Researchers develop automatic drawing machine to produce paper metamaterials

Researchers have developed an automatic drawing machine that uses pens and pencils to draw metamaterials on paper. They demonstrated the new approach by using it to make three metamaterials that can be used to manipulate the microwave region of the electromagnetic spectrum.

Metamaterials are artificially created composite materials that derive their properties from patterned microstructures rather than the chemical composition of the materials themselves. The precise shape, geometry, size, orientation and placement of the structures can be used to manipulate electromagnetic waves in ways not possible with conventional materials.

“Metamaterials, especially those used as absorbers, usually need to be thin, light, wide and strong, but it is not easy to create thin and light devices using traditional substrates,” said research team leader Junming Zhao of Nanjing University in China. “Using paper as a substrate can help meet these requirements while also amenable to metasurfaces that conform to the surface or that are mechanically reconfigurable.”

In the diary Optical Materials Express, the researchers describe their new technique, which uses a conductive ink pen to draw wires and mechanical pencils to draw resistors and resistive films. They incorporated this process into a computer-controlled drafting machine to make it more automatic and accurate.

“Although paper-based metamaterials have previously been made using inkjet printing technology, our painting technique is cheaper, simpler and more flexible,” Zhao said. “Our method could be useful for creating reconfigurable antennas and metal elements, as well as metamaterial devices that absorb incident electromagnetic energy from cell phones or other sources.”

Automated drawing

The new drawing machine uses pens with ink containing conductive material or normal mechanical pencils with different graphite contents. It has three stepper motors, two of which control the movement of the pen or pencil in the horizontal plane and the other one raises or lowers the writing instrument in the vertical plane. The parameters of the drawing machine, such as the speed of movement, are controlled by a computer.

“Some of the paper we tried was not very compatible with the pencils or conductive ink pens, resulting in poor conductivity of the drawn patterns,” Zhao said. “After some testing, we found that the best performance came from using 0.22mm paper, which is readily available and very compatible with pencils and conductive ink.”

The researchers used the conductive ink pen to draw patterns on paper and found that the patterns had a good conductivity of 3×10⁶ Siemens per meter. They also tested pencils with different amounts of graphite, draw times, and draw pressures to characterize how these factors affect electrical resistance. This allowed them to calculate the conditions needed to draw patterns of specific resistance.

Creating paper metamaterials

Using their new painting technique, the researchers designed and fabricated three different paper metamaterials: a polarization converter, an absorber, and a conformal encoding metasurface. They showed that the polarization converter can rotate the linear polarization by 90° with conversion efficiency above 90% from 3.1 to 6.6 GHz. The absorber they made had a mass of only 58.3 grams and achieved 90% absorption between 2.1 GHz to 10.5 GHz.

The researchers also created a conformal encoding metasurface that can be used to reduce the radar cross-section used to cloak the radar signal in military aircraft and ships. This metasurface had two structural units with a 180° reflection phase difference relative to each other, which allowed them to act as “0” and “1” elements for 1-bit encoding. When bent around a curved surface, this metasurface achieved a 10 dB reduction in the radar cross section in the frequency band from 8.94 to 11.59 GHz.

“We hope that in the future we can use the painting technology to design and manufacture meta-devices that can be worn or applied to the skin to achieve electromagnetic shielding and other functions,” Zhao said. “We also plan to design mechanically reconfigurable metamaterials that take advantage of the fact that paper can bend and fold.”