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The reptilian millirobot changes from caterpillar motion to accordion motion after reprogramming. Credit: City University of Hong Kong
An easy way to create millirobots by coating objects with a glue-like magnetic spray was developed in a joint research led by a scientist from City University of Hong Kong (CityU). Pushed by the magnetic field, coated objects can crawl, walk or roll on different surfaces. Because the magnetic coating is biocompatible and can be disintegrated into powders when needed, this technology demonstrates the potential for biomedical applications, including catheter navigation and drug delivery.
The research team is led by Dr. Shen Yajing, Associate Professor of the Department of Biomedical Engineering (BME) at CityU in collaboration with the Shenzhen Institutes of Advanced Technology (SIAT), Chinese Academy of Sciences (CAS). The research results were published in the scientific journal Science Robotics, entitled “An agglutinated magnetic spray transforms inanimate objects into millirobots for biomedical applications”.
An easy way to create millirobots by coating objects with a magnetic spray was developed in a joint research led by a CityU scientist. Since the magnetic coating is biocompatible and can be disintegrated into powder when needed, this technology demonstrates the potential for biomedical applications. Credit: City University of Hong Kong
Transforming objects into millirobots with a “magnetic coating”
Scientists have developed millirobots or insect-scale robots that can adapt to different environments for exploration and biomedical applications.
Dr. Shen’s research team devised a simple approach to building millirobots by coating objects with a glue-like composite magnetic spray called M-spray. “Our idea is that by wearing this ‘magnetic coating’, we can turn any object into a robot and control its locomotion. The M-spray we developed can attach to the targeted object and “activate” the object when guided by a magnetic field, “explained Dr. Shen.
Dr. Shen (center) and the CityU team. Credit: City University of Hong Kong
Composed of polyvinyl alcohol (PVA), gluten and iron particles, M-spray can adhere to the rough and smooth surfaces of an object (1D), two-dimensional (2D) or three-dimensional (3D) instantly, stably and firmly. The film that has formed on the surface is about 0.1-0.25mm thick, thin enough to preserve the original size, shape and structure of the objects.
After coating the object with M-spray, the researchers magnetized it with single or multiple directions of magnetization, which could control how the object moved from a magnetic field. Then they applied heat to the object until the coating solidified.
In this way, when guided by a magnetic field, objects can be transformed into millirobots with different modes of locomotion, such as crawling, flipping, walking and rolling, on various surfaces from glass, leather, wood to sand. The team demonstrated this by converting cotton thread (1D), origami (2D flat plane), polydimethylsiloxane (PDMS) film (2D curved / soft surface) and plastic tube (3D round object) into soft reptile robot, multi robot – walking robot foot and rotating robot respectively.
Reprogramming on request to change the mode of locomotion
What makes this approach special is that the team can reprogram the locomotion mode of the millirobot on demand.
Mr. Yang Xiong, the co-first author of this article, explained that conventionally, the initial structure of the robot is usually fixed once built, thereby limiting its versatility in motion. However, by fully wetting the solidified M-spray coating to make it sticky like glue and then applying a strong magnetic field, it is possible to change the direction of distribution and alignment of the magnetic particles (axis of easy magnetization) of the M-spray coating.
The reptilian millirobot changes from caterpillar motion to accordion motion after reprogramming. Credit: City University of Hong Kong
Their experiments showed that the same millirobot can switch between a different mode of locomotion – for example, from a faster 3D caterpillar movement in a spacious environment to a slower 2D accordion movement to pass through a narrow space.
Navigation skills and disintegrating properties
This reprogrammable activation function is also useful for navigating to targets. To explore the potential in biomedical applications, the team conducted experiments with a catheter, which is widely used for insertion into the body to treat disease or perform surgical procedures. They showed that the M-spray coated catheter can perform tight or smooth bends. And the impact of blood / liquid flow on the range of motion and stability of the M-spray coated catheter was limited.
By reprogramming the M-spray coating of different sections of a cotton thread according to the delivery activity and the environment, they further demonstrated that it is possible to achieve quick steering and smoothly pass through a narrow and uneven structure. Dr. Shen pointed out that from a clinical application standpoint, this can prevent unexpected immersion into the throat wall during insertion. “Activity-based reprogramming offers promising potential for catheter manipulation in the esophagus, vessel and urethra complex where navigation is always required,” he said.
Another important feature of this technology is that the M-spray coating can be disintegrated into powder on demand by manipulating a magnetic field. “All the raw materials of M-spray, namely PVA, gluten and iron particles, are biocompatible. The disintegrated coating could be absorbed or excreted by the human body, ”said Dr. Shen, noting that the side effect of M-spray disintegration is negligible.
Successful drug delivery in the rabbit’s stomach
To further test the feasibility and effectiveness of the M-spray-enabled millirobot for drug delivery, the team conducted in vivo tests with rabbits and M-spray-coated capsules. During the delivery process, the rabbits were anesthetized and the location of the capsule in the stomach was monitored by radiological imaging. When the capsule reached the targeted region, the researchers disintegrated the coating by applying an oscillating magnetic field. “The controllable disintegration property of M-spray allows the drug to be released in a targeted location rather than dispersing in the organ,” added Dr. Shen.
Although the M-spray coating will begin to disintegrate in about eight minutes in a highly acidic environment (pH level 1), the team showed that an additional layer of PVA on the surface of the M-spray coating could prolong it to about 15 minutes. And if you replace the iron particles with nickel particles, the coating could remain stable in a highly acidic environment even after 30 minutes.
“The results of our experiment indicated that several millirobots could be built with the M-spray adapting to various environments, surface conditions and obstacles. We hope that this construction strategy will contribute to the development and application of millirobots in various fields, such as active transport, mobile sensors and devices, especially for operations in confined spaces, ”said Dr Shen.
Reference: 18 November 2020, Robotic science.
DOI: 10.1126 / scirobotics.abc8191
Dr. Shen of CityU and Dr. Wu Xinyu of SIAT of CAS are the corresponding authors of the article. The first authors are Mr. Yang Xiong, a doctoral student of BME at CityU and Dr. Shang Wanfeng of SIAT CAS. Other co-authors are Dr. Lu Haojian, Dr. Liu Yanting, Mr. Yang Liu and Miss Tan Rong, recent graduates and PhD students of Dr. Shen’s team.
The research was supported by the National Science Foundation of China and the Hong Kong Research Grants Council.
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