A winged microchip about the size of a grain of sand has become the smallest-ever man-made flying machine.
Scientists say the ‘microfliers’ could monitor air pollution, airborne disease and environmental contamination.
The tiny device does not have a motor or engine. Instead, it catches flight on the wind – just like a maple tree’s propeller seed – and spins like a helicopter through the air toward the ground.
By studying maple trees and other types of wind-dispersed seeds, engineers optimised the microflier’s aerodynamics to ensure that when dropped from high up, it falls at a slow velocity in a controlled manner.
Engineers explained in the journal Nature how that stabilises its flight, ensuring dispersal over a broad area and increasing the amount of time it interacts with the air, making it ideal for monitoring air pollution and airborne disease.
They said the microfliers also can be packed with ultra-miniaturised technology, including sensors, power sources, antennas for wireless communication and embedded memory to store data.
Bioelectronics pioneer Professor John Rogers, who led the development, said: ‘Our goal was to add winged flight to small-scale electronic systems, with the idea that these capabilities would allow us to distribute highly functional, miniaturised electronic devices to sense the environment for contamination monitoring, population surveillance or disease tracking.
‘We were able to do that using ideas inspired by the biological world.
‘Over the course of billions of years, nature has designed seeds with very sophisticated aerodynamics.
‘We borrowed those design concepts, adapted them and applied them to electronic circuit platforms.
‘Evolution was likely the driving force for the sophisticated aerodynamic properties exhibited by many classes of seeds.
‘These biological structures are designed to fall slowly and in a controlled manner, so they can interact with wind patterns for the longest-possible period of time.
‘This feature maximises lateral distribution via purely passive, airborne mechanisms.’
To design the microfliers, Prof Rogers and his colleagues at Northwestern University in the United States, studied the aerodynamics of seeds of several plants, drawing its most direct inspiration from the tristellateia, a flowering vine with star-shaped seeds.
Tristellateia seeds have bladed wings that catch the wind to fall with a slow, rotating spin.
The team designed and built many different types of microfliers, including one with three wings, optimised to similar shapes and angles as the wings on a tristellateia seed.
To pinpoint the most ideal structure, Professor Yonggang Huang, who led the study’s theoretical work, conducted full-scale computational modelling of how the air flows around the device to mimic the tristellateia seed’s slow, controlled rotation.
Based on this, Prof Rogers’ group then built and tested structures in the lab, using advanced methods for imaging and quantifying patterns of flow.
The resulting structures can be formed across a wide variety of sizes and shapes, some with properties that Prof Rogers says can give nature “a run for its money.”
Prof Rogers: ‘We think that we beat nature. At least in the narrow sense that we have been able to build structures that fall with more stable trajectories and at slower terminal velocities than equivalent seeds that you would see from plants or trees.
‘We also were able to build these helicopter flying structures at sizes much smaller than those found in nature.
‘That’s important because device miniaturisation represents the dominating development trajectory in the electronics industry, where sensors, radios, batteries and other components can be constructed in ever smaller dimensions.’
To manufacture the devices, Prof Rogers’ team drew inspiration from a child’s pop-up book.
His team first fabricated precursors to flying structures in flat, planar geometries. Then, they bonded the precursors onto a slightly stretched rubber substrate.
When the stretched substrate is relaxed, a controlled buckling process occurs that causes the wings to ‘pop up’ into precisely defined three-dimensional forms.
Prof Rogers said: ‘This strategy of building 3D structures from 2D precursors is powerful because all existing semiconductor devices are built in planar layouts.
‘We can thus exploit the most advanced materials and manufacturing methods used by the consumer electronics industry to make completely standard, flat, chip-like designs.
‘Then, we just transform them into 3D flying shapes by principles that are similar to those of a pop-up book.’
He explained that the microfliers comprise two parts: millimetre-sized electronic functional components and their wings. As the microflier falls through the air, its wings interact with the air to create a slow, stable rotational motion.
The weight of the electronics is distributed low in the centre of the microflier to prevent it from losing control and chaotically tumbling to the ground.
Prof Rogers’ team included sensors, a power source that can harvest ambient energy, memory storage and an antenna that can wirelessly transfer data to a smartphone, tablet or computer.
The team outfitted one device with all of the elements to detect particulates in the air. In another example, they incorporated acidity sensors that could be used to monitor water quality and photodetectors to measure sun exposure at different wavelengths.
Prof Rogers says that large numbers of devices could be dropped from an aircraft or building and broadly dispersed to monitor the air after a chemical spill or to track levels of air pollution at various altitudes.
He said: ‘Most monitoring technologies involve bulk instrumentation designed to collect data locally at a small number of locations across a spatial area of interest.
‘We envision a large multiplicity of miniaturised sensors that can be distributed at a high spatial density over large areas, to form a wireless network.’
Prof Rogers even has a plan to deal with any electronic litter. His team already develops transient electronics that can harmlessly dissolve in water after they are no longer needed – as shown in recent work on bioresorbable pacemakers.
Now his team is using the same materials and techniques to build microfliers that naturally degrade and disappear in ground water over time.
Prof Rogers added: ‘We fabricate such physically transient electronics systems using degradable polymers, compostable conductors and dissolvable integrated circuit chips that naturally vanish into environmentally benign end products when exposed to water.
‘We recognise that recovery of large collections of microfliers might be difficult. To address this concern, these environmentally resorbable versions dissolve naturally and harmlessly.’
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