The medical promise of nanorobots for diagnostics or targeted drug delivery is great but cannot be realized without development of an inexpensive propulsion system for these devices. New developments may now propel nanoswimmers closer to reality with the aid of bacterial flagella.

An international research team has demonstrated a method for plating silica onto flagella, the helix-shaped Trajectory of a templated helical silica nanoswimmer manually controlled to move in an approximate figure-eight pattern; scale bar is 5 μm. Credit: Jamel Ali/Drexel Universitytails found on many bacteria, to produce nanoscale swimming robots. These biotemplated nanoswimmers spin their flagella when exposed to rotating magnetic fields and can perform nearly as well as living bacteria.

Realization of nanoswimming depends on an understanding of the Reynolds number, the dimensionless quantities that relates fluid velocity, viscosity and the size of objects in the fluid. With a Reynolds number of one-millionth our own, bacteria must use nonreciprocal motion in the near absence of inertial forces. Using helical tails made of flagellin protein enables many species of bacteria to navigate these microscopic conditions with relative ease.

Instead of using relatively high-cost self-scrolling nanobelts or lasers, the researchers used a bottom-up approach, first culturing a strain of Salmonella typhimurium and removing the flagella. Alkaline solutions were then used to fix the flagella into their desired shape and pitch, at which point the proteins were plated with silica. Nickel deposited on the silica templates allows them to be controlled by magnetic fields.

When exposed to a magnetic field, the nanorobots kept up the pace with their bacterial counterparts and were projected to be able to cover 22 micrometers, more than four times their length, in a second. In addition to this, the team was able to steer the nanoswimmers into figure-eight paths.

In addition to the potential for nonconducting nanoscale helices in the area of targeted cancer therapeutics, the technology might in future be used to plate conductive materials to flagella and produce helical materials for electronics and photonics.

Researchers from Drexel University in Philadelphia, Penn., Southern University of Science Technology in Shenzhen, China and Southern Methodist University in Dallas, Texas, participated in this development.

The research is published in APL Materials.