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

TL;DR

This paper explores using upconversion nanoparticles with continuous-wave lasers for multi-photon polymerization, enabling tunable feature sizes in 3D printing with cost-effective light sources.

Contribution

It demonstrates how NIR excitation intensity can control voxel size in upconversion nanoparticle-based photopolymerization, offering a cheaper alternative to femtosecond laser-based two-photon printing.

Findings

Voxel size can be tuned from 1.3 to 2.8 μm transversely.

Axial voxel size can be adjusted from 7.7 to 59 μm.

Tunable feature sizes achieved without changing polymerization degree.

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 limits 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 {\mu}m and the axial size from 7.7 to 59 {\mu}m by adjusting the NIR power without changing the degree of polymerization. This work opens up new opportunities for speeding up the fabrication while preserving the minimum feature size with cheap light sources.