From its bow to stern, this tiny boat is 30 micrometers, which is about 1/3rds of a hair's diameter. And we are to thank Leiden University physicists Rachel Doherty, Daniela Kraft, and colleagues.
The image you see is captured using an electron microscope and there's a good reason for its creation. To explain that, we have to delve into the world of so-called microswimmers. Kraft's research group works on them; in short, they are particles that move inside water, we can follow such materials using a microscope.
We can divide microswimmers into two main categories, biological microswimmers, such as bacteria or viruses, and non-biological swimmers. See, there was one small problematic aspect to understanding how these microswimmers work though, the latter category was limited to spherical shapes thus far. While on the other hand, biological microswimmers can come in many shapes, so it's definitely a limiting aspect.
But 3D printing can change this, as demonstrated by researchers in their paper. They printed a variety of shapes, for example, a spiral-shaped particle, and found that it rotates around itself when it is propelled through the water.
Judging from the picture, the boat has a few hundred times smaller details than the length of the boat. Kraft's team used the so-called two-photon polymerization (2PP). This is a relatively recent method. The starting material is a slab of monomer. A source of near-infrared lasers directed through lenses induces a polymerization reaction and molds it into shape.
So in this context, the printing process is actually directing a laser's focal point around. This is where using near-infrared comes in handy as this material is fairly transparent. The unpolymerized part gets dissolved away at the end.
Light-induced polymerization often requires UV light. In the 2PP method, two infrared photons with half the wavelength of the required UV frequency. These photons merge and form the required UV to polymerize the monomer.