Colorimetric pH Sensing and 3D Printable Hydrogels with Laponite^® as Universal Rheology Modifier—A Value-Added Supplement for e-Skins and e-Textiles

Hydrogels contain water that often incorporates various solutes; unless hydrogels are encapsulated by a barrier, the solutes in the hydrogels reach an equilibrium with the environment. The most attractive feature of hydrogels is that the aqueous environment becomes a substantial part of the material itself, while the hydrogels maintain their mechanical consistency with a physical boundary. For these reasons, hydrogel is an ideal sensor platform in aqueous media. Colorimetric assays deliver information intuitively; keeping chromophores free from leaching has been a challenge in aqueous media. Direct ink writing is a versatile freeform manufacturing method for hydrogels; a generalizable formula to enable 3D printability can be impactful.<br/>In this study, a dual network hydrogel of polyacrylamide and alginate with a backbone-incorporated modified pH sensing chromophore, methacrylated phenol red (MAPR), is printed by incorporating Laponite^® as a universal rheological modifier. Here, a co-polymerization of MAPR with polyacrylamide enables colorimetric response to pH, ranging from yellow (pH=4) to fuchsia (pH=10) without leaching of the chromophore. Rheological analysis of the precursor solutions reveals that a critical content of Laponite^® to achieve substantial shear thinning behaviour to be , 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 content. For printing, 8 wt% of Laponite^® provides the optimal shear thinning and viscosity at rest for the best printing fidelity. While strong anisotropicity in printed strands leads to a contrast in mechanical properties with respect to the printing directions, optimized printing produces hydrogels that stretches 18 times the original length. This paves a way to incorporate structure based toughening mechanism by optimizing the raster direction of 3D printing.<br/>Our 3D printed colorimetric pH sensing hydrogels combine sensitivity, intuitive data interpretation, mechanical strength, and structural integrity to operate in various aqueous environments while having arbitrary shapes to address various needs in environmental, underwater, and healthcare applications. We believe that our colorimetric hydrogel platform wherein the chromophore with environmental sensitivity is covalently integrated in the main chain of the polymeric backbones provides a robust and versatile platform to incorporate multifunctional sensitivity. These colorimetric hydrogel sensors can seamlessly blend into e-skins, e-textiles, actuators, coatings, optics, electronics, and water harvesters for boundless future applications<br/>* Rayan Basodan and Ozge Zengin Akca contributed equally to this work.