WEB 2-photon lithography of photoresist/cellulose nanocrystal nanocomposites for tunable mechanical metamaterials
Microlattices are an emerging class of metamaterials combining desirable properties like high specific strength and impact energy absorption. Two-photon polymerization is a promising nanofabrication technique that allows producing metamaterials up to the millimeter scale with a submicron resolution. Here, we aim at improving the mechanical properties, i.e. stiffness, strength, and hardening modulus, of photoresists used for 2-photon lithography by mixing them with cellulose nanocrystals (CNC). Cellulose is a highly abundant biopolymer and is mostly found in the cell walls of plants. CNC are crystalline nanofibres produced by a treatment of plant pulp. They are transparent to light in the visible and near-infrared range, possess outstanding mechanical properties, and can be modified chemically to add functionality. They are therefore a highly attractive additive for tuning properties of photoresists.
Cellulose nanocrystals produced by sulfuric acid hydrolysis of wood pulp were suspended in an organic solvent and mixed with a commercial photoresist (IP-S, Nanoscribe GmbH, Germany) in different concentrations (4.5, 13, and 20wt%). Process parameters were identified that allow reliably producing structures by 2-photon lithography. It was found that the addition of nanocellulose leads to increased light scattering resulting in a resolution of 2µm. Micropillars (diam. 13µm, aspect ratio 3) and microtensile specimens (gauge section 4x8x20µm) with varying cellulose concentrations were prepared and tested as a function of strain rate (0.001 to 10/s). It was found that the presence of CNC significantly increased stiffness, strength and hardening modulus (30% for 4.5wt% and 70% for 13wt% CNC) under compressive loading. For tensile experiments, only small differences were observed, possibly due to orientation of the CNC normal to the loading direction. Finally, cellular structures were produced and tested mechanically to demonstrate the ability to print metamaterials using the new class of CNC nanocomposites. Further work is ongoing to measure and better control nanofiber orientation.
This study demonstrates the feasibility of using cellulose nanocrystals for tuning the material properties of photoresists. This allows producing microlattices with improved mechanical properties and opens up the potential to reach so far unpopulated regions in property space.
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