In the simplest manner, the formation of hybrid hydrogels can be monitored with the naked eye. For instance, hydrogels lose their transparency and turned opaque white after introduction of liposomes (Figure ), CNCs or silica NPs. Nanoclays gave a whitish or yellowish coloration, carbon nanotube/liposome complexes (CLC) and silver-coated copper NPs (Ag@Cu NPs) resulted in a gray coloration (Figure ) whereas the hydrogels containing PEDOT NPs appeared dark blue. The presence of Au nano-objects (e.g., rods, particles), PDA NPs or PDA-coated HAP induced darkening of the dispersion (Figure ). The intensity of the color usually correlated with the NP content (Figure ), and a uniform hue indicated a homogeneous mixture implying the absence of macroscopic aggregation and/or caking. On the other hand, the gradations of purple color from the outer layer to the core of the PNIPAAm-catechol@Au hybrid hydrogel suggested a negative concentration gradient, with Au NPs predominantly accumulating near the surface (Figure ). Figure 14: (a) Blank (left) and liposome-loaded (right) CS hydrogels at 37 °C. Adapted with permission. Copyright 2018 Elsevier. (b) Photographs of pure Gel (left) and Ag@Cu NPscontaining Gel (right) hydrogels. Adapted with permission. Copyright 2021, Wiley-VCH.