First published September 24, 2021.
In September 2021, engineers at Northwestern University in Illinois unveiled what they described as the smallest flying structure ever made by humans: a winged microchip, nicknamed a "microflier", with the smallest versions hardly bigger than a grain of sand. It has no motor and no battery-driven rotor. Instead, it is shaped so that it spins slowly as it falls, the way a maple seed twirls to the ground. The idea is to scatter large numbers of these tiny devices from a height so they can carry sensors through the air and report on pollution, contamination or the spread of airborne disease.
What the microflier is
A microflier is a thin electronic circuit folded into a three-dimensional, wing-shaped body. The research was published in the journal Nature on 23 September 2021, in a paper titled "Three-dimensional electronic microfliers inspired by wind-dispersed seeds", and it made the cover of that issue. The project was led by John A. Rogers, a materials scientist at Northwestern who is known for soft, flexible and skin-mounted electronics.
The devices come in a range of sizes. The larger ones are a few millimetres across and can carry working electronics; the smallest are close to the size of a speck of dust. All of them share the same basic trick: their wings catch the air during descent and set the whole structure rotating, which slows its fall and keeps it stable.
How it flies: a lesson from seeds
Many plants spread their seeds by wind. Maple seeds, sometimes called helicopters or whirligigs, have a single wing that makes them spin. Dandelion seeds use a feathery parachute. Other species have several small wings arranged around a central core. All of these designs do the same job: they keep the seed in the air longer, so a breeze can carry it further from the parent plant.
The Northwestern team studied these natural shapes and then used computer modelling to test many wing geometries, looking for the slowest and steadiest descent. Their designs reportedly borrow most closely from a vine whose star-shaped seeds whirl as they fall. Because the shape is engineered rather than grown, the team could tune the angle and curvature of each wing.
The result surprised even its makers. Rogers told ScienceAlert: "We think that we beat nature." He meant it in a narrow sense: their structures fell more stably and more slowly than comparable seeds, and they could be made much smaller than any seed that disperses this way.
It is worth being precise about the word "flying". The microflier does not take off, hover or steer itself. It glides downward under control. Early versions of this story described it as a tiny helicopter with rotor blades, but there is no rotor: the spinning comes entirely from air flowing over the wings as the device drops.
How you build something this small
Making a three-dimensional object at this scale is hard, because most chip-making methods produce flat layers. Rogers' laboratory has spent years developing a technique often compared to a children's pop-up book. The circuit and wings are first made flat, using standard semiconductor processes, and placed on a stretched rubber base. When the tension is released, the flat pattern buckles upward and locks into a 3D shape.
This approach matters because it lets engineers use the same mass-production tools as the electronics industry. In principle, thousands of microfliers could be made at once on a single wafer, which is what you would need for the large-scale scattering the team imagines.
What it can carry and what it could do
Despite their size, the larger microfliers can hold sensors, a power source, an antenna for wireless communication and a small amount of memory to store data. One of the larger demonstration versions carried sensors that respond to ultraviolet light, and the team described the platform as adaptable to other kinds of environmental sensing.
The applications the team has suggested include:
- Air quality mapping: dropping a cloud of sensors to measure pollution at different heights as they fall.
- Contamination monitoring: checking an area after a chemical leak, or tracking clean-up after an oil spill.
- Disease tracking: sampling the air for signs of airborne pathogens.
- Population surveillance: a phrase Rogers used himself, generally understood to mean monitoring the conditions people are exposed to.
Commentators have also pointed out that any technology able to spread thousands of tiny sensors over a wide area could interest militaries and intelligence services. That concern is speculative; the published research focuses on environmental and health monitoring.
The litter problem, and devices that dissolve
The obvious drawback of scattering electronics across the countryside is that someone has to pick them up, and with devices the size of sand grains, that is close to impossible. The researchers addressed this directly. They have built versions from transient materials: degradable polymers, compostable conductors and silicon chips thin enough to dissolve in water. After a period outdoors, these microfliers are designed to break down into harmless products rather than leaving permanent waste.
Transient electronics are a long-running theme of Rogers' work, which also includes medical implants that dissolve in the body after they are no longer needed. Even so, it remains to be shown how quickly and completely such devices would break down in real soil, rivers and oceans, and regulators would want evidence before allowing large releases.
Where the research stands
The 2021 paper was a proof of concept: it showed that winged electronic structures can be designed, manufactured and made to fall slowly and stably while carrying useful hardware. It did not show a fleet of microfliers already monitoring the air over a city. Practical questions remain, including how far the devices travel in real winds, how their data is collected, how they are powered for long enough to be useful, and how their environmental footprint compares with conventional monitoring stations.
What makes the project interesting is the way it combines biology and engineering. Plants solved the problem of slow, wind-borne descent millions of years ago; this team copied the principle, improved on it in a narrow sense, and shrank it to a scale nature never reached. If the remaining practical problems can be solved, seed-like sensors could become a cheap way to see what is in the air we breathe.
Sources: ScienceAlert, 23 September 2021; John A. Rogers (Wikipedia); Kim et al., "Three-dimensional electronic microfliers inspired by wind-dispersed seeds", Nature 597 (2021); Northwestern University news release, September 2021.
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