Flying Microchip Takes Inspiration From Wind-dispersed Seeds
The researchers at Northwestern University say they have created the smallest man-made flying structures.
Science has long looked to nature for inspiration. Now, researchers at Northwestern University are following suit, modeling a new breed of flying microchip after maple tree seeds in free fall.
The flying microchip next to an ant for scale. Image used courtesy of Northwestern University
After analyzing the aerodynamics of a variety of wind-dispersed seeds, the team found that the tristellateia plant's star-shaped seeds have bladed wings that allow it to slowly rotate and spin when it catches the wind. This fall pattern sparked the team's ideas about "microflier" chips that can, perhaps, be used for monitoring airborne contaminants, surveilling populations, and tracking diseases.
How were the researchers able to create the so-called "smallest man-made flying structures?"
A Flying Chip Modeled After a Wind-borne Seed
Before landing on the tristellateia seed's helicopter-like structure, lead researcher John A. Rogers and his team prototyped several different types of microfliers. The team used computational fluid dynamics (CFD) and analytical approaches to identify how air would flow around the microchip to replicate the slow and controlled velocity of a tristellateia seed.
Close up of the flying microchip. Image used courtesy of Nature
Aboard these devices, which are roughly the size of a grain of salt, the researchers have been able to integrate miniaturized technology including sensors, power sources, antennas for NFC communication, and embedded memory.
In a Vice interview, Rogers explains, "The aerodynamics start to break down as you decrease sizes below about a millimeter. Below that size scale, everything looks and falls like a sphere. There are really no flow-driven rotational motions."
He adds, "All of the advantages of the helicopter design begin to disappear below a certain length scale, so we pushed it all the way, as far as you can go or as physics would allow, and that size is, in fact, much smaller than you would see with seeds.”
Electronics Onboard Pop-up Wings
In their paper published in Nature, the researchers explained that the hard part was the aerodynamics, and the easy part was the electronics. The flying microchip has two main parts: the millimeter-sized electronic components and the blade-like wings.
The microflier's wings are designed to mimic the helicopter-like blades of a tristellateia seed. Image used courtesy of Northwestern University
For their fliers, the researchers developed semiconductor devices based on silicon nanomembranes with a thickness of 200 nm as the active material. The devices they created on the flier include n-channel MOSFETs ( L/W of 20/80μm, SiO2 gate dielectrics, and metal electrodes) along with diodes formed from materials similar to those of the MOSFETs. Beyond this, they integrated external components such as NFC coils, MCUs, and a variety of sensors including pH sensors and photodetectors.
The electronic microfliers are in the millimeter range for size—roughly the size of a grain of sand. Image used courtesy of Northwestern University
For the mechanical housing, the team created flat, planar precursors bonded to a stretched rubber substrate. The substrate buckles when it relaxes, causing the wings on the microflier to "pop up" like a children's book into 3D forms that look similar to maple seed blades.
The team placed the electronics low in the middle of the microflier to evenly distribute weight and prevent the structure from losing control while in flight. So far, Rogers and his team have demonstrated how the device's antenna can wirelessly transfer sensor data to a mobile phone, tablet, or computer, powered by a device that harvests ambient energy.
Biodegradable Sensors for Environmental Monitoring
In practice, these devices are designed to gather environmental data, including air pollution and the spread of disease. The researchers envision large numbers of the flying microchips dropped from a plane or building, dispersing broadly and floating slowly and gently to the ground, collecting data all the while.
In a lab setting, the Northwestern team demonstrated how the microflier can detect particulates in the air, monitor water quality with pH sensors, and measure sun exposure at various wavelengths via photodetectors.
Bioresorbable electronics. Image used courtesy of Northwestern University
In order to make the flying microchips sustainable and safe for the environment, the researchers also propose that the fliers be made from biodegradable, bioresorbable electronics. These materials dissolve completely in water, leaving behind no unwanted chemicals. Once these fliers reach the ground, they will eventually be washed away by the next rain.