A review on swelling theories of pH-sensitive hydrogels

The pH-sensitive hydrogels are a unique class of three-dimensional hydrophilic polymers containing ionic pendant groups on their polymeric network. In response to the external pH variation, these materials span the volume of an aqueous/biological solution and swell without dissolution. Nowadays, the pH-sensitive hydrogels have attracted the growing interest of many researchers in various fields of researches such as drug delivery, tissue engineering, soft actuators, etc. which makes the simulation of their behavior crucial for optimum utilization. The essential prerequisite for the simulation of these materials is the swelling theory. In this article, we present a review of the fundamental phenomena in swelling theories of the pH-sensitive hydrogels. We also classify the swelling theories into two groups including the monophasic and multiphasic theories. Then, we briefly examine the most highlighted swelling theories in each group. The experimental data is essential for material parameters estimation as well as verification of the constitutive equations. Thus, we introduce and discuss the most highlighted experiments on the swelling and deswelling behavior of the pH-sensitive hydrogels as well.