Self-Propelled Nanorobots

University of Colorado, 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. The tiny synthetic nanorobots are incredibly effective at escaping cavities within mazelike environments.

The nanoswimmers came to the attention of the theoretical physics community about 20 years ago and people imagined a wealth of real-world applications. 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, 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 that allows the particle to take energy from the environment and convert it into directional motion, also known as self-propulsion.

In contrast, passive particles that move about randomly (a kind of motion known as Brownian motion) are known as Brownian particles. 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 Janus particles were 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 the particles are incredibly small — about 250 nanometers or just wider than a human hair (160 nanometers) but still much smaller than the head of a pin (1-2 millimeters) — 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 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.

This hurdle was overcome by using refractive index liquid in the porous medium — 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 team to track three-dimensional trajectories of the particles and create visual representations, without which it would not be possible to better understand the movement and behavior of either individuals or groups of nanoswimmers.

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