Diseases that affect the back of the eye (or retina) are difficult to treat because it is challenging to deliver drugs through the dense, vitreous tissue of the eye. A team of researchers led by Tian Qiu and Peer Fischer of the Max Planck Institute for Intelligent Systems in Stuttgart, Germany, has now made the first nanobots that can easily “swim” through this tissue. The devices, which are magnetic and measure 500 nm across, are shaped like propellers and are coated with a liquid layer that prevents them from adhering to eye tissue while they travel through it. They may be loaded with drugs or other therapeutic agents and could be steered to the targeted area using applied magnetic fields.

“Propelling objects through biological tissue is challenging, even though the majority component is water, because it consists of dense molecular networks that make it difficult for particles to penetrate,” explains Fischer. “The vitreous humour of the eye for instance contains a dense network of hyaluronan and collagen molecules. This network acts as a barrier to particles.”

Cork-screw-like propellers

The researchers made their nanopropellers using a vacuum-based technique called glancing angle deposition. They first patterned silica-based nanoparticles on a wafer and then placed this wafer in a vacuum chamber at an extreme angle while depositing either iron or nickel and silica material. “The shallow angle causes shadowing so that material only deposits on the nanoparticles,” Fischer tells Physics World. “Rotating the substrate during growth produces cork-screw-like helical propeller structures.”

The devices were then coated with a biocompatible, clinically-approved two-layered non-stick material containing liquid fluorocarbon. This coating is inspired by a liquid layer found on the “lip” of the carnivorous Nepenthes pitcher plant that uses it to catch insects. It greatly reduces the adhesive force between the nanorobots and the biological protein network in eye tissue and allows the devices to travel almost unimpeded through it.

The nanoparticles measure 500 nm across and are 2 μm long. This is comparable to the mesh size of the biopolymeric network and means that they are small enough to slip through without damaging this tissue.

Many tissues

The Max Planck scientists injected tens of thousands of these nanobots into a dissected pig eye using a needle. They then applied a magnetic field that rotates the bots so that they swim (at a speed of around 10 μm/s) in a specific direction through the eyeball and towards the retina, where they finally land. They monitored the devices’ trajectory using optical coherence tomography – a safe imaging technique routinely employed in diagnosing eye diseases. The whole process takes around 30 minutes, which is much shorter than the time it would take for a drug molecule to naturally diffuse to the retina.

The team, reporting its work in Science Advances 10.1126/sciadv.aat4388, says that it is now busy working on nanopropellers that can travel through dense tissues other than that of the eye – and deliver drugs to these tissues.