high surface tension and its viscosity (flow properties) is difficult to control, its use in coatings has been limited until now. Pyeon and his colleagues coated the gallistan particles with the binder polyvinylpyrrolidone (PVP), thereby reducing the surface tension. They also added a synthetic substance called laponite, which made the ink less viscous.

The study was selected as the cover story for the October 2025 issue of the journal "Small". © KAIST

In this way, they obtained an ink that adheres reliably to various materials and can be easily applied using conventional printing methods or even a brush. It dries evenly without forming stains or cracks and is ready for use without any further processing. Electrodes printed with this ink could also be stretched 120 times their original length without losing their conductivity. Moreover, they lasted for almost a year.

Invisibility cloak against radar waves

To demonstrate the potential applications, the researchers used their newly developed ink to create a metamaterial absorber that interacts with electromagnetic waves. They printed tiny, net-like structures with their special ink and measured the extent to which these structures absorbed different wavelengths.

The result: "In its unstretched state, our metamaterial absorber absorbed electromagnetic waves with a frequency of 5.68 gigahertz," the team reports. This corresponds to the wavelength of radar waves. For a narrowly defined frequency range, a corresponding metamaterial could therefore make objects disappear from radar.

Liquid metal composite particle (LMCP) ink. a) Comparison of the characteristics of printing inks based on liquid metal (LM), including pure LM inks,^{[5, 9, 10]} conventional liquid metal particle (LMP) inks,^{[14-16, 18, 19]} conventional LMP slurry inks,^[21-23] liquid metal-embedded elastomer (LMEE) inks,^[25-28] and the current LMCP ink. The viscosities (µ) at a shear rate of approximately 1 s⁻¹ are compared for each ink. Post-processing involves sintering, heat-assisted solvent drying, and elastomer curing. The detailed fabrication process of the LMCP ink is illustrated in Figure S1 (Supporting Information). b) Demonstration of the versatility of LMCP ink with various conventional printing techniques, including inkjet printing, direct writing, brushing, doctor blade coating, and roll-to-roll printing (from left to right). Souce:출처: Versatile Liquid Metal Composite Inks for Printable, Durable, and Ultra-Stretchable Electronics / First published: 11 June 2025 / nano-micro small)

Further experiments showed that the covered area can be adjusted by stretching the material: the more it was stretched, the lower the resonant frequency became. Depending on the stretch and the printed pattern, different frequency ranges of electromagnetic radiation could be covered. However, the metamaterial absorber was ineffective against visible light. To become invisible to the human eye, the material would have to interact at frequencies approximately 100,000 times higher.

Wearable Electronics

But even apart from its invisibility properties, the novel ink offers a wide range of applications. "Its adaptability makes the ink a promising material for diverse applications in stretchable and flexible electronics, including wearable devices, soft robotics, energy conversion and storage systems, and nanogenerators," the researchers write. (Small, 2025, doi: 10.1002/smll.202501829)

Source: Korea Advanced Institute of Science and Technology (KAIST)