Scientists have created robots that are only nanometres in length, small enough to manoeuvre inside the human body and possibly inside cells.
The tiny screw-shaped propellers can move in a gel-like fluid, mimicking the environment inside a living organism.
The filament that makes up the propeller, made of silica and nickel, is only 70 nanometres in diameter; the entire propeller is 400 nanometres long. A nanometre is one billionth of a metre.
"If you compare the diameter of the [nanopropellers] with a human blood cell, then the [propellers] are 100 times smaller," said Peer Fischer, a member of the research team and head of the Micro, Nano, and Molecular Systems Lab at the Max Planck Institute for Intelligent Systems.
The team already knew that tiny propellers moved well through water, but to test if they could move through living organisms, they chose hyaluronan, a material that occurs throughout the human body, including the synovial fluids in joints and the vitreous humour in your eyeball.
The hyaluronan gel contains a mesh of long proteins called polymers; the polymers are large enough to prevent micrometre-sized propellers from moving much at all.
But the openings are large enough for nanometre-sized objects to pass through. The scientists were able to control the motion of the propellers using a relatively weak rotating magnetic field.
The team expected that they would have trouble controlling the motion of the nanopropellers, since at their size they start to be governed by diffusion, just as if they were molecules.
But because the nanopropellers are the same size as the mesh in the gel, they "actually display significantly enhanced propulsion velocities, exceeding the highest speeds measured in glycerin as compared with micro-propellers, which show very low or negligible propulsion," said study co-author Associate Professor Alex Leshanksy at the Technion-Israel Institute of Technology.
While the nanopropellers are astonishing for their technical complexity, the real significance is how they might affect medicine.
"One can now think about targeted applications, for instance in the eye where they may be moved to a precise location at the retina," said Fischer.
Scientists could also attach "active molecules" to the tips of the propellers, or use the propellers to deliver tiny doses of radiation.
The work was published in the journal ACS Nano.