Enhancing the Mechanical Properties of 3D-Printed Waterborne Polyurethane-Urea and Cellulose Nanocrystal Scaffolds through Crosslinking

In this work, shape-customized scaffolds based on waterborne polyurethane-urea (WBPUU) were prepared via the combination of direct ink writing 3D-printing and freeze-drying techniques. To improve the printing performance of the ink and guarantee a good shape fidelity of the scaffold, cellulose nanocrystals (CNC) were added during the synthesis of the WBPUU and some of the printed constructs were immersed in CaCl2 prior to the freeze-drying process to promote ionic crosslinking between calcium ions and the polyurethane. The results showed that apart from allowing the ink to be successfully printed, obtaining scaffolds with good shape fidelity, the addition of the CNC resulted in a greater homogeneity of the porous structure as well as an increase of the swelling capacity of the scaffolds. Additionally, the CNC has a reinforcement effect in the printed systems, presenting a higher compression modulus as the CNC content increases. In the case of samples crosslinked by calcium ions, a rigid shell was observed by scanning electron microscopy, which resulted in stiffer scaffolds that presented a lower water absorption capacity as well as an enhancement of the thermal stability. These results showed the potential of this type of post-printing process to tune the mechanical properties of the scaffold, thus widening the potential of this type of material.

Automated summary

In this study, shape-customized scaffolds based on waterborne polyurethane-urea (WBPUU) were prepared using direct ink writing 3D-printing and freeze-drying techniques. The addition of cellulose nanocrystals (CNC) during the synthesis of WBPUU and the immersion of printed constructs in CaCl2 prior to freeze-drying promoted ionic crosslinking and enhanced the printing performance and shape fidelity of the scaffolds. The CNC addition resulted in a more homogeneous porous structure and increased swelling capacity of the scaffolds. The CNC also reinforced the printed systems, increasing the compression modulus. Crosslinked samples exhibited a rigid shell, leading to stiffer scaffolds with lower water absorption and improved thermal stability. These findings demonstrate the potential of post-printing processes for tuning the mechanical properties and widening the applications of these scaffolds.