If you remember the 1966 science fiction film Fantastic Voyage, you'll recall how miniaturized government agents traveled through blood vessels in a tiny submarine, in their attempt remove a blood clot from a scientist's brain. Synthetic nanomotors that can do the same job have been the subject of numerous research efforts and now University of California, San Diego (UCSD) researchers report that they've created powerful biodegradable "microswimmers" that can deliver drugs more precisely, derived from common plants like passion fruit and wild banana.

Creating nano-sized machines that can move through bodily fluids is extremely challenging, because physics works differently at the nanoscale level. While powering a nanovehicle adequately and steering it to the right place are problems in themselves, getting such minute machines to move through blood, for instance, means they'll need to overcome viscous forces or the stickiness and thickness of blood at that level.

"It is like swimming through honey for a human," Wei Gao, a researcher at UCSD's Nanoengineering Lab tells Gizmag.

Drawing inspiration from nature, the team developed magnetically-propelled synthetic nanomotors that moved in a corkscrew fashion, but these required expensive equipment and fabrication processes that made them impractical for large scale production. Exploring natural plant structures they could use instead, led them to examine the water transportation cells or Xylem tissue in plants like African Lily, Indian Hawthorn and others.

After removing spiral microstructures about the width of a cotton fiber from the plant's stems and coating them with fine layers of nickel and titanium, they ended up with microswimmers they could control magnetically.

"Under a rotating magnetic field, the microswimmers will start to rotate," Joseph Wang, the Head of the Lab, tells us. "The helical structures can transform the rotation around their helical axis into a translational corkscrew motion."

Not only can these helical microswimmers move more powerfully through biological fluids, thanks to this unique propulsion behavior, but they are also more efficient and can be propelled really fast by varying the strength of the magnetic field.

"Our plant-based microswimmer can swim at a high speed of 250 μm/s, making it one of the fastest magnetic propelled motors," states Gao.

That's effectively five times its body length per second, equivalent to Usain Bolt's World Record of running 100 meters in 9.58 seconds. Compared to current nanoparticle-based drug delivery systems that rely on diffusion, that's blindingly fast.

The plant base also makes it easy to embed drugs into the structure or have it carry drug-loaded nanoparticles and trigger their release via heat, temperature or light. Not only are these microswimmers biocompatible and biodegradable, it's simple to produce huge quantities of them quickly and inexpensively. For instance, one leaf of the Indian Hawthorn can produce around 4,500 microswimmers while a 30-cm (11.8-in) bit of the African lily's stem yields over 1,500,000 of them.

Propelling them magnetically enables very precise steering, allowing drugs to be delivered into deep tumor tissue at the right location and time. Since there aren't any toxic-fuel-laden nanomotors being sent into the body, there's no question of side effects.

"The majority of intravenously administered therapeutic nanoparticles are also reaching normal tissues, resulting in considerable adverse side effects," explains Gao. "In our case, we can control the movement of the swimmer and directly deliver the drug into the cancer cells."

There's more work to be done, before they can be deployed for actual drug delivery.

"For propulsion in the real human body, the microswimmer will face much more complex environments," Gao tells us. "It needs to overcome various challenges such as a fast-moving blood stream and obstacles such as cells and proteins. Currently, we are still far away from a real nanosubmarine, as in the movie. However, we are getting closer and closer. "

Additional potential applications include removing contaminants from the environment, manufacturing nanoscale devices, biosensing and more. The scientists' paper describing the development was recently published in the journal Nano Letters.

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