By combining magnetic materials with magnetic fields, robotics researchers continue to develop machines that can be remotely manipulated in all kinds of useful ways, such as somersault through the colon or crawl through blood vessels to deliver drugs. Scientists at the City University of Hong Kong have developed a new type of spray-on coating they say can give regular objects these kinds of capabilities, with particular potential in biomedical applications.

The research focuses on expanding the use of insect-scale robots measured in mere millimeters, the tiny size of which lends itself to applications in the human body. Back in 2018, we looked at an interesting example of these types of “millirobot,” where scientists embedded magnetic microparticles into a rubbery silicon robot body, which could then be made to walk, crawl, jump and roll via application of an external magnetic field.

Rather than build a magnetic millirobot from the ground up, the authors of the new study set out to develop a tool that could be used to construct magnetic millirobots from regular objects. This tool comes in the form of a magnetic coating called M-spray, which is made of polyvinyl alcohol, gluten and iron particles, and can adhere to smooth and textured surfaces of all kinds of materials.

“Our idea is that by putting on this ‘magnetic coat’, we can turn any objects into a robot and control their locomotion,” says Dr Shen Yajing, who led the research team. “The M-spray we developed can stick on the targeted object and ‘activate’ the object when driven by a magnetic field.”

Dr Shen Yajing (middle) and fellow researchers behind the M-spray, Tan Rong (left) and Yang Xiong (right)

City University of Hong Kong

The film formed by the M-spray is less than a quarter of a millimeter thick, which the team says is key to maintaining the form and size of the original object. The team demonstrated the approach using cotton threads, thin films and plastic pipes as their starting objects, which became soft tiny robots capable of walking, crawl and rolling with the help of a magnetic field. But interestingly, the mode of locomotion isn’t set in stone once the coating is applied.

The locomotion mode can actually be reprogrammed on demand by wetting the solidified coating, turning it into a glue-like substance. Then, by applying a strong magnetic field, the magnetic particles within the coating can be redistributed and realigned, changing the way the robot reacts to the magnetic field.

The team demonstrated this by having the same millirobot change from a caterpillar-like movement to a slower concertina-like movement, as a way of squeezing through a narrow gap. In another experiment, the researchers coated a catheter in the M-spray and reprogrammed its locomotion mode on the fly to have it perform both smooth and sharp turns, which could help avoid injury when these medical devices are inserted into the human body.

In vivo experiments followed, involving capsules coated with M-spray and anesthetized rabbits. The rabbits were administered the capsules and the team tracked them as they moved through the stomach with radiology imaging, before dissolving the coating when the capsules reached a targeted location. This is made possible by the makeup of the M-spray that enables it to disintegrate into powder under a magnetic field or acidic environments.

“All the raw materials of M-spray, namely PVA, gluten and iron particles, are biocompatible,” says Shen. “The disintegrated coating could be absorbed or excreted by the human body.”

Beyond biomedical applications, the team sees a range of potential uses for the technology.

“We hope this construction strategy can contribute to the development and application of millirobots in different fields, such as active transportation, moveable sensor and devices, particularly for the tasks in limited space,” said Dr Shen.

The research was published in the journal Science Robotics, while you can see a demonstration of the robots in the video below.

Novel magnetic spray transforms objects into millirobots for biomedical applications

Source: City University of Hong Kong

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