Almost one hundred years ago, a passive listening device or bug was developed which required no local power source. This made the devices very hard to detect because it could be planted practically anywhere. The device consisted of a dipole antenna with a carbon microphone connected in series with the elements. An external RF transmitter operating at the corresponding frequency illuminated the dipole to make it resonate and sound waves reaching the microphone caused its impedance to vary and the antenna radiation characteristics to change. The effect provided a reflected RF signal amplitude modulated by the sound waves hitting the microphone diaphragm. Since there was no GHz technology available at that time, the small antennae needed to be tuned to the transmission frequency in order to maximize the effect.
This is no longer a problem today. At 2.4 GHz typical for Wi-Fi and Bluetooth, the wavelength λ = 12.5 cm. There is no need for electronic components such as coils and capacitors to make the antenna more compact. Researchers at the University of Washington have incorporated printed flexible antennae into simple mechanical mechanisms so that the movement of cogs or switching of contacts momentarily de-tunes the antenna and changes the RF backscatter pattern. The antenna is printed using copper-rich conductive filament material. Since 2.4 GHz radio signals are almost everywhere, it only needs a suitable, sensitive AM receiver tuned to this frequency to pick up the modulated backscatter signal and you can transfer information without a single electronic component, a simple input device like a push button can be implemented really easily.
It’s not strictly true that there are no electronics involved; an antenna is after all an electronic component. The hype surrounding the research project is quite sensational particularly as it is trumpeted as ‘3-D printed objects connect to Wi-Fi without electronics’. One thing you can be sure of is that these clever little mechanical trinkets are certainly not hooking up using Wi-Fi messaging protocols, a mechanical IP communication device truly would be miraculous. The system employs a specialized and sensitive AM receiver designed to demodulate amplitude changes in reflected signals, which certainly rules out ‘any Wi-Fi receiver’ as claimed in the video. The receiver could probably be implemented using a transceiver chip like the MAX2829 to supply a baseband output signal. It’s still early days and although the information transfer is slow and in one direction only, the technology is cheap and looks quite promising.
You can read more in this PDF file and also the University of Washington have provided more information and files describing the printed 3D models here.