Currently, RFID tags are available in a number of configurations, including battery-assisted and ‘passive’ varieties. Both types of tags contain a small antenna which communicates with a remote reader by backscattering the RF signal, sending it a simple code or set of data that is stored in the tag’s small integrated chip. Battery-assisted tags include a small battery that powers this chip. Passive RFID tags are designed to harvest energy from the reader itself, which naturally emits just enough radio waves within FCC limits to power the tag’s memory chip and receive a reflected signal.
MIT previously designed an RFID tag-antenna that changes the way it transmits radio waves in response to moisture content in the soil, and an antenna to sense signs of anaemia in blood flowing across an RFID tag.
But, according to the team, there are drawbacks to such antenna-centric designs. “With antenna-based sensors, there’s more chance you’ll get false positives or negatives,” explains Sai Nithin Reddy Kantareddy of MIT.
Instead of manipulating a tag’s antenna, MIT have now tailored the memory chip. They used off-the-shelf integrated chips that are designed to switch between an RF energy-based mode, and a local energy-assisted mode; and worked each chip into an RFID tag with a standard radio-frequency antenna.
The researchers built a simple circuit around the memory chip, enabling the chip to switch to a local energy-assisted mode only when it senses a certain stimuli. When in this assisted mode, the chip emits a new protocol code, distinct from the normal code it transmits when in a passive mode. A reader can then interpret this new code as a signal that a stimuli of interest has been detected.
Kantareddy claims this is more reliable than antenna-based designs because it essentially separates a tag’s sensing and communication capabilities.
In a demonstration, the team set up commercially available glucose-sensing electrodes, filled with the electrolyte glucose oxidase. When the electrolyte interacts with glucose, the electrode produces an electric charge, acting as a local energy source, or battery, the team explains.
The researchers attached these electrodes to an RFID tag’s memory chip and circuit. When they added glucose to each electrode, the resulting charge caused the chip to switch from its passive RF power mode, to the local charge-assisted power mode. The more glucose they added, the longer the chip stayed in this secondary power mode.
“Since you’re getting energy from RF and your electrodes, this increases your communication range,” Kantareddy says. “With this design, your reader can be 10 metres away, rather than 1 or 2. This can decrease the number and cost of readers that, say, a facility requires.”
The team now intends to develop an RFID carbon monoxide sensor by combining the design with different types of electrodes engineered to produce a charge in the presence of the gas.
