Sometimes, in the pursuit of efficiency, we can make big gains by taking cues from nature. Wing design is one place where nature excels, and we’ve been cribbing its notes for everything from wind turbines to boat propellers to control surfaces on planes. Now a team of scientists, led by John A. Rogers from Northwestern, have put biomimetic wings on a microchip, creating the smallest flying structure that humans have ever made.
“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, miniaturized electronic devices to sense the environment for contamination monitoring, population surveillance or disease tracking,” Rogers said. “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.”
During the design process for these “microfliers,” the researchers closely studied the design of different kinds of seeds. They watched maple helicopters’ flight pattern as they spin and flutter toward the ground, and compared it with the gently spinning descent of Tristellateia seeds. Like seeds, the researchers designed their devices around the payload, a tiny flake of ultra-miniaturized electronics. The result looks like nothing quite so much as extremely fancy biomimetic glitter:
Rogers and his team designed and built many different types of microfliers, at first relying on flat, planar geometries, like origami. Then, inspired by children’s pop-up storybooks, they tried bonding those stiff geometric structures onto elastic materials that were held under tension. By way of a “controlled buckling process,” they formed precise three-dimensional forms when the stretchy substrate relaxed.
In collaboration with mechanical engineering professor Yonggang Huang, also of Northwestern, the development team for these tiny fliers used statistics and computing power to conduct many different computational fluid dynamics simulations. “The computational modeling allows a rapid design optimization of the fly structures that yields the smallest terminal velocity,” said Huang. “This is impossible with trial-and-error experiments.”
They compared many microfliers in each generation of testing and then selected the designs that performed the best against specific objectives. This let the team iterate toward shapes that worked, without necessarily having to work out a specific mathematical description of each and every curve beforehand. The resulting structures embody a wide variety of shapes and sizes, but the team focused most closely on the graceful, three-winged Tristellateia seeds.
It’s tough to beat nature when it comes to efficiency, but Rogers thinks the team has done it, “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,” he said. “We also were able to build these helicopter flying structures at sizes much smaller than those found in nature.”
Beyond their wing designs, the group also demonstrated several different options for the electronics payload. One build designed for monitoring particulate in the air column included sensors, a power source capable of harvesting ambient energy, a bit of memory, and an antenna for transmitting whatever data it gathers. Another, meant for monitoring water quality, used a pH sensor and photodetectors that could measure sun exposure at different wavelengths.
Rogers envisions a use case for these microfliers that might involve dispersing them from planes or casting them out to sea in huge numbers. On the one hand, it feels a little bit tinfoil-hat to be talking about a swarm of intelligent, flying surveillance debris that phones home and then vanishes by dissolving into the water. On the other hand, though, it seems ironic to put a whole bunch of electronics litter into the air and water in order to test them for pollution. Rogers is on that problem too. His team recently made headlines after demonstrating temporary, bioresorbable pacemakers.
The lab already develops “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,” he explained. “We recognize that recovery of large collections of microfliers might be difficult. To address this concern, these environmentally resorbable versions dissolve naturally and harmlessly.”
The research is featured on the cover of the Sept. 23 issue of Nature. (Images and video: Northwestern University.)