A flashlight beam in fog draws a cone, but it can't carve a figure out of a cloud. A hologram, though, is like a stencil for light: it knows in advance where to gather into bright points and where to remain shadow. Now such a stencil makes liquid resin solidify throughout the entire volume at once, without layer-by-layer printing.
The problem is that light scatters in the resin, and the chemical curing reaction spreads like a drop of ink on wet paper. The engineers took a clever approach: they pre-distorted the hologram so that the light spots become sharper where chemistry would have smeared them more. This compensates for the blur before it even happens.
As a result, within seconds, parts with a cross-section of ten micrometers, thinner than a human hair, grow in a cuvette smaller than a fingernail. The entire setup is stationary: no laser scanning layers, no platform lowering into a vat. Only the hologram changes dozens of times per second, and the resin obediently solidifies in the right places.
This approach will be useful for tiny lenses, gratings, and scaffolds that today are assembled piece by piece or cannot be made at all. Instead of lengthy fiddling with microscopic parts, a single flash of light and the shape is ready.
Ten micrometers is about one-fifth the thickness of a human hair. A part of that size can't be clearly seen without a magnifying glass, yet it was printed in seconds.