The
reptile millirobot changes from caterpillar motion to concertina motion after
reprogramming.
Courtesy: City University of Hong Kong.
An easy
way to make 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). Driven by the magnetic field, the coated objects can crawl, walk,
or roll on different surfaces. As 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 findings have been published in the scientific journal Science
Robotics, titled “An agglutinate magnetic spray transforms inanimate objects
into millirobots for biomedical applications.”
Transforming
objects into millirobots with a “magnetic coat”
Scientists
have been developing millirobots or insect-scale robots that can adapt to
different environments for exploration and biomedical applications.
Dr. Shen’s research team came up with a simple approach to construct millirobots by coating objects with a composited glue-like magnetic spray, called M-spray. “Our idea is that by putting on this ‘magnetic coat’, we can turn any objects into a robot and control their locomotion. The M-spray we developed can stick on the targeted object and ‘activate’ the object when driven by a magnetic field,” explained Dr. Shen.
Composed
of polyvinyl alcohol (PVA), gluten, and iron particles, M-spray can adhere to
the rough and smooth surfaces of one (1D), two (2D) or three-dimensional (3D)
objects instantly, stably, and firmly. The film it formed on the surface is
just about 0.1 to 0.25mm thick, which is thin enough to preserve the original
size, form, and structure of the objects.
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 on the object until the coating was
solidified.
In this
way, 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, 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.
The
reptile millirobot changes from caterpillar motion to concertina motion after
reprogramming.
Courtesy: City University of Hong Kong.
Their
experiments showed that the same millirobot could switch between different
locomotion modes, such as from a faster 3D caterpillar movement in a spacious
environment to a slower 2D concertina movement for passing through a narrow
gap.
Navigating
ability and disintegrable property
This
reprogrammable actuation feature is also helpful for navigation towards
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 on 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 capsule 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
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 environment, 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.