Fantastic Voyage: Robotic Nano-swimmers could Swim through Bloodstream, Eyeball Fluid
Anyone remember the 1966 science-fiction movie Fantastic Voyage, where a submarine gets shrunk down so that it’s crew can travel through the human body, allowing the crew to perform surgery in the brain?
Researchers from the Max Planck Institute in Stuttgart have now made a first step towards a micro-robotic version of this scenario.
They have successfully developed swimming bodies that meet two requirements. They are simultaneously small enough to be used in bodily fluids or even individual cells, and they are able to navigate through complex biological fluids.
As you can imagine, tiny submarines like this that could navigate through the body could do amazing things such as precisely deliver drugs to a targeted location, a point on the retina for instance.
They could make it possible to carry out gene therapy in a specific cell. Making some surgical procedures minimally invasive. They might even be able to remove cancer tumours before they became dangerous, or do the work of antibodies in a compromised immune system.
But there are two basic constraints to realizing these dreams.
Cooperating with researchers at the Technion in Israel and the Technical University in Dortmund, the group describes in a recent paper a form of artificial scallop just a few hundred micrometers in diameter, designed to travel in liquids by simply opening and closing its shells. This is not as obvious as it sounds.
“The shell is only a few times larger than the thickness of a human hair,” says Fischer. “A liquid like water is about as viscous for these devices as honey or even tar is for us.”
Additionally, with so much friction in fluids, symmetrical movements, such as the reciprocal opening and closing of a scallop shell, would not result in any forward propulsion. The back-and-forth movements caused by the opposing movements would simply cancel each other out and not move anywhere.
But researchers tested their swimmer in appropriate model fluids, having characteristics distinguishing them from water.
“Most bodily fluids have the property that their viscosity changes depending on the speed of movement,” says Fischer. “In synovial fluid found in joints, for example, hyaluronic acid molecules arrange themselves into network-like structures that result in a high viscosity. But as soon as something moves through this fluid, the molecular mesh breaks apart and the fluid becomes less viscous”.
The Magnetic Control Solution
The scientists control the scallop so that it opens much faster than it closes.
“This temporally asymmetric pattern of movement causes the fluid to be less viscous during opening than during the subsequent closing stroke,” says team member Tian Qiu.
The distance the scallop travels when opening is therefore not the same as the distance it moves backward when closing, and this results in net forward propulsion. This marks the first time an artificial device of this size has ever been able to move through fluids by means of symmetrical motion cycles, says Tian Qiu.
Debora Schamel / MPI for Intelligent Systems
To control their micro-scallop, tiny rare-earth magnets were integrated in the two scallop shells. This enables them to control how the scallop shells open and close, and ultimately how the device moves, by applying an external magnetic field.
The discovery that micro-devices can swim through some liquids with symmetrical movements does not just apply to magnetically-driven micro-robots. A scallop-shaped miniature submarine could actually be driven by an actuator that responds, for example, to temperature changes.
“We’re interested in the next step, for example whether we can also guide this robot through the extracellular matrix of a tissue,” says Peer Fischer.
In fact, the team has already written a paper on an even smaller device, in the form of a corkscrew-shaped nanohelix.
Such helical structures have been around for a while. However, until recently their production was limited to sizes of tens of micrometres or more.
Now, for the first time, the researchers in Stuttgart have succeeded in devising a suitable propeller with a diameter of around 100 nanometres, or one-tenth of a micrometre. The miniature swimmer measures just 400 nanometres in length.
Tian Qiu, Tung-Chun Lee, Andrew G. Mark, Konstantin I. Morozov, Raphael Münster, Otto Mierka, Stefan Turek, Alexander M. Leshansky und Peer Fischer Swimming by reciprocal motion at low Reynolds number Nature Communications, 4 November 2014; DOI: 10.1038/ncomms6119