Laponite-Enabled\nFreeform Manufacturing of Tough Hydrogels\nwith Colorimetric pH Sensitivity

Hydrogels are ideal sensor platforms in aqueous media.\nColorimetric\nassays deliver information intuitively; preventing chromophore leaching\nhas been a challenge in aqueous media. Direct ink writing is a versatile\nfree-form manufacturing method for hydrogels; a generalizable formula\nto enable three-dimensional (3D) printability can be impactful. In\nthis study, a dual-network hydrogel of polyacrylamide and alginate\nis 3D printed by incorporating Laponite as a universal aqueous rheological\nmodifier. Copolymerization of methacrylated phenol red with polyacrylamide\nenables colorimetric response to pH, ranging from yellow (pH = 5)\nto fuchsia (pH = 10). Rheological analysis of the precursor solutions\nreveals that a critical quantity of Laponite to achieve substantial\nshear thinning behavior exists at 4–4.5 wt %, which is consistent\nwith a pervaded volume-based model. X-ray diffraction confirms the\nformation of 5-layer thick Laponite aggregates when their content\nexceeds the critical quantity. For 3D printing, 8 wt % of Laponite\nprovides the optimal shear thinning and viscosity at rest for the\nbest print fidelity. Strong anisotropicity in printed strands leads\nto a contrast in the mechanical properties with respect to the printing\ndirections. Optimized printing produces hydrogels that can stretch\n18 times their original length. Our colorimetric pH sensing hydrogel\nshowcases a scalable freeform manufacturing-enabled platform technology\nwith chemically bound chromophores.