With the continuous shrinkage of sensors and robots, the power source has become an important issue. Despite using methods such as magnetism and electric fields as power sources, they are also prone to limiting their autonomy. This time, a renowned American university has developed a super-small battery for cell-sized autonomous robots. The applications of these robots include drug delivery inside the body, detecting natural gas pipeline leaks, and more.
Traditional battery materials mostly rely on wet chemistry preparation, which cannot be significantly miniaturized. Batteries made of lithium and ceramic materials are also limited by volume and efficiency, restricting most microbatteries to millimeter-scale or sub-square millimeter size, unable to directly power picoliter (pL, one trillionth of a liter) or cell-sized robots.
Researchers from the Massachusetts Institute of Technology (MIT) in the United States have used photolithography techniques to create a high-energy-density microbattery that can be placed on cell-sized autonomous robots. This ensures these robots have enough power to operate autonomously in limited environments such as pipelines, underground, inside the body, and on ships. The research paper was published in mid-August in the “Science Robotics” journal.
In order to create more autonomous micro-robots, the researchers opted for zinc-air batteries. This type of battery boasts high energy and energy density, providing a longer lifespan compared to many other types of batteries.
In a clean room, using standard microfabrication technology on a 2-inch (diameter 5.08 cm) silicon wafer, the scientists manufactured about 10,000 (100×100 array) identical picoliter-scale “zinc-air batteries” in one go.
The top layer of these batteries consists of a zinc (Zn) anode (thickness 1.5μm) and a platinum (Pt) cathode embedded on an SU-8 (a corrosion-resistant polymer) insulating layer. The dimensions are only 100μm in length and width, with a thickness of only 0.002mm (equivalent to the thickness of a human hair), printed on a silicon wafer substrate.
When these electrodes interact with oxygen molecules in the air, zinc oxidizes, releasing electrons flowing towards the platinum electrode, generating electricity. During the reaction process, a layer of zinc phosphate precipitate forms on the zinc anode surface (which does not affect its operation), while the platinum cathode catalyzes water into hydroxide ions and oxygen.
The researchers submerged the 100μm-sized zinc-air battery in a neutral phosphate-buffered saline solution with a concentration of 0.15M to observe how much current, voltage, and discharge duration it could generate. The results showed that the average voltage of the battery ranged from 0.3 to 0.5V, and with an increase in discharge rate from 0.1 mA/cm to 0.4 mA/cm, the average output power increased by 1 to 3nW.
Moreover, the discharge duration of the battery remained between 2,000 and 3,000 seconds. However, when the discharge rate increased to 0.4 mA/cm, the discharge duration shortened to 1,000 to 2,000 seconds.
Furthermore, the researchers also created 50μm, 20μm, and even smaller zinc-air batteries to observe whether these smaller batteries could maintain their performance and be used in even smaller robots. The results showed that the smaller batteries could still maintain a certain level of performance.
After confirming the battery’s efficiency, the researchers mounted them on micro-robots to observe their overall performance. The results demonstrated that the picoliter-scale zinc-air battery provided stable power for the micro-robots’ circuits, sensors, clock circuits (allowing the robots to track time), and other actuators, giving them good autonomy.
The researchers mentioned that this picoliter-scale zinc-air battery, compared to other reported microbatteries, boasts a higher energy density. However, there is still significant room for improvement in the power density of the battery. Future developments will focus on enhancing the battery to provide more functions for cell-sized robots, expanding their applications to various fields.
Professor Michael Strano from the Department of Chemical Engineering at MIT expressed to the university newsroom, “We believe that this result is very beneficial for robotic technology. We have begun synthesizing some parts and devices on the battery, which will establish the functionality of the robots.”
Strano added, “Manipulating micro-robots manually can acquire the necessary energy externally. However, if you want a small robot to enter spaces beyond human control, it needs a certain level of autonomy, and thus, the battery becomes essential. We are currently manufacturing the basic components of the robots so that they can work at the cellular level.”
This research was funded by the U.S. Army Research Office, the U.S. Department of Energy, the National Science Foundation, and MathWorks Engineering Scholarship.