Control of Molecular Alignment in 3D via Two-Photon Lithography

Two-photon lithography (TPL) has effectively leveraged the unique characteristics of photopolymerizable liquid crystals (i.e. reactive mesogens), leading to significant advancements in areas like soft microrobotics, microfluidics, and microoptics, as well as analytical sensing and anti-counterfeiting [1-3]. One of the current challenges of TPL is to impart the objects with further functionalities (often called “4D printing"), such as the ability to dynamically respond to external stimuli (i.e., temperature, light, pH, humidity, etc.) or to spatially modulate their properties (i.e., optical, mechanical, etc.) at the microscale [4, 5]. In this context, precise 3D control over the molecular alignment is crucial to fully unlock the power of TPL because it directly influences the performance of the printed structures. While various modular techniques like photoalignment, micro- and nano-rubbing, and microchannels from direct laser writing have enabled the creation of 2D director patterns, achieving more complex 3D director arrangements typically necessitates multistep procedures or specialized setups, such as 3D microscaffolding, applying electric fields during printing, or even assembling components printed with different orientations. [2, 6] The ability to adjust molecular alignment in situ and in a single step would represent a significant breakthrough, as it would confer enhanced flexibility in patterning the director field, leading to advanced functionalities in the 3D/4D-printed microstructures. Here we present a single-step simple method, based on the standard TPL workflow, to reorient the mesogenic moieties (i.e. nematic director) in situ, with subdiffraction-limited spatial resolution. Such directortuning mode (DiTuM) relies only on optimized TPL printing parameters to pattern the director field in 3D, circumventing the necessity for external driving fields, intricate surface alignment, 3D microscaffolds or other multi-step fabrication procedures. A TPL-induced “easy axis”, i.e. a preferred orientation for the director, is locally imposed by adjusting the laser scan speed and direction. It competes with the initial director of the reactive mesogens, enabling the creation of potentially convoluted 3D director and optical axis patterns. [7] With potential applications in authentication and anti-counterfeiting in mind, we propose security labels whose intricate polarized optical response arises from the 3D director pattern, designed to have a sophisticated relationship with the microprint's size, shape and fine details. References [1] X. Wan, Z. Xiao, Y. Tian, M. Chen, F. Lui, D. Wang, Y. Liu, P. J. Da Silva Bartolo, C. Yan, Y. Shi, R. R. Zhao, H. J. Qi, K. Zhou, Adv. Mater. 2024, 36, 2312263. [2] M. del Pozo, J. A. Sol, A. P. Schenning, M. G. Debije, Adv. Mater. 2022, 34, 2104390. [3] S. Donato, S. Nocentini, D. Martella, S. Kolagatla, D. S. Wiersma, C. Parmeggiani, C. Delaney, L. Florea, Small 2023, 20, 2306802 [4] T. Ritacco, D. M. Aceti, G. De Domenico, M. Giocondo, A. Mazzulla, G. Cipparrone, P. Pagliusi, Adv. Opt. Mater. 2022, 10, 2101526. [5] T. Ritacco, A. Mazzulla, R. Beccherelli, P. Pagliusi, Liq. Cryst. 2024, 51, 2232 [6] L. Y. Hsu, S. Gomez Melo, C. Vazquez-Martel, C. A. Spiegel, F. Ziebert, U. S. Schwarz, E. Blasco, Sci. Adv. 2024, 10, adq2597 [7] T. Ritacco, A. Mazzulla, M. Giocondo, G. Cipparrone, P. Pagliusi Adv. Sci. 2025, 2415159