3D Printing Beyond Conventional Rheological Limits: Two‐Step Crosslinking of Low‐Viscosity/Moduli Inks Into Tough Hydrogels

Abstract

A careful balance between extrusion flowability and rapid viscoelastic recovery is essential for direct ink writing (DIW). The ink must flow easily under shear during extrusion and quickly recover a solid‐like structure after extrusion to maintain the printed architecture. Conventional ionic crosslinking methods to tune the rheological features often lead to inhomogeneity, nozzle clogging, and printing failure. To overcome these challenges, we introduce a two‐step ionic crosslinking strategy for weakly structured graphene oxide (GO)/sodium alginate (SA) inks. First, to stabilize each printed layer of weak hydrogels while preventing nozzle clogging, each extruded layer is immediately sprayed with an ionic solution upon deposition to induce partial crosslinking. Second, the fully printed partially crosslinked scaffold is submerged in an ionic bath to complete the crosslinking. Using this method, flowing hydrogels are turned into tough or brittle 3D‐printed structures by methodically fine‐tuning materials’ qualities, affected by GO content, SA concentration, and crosslinker valency. Moreover, the effect of salt type on the hydrogels’ shape retention and printing quality, as well as on the morphological features of the resulting cryogels, is investigated. All in all, this study introduces a two‐step ionic crosslinking technique that separates printability from high rheological requirements to overcome complications in the DIW process.