Scientists have developed a new technique for fabricating metamaterials from sheets of paper, using a computer to guide the movement of conductive ink pens and mechanical pencils.
Demonstrating the fabrication of three types of metamaterials — including one that could potentially be used to hide planes and ships from radar signals — the researchers reckon that their process could make the production of flexible metamaterials cheaper and easier.
Metamaterials are artificial structures usually composed of large numbers of tiny resonators mounted on a substrate, whose electromagnetic response relies on the resonators’ parameters and arrangement rather than the properties of the constituent materials. Such exotic materials have enabled devices not realizable with materials found in nature, such as invisibility cloaks and flat “metalenses.” The possibility of fabricating them from substrates of paper rather than other dielectrics promises to make such devices smaller, lighter, and foldable — as well as cheaper and more environmentally friendly, thanks to the absence of chemical etching.
In the new research, Junming Zhao, Tian Jiang and colleagues at Nanjing University have devised a new automated scheme for making such paper-based metamaterials that replaces well-established but pricey ink-jet printing with a computer-controlled drawing machine. The machine uses three stepping motors to move a pen across a sheet of paper, as well as in and out of contact with it, such that the pen — containing conductive ink — traces out a grid of conductors on the paper’s surface.
After spending an hour or two in an electric oven for the ink to dry, the paper then undergoes a second round of drawing. In round two, the pen is replaced with a mechanical pencil containing a narrow, retractable length of lead, made from a specific mixture of graphite and clay, to add resistors to the metamaterial grid.
Drawing resistors in this way is tricky, Zhao and colleagues point out, since obtaining a given resistivity requires control of many factors, including the graphite-clay ratio, the paper’s texture, the speed and tightness of the drawing path and the resistors’ dimensions. Another key variable is the pressure applied to the pencil, which the researchers optimized by measuring the resistance of squares drawn at different pressures and then plotting the resistance-pressure relationship.
The researchers chose commercially available paper with a thickness of 0.22 mm and a relative permittivity of 2.3. They say that metamaterials based on this paper substrate can be made more durable by sealing them inside plastic films — specifically, films made from 0.065-mm-thick polyethylene terephthalate.
The team used its drawing machine to fabricate three types of paper-based metamaterials. One was a polarization converter, which the researchers found was able to rotate the plane of linearly polarized microwaves by 90° — yielding 90 percent conversion efficiency over a 3.5-GHz frequency range. The second metamaterial instead acted as a broadband absorber, weighing in at just 58.3 g but absorbing 90 percent of the radiation it was exposed to between the frequencies of 2.1 GHz and 10.5 GHz.
Last but not least, the group demonstrated what is known as a conformal coding metasurface, which could be applied to an object to reduce the power of reflected radar waves — thereby making the object harder to detect. The surface contains two layers, one of which is drawn by the ink pen and comprises large numbers of C-shaped split resonant rings with two distinct orientations, so as to encode 0s and 1s in the phase of the reflected waves. The other layer is drawn by the pencil to create resistive loops that absorb incoming energy.
With the metasurface bent around a metallic cylinder, it not only absorbed some of the energy from incoming waves but also reflected those waves across a much wider range of solid angles than an uncovered cylinder would. The coding, meanwhile, allowed for a continual variation in the surface’s reflection properties — the random sequence of 0s and 1s used in the experiment yielding diffuse reflection. The combined effect of these mechanisms was to reduce the reflected power by a factor of ten compared with that from a bare cylinder, for incident radiation between about 9 and 12 GHz.
The researchers found that the experimental results for all three types of metamaterials agreed well with simulation data. This consistency, they argue, “demonstrates the feasibility and reliability of the drawing technique.” The performance of the conformal surface also shows the promise of paper as a flexible substrate, they said.
Zhao said that while paper is lightweight and flexible, it is also fragile and less durable than other types of dielectrics. But he said that if these limitations can be overcome, a number of applications beckon, such as reconfigurable antennas and metalenses. He also envisages devices that absorb radiation from mobile phones and other appliances to reduce the potentially harmful effect on users. Such protective devices, he suggests, could be attached to a person’s skin via small amounts of adhesive sprayed onto the paper.