Laponite-Enabled Freeform Manufacturing of Tough Hydrogels with Colorimetric pH Sensitivity

Hydrogels are ideal sensor platforms in aqueous media. Colorimetric assays deliver information intuitively; preventing chromophore leaching has been a challenge in aqueous media. Direct ink writing is a versatile free-form manufacturing method for hydrogels; a generalizable formula to enable three-dimensional (3D) printability can be impactful. In this study, a dual-network hydrogel of polyacrylamide and alginate is 3D printed by incorporating Laponite as a universal aqueous rheological modifier. Copolymerization of methacrylated phenol red with polyacrylamide enables colorimetric response to pH, ranging from yellow (pH = 5) to fuchsia (pH = 10). Rheological analysis of the precursor solutions reveals that a critical quantity of Laponite to achieve substantial shear thinning behavior exists at 4–4.5 wt %, which is consistent with a pervaded volume-based model. X-ray diffraction confirms the formation of 5-layer thick Laponite aggregates when their content exceeds the critical quantity. For 3D printing, 8 wt % of Laponite provides the optimal shear thinning and viscosity at rest for the best print fidelity. Strong anisotropicity in printed strands leads to a contrast in the mechanical properties with respect to the printing directions. Optimized printing produces hydrogels that can stretch 18 times their original length. Our colorimetric pH sensing hydrogel showcases a scalable freeform manufacturing-enabled platform technology with chemically bound chromophores.