A new type of ink based on liquid metal could help develop flexible invisibility cloaks in the future. It can be printed on various materials, remains soft and can be stretched to 120 times its original dimensions – while still maintaining its electrical conductivity and stability. Appropriately printed surfaces can redirect electromagnetic waves. In this way, for example, robots could be made invisible on radar images. The liquid metal ink could also be used for portable electronics.
Metamaterials can redirect light and other types of electromagnetic waves so that objects become invisible or can no longer be detected using infrared cameras and radar. However, an “invisibility cloak” like Harry Potter is still a long way off. Previous variants can only hide very small, stationary objects and are usually limited to a small range of the electromagnetic spectrum – for example only certain wavelengths of visible light or certain radar frequencies.
Liquid metal printable ink
A team led by Jeongsu Pyeon from the Korea Institute of Science and Technology in Daejeon has now developed a technology that could enable new types of invisibility cloaks – and at the same time open up numerous applications for wearable electronics. “We have developed an ink that is based on liquid metal and is suitable for producing metamaterial absorbers, i.e. structures that can effectively absorb or redirect electromagnetic waves,” reports the research team.
The researchers used Galinstan, an alloy made up of the metals gallium, indium and tin, which is liquid at room temperature, as the base substance for their ink. Since this alloy has a very high surface tension and its viscosity, i.e. its flow properties, is difficult to control, its use for coatings has so far been limited. Pyeon and his colleagues now coated the Gallistan particles with the binding agent polyvinylpyrrolidone (PVP) and in this way reduced the surface tension. They also added a synthetic substance called laponite, which made the ink thinner.
This gave them an ink that adheres reliably to various materials and can be easily applied using conventional printing methods or with a brush. It dries evenly without forming stains or cracks and can be used without any further processing steps. Electrodes printed with this ink could also be stretched up to 120 times their length without losing their conductivity. They also lasted almost a year.
Invisibility cloak against radar waves
To demonstrate the possible applications, the researchers used their newly developed ink to produce a metamaterial absorber that interacts with electromagnetic waves. To do this, they printed tiny net-like structures with their special ink and measured the extent to which they absorbed different wavelengths. The result: “In the unstretched state, our metamaterial absorber absorbed electromagnetic waves with a frequency of 5.68 gigahertz,” reports the team. This corresponds to the length of radar waves. For a narrow frequency range, a corresponding metamaterial could make objects disappear from radar.
Further tests showed that the covered area could be adjusted by stretching the material: the more it was stretched, the lower the resonance frequency became. Depending on the stretch and the printed pattern, different frequency ranges of electromagnetic radiation could be covered. However, the metamaterial absorber was unable to do anything against visible light. To become invisible to our eyes, the material would have to interact with frequencies about 100,000 times higher.
But even independent of invisibility cloaks, the new ink offers a wide range of possible applications. “Its adaptability makes the ink a promising material for a variety of applications in stretchable and flexible electronics, including wearable devices, soft robotics, energy conversion and storage systems, and nanogenerators,” the researchers write.
Source: Jeongsu Pyeon (Korea Advanced Institute of Science and Technology) et al., Small, 2025, doi: 10.1002/smll.202501829