2.6. Mucoadhesion Study by Tensile Strength Method

Investigation of the mucoadhesive bond strength between the surfaces of the co-hydrogels and the surfaces of the artificial mucus and the pig small intestine was carried out using the tensiometric technique [14,26,27,28]. In a typical tensile experiment, the force necessary for detachment of the two surfaces is recorded as a function of elongation observed at the polymer–mucus interface. The stress is equal to the force divided by the total initial area, and the work of adhesion is calculated as the area under the force-elongation curves. Here, we compared the maximum detachment stresses (MDSs) (maximum forces required to separate the two surfaces divided by the areas) among the different compositions of the co-hydrogels with the mucosal surfaces. A TA.XTplus texture analyzer (Stable Micro Systems, Godalming, Surrey, UK) was used for doing the experiments and the Exponent software from Stable Micro Systems was used for analyzing the data. For the mucoadhesion study with the mucus made of the porcine stomach mucin Type II [42], a super strong waterproof black adhesive double-sided foam mounting tape was used. A piece of 25 mm × 25 mm of the double-sided tape was placed on the center of the platform (bottom), and then covered with a 25 mm × 25 mm filter paper. Also, a piece of 10 mm × 10 mm double-sided tape was adhered to the center of the round probe (top) to which 10 mm × 10 mm filter paper was mounted (see Figure S1 in the Supplementary Information.) A 30 µL co-hydrogel sample was placed onto the 10 mm × 10 mm filter paper, and a 185 µL mucus sample was applied onto the 25 mm × 25 mm filter paper. The samples were allowed to homogenize for 5 min in the filter papers before running each test. For the mucoadhesion study with the pig small intestine, a 25 mm × 25 mm pig small intestine tissue with internal surface (the surface covered with mucosal layer) up was placed on the surface of the texture analyzer’s platform [14,43]. A zinc plated cut washer with 3/8 inches in inner-diameter was placed on top of the small intestine sample to secure the pig small intestine tissue sticking on the platform surface (see Figure S2). A 0.5 inch in diameter double sided tape was adhered to the center of the TA-10 probe. The bottom side of the double sided tape was covered with a 0.5 inch in diameter filter paper, which was then soaked with the co-hydrogel sample (see Figure S2). When running the tensile strength experiment, once a trigger force of 150 g was detected on the surface of the sample, the probe proceeded to compress the sample until a 250 g force was reached. The force is then held for 60 s to let the co-hydrogels have enough contact time with the mucus surface. The probe then withdrew to a maximum distance of 11 mm above the sample. The test of each sample was repeated at least 4 times to ensure repeatability.