At present, 3D printing is an increasingly widespread and accessible technology, typically involving the formation of solid polymeric materials in a specific region, either by extruding pre-formed polymers or by generating them in situ from their corresponding monomers, the molecules that make up polymers. However, these techniques often suffer from several drawbacks, such as long printing times or low resolution, preventing the production of printed materials with micrometric dimensions. To address these issues, polymer formation through light irradiation could be a promising solution, as photopolymerisation reactions tend to be faster and can be induced with precise spatiotemporal control.

Most light-induced polymer material formation processes occur under irradiation from a single light source, which limits their temporal and spatial precision. For example, some photoactivated reagents may diffuse beyond the illuminated zone, or their lifetime may exceed irradiation time, thus limiting the spatiotemporal resolution of the photopolymerisation process. Additionally, the maximum spatial resolution achievable with conventional optics is diffraction-limited, preventing such reactions from being confined to the nanometric scale.

To overcome these challenges, controlling photopolymerisation reactions using two different-coloured light sources has been proposed by several research groups, thus enabling the development of new lithographic and 3D printing techniques with enhanced capabilities. Although it is a promising solution, very few reactions of this type are currently known to exist.
Researchers from the UAB Department of Chemistry, led by Jordi Hernando, worked in collaboration with Prof Christopher Barner-Kowollik’s group at Queensland University of Technology, Australia, to develop a new photopolymerisation reaction controlled antagonistically by two different colours of light. Specifically, one light beam promotes the formation of the polymeric material, while another beam halts the reaction.

This chemical process involves photoinduced curing via an oxo-Diels–Alder cycloaddition between two reactants. “On the one hand, we use prepolymers that are activated by ultraviolet light, and on the other hand, curing agents that change from a reactive to a non-reactive state depending on whether they are irradiated with ultraviolet or red light,” explains Jordi Hernando.

With this new method developed by UAB researchers, light beams with distinct irradiation patterns can be used and therefore polymer resin curing occurs only in regions illuminated exclusively with ultraviolet light, while no solid material forms in areas exposed to both colours, making it possible to delimit the area where the polymer solidifies. With this system researchers were able to obtain solid polymeric materials in the laboratory with controlled shapes and with resolutions below millimetres. “We are now exploring the use of this new methodology to improve the performance of 3D printing processes and reach sub-micrometre resolutions, which represents a significant step for this technology,” says Jordi Hernando.

The study included the involvement of researchers from the UAB Department of Chemistry Arnau Marco, Marc Villabona, Gonzalo Guirado and Rosa M. Sebastián, and the collaboration of researchers from the Institute of Nanotechnology (INT) in Karlsruhe, Germany, and from Queensland University of Technology in Australia. The research was recently published in the journal Advanced Functional Materials.