CU Boulder researchers have discovered that minuscule, self-propelled particles called
“nanoswimmers” can escape from mazes as much as 20 times faster than other, passive
particles, paving the way for their use in everything from industrial clean-ups to medication
delivery.
The findings, published in the Proceedings of the National Academy of Sciences, describe
how these tiny synthetic nanorobots are incredibly effective at escaping cavities within maze-like
environments.
These nanoswimmers could one day be used to remediate contaminated soil, improve water
filtration or even deliver drugs to targeted areas of the body, like within dense tissues.
These nanoswimmers came to the attention of the theoretical physics community about 20
years ago, and people imagined a wealth of real-world applications.
But unfortunately these tangible applications have not yet been realized, in part because
it’s been quite difficult to observe and model their movement in relevant environments—until
now.
These nanoswimmers, also called Janus particles (named after a Roman two-headed god), are
tiny spherical particles composed of polymer or silica, engineered with different chemical
properties on each side of the sphere.
One hemisphere promotes chemical reactions to occur, but not the other.
This creates a chemical field which allows the particle to take energy from the environment
and convert it into directional motion—also known as self-propulsion.
In contrast, passive particles which move about randomly (a kind of motion known as
Brownian motion) are known as Brownian particles.
They’re named after 19th century scientist Robert Brown, who studied such things as the
random motion of pollen grains suspended in water.
The researchers converted these passive Brownian particles into Janus particles (nanoswimmers)
for this research.
Then they made these self-propelled nanoswimmers try to move through a maze, made of a porous
medium, and compared how efficiently and effectively they found escape routes compared to the passive,
Brownian particles.
The results were shocking, even to the researchers.
The Janus particles were incredibly effective at escaping cavities within the maze—as
much as 20 times faster than the Brownian particles—because they moved strategically
along the cavity walls searching for holes, which allowed them to find the exits very
quickly.
Their self-propulsion also appeared to give them a boost of energy needed to pass through
the exit holes within the maze.
While these particles are incredibly small, around 250 nanometers—just wider than a
human hair, but still much, much smaller than the head of a pin —the work is scalable.
This means that these particles could navigate and permeate spaces as microscopic as human
tissue to carry cargo and deliver drugs, as well as through soil underground or beaches
of sand to remove unwanted pollutants.
The next step in this line of research is to understand how nanoswimmers behave in groups
within confined environments, or in combination with passive particles.
One of the main obstacles to reaching this goal is the difficulty involved in being able
to observe and understand the 3D movement of these tiny particles deep within a material
comprising complex interconnected spaces.
The researcher overcame this hurdle by using refractive index liquid in the porous medium,
which is liquid that affects how fast light travels through a material.
This made the maze essentially invisible, while allowing the observation of 3D particle
motion using a technique known as double-helix point spread function microscopy.
This enabled the researcher to track three-dimensional trajectories of the particles and create visual
representations, a major advancement from typical 2D modeling of nanoparticles.
Without this advancement, it would not be possible to better understand the movement
and behavior of either individuals or groups of nanoswimmers.