Science fiction fans have long been enamored with the idea of microscopic submarines that could navigate the human body. A recent publication in JACSJournal of the American Chemical Society demonstrates that this futuristic idea is not really so far off.
In the Fantastic Voyage, both a film and novel from the 1960s, a miniature submarine dramatically braves the perils of a man’s bloodstream to find and break apart a life-threatening blood clot. The submarine is made by a purely fictional machine that can shrink individual atoms. Such a feat is of course scientifically impossible, at least insofar as we currently understand the laws of physics. Nevertheless, tiny controllable vehicles can be made using normal-sized atoms, as the authors of this JACS paper have shown.
The vehicles that Joseph Wang and coworkers developed are really nanowires, made from gold, silver and nickel. Each nanowire is only 200 nanometersThe thickness of a piece of paper is about 100,000 nanometers thick, and about 6000 nanometers long. One end of the nanowire is made of pure gold, and the other end (the “tail”) is nickel. The two ends are connected by a thin, bendable segment of silver.
Elemental nickel is magnetic, so when these nanowires are exposed to a rotating magnetic field, the nickel tail responds by trying to spin around and around. Although the nickel tail is the magnetic portion of the wire, the key feature of this design is really the flexible silver midsection. If the silver midsection wasn’t bendable, then the rotation of the nickel tail would just cause the entire wire to spin all at once like a stiff log. Instead, the circular movement of the nickel end starts to twist and deform the silver middle, which in turn tugs on the gold end. Thus, once the entire nanowire is in motion, it looks more like a rotating corkscrew than a rotating log. This corkscrew-type movement is essential for producing forward or backward movement.
In fact, before this publication, other researchers had designed magnetic nano-corkscrews that could swim through water in a similar fashion. However, making miniature corkscrews is a lot more difficult than making mini wires, so this paper represents a major conceptual simplification that could really advance the practicality of these nano-swimmers.
The preparation of these wires is relatively simple. They are made by the sequential deposition of the three metals into a hollow template made of aluminum. Gold particles are first deposited, followed by silver, and lastly nickel. The aluminum template is then removed by dissolving it with a strong base, revealing the full nanowire. Next, the wire is dipped in a solution of hydrogen peroxide, which partially dissolves the thin silver segment, making it even thinner and super flexible.
The wires can swim at speeds up to a couple of centimeters per hour, which is actually pretty fast considering their tiny size. To scale, this speed is analogous to a six-foot snake slithering four miles in one hour – a little faster than most people walk. By changing the relative length of the gold and nickel ends, the researchers were able to choose whether the nanowire would travel “forward” (gold end first) or “backward” (nickel end first). Importantly, the authors demonstrated that the wires can swim in even very salty water that mimics biological fluids. Eventually, devices based on these swimmers could be used for selective drug delivery in the human body, perhaps by attaching drug molecules to the gold end of the wire.