Fig. 6. (a) Images of individual device components before introduction into solutions of variable pH. The CAP coating is visible on the sampling aperture of each cap. (b) Images of assembled device inside solutions of different pH for different time lengths. Green solution simulates saliva (pH 7.0 for 1 minute), red solution simulates gastric acid (pH 3.0 for 3 hours), and blue solution simulates intestinal fluid (pH 7.0 for 3 hours). (c) Images of sampling aperture and hydrogel after exposure. As shown, the coatings of the caps associated with saliva and gastric acid remained intact, however, the enteric coating of the intestinal fluid fully dissolved which caused the hydrogel to absorb the blue dye. Scale bar 6 mm.