Abstract
Three-dimensional (3D) printing has exploded in interest as new technologies have opened up a multitude of applications1,2,3,4,5,6, with stereolithography a particularly successful approach4,7,8,9. However, owing to the linear absorption of light, this technique requires photopolymerization to occur at the surface of the printing volume, imparting fundamental limitations on resin choice and shape gamut. One promising way to circumvent this interfacial paradigm is to move beyond linear processes, with many groups using two-photon absorption to print in a truly volumetric fashion3,7,8,9. Using two-photon absorption, many groups and companies have been able to create remarkable nanoscale structures4,5, but the laser power required to drive this process has limited print size and speed, preventing widespread application beyond the nanoscale. Here we use triplet fusion upconversion10,11,12,13 to print volumetrically with less than 4 milliwatt continuous-wave excitation. Upconversion is introduced to the resin by means of encapsulation with a silica shell and solubilizing ligands. We further introduce an excitonic strategy to systematically control the upconversion threshold to support either monovoxel or parallelized printing schemes, printing at power densities several orders of magnitude lower than the power densities required for two-photon-based 3D printing.
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Data availability
The data supporting this study are available from the corresponding author on reasonable request.
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Acknowledgements
S.N.S. acknowledges the support of the Arnold O. Beckman Postdoctoral Fellowship. M.S. acknowledges financial support through a Doc.Mobility Fellowship from the Swiss National Science Foundation (project no. P1SKP2 187676). A.O.G. acknowledges the support of a National Science Foundation Graduate Research Fellowship under grant DGE-1656518 and a Stanford Graduate Fellowship in Science & Engineering (SGF) as a Scott A. and Geraldine D. Macomber Fellow. We thank C. J. Brinker from the University of New Mexico and S. Kommera from Stanford University for their helpful discussions, V. A. Lifton from Evonik for supplying the Aerosil 200, and A. Sellinger and A. Lim from Colorado School of Mines for performing the TGA experiments. This research is financed through the support of the Rowland Fellowship at the Rowland Institute at Harvard University, the Harvard PSE Accelerator Fund and the Gordon and Betty Moore Foundation. Portions of this work were performed at: the Harvard Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF award no. 1541959; the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under award ECCS-2026822; the Stanford ChEM-H Macromolecular Structure Knowledge Center. The STL file for 3DBenchy – The jolly 3D printing torture-test by CreativeTools.se by CreativeTools is licensed under the Creative Commons - Attribution - No Derivatives license. The image of the gear we present in Fig. 4 and Extended Data Fig. 2 was reproduced with permission from Alamy Inc./Natalia Lukiianova.
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Contributions
D.N.C. and S.N.S. conceived the project. S.N.S., D.A., M.K.G. and T.H.S. developed the UCNCs. M.K.G. conducted the microscopy characterization of UCNCs. D.N.C., S.N.S. and T.H.S. developed the 3D printing resins. D.N.C., M.S., A.O.G. and R.C.S. constructed the 3D printers and optical setups. T.H.S. and A.O.G. generated the parts presented in this manuscript. D.N.C. directed the project.
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Harvard University has filed several patents on the basis of this work. S.N.S., R.C.S. and D.N.C. are co-founders of Quadratic3D, Inc. S.N.S. is the Chief Technological Officer, D.N.C. is the Chief Scientific Advisor and R.C.S. is an advisor to Quadratic3D, Inc.
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Extended data figures and tables
Extended Data Fig. 1 UCNC characterization.
a, TEM of the UCNCs. The scale bar represents a length of 50 nm. b, Absorption spectra of the annihilators used in this work. c, Photograph of UCNCs dispersed in water with and without the addition of MPEG-Silane. A precipitate rapidly forms (<1 h) in the vial without MPEG-silane, probably owing to nanocapsule aggregation. This aggregation is irreversible. TGA of capsule paste (d) and capsule constituents in nitrogen (e). In panel d, the temperature was held at 100 °C until the capsule paste mass remained constant. f, Emission–absorption overlap between the upconverted emission (Br-TIPS-anthracene), the photoinitiator (Ivocerin) and the light blocker (Sudan I). g, UCNCs and F127 micelles24 dispersed in various solvents. UCNCs and F127 micelles were both synthesized in water and added at a 1:30 ratio to the listed solvents. They were then excited at 635 nm and imaged through a 550-nm short-pass filter. The tap-water sample was dispersed in water directly from the tap and left uncapped for 20 min before taking the image. Acrylic acid and PEGDA were each used to assess capsule durability in acrylate-based monomers for printing resins.
Extended Data Fig. 2 Printing schematics.
a, Photograph of the FDM printing setup, which moves a laser spot in three dimensions. The original instructions for the FDM printer are found at https://www.kosselplus.com/, with our modifications presented above. Cartoon schematic (b) and photograph (c) of the DMD printing system, which allows for stationary, parallel excitation at one time. d, Cartoon depiction of the UCNC-facilitated printing process using parallel excitation. e, Emission spectra of the light sources used to generate prints (black: 637-nm fibre-coupled laser for the monovoxel excitation printer; red: 625-nm LED for the parallel excitation printer). f, The file images projected by the DMD to print the Stanford University logo and gear presented in the main text.
