RFID tags are being used by retailers and manufacturers to track their products in shops and warehouses. These tags are also used to keep tabs on products throughout a supply chain. RFID tags are used to trace everything from casino chips and cattle to amusement park visitors and marathon runners. Often, these tags come in the form of paper-based labels outfitted with a simple antenna and memory chip. When slapped on a milk carton or jacket collar, RFID tags act as smart signatures, transmitting information to a radio-frequency reader about the identity, state, or location of a given product.
A group of engineers at the Auto-ID Lab at MIT are now flipping the technology toward a new function: sensing. They have developed a new ultra-high-frequency, or UHF RFID tag-sensor configuration that senses spikes in glucose and wirelessly transmits this information to a reader. In the future, the team plans to tailor the tag to sense chemicals and gases in the environment, such as carbon monoxide.
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.
Recently, researchers have been experimenting with ways to turn passive RFID tags into sensors that can operate over long stretches of time without the need for batteries or replacements. These efforts have typically focused on manipulating a tag’s antenna, engineering it in such a way that its electrical properties change in response to certain stimuli in the environment. As a result, an antenna should reflect radio waves back to a reader at a characteristically different frequency or signal-strength, indicating that a certain stimuli has been detected.
For instance, this group previously designed an RFID tag-antenna that changes the way it transmits radio waves in response to moisture content in the soil. The team also fabricated an antenna to sense signs of anemia in blood flowing across an RFID tag.
But there are drawbacks to such antenna-centric designs, the main one being “multipath interference,” a confounding effect in which radio waves, even from a single source such as an RFID reader or antenna, can reflect off multiple surfaces.
To overcome this they took a new approach: Instead of manipulating a tag’s antenna, they tried tailoring its memory chip. They purchased off-the-shelf integrated chips that are designed to switch between two different power modes: an RF energy-based mode, similar to fully passive RFIDs; and a local energy-assisted mode, such as from an external battery or capacitor, similar to semipassive RFID tags.
The team worked each chip into an RFID tag with a standard radio-frequency antenna. In a key step, 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 (commercially called battery-assisted passive mode, or BAP), 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.
This chip-based design can create more reliable RFID sensor than antenna-based designs because it essentially separates a tag’s sensing and communication capabilities. In antenna-based sensors, both the chip that stores data and the antenna that transmits data are dependent on the radio waves reflected in the environment. With this new design, a chip does not have to depend on confounding radio waves in order to sense something.
As a demonstration, the researchers developed an RFID glucose sensor. They 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 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.
A reader, sensing this new power mode, can interpret this as a signal that glucose is present. The reader can potentially determine the amount of glucose by measuring the time during which the chip stays in the battery-assisted mode: The longer it remains in this mode, the more glucose there must be.
While the team’s sensor was able to detect glucose, its performance was below that of commercially available glucose sensors. The goal was not necessarily to develop an RFID glucose sensor, but to show that the group’s design could be manipulated to sense something more reliably than antenna-based sensors.
The design is also more efficient. A tag can run passively on RF energy reflected from a nearby reader until a stimuli of interest comes around. The stimulus itself produces a charge, which powers a tag’s chip to send an alarm code to the reader. The very act of sensing, therefore, produces additional power to power the integrated chip.
Going forward, scientists plans to develop an RFID carbon monoxide sensor by combining this design with different types of electrodes engineered to produce a charge in the presence of the gas.
