[Plasteel is almost here.] It’s a laser-printed material: as in a laser targets precise points within a drop of liquid and hardens those point. When the structure is complete, the remaining liquid is washed away. Right now, structures are measured in micrometers and millimeters.
[Plasteel is almost here.]: http://arstechnica.com/science/2014/02/new-laser-printed-material-is-lighter-than-water-as-strong-as-steel/