The Internet of Things (IoT) represents a convergence of embedded controllers, smart sensors and wireless communication technology. While computers, sensors and RF circuits continue to shrink, antennas require a certain thickness in order for them to transmit and receive signals efficiently. Engineers at Drexel University's Wireless Systems Laboratory have developed antennas, transmission lines and RFID tags that are less than one micron thick—about one one-hundredth the thickness of a standard sheet of paper—that actually outperform their bulky copper counterparts. Even better: these antennas can be sprayed onto a surface, enabling low-cost antennas that are thin and flexible enough to be attached to virtually any object.

(Image courtesy of Drexel University.)

MXene

The promising two-dimensional material, a variation of titanium carbide known as MXene (pronounced "Maxine"), has a high conductivity—even better than carbon nanotubes or graphene—which enables it to carry signals with minimal attenuation. Here's a short video that shows the spray-on process:

Metallic antennas are commonly made of copper, silver or aluminum. To effectively propagate Wi-Fi or Bluetooth signals, metallic antennas must be at least five microns thick. For reasons still unknown to the researchers, carbon nanotubes and graphene appear to work with one-tenth of that thickness, but these materials have relatively low conductance. MXene, on the other hand, has a very high conductivity, which, coupled with the spray-on process, makes it a good choice for embedded antennas.

(Image courtesy of Drexel University.)

Performance

Earlier experiments demonstrated titanium carbide's suitability for electromagnetic interference shielding, where a thin coat of the substance performed as well as traditional metal shields. Drexel scientists extrapolated on that research to develop MXene antennas, waveguides and ultrathin passive RFID tags.

Engineers fabricated MXene dipole antennas, 62mm in length to match the 2.4GHz frequencies used by Bluetooth and Wi-Fi, with thicknesses ranging from 62nm (0.062 microns) to 1.4 microns. The dipoles were sprayed onto various substrates, including paper and polyethylene terephthalate (PET). Simulation and testing showed that MXene antennas that were just 1.4 microns thick performed 50 times better than graphene antennas that were five times bulkier—and the MXene antennas performed 300 times better than those made with silver ink.

One might expect similar results when using MXene as a waveguide transmission line, and sure enough, the material proved to be a nearly perfect substance for the job. MXene waveguides exhibited a paltry 0.1dB of attenuation—better than graphene at 7 microns and silver at 100 microns. Engineers tested the MXene transmission lines in both the bent and unbent conditions, and observed little difference between the two. They did note, however, that repeated flexing increased the antennas’ resistance. More work is needed to improve the material’s durability.

Finally, one micron-thick MXene antennas, which were affixed to passive RFID chips and tested at various frequencies and distances, were found to be readable at ranges of six to eight meters (20-26 feet)—pretty typical for passive RFID tags with omnidirectional antennas.

Who Needs Batteries?

The small size and flexibility offered by spray-on antennas allow them to be placed virtually anywhere, and the fact that they perform better than antennas made with thicker materials means that they can transmit information more efficiently, which lowers the power requirements of the devices. When the researchers figure out how to improve MXene's durability, we'll see more products that employ energy harvesting, rather than batteries, as their power source.

If you could make wireless sensors that were small, cheap and low-power, where would you put them?

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