ABSTRACT
Therefore, the concept of using pH as a trigger to release a drug at the intestinal absorptive site is based on varying pH down the GI tract. Gastric-irritating or pH and protease labile protein therapeutics can be protected by the collapsed/compact gel matrix in gastric environment (pH ~ 1.2) while being released in the intestinal lumens (pH ~ 7.8) through matrix swelling or erosion (Ariel, 2009). In fact, all of the pH-sensitive polymers contain pendant acidic or basic groups that either accept or release protons in response to changes in environmental pH (Lim and Sun, 1981). The onset of pH-responsive swelling is controlled by the ionization of the pendant groups, which change water activity of the gel network near the pKa or pKb, where the ionic pendants increase
the hydrophilicity of the polymer and generate an electrostatic repulsion between the polymer chains, leading to the swelling of the polymer network (Ariel, 2009). The swelling of the hydrogel increases as the external pH increases in the case of weakly acidic (anionic) groups but decreases if the polymer contains weakly basic (cationic) groups. These polymers with a large number of ionizable groups are known as polyelectrolytes.
