Diving into the bloodstream

In the 1966 science fiction film "The Fantastic Voyage", scientists have their submarine shrunk and injected into the body of another researcher. Truly a fiction, at least as far as shrinking people is concerned. It's a different story with machines. Here, science is making promising progress.

The core of the plot of the feature film "Fantastic Voyage" is quickly told: In 1965, the Czech scientist Dr. Beneš defects to the West, but becomes the victim of an attack and suffers life-threatening injuries. A blood clot in the brain has to be removed, which seems impossible by conventional means. However, a new development makes it possible to shrink people and machines down to the size of a microbe. To save Dr Beneš's life, a team of experts and a special submarine are miniaturised and injected into the patient's bloodstream. A race against time begins because man and machine grow to normal size after 60 minutes and have to be taken out of the body beforehand.

An exhilarating story of pure fantasy. Or is it? Shrinking humans may be a far way off but the use of microscopic machinery is already here. Unlike in the movie, these aren't 'shrunk' but rather produced in its needed size directly in their constituent parts of metal and plastic polymers.  

To move a micromachine from the injection site to a specific location in the body, an external driving force is needed – usually magnetic fields used to propel magnetic metals. For sufficiently soft, flexible and mobile devices, and to be able to absorb and transport active pharmaceutical ingredients, polymers are needed. That nothing remains in the organism that could cause harm, the polymer must dissolve after use, and everything that remains from the intervention  excreted.

The core of the ETH method is the so-called three-dimensional (3-D) lithography. To explain: Lithography is the oldest flat printing process. In this process, the words, texts or images are engraved laterally reversed to a stone (Greek: lithos) and transferred to paper or another receiving medium with pressure in a press, and in the correct orientation with ink. This long-lived process dominated well into the twentieth century.

So the ETH method uses high-precision 3-D printing technology to produce a mold on a micrometre scale, riddled with the finest of channels; it represents the negative. Where necessary, the researchers fill in metals by means of electrochemical deposition, and plastic is applied to other channels, thus creating the mini-machine. The mold is then dissolved in a solvent.

Using this 3-D lithography, the ETH researchers say they have been able to produce tiny vehicles with plastic chassis and magnetic metal wheels driven by a rotating magnetic field, either on a glass surface or, depending on the polymer in question, in or on the surface of a liquid.

Initial mobility tests have proven successful and promising. However, before a modern form of the "fantastic journey" through the human body can be established in medical routine, some development is still needed. The ETH scientists are currently working on the further development of their two-component micromachines, whereby they are willing to test the use of different materials. In addition, they will try to produce more complex shapes and machines, including those that canfold together and unfold again. The researchers also seek to expand the range of their miniaturised "transporters" for specific purposes. In addition to drug-delivering ferries, future applications include micromachines that can be used to treat dangerous aneurysms bulges in blood vesselsor to perform other operations. Specifically, the researchers envisage the development of foldable stents to keep blocked vessels open or to prevent them from becoming blocked again.

Alcântara et al., Mechanically Interlocked 3D Multi-Material Micromachines, Nature Communications (2020), doi: 10.1038/s41467-020-19725-6 [https://doi.org/10.1038/s41467-020-19725-6]