Apr 12, 2022Researchers at Northwestern University are undertaking the next phase of a three-year project to create a tiny device that flies, collects sensor readings in ambient air and transmits the results via Near Field Communication (NFC). With specialized wings, this device is designed to be dropped from an aircraft and take flight.
The group published their first peer-reviewed paper on this topic in the journal Nature in September 2021, with more work underway that will be described in a subsequent paper this summer. The entire sensor device, with wings, NFC software-on-a-chip (SoC) and sensor technology, measures a few millimeters in size, with the option to scale down to about the size of a grain of sand using other wireless technologies. When the tag is released into air currents, its 3D wings allow it to drift with those currents in a combination of rotational and gliding motions.
The NFC Micro-flier
The team's goal is to design a battery-free piece of technology that could be dispersed around an environment to capture ambient sensor information, such as pollutants and weather patterns, as it glides down to Earth. The NFC technology could be leveraged to capture the accumulated sensor data after the tag lands on the ground. One unique element of this device is its use of a suite of sensor technologies designed for monitoring physiological parameters of the human body, says John Rogers, a Northwestern University bioelectronics professor and the white paper's co-author.
The university's researchers have been building and testing such biosensors for the past decade or more, including small body-conforming devices or implants to address patient needs, or to conduct basic biology research on humans or animals. "We have a range of NFC devices," Rogers says, which can forward sensor data via a 13.56 MHz transmission compliant with ISO 14443. The tag not only responds to transmissions from a reader, such as the kind built into most smartphones, but could also transmit its own unique ID number, along with the sensor data that has been collected.
When it comes to tracking body conditions, the tag and sensor can be directly implanted into a person or be attached to skin. "You don't have to worry about dispersal in these cases—you just place them where you need them," Rogers states. "Our goal with this most recent work was to ask the question, 'Would there be ways to leverage that foundation of technology and body-integrated devices for monitoring health status, but instead for monitoring environmental processes?'" That was the starting point for the micro-flier development, he recalls.
John Rogers
According to Rogers, tags used for environmental monitoring would need to be distributed at scale. "So that was the question," he says, "that we were seeking to answer: 'What would be the mechanisms for dispersal?'" That led to research into the aerodynamics of passive flight. The researchers looked to nature for help. They modeled their research, in part, on the wings of maple seeds that fall from trees and let the wind carry them far from the branches. Beyond simply applying one wing, they used three, in a helicopter configuration with three dimensional shapes to guide passive flight.
At the center of the device is an integrated circuit. As the micro-flier falls through the air, its wings interact with the currents to create a slow, stable rotational motion. The weight of the electronics is distributed low in the center of the micro-flier to prevent it from losing control and tumbling in a chaotic pattern. There is no battery, but the chip's built-in power source can harvest ambient energy from sunlight for storing the collected sensor data.
The photodiodes generate a photocurrent with a magnitude that correlates with the exposure intensity. This current charges the device in such a way that the accumulated charge can define voltage on a supercapacitor on the chip that indicates exposure dose and, thus, information about the size and concentration of particulates in the air. When the tag is interrogated, its response yields a collection of accumulated voltages, each corresponding to doses at different wavelengths, Rogers says. "So indirectly," he states, "it's telling you about particulate density and size in the environment from that spectral information."
Once the sensor hits the ground, it waits for the data transfer. One way to collect data, Rogers says, would be with a low-flying drone equipped with an NFC reader. As tags fall, the drone could sweep over an area to read tag data, which could then be uploaded to a server. "Conceptually," he explains, "you're collapsing that 3D volume of data," which is captured as the tag descends through the atmosphere, "to a 2D surface that you can then scan."
To date, the NFC micro-flier devices have been tested in a laboratory environment, in a wind tunnel rather than in the field. To test how well the device could capture particulate data, the team used a dust-generation chamber with incense sticks, smoke candles and corn starch. A series of four fans, placed on the bottom of the chamber, produced the airflows through which the micro-fliers could glide.
The next stage of development could involve outdoor testing of the devices, as well as further wing engineering. "We are working on different types of wing structures," Rogers says, "inspired by different types of seeds." There is more than one way to take flight in the natural world, he notes. For instance, while early research focused on helicopter-type motion during freefall, "You can imagine dandelion-type, parachute-type structures and other kinds of gliders," which are other potential methods to bring the sensors to the ground in a dispersed manner.
NFC is not the only way to capture data from the device. The team has also explored color-changing chemistries that could be visible to a camera mounted on a drone. Yellow, for example, could indicate that a contaminant was detected in the upper atmosphere. "Some of our work is focused on developing those colorimetric approaches as well," Rogers says. "We have a number of approaches that we're pursuing, each one useful for different applications, with additional possibilities for combined use." He adds, "I think, ultimately, digital wireless is the way to go," via NFC technology, due to the specific data that can be collected, including the sensor's unique ID and the exact sensor results transmitted.
The technology could come in a biodegradable version, something that builds off the work the team has done in temporary implants based on bioresorbable electronics. "We have a whole set of materials that can support sophisticated forms of electronics," Rogers states, "but have, as a unique defining characteristic, the ability to dissolve in water." In this effort, the researchers have worked with a commercial silicon foundry to build biodegradable silicon integrated circuits, which in principle could support NFC functionality.
In the long term, Rogers speculates that the devices could monitor a wide variety of conditions, including weather patterns, chemical spills, and the propagation of pathogens or infectious diseases. In the meantime, the team is continuing to study air flow and the patterns of movement air may take as tags pass through the upper atmosphere, where sensing takes place. Going forward, the researchers hope to connect with technology companies to accomplish the next step of developments.
Exhibitors at RFID Journal LIVE! 2022 will offer solutions for monitoring pollution and weather, as well as NFC tags. To learn more, visit the event's website.
