EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Microlattices are an emerging class of metamaterials combining desirable mechanical properties like high specific strength, ductility and energy absorption. Two-photon lithography (TPL) is a promising nanofabrication technique that allows producing structures up to the millimeter scale with submicron resolution. Here, we aim at 3D printing and characterizing miniaturized structures at micrometer resolution using nanocellulose-based composites for improved mechanical performance. Cellulose is an abundant biopolymer mostly found in plant cell walls. CNC are produced by treatment of plant pulp and are crystalline nanofibres that are transparent to light in the visible and near-infrared range, possess outstanding mechanical properties, and can be modified chemically to add functionality. This makes them an attractive material for tuning material properties of microlattices.

CNC produced by sulfuric acid hydrolysis of wood pulp were suspended in an organic solvent and mixed with an acrylic-based commercial photoresist (IP-S, Nanoscribe, Germany) in different concentrations (4.5 and 13 wt%). Process parameters were identified that allow synthesizing structures by TPL. We found that high quality structures can be produced where the maximum resolution of 2µm is limited by additional light scattering caused by the CNCs. We synthesized micropillars (diameter 13 µm, aspect ratio 3) with varying CNC concentrations and compressed them using an in situ micro-indenter. We observed that the presence of CNC significantly increases stiffness, strength and hardening modulus (30% for 4.5wt% and 70% for 13wt% CNC) under compressive loading. Finally, microscale honeycomb and cellular structures were produced and tested mechanically to demonstrate the ability to print metamaterials with improved mechanical properties using this new class of CNC nanocomposites.

This study demonstrates the feasibility of using cellulose nanocrystals for synthesizing microlattices with improved mechanical properties and opens up the potential to reach so far unpopulated regions in material property space.