AbstractThe range of applications for additive manufacturing is expanding quickly, includi
ng mass production of athletic footwear parts1, dental ceramics2 and aerospace components3
as well as fabrication of microfluidics4, medical devices5, and artificial organs6. The l
ight-induced additive manufacturing techniques7 used are particularly successful owing to
their high spatial and temporal control, but such techniques still share the common motifs
of pointwise or layered generation, as do stereolithography8, laser powder bed fusion9, a
nd continuous liquid interface production10 and its successors11,12. Volumetric 3D printin
g13,14,15,16,17,18,19,20 is the next step onward from sequential additive manufacturing me
thods. Here we introduce xolography, a dual colour technique using photoswitchable photoin
itiators to induce local polymerization inside a confined monomer volume upon linear excit
ation by intersecting light beams of different wavelengths. We demonstrate this concept wi
th a volumetric printer designed to generate three-dimensional objects with complex struct
ural features as well as mechanical and optical functions. Compared to state-of-the-art vo
lumetric printing methods, our technique has a resolution about ten times higher than comp
uted axial lithography without feedback optimization, and a volume generation rate four to
five orders of magnitude higher than two-photon photopolymerization. We expect this techn
ology to transform rapid volumetric production for objects at the nanoscopic to macroscopi
c length scales.MainWe present a volumetric 3D printing process in which the structure of
the entire resin volume is retained and complex multicomponent objects are manufactured an
d stabilized by the surrounding viscous fluid matrix. In contrast to sheet-based methods,
support structures for overhanging features, requiring elaborate post-processing, are no l
onger required, layer-interface-associated anisotropies vanish and fragile, soft objects c
an be solidified. This method represents one-step fabrication of complete systems without
requiring later assembly yet still incorporating moving parts.Until now, two different lig
ht-based, volumetric technologies have received the most attention. To fabricate high-reso
lution microscale objects, two-photon photopolymerization is the state of the art and has
realized objects with sub-100-nm feature sizes16,17. A major limitation is the volume gene
ration rate, which is typically well below 1–20 mm3 h−1 (refs. 18,19), owing to the underl
ying nonlinear absorption process to harden a localized volume within the resin space. For
volumetric additive manufacturing of macroscopic objects, computed axial lithography rota
tes a homogeneous resin volume while multiple images are projected into the target materia
l at defined angles14,20. The superposition of expositions leads to a cumulated dose distr
ibution of formed radicals, which solidify centimetre-sized objects within 30–120 s and le
ave other areas below the polymerization threshold. The technique requires a nonlinear res
ponse of the resin to define the threshold, which is at present mediated by an oxygen inhi
bition process. The resolution of printed objects has been reported as 300 μm, limited by
dose fluctuations caused by light that passes through partially or already polymerized are
as during the printing process. Recently, optimization and inclusion of a feedback system
trying to compensate these effects during a second print of the same object resulted in 80
-μm positive and 500-μm negative feature sizes15.In our approach, we eliminate the nonline
arities mentioned by using two intersecting light beams of different wavelengths to solidi
fy localized regions. This approach, known as dual-colour photopolymerization (DCP), was p
roposed early on by Swainson21. Curing is mediated by a two-colour photoinitiator added to
the resin, which is activated by a first wavelength, while absorption of the second wavel
ength either (1) initiates or (2) inhibits photopolymerization. The latter pathway frequen
tly utilizes two-component photoinitiators and has been applied with success to two-colour
photolithography22,23, continuous layer-by-layer printing12, and patterning of thin volum
es24. Efficient route (1) photoinitiators required for deep-volume DCP are, however, not y
et common. Molecular photoswitches25 are valuable candidates26, especially because each sw
itching state can be precisely tailored with respect to its photochromic properties, absor
ption spectra, lifetime and chemical reactivity. Here, we leverage the potential of DCP to
realize a true volumetric 3D printing technology by combining a novel dual-colour photoin
itiator with a new projection light system for DCP, designed to achieve rapid, high-resolu
tion printing.