After coating the object with M-spray, the researchers magnetized it with single or multiple magnetization directions, which could control how the object moved by a magnetic field. Then they applied heat to the object until the coating was solidified.
In this way (single or multiple magnetization directions), when driven by a magnetic field, the objects can be transformed into millirobots with different locomotion modes, such as crawling, flipping, walking, and rolling, on various surfaces from glass, skin, and wood to sand. The team demonstrated this feature by converting cotton thread (1D), origami (2D flat plane), polydimethylsiloxane (PDMS) film (2D curved/soft surface), and plastic pipe (3D round object) into soft reptile robot, multi-foot robot, walking robot and rolling robot respectively.
On-demand reprogramming to change locomotion mode
What makes this approach special is that the team can reprogram the millirobot’s locomotion mode on demand.
Mr. Yang Xiong, the co-first author of this paper, explained that conventionally, the robot’s initial structure is usually fixed once it is constructed, hence constraining its versatility in motion. However, by wetting the solidified M-spray coating fully to make it adhesive like glue and then by applying a strong magnetic field, the distribution and alignment direction of the magnetic particles (easy magnetization axis) of the M-spray coating can be changed.
Navigating ability and disintegrable property
This reprogrammable actuation feature is also helpful for navigation toward targets. To explore the potential in biomedical applications, the team carried out experiments with a catheter, which is widely used for inserting into the body to treat disease or perform surgical procedures. They demonstrated that the M-spray coated catheter could perform sharp or smooth turns. And the impact of blood/liquid flow on the motion ability and stability of the M-spray coated catheter was limited.
By reprograming the M-spray coating of different sections of a cotton thread based on the delivery task and environment, they further showed that it could achieve a fast-steering and smoothly pass through an irregular, narrow structure. Dr. Shen pointed out that from the view of clinical application, this can prevent the unexpected plunging in the throat wall during insertion. “Task-based reprogramming offers promising potential for catheter manipulation in the complex esophagus, vessel, and urethra where navigation is always required,” he said.
Another important feature of this technology is that the M-spray coating can be disintegrated into powders on-demand with the manipulation of 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, stressing the side effect of the disintegration of M-spray is negligible.
Successful drug delivery in rabbit stomach
To further verify the feasibility and effectiveness of the M-spray enabled millirobot for drug delivery, the team conducted in vivo test with rabbits and capsules coated with M-spray. During the delivery process, the rabbits were anesthetized, and the position of the capsule in the stomach was tracked by radiology 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 enables the drug to be released in a targeted location rather than scattering in the organ,” Dr. Shen added.
Though the M-spray coating will start to disintegrate in about eight minutes under a strongly acidic environment (pH level 1), the team showed that an additional PVA layer on the surface of the M-spray coating could prolong it to about 15 minutes. And if replacing the iron particles with nickel particles, the coating could keep stable in a strongly acidic environment even after 30 minutes.
“Our experiment results indicated that different millirobots could be constructed with the M-spray adapting to various environments, surface conditions, and obstacles. 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.
