Wireless sensors are ubiquitous in real life and play a vital role in military, agricultural production, ecological monitoring and climate change, medical and industrial fields. For example, wireless sensors can be used to monitor crop irrigation, soil air, the environment and migration of livestock and poultry, large surface monitoring, and so on. At the same time, several sensors can be used to monitor rainfall, river water level and soil moisture, describe ecological diversity, and thus carry out animal habitat ecological monitoring.
Typically, this type of environmental monitoring requires the deployment of hundreds or even thousands of sensors and the formation of a network of interconnected wireless sensors through wireless communication. However, physically placing hundreds of sensors on large areas is currently a time-consuming and expensive task. How to disperse a large number of low-cost sensors over a wide geographical area to achieve wide-area sensing remains a huge challenge.
In nature, plant seeds are generally very light and tend to have very good aerodynamic structures. The seeds of dandelions are light and small, with a ring of white fuzz at the top, and when they mature, they are blown by the wind, and they fly like parachutes into the distance. Studies have shown that under the right environment, dandelion seeds can take root and grow hundreds of kilometers away without fuel or electricity. For a long time, people have tried to use the geometry of plant seeds to design new aircraft that are as powerful, effective and safe as dandelion seeds. Imagine if wireless sensors can be mounted on such aircraft, it can be like sowing seeds, throwing a large number of micro-flying equipment from drones or high places and widely dispersing, so as to achieve large-area wireless sensor network deployment, providing a better space range for future IoT technology.
Figure 1. Dandelions flutter in the wind.
Inspired by dandelions using wind to spread seeds, the team of Professors Vikram Iyer and Shyamnath Gollakota of the University of Washington in the United States has developed a battery-free miniature wireless sensor device that can be dispersed in the wind. Weighing 30 mg, the device is designed on a flexible substrate using programmable off-the-shelf components, providing scalability and flexibility for a variety of sensing and computing applications. At the same time, the system is powered by lightweight solar cells and an energy-harvesting circuit that is robust to low-light and variable light conditions and has a backscattered communication link that supports data transmission. To achieve the large-area dispersion and upright landing required for solar energy harvesting, the research team developed a dandelion-inspired thin-film porous structure with a terminal velocity of 0.87 ±0.02 meters per second and an aerodynamic stability of more than 95% with an upright landing probability. Outdoor environmental test results show that these devices can travel 50-100 meters in a breeze-to-stroke situation. Finally, the research team also showed how to adjust the porosity and diameter of the structure to achieve dispersion variation across devices.
The work demonstrates a fully functional, airborne wireless sensing system and evaluates it in outdoor environments and real-world sensing applications. The study was successfully published in Nature under the title "Wind dispersal of battery-free wireless devices".
To simulate the behavior of dandelion seeds, miniature sensor systems need to solve several key challenges.
First, a lightweight drag-increasing structure similar to dandelion seeds needs to be designed and manufactured so that wireless sensor devices can slowly fall to the ground and be blown around by the breeze. At the same time, to stay light, the research team used solar panels instead of bulky batteries to power the electronics, which created new challenges. Because solar energy collection requires the panels to face the sun, and increases the aerodynamic stability requirements of the resistance structure so that it can passively reorient and fall vertically.
Figure 2. Miniature wireless sensing systems use solar panels (black rectangular parts) to power electronic components.
To simulate the structure of dandelion seeds, the researchers performed a two-dimensional projection of dandelion seeds, and by laser means designed and prepared 75 thin film porous configurations, studying their terminal landing speed to optimize the structure of the flight equipment, so that they can always flip and fall in an upright direction like dandelion seeds.
Figure 3. A variety of thin film porous configurations that mimic the structure of dandelion seeds
It is worth noting that solar energy is an intermittent energy source. Without a battery, the system cannot store power, which means that the sensor stops working after the sun goes down. The key challenge is that when the sun rises the next morning, the system needs a little bit of energy to start. To solve this problem, the research team designed a miniature capacitor and a miniature circuit in the sensor's electronic components. Capacitors can store some energy at night, while miniature circuits can measure how much energy is stored, and once the sun rises and more energy comes in, beyond a certain set threshold, it will trigger the rest of the system. The circuit is robust to low-light and variable light conditions and has backscattered communication links that support data transmission.
Figure 4. Circuit and electronic component design for miniature wireless sensing systems
To measure how far these devices travel in the wind, the researchers threw them down from different heights. It was found that the miniature wireless sensor device was capable of traveling 50-100 meters in light and stroke conditions. And, using drones can release thousands of such devices at once, blowing them apart in the wind, and 95 percent able to keep them vertically falling, keeping the solar panels facing the sun.
In addition, due to the use of a battery-free system, nothing on this device will run out of power. In principle, as long as no physical failure occurs, the device can continue to operate.
However, in this way, electronic devices will be scattered in the ecosystem for a long time, which will have a certain impact on the ecological environment. Currently, researchers are investigating how to build these systems using renewable green materials to make them easily biodegradable.
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bibliography:
Iyer, V., Gaensbauer, H., Daniel, T.L. et al. Wind dispersal of battery-free wireless devices. Nature (2022). https://doi.org/10.1038/s41586-021-04363-9
Source: Frontiers of Polymer Science