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Chitin nanocrystal based aqueous inks for 3D printing via direct ink writing

Chitin nanocrystal (ChiNC), a kind of rod-like nanoparticle, exhibits a high elastic modulus, high mechanical strength, good biocompatibility, and biodegradability. Its aqueous dispersion behaves with a prominent shear thinning and concentration-related sol-gel transition characteristics, which has feasibility of extrusion-based 3D printing, that is, direct ink writing (DIW). Based on this, herein, ChiNC was chosen as matrix material to explore its printability via DIW. A kind of waterborne polycitrate prepolymer (pre-POFC) was introduced into the ChiNC aqueous dispersion inks. On the one hand, pre-POFC was as rheology modifier to tune the flow behavior of the inks; on the other hand, the cured POFC component was as adhesive agent to glue ChiNC nanoparticles, aiming to improve water-stability and remove brittleness of the ChiNC-based materials. The results showed the optimal loading ratio of POFC component was 30–50 wt% determined via water-stability tests. The rheological analyses showed that the ChiNC/pre-POFC aqueous inks exhibited a prominent shear thinning behavior, thixotropy, modulus recovery ability, and higher dynamical yield stress (σy), meaning such systems had a good DIW printability theoretically. Actually, DIW results showed that only ChiNC/pre-POFC-6/4–30wt% and ChiNC/pre-POFC-5/5–30wt% inks exhibited optimal printability, in consideration of minimal contraction and superior shape retention after dehydration and thermocuring. However, the mechanical properties of printed films were weaker than that of cast films due to the limited printing precision and lower density.