Direct Laser Writing of Silica Nanoparticle Composites for Mechanical Reinforcement of Hydrogel Networks

The desire to create 3D micro and nanostructures has been of great interest in the fields of microelectromechanical systems (MEMS), metamaterials and micro/nanofluidics among others. Direct Laser Writing (DLW) by two-photon polymerisation (TPP) allows for the fabrication of precise and complex (sub)micrometric structures with features that can go below 100 nm.¹ In recent years, TPP has been employed for the fabrication of microstructures using a wide range of materials, including hydrogels,¹ ionogels,² poly(ionic liquids)s³ and biopolymers.⁴<br/><br/>While TPP-based fabrication allows for the realisation of a wide range of 3D designs, challenges associated with collapsing of the structure or of structural features can be encountered in particular when dealing with self-supporting, high-aspect ratio structures, or designs that contain large overhangs. These challenges are caused by insufficient mechanical stability of the final polymer structure and can appear during development and drying stages.⁵ One method that demonstrated improved mechanical strength of the polymeric material was achieved through the incorporation of nanofillers in the photoresists. Xiong <i>et al.</i> reported the development of a TPP compatible multi-walled carbon nanotube (MW-CNT) - thiol-acrylate composite resin. Addition of a small concentration of MW-CNTs (0.1-0.2 wt%) inside a proprietary acrylate-based photoresist, dramatically improved the capability of creating over-hanging structures and resulted in a decrease in polymer shrinkage post-development from 4% to 2.4%.⁶<br/><br/>Therefore, strategies for developing nanomaterial-composite photoresists will advance the state-of-the-art in 2PP fabrication and will further contribute to tuning of the mechanical properties of the fabricated structures. Additionally, we believe that by using a nanocomposite approach, less chemically crosslinked polymer networks can be fabricated by 2PP, offering the possibility of fabricating soft hydrogel networks reinforced by nanofillers.<br/><br/>Hydrogel reinforcement through the inclusion of SiO₂ nanoparticles (SiNPs) have been achieved on the macroscale, for a range of hydrogel networks including alginate/polyacrylamide, where incorpration of ~9 wt% SiNPs in the polymer network resulted in an increase in the Young's modulus of over 600%, and an increase in the fracture stress by ~300%.⁷<br/><br/>Herein we employ SiNPs for the mechanical reinforcement of hydrogels fabricated via TPP. SiNPs are synthesized via the Stober method, which is the ammonia catalysed reaction of tetraethylorthosilicate (TEOS) with water in low molecular weight alcohols. SiNPs of sizes ranging from 100 nm to 750 nm were synthesized by varying the catalyst. Following successful purification and analysis, SiNPs were included in photoresists for 2PP based on poly(ethylene glycol) phenyl ether acrylate. SiNPs of the various sizes were incorporated in concentrations from 5 to 15 wt% in the photoresists. AFM measurements of 4 μm cylinder structures were carried out to obtain the Young's modulus in the dry and hydrated states.<br/><br/>Future investigations focus on expanding the library of 2PP-fabricated hydrogel materials reinforced via the approach presented herein, to allow for the realisation of functional and responsive hydrogel actuators, of improved actuation reversibility and life-time.<br/><br/><br/>1.M. Carlotti and V. Mattoli, <i>Small</i>, 2019, 15, 1902687.<br/>2.C. Delaney, J. Qian, X. Zhang, R. Potyrailo, AL. Bradley, L. Florea, <i>Journal of Materials Chemistry C</i> 9, 2021, 11674-11678.<br/>3.A. Tudor, C. Delaney, ... L. Florea, Materials Today 21, 807-816.<br/>4.B. van der Sanden, L. Gredy, D. Wion, O. Stephan. <i>Acta Biomaterialia</i>, 2021, 130, 172-182.<br/>5.J. Purtov, A. Verch, P. Rogin and R. Hensel, <i>Microelectronic Engineering</i>, 2018, 194, 45-50.<br/>6.W. Xiong, Y. Liu, L. Jiang, Y. Zhou, D. Li, L. Jiang, J. Silvain, Y. Lu, <i>Advanced Materials</i>, 2016, 28, 2002-2009.<br/>7.X. Xu, S. Lü, C. Gao, X. Wang, X. Bai, N. Gao and M. Liu, <i>Chemical Engineering Journal</i>, 2014, 240, 331-337.<br/><br/>This research received funding from the European Research Council (ERC) Starting Grant (No. 802929 - ChemLife).