How Do Motorless Microfliers Stay Airborne?

TL;DR

Motorless microfliers stay airborne longer by spinning or exploiting airflow, slowing their descent instead of generating powered lift. Inspired by wind-dispersed seeds and manufactured with semiconductor processes, these three-dimensional devices can fall two to three times slower than flat versions, carry tiny sensing components, and dissolve in room-temperature water in about one hour when made from biodegradable polymers.

Transcript

Go ahead and open up your palm. Imagine there is an object resting on it that is only as thick as two or three strands of hair. It has no motor. It has no battery. Yet, it catches the wind and drifts down from the sky even more slowly than a snowflake. You might think it's a tiny drone, but it isn't. Inside, there is no motor or battery to ... Read More

Key Insights

• The microflier is a passive device that slows its descent rather than producing powered lift. Removing motors and batteries avoids the scaling problem that makes conventional flying machines increasingly difficult to miniaturize, while longer airborne time allows the wind to transport the object farther.
• Wind-dispersed seeds are biological models for passive flight. Dandelion seed hairs create a detached, donut-shaped vortex ring above the seed head, forming an aerodynamic cushion that slows descent with four times the efficiency of a parachute carrying the same weight.
• The 2021 microflier was inspired by the star-shaped seed Tristellateia. Researchers imitated its geometry so the artificial device could rotate naturally as it descended, demonstrating that careful observation of biological structures can provide solutions to engineering problems involving flight at extremely small scales.
• Semiconductor photolithography enables microscopic devices to be produced in large batches. A light-reactive liquid is spread across a spinning silicon wafer, dried, patterned through a stencil, exposed to light, and developed, leaving hundreds of identical shapes that would be impossible to cut and assemble individually by hand.
• Flat fabrication initially prevented the devices from rotating. Seed-inspired flight requires wings twisted at different angles so airflow acts differently on each wing, but conventional semiconductor processing builds layered patterns on a plane, producing structures with no effective twist for the air to catch.
• Mechanical buckling converts flat microstructures into three-dimensional fliers. Researchers attached patterned components to selected locations on a stretched rubber sheet, then released the tension so contraction folded designated sections upward automatically, removing the need to manipulate barely visible parts with tweezers.
• Three-dimensional geometry reduces the devices' descent speed by two to three times compared with the same components pressed flat. Some experimental structures descended more slowly than a snowflake, increasing the time available for wind transport without requiring a motor, battery, or propeller.
• Biodegradable construction addresses the problem of retrieving widely dispersed microscopic devices. A microflier made from biodegradable polymer disappears without a trace after about one hour in room-temperature water, replacing physical recovery with controlled dissolution after the device has served its purpose.

Questions & Answers

Q: How do motorless microfliers stay in the air?

Motorless microfliers do not climb or hover through powered lift. Instead, their three-dimensional wings interact with moving air and cause the body to spin while descending. This rotation slows the fall, sometimes below the descent rate of a snowflake. A slower fall keeps the device airborne longer and lets the wind carry it farther without a motor, battery, or propeller.

Q: Why are seeds useful models for microscopic flying devices?

Wind-dispersed seeds already solve the problem of traveling through air without engines or batteries. Dandelion seeds, for example, create a detached, donut-shaped vortex ring above their hairs. This air structure supports the seed and slows its descent with four times the efficiency of a parachute carrying the same weight. The artificial microflier similarly draws inspiration from the star-shaped Tristellateia seed.

Q: How are microscopic motorless fliers manufactured?

The devices are patterned with a semiconductor manufacturing process called photolithography. A light-reactive liquid is deposited on a smooth silicon wafer and spread into a very thin film by rapid spinning. After drying, a stencil defines the desired shape during light exposure. Chemical development leaves the patterned structure, and one wafer can produce hundreds of identical components simultaneously.

Q: Why did the first flat microfliers fail to spin?

Semiconductor fabrication builds patterns on a flat surface, so the first completed devices were essentially flat sheets. Natural seeds rotate because their wings are twisted at different angles, causing airflow to act differently across the structure. The flat artificial wings lacked this twist, leaving the air with no suitable geometry to catch. Consequently, the devices fell without showing the intended rotation.

Q: How were flat microstructures turned into three-dimensional fliers?

Researchers stretched a rubber sheet and attached flat patterned components at selected locations on its taut surface. When they released the sheet, it rapidly contracted toward its original length. That mechanical force made predetermined sections fold and rise automatically, much like a pop-up structure. This method assembled barely visible devices without tweezers and preserved the advantages of producing hundreds through a flat wafer process.

Q: How much does three-dimensional structure slow a microflier?

The three-dimensional structure makes the device descend two to three times more slowly than the same components in a flat configuration. Some structures created by the research team fell even more slowly than a snowflake. The improvement comes from geometry rather than powered propulsion, and the added airborne time increases the distance over which ambient wind can transport the device.

Q: What role did Bong Hoon Kim have in the microflier research?

Bong Hoon Kim appeared first among the four co-lead authors of the paper published in Nature volume 597 on September 23, 2021. His contribution involved working at the front of the experiments, including fabricating devices, dropping them, and measuring descent data. The project was an international collaboration led by John Rogers' Northwestern University team, with John Rogers serving as corresponding author.

Q: Why were biodegradable microfliers developed?

Microscopic fliers dispersed through the environment can be difficult or impossible to retrieve after use. The follow-up research addressed this limitation by replacing recovery with disappearance. A version made from biodegradable polymer dissolves without a trace in room-temperature water in about one hour. This design allows the tiny flying structure to vanish after deployment instead of remaining where it lands.

Summary & Key Takeaways

• The microflier replaces powered flight with controlled descent. Inspired by seeds that remain aloft without motors or batteries, its three-dimensional wings interact with air and make the body rotate. This rotation reduces its falling speed, allowing wind to carry it farther and giving embedded sensing components more time in the air.

• Semiconductor photolithography makes hundreds of identical microscopic components on one wafer, but the process initially produces flat devices that simply fall. Researchers solved this limitation by attaching patterned components to stretched rubber. When released, the rubber contracts and forces selected sections upward, automatically transforming each flat pattern into a functional three-dimensional structure.

• Bong Hoon Kim was listed first among four co-lead authors of the international research published on September 23, 2021, with John Rogers as corresponding author. After returning to Korea and joining DGIST in 2022, Kim continued the work, reducing the device to about 200 micrometers and developing biodegradable versions that dissolve in water.