Extended Data Fig. 3 3D printing with UCNCs.
a, Formlabs print simulation of the same Benchy STL. The file was imported into the free software PreForm 3.18.0 and simulated for printing on a Form 3B printer at 50-μm layer height. The boat was scaled to match the dimensions of the Benchy printed on our printer. The boat, without support structures, required 200 layers and 0.14 ml of resin. At this point, we used the ‘one-click-print’ function to generate the printable structure with support structure. This resulted in an object with 289 layers at 50-μm layer height, using 0.85 ml of resin. The STL file image (b) and a photograph (c) of the Harvard University logo presented in Supplementary Video 3. The Harvard University logo presented here was printed using a different resin formulation with bis(5-2,4-cylcopentadien-1-yl)-bis(2,6-difluoro-3-(1H-pyrrol-1-yl)-phenyl) titanium (titanocene, Gelest) as a photoinitiator instead of Ivocerin. This resin was prepared with 1.9 wt% Aerosil 200, 3 wt% titanocene, 13 wt% Br-TIPS-anthracene capsules, 0.03 wt% Sudan I and 5 ppm TEMPO. This resin formulation limited printing resolution,hence the emphasis on the use of the resin presented in the main text and Methods. d–f Side-by-side comparisons of the STL file schematic to the corresponding Benchy photograph from the same perspective. The boats presented in panels d-f were printed using the resin formulation described in the Methods section. g, A representative image of a gear under the microscope shows that round and straight features are generally smooth. The images were taken after washing away excess resin and allowing the gear to dry under an ambient atmosphere in the dark. h, An overprinted boat gives a lack of discernable features. i, An underprinted boat shows missing features and damage from the wash process. Both issues are remedied by altering the print speed and irradiation power. The boats presented here were printed using the same resin formulation as the Harvard University logo presented in panel c.
Extended Data Fig. 4 Printing in the quadratic regime.
The z distance from the focal point of the voxel in which the printing in the quadratic regime is approximated. We assume an approximately constant illumination profile.
Extended Data Fig. 5 Printing resolution note 1: blurring of the upconverted voxel in the printing resin.
Cartoon depiction (not to scale) of the blurring that occurs between the shape of the upconverted voxel and the photoinitiation in resin caused by absorption of the upconverted light. Although upconverted light generates light in nanocapsules that emits isotropically from a focal point, this light must subsequently be absorbed by the photoinitiator to cause polymerization. This emission and reabsorption introduce a blurring function to the print that is dependent on the Beer–Lambert law for the distance that the light travels before reabsorption.
Supplementary information
41586_2022_4485_MOESM1_ESM.txt
Supplementary Data 1 Original STL Benchy file This STL file was uploaded to the commercially available software Simplify3D to generate the Benchy print. The STL file for 3DBenchy – The jolly 3D printing torture-test by CreativeTools.se by CreativeTools is licensed under the Creative Commons - Attribution - No Derivatives license.
41586_2022_4485_MOESM2_ESM.txt
Supplementary Data 2 Edited STL Benchy file This gcode file was used to generate the Benchy print presented in this manuscript. The STL file was imported into the software Simplify3D, the dimensions of Benchy were scaled and exported as a gcode file to control the monovoxel excitation printer. The height of the original STL file was adjusted to account for the refractive index of the resin.
41586_2022_4485_MOESM3_ESM.mp4
Supplementary Video 1 Resin viscosity comparison A relative viscosity comparison of resin with Aerosil 200 (R, right/bottom cuvette) with PETA (P, left/top cuvette).
41586_2022_4485_MOESM4_ESM.mp4
Supplementary Video 2 Monovoxel excitation printer time-lapse video This time-lapse video demonstrates the monovoxel-excitation-based printing of Benchy using the Br-TIPS-anthracene annihilator-based resin. The photograph at the end of the video shows representative contents of a cuvette containing the Benchy print, along with uncured resin to be later washed off of the print. The actual time to generate the print is 1 h 50 min.
41586_2022_4485_MOESM5_ESM.mp4
Supplementary Video 3 Harvard University logo printed with the monovoxel printer This video shows the front and profile views of the Harvard University logo printed inside a 1-cm-path-length polystyrene cuvette using monovoxel-excitation-based printing.
41586_2022_4485_MOESM6_ESM.mp4
Supplementary Video 4 Parallel excitation printer time-lapse video This time-lapse video demonstrates the DMD-based printing of a gear with the TIPS-anthracene annihilator-based resin, with a photograph at the end showing the contents of the Petri dish after a print. The actual time to generate the print is 8 min.
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Sanders, S.N., Schloemer, T.H., Gangishetty, M.K. et al. Triplet fusion upconversion nanocapsules for volumetric 3D printing. Nature 604, 474–478 (2022). https://doi.org/10.1038/s41586-022-04485-8
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DOI: https://doi.org/10.1038/s41586-022-04485-8
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