With the exception of experimental non-planar techniques, all 3D printing processes fabricate objects by adding one layer at a time. The major drawback with this technology is "stepping" caused by a curved or angled surface relative to the Z axis. One can reduce the visibility of those steps by decreasing the layer thickness, but it can't be eliminated entirely. To avoid stepping altogether and also provide some other unique benefits, engineers from Stanford and Harvard created a new volumetric 3D printing process that doesn't rely on layers at all.
With SLA (stereolithography) 3D printing, special resin hardens when exposed to a particular light spectrum (usually 405nm UV light). Most printers either project that UV light or use a UV laser. But those must cure the resin in layers. If one shot a laser through a volume of resin, it would cure a line along the entire length of the laser's path through the resin—assuming it could penetrate with enough energy. To avoid that, printers lower a platform into the resin vat and leave only a small gap. That gap is equal to the layer thickness. Even if you attempted to refocus the laser to specific depths, the unfocused beam would still carry enough energy to cure the resin along its path.
This new technology circumvents that issue by using a low-power red laser that doesn't carry enough energy to cure the resin. But the resin contains a suspension of cutting-edge nanocapsules that emit blue light when energized by the red laser. That blue light cures the resin, but the red laser only has enough energy to activate the nanocapsules at its focal point. This means that resin only cures around the focal point of the laser, which is adjustable. The printer can emit the laser from any side of the resin vat and set the focal point to a specific depth, meaning it can cure any single point within the volume.
The resin, in this case, is more like Jell-O than conventional 3D printing resin. This ensures that a cured point will stay suspended in place, eliminating the need for support structures and enabling very complex geometry. Combine all of these factors together and you have a 3D printing process that avoids layer stepping, doesn't waste time or material on support structures, and creates detailed geometry.