Light-sheet 3D microprinting via two-colour two-step absorption

Abstract

need further improvement. First, the total sample fabrication time is presently dominated by sample handling, that is, print setup, sample transport and development. These steps can be automated, for example, by using microfluidics 42 , or by automatically immersing the sample in liquid containers 43 . Second, the photoresin as presented here limits the total exposed volume. After prolonged printing, out-of-focus areas polymerize due to proximity effects. Such accumulation or depletion effects can likely be suppressed by using further improved photoresin formulations. Directions for optimization can be deduced from the rate model analysis presented here. In addition, the in situ replenishment of the used photoresin by using microfluidics, polymerization inhibition by an oxygen-permeable membrane (as used in CLIP 8 ), or volumetric polymerization inhibition patterning 10 can potentially further suppress out-of-focus polymerization. Third, the red solid-state laser can be replaced by inexpensive high-power laser diodes, as is already the case for the blue laser. Compared with the amplified femtosecond pulsed lasers used in FP-TPL, the continuous-wave lasers used in this work are already much less expensive and easier to operate. The development of two-colour two-step photoinitiators improved with respect to sensitivity and tuned to readily available laser wavelengths would be highly desirable for this aspect.The underlying logical-AND-type functionality is not limited to light-sheet 3D printing, but could also prove beneficial in combination with CAL [5][6][7] , where a two-colour projection system was recently demonstrated for multimaterial 3D printing 44 . Related CAL setups could be used to simultaneously expose the photoresin using two-colour two-step absorption, with the two colours impinging from different directions, thereby reducing the proximity effect and voxel size.

Challenges and Opportunities in 3D Laser Printing Based on (1 + 1)-Photon Absorption

“…Photoinitiator molecules (green subpanel) are optically excited by ultraviolet (UV) photons emitted by excited upconversion NPs. − Alternatively, the energy from upconversion NPs can be transferred to photoinitiator molecules via a nonradiative energy transfer, e.g., Förster resonance energy transfer (not shown). (b) In two-step absorption, , a reluctant photoinitiator is optically excited via an excited singlet to an inactive triplet state. In this state, the photoinitiator can absorb a second photon, which provides sufficient energy to trigger a polymerization reaction.…”

“…We term this process “two-step absorption” . If the colors of the two involved photons are the same, we refer to the process as “one-color two-step absorption” . In this case, the local excitation rate is proportional to the square of the light intensity at this position.…”

“…In this case, the local excitation rate is proportional to the square of the light intensity at this position. If the colors of the two photons are dissimilar, we speak of “two-color two-step absorption” . Here, the excitation rate is proportional to the product of the two corresponding light intensities (as for the case of upconversion luminescence).…”

“…From the above (1 + 1)-photon absorption mechanisms, 3D printers exploiting either upconversion luminescence or triplet–triplet annihilation print with rates of 1 voxel/s up to 40 voxels/s. ,, In contrast, using two-step absorption has led to the largest printing rates . In particular, using light-sheet 3D printing via two-color two-step absorption, printing rates in the range from 10 5 voxels/s to 10 7 voxels/s have been demonstrated. , As a reference, common tabletop filament-extruder 3D printers achieve printing rates of merely a few hundred voxels/s …”

“…17 Here, the excitation rate is proportional to the product of the two corresponding light intensities (as for the case of upconversion luminescence). In the recently published examples of two-step absorption, 17,27 the lifetime of the triplet (intermediate) state has been shown to be in the range of 100 μs or below. Generally, a long triplet lifetime is beneficial for obtaining high photoresin sensitivity.…”