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. 2021 Dec 1;21(23):8049. doi: 10.3390/s21238049

Figure 4.

Figure 4

Colorimetric indicator. (i) Reversible changes of (a) color and (b) stopband with changing RH. (ii) (a) UV-vis spectra of the films under different RH. (b) Relationship between the stopband and RH. (iii) Scanning electron microscope (SEM) images of the fabricated hydrogel. P(St–MMA–AA) PC seen from the (a) top and (b) cross-view. PAAm–P(St–MMA–AA) observed from the (c) top and (d) cross-view. (iv) Microscope images of the hydrogel in its (a) dry state and (b) wet state. (c) Illustration of the structure change of hydrogel before and after fully wetted in water. Adapted with permission from ref. [11]. Copyright 2008 Royal Society of Chemistry.