Multi-photon polymerization using upconversion nanoparticles for tunable feature-size printing

Abstract

The recent development of light-based 3D printing technologies has marked a turning point in additive manufacturing. Through photopolymerization, liquid resins can be solidified into complex objects. Usually, the polymerization is triggered by exciting a photoinitiator with ultraviolet (UV) or blue light. In two-photon printing (TPP), the excitation is done through the non-linear absorption of two photons; it enables printing 100-nm voxels but requires expensive femtosecond lasers which strongly limit their broad dissemination. Upconversion nanoparticles (UCNPs) have recently been proposed as an alternative to TPP for photopolymerization but using continuous-wave lasers. UCNPs convert near-infrared (NIR) into visible/UV light to initiate the polymerization locally as in TPP. Here we provide a study of this multi-photon mechanism and demonstrate how the non-linearity impacts the printing process. In particular, we report on the possibility of fine-tuning the size of the printed voxel by adjusting the NIR excitation intensity. Using gelatin-based hydrogel, we are able to vary the transverse voxel size from 1.3 to 2.8 μm and the axial size from 7.7 to 59 μm by adjusting the NIR power without changing the degree of polymerization. This work opens up new opportunities to construct 3D structures with micrometer feature size by direct laser writing with continuous wave inexpensive light sources.

Keywords: additive manufacturing; hydrogels; light-based 3D printing; multi-photon polymerization; photopolymerization; upconversion nanoparticle.

Figure 1:

Comparison between two-photon polymerization and multi-photon polymerization with UCNPs. (A) A femtosecond NIR beam is focused by a microscope objective through a square capillary (100 μm × 100 μm) into the resin. The printed voxel size can be tuned by adjusting the NIR intensity which produces under or over-polymerized parts, in other words, voxels with different degrees of polymerization. (B) UCNPs provide another solution for multi-photon polymerization using a CW laser. The NIR beam is focused into the photosensitive resin. The beam size of the upconverted fluorescence depends on the NIR intensity which enables printing voxels of different sizes with the same degree of polymerization.

Figure 2:

Characterization of UCNPs. (A) Transmission electron microscopy image of a typical UCNP used in our system. (B) Fluorescent voxel in a 10 mm × 10 mm cuvette containing aqueous UCNPs produced by a CW focused laser beam (intensity at the focal plane: 19 kW/cm², wavelength: 976 nm). (C) Emission spectrum of UCNPs (blue) upon NIR light illumination and absorption spectrum of LAP (orange). a.u., arbitrary units. (D) Energy level diagram of NaYF₄: Yb³⁺, Tm³⁺. Solid arrows represent the photon absorptions or emissions, and dashed arrows represent different energy transfer processes. (E) Emission spectra of UCNPs in the gelatin solution at the excitation intensity of 50, 150 and 480 kW/cm². (F) The power-dependent emission curve at different fluorescence peaks marked in (E). Each curve is normalized to show the nonlinearity difference.

Figure 3:

Characterization of the NIR threshold dose and the printed voxel size. (A) Schematic illustration of the method for printing and characterizing a ‘voxel’. Because of the small refractive index mismatch between crosslinked and unpolymerized gelMA, the beam is scanned on the x-axis to increase the phase change and improve the imaging contrast in the yz-plane. (B) For the characterization, the prints are imaged with a DPC microscope. The acquired images enable the reconstruction of the phase of the samples with sufficient contrast to extract features like the voxel size. (C) DPC images of voxels (marked with arrows) with different doses of NIR light (NIR intensity: 1.1 × 10⁵ W/cm², exposure time: 6–12 s). The dose used to print the voxel indicated by the red arrow corresponds to the polymerization threshold dose at this excitation intensity. (D) Profile along y-direction across the center of the voxels (highlighted in yellow), and (E) the corresponding CNR. The gray dotted line indicates the CNR of the image we choose for the polymerization threshold, and the red dotted line indicates the threshold dose. (F) Polymerization threshold dose versus the excitation intensity. (G) Reconstructed phase of voxels printed at different excitation intensities with CNR ∼15 in DPC images (for a similar refractive index change). Lateral (H) and axial (I) size of the fluorescence and the printed voxels versus the excitation intensity. The dashed line marks the focal spot size of the NIR beam in each direction.

Figure 4:

Fabrication of a butterfly of tunable feature sizes with UCNP-assisted multi-photon printing. (A) Butterfly model and its projections in two orthogonal directions. The model is printed at different excitation intensities based on the axial feature size. The NIR light dose is adjusted for each voxel to change its size while preserving a uniform degree of polymerization across the whole structure. DPC images of xy-plane (B) and yz-plane (C) show the scanning range and the feature size, respectively. The body and the antenna of the butterfly are printed with a larger scanning range, resulting in higher contrast in the xy-plane projection.