pH sensitive or pH responsive polymers are materials which will respond to the changes in the pH of the surrounding medium by varying their dimensions. Materials may swell, collapse, or change depending on the pH of their environment. This behavior is exhibited due to the presence of certain functional groups in the polymer chain. pH-sensitive materials can be either acidic or basic, responding to either basic or acidic pH values. These polymers can be designed with many different architectures for different applications. Key uses of pH sensitive polymers are controlled drug delivery systems, biomimetics, micromechanical systems, separation processes, and surface functionalization. #Types pH sensitive polymers can be broken into two categories: those with acidic groups (such as -COOH and -SO3H) and those with basic groups (-NH2). The mechanism of response is the same for both, only the stimulus varies. The general form of the polymer is a backbone with functional "pendant groups" that hang off of it. When these functional groups become ionized in certain pH levels, they acquire a charge (+/-). Repulsions between like charges cause the polymers to change shape. Polyacids Polyacids, also known as anionic polymers, are polymers that have acidic groups. Examples of acidic functional groups include carboxylic acids (-COOH), sulfonic acids (-SO3H), phosphonic acids, and boronic acids. Polyacids accept protons at low pH values. At higher pH values, they deprotonate and become negatively charged. The negative charges create a repulsion that causes the polymer to swell. This swelling behavior is observed when the pH is greater than the pKa of the polymer. Examples include polymethyl methacrylate polymers (pharmacologyonline 1 (2011)152-164) and cellulose acetate phthalate. Polybases Polybases are the basic equivalent of polyacids and are also known as cationic polymers. They accept protons at low pH like polyacids do, but they then become positively charged. In contrast, at higher pH values they are neutral. Swelling behavior is seen when the pH is less than the pKa of the polymer. Natural polymers Although many sources talk about synthetic pH sensitive polymers, natural polymers can also display pH-responsive behavior. Examples include chitosan, hyaluronic acid, alginic acid and dextran. Chitosan, a frequently used example, is cationic. Since DNA is negatively charged, DNA could be attached to chitosan as a way to deliver genes to cells. Alginic acid, on the other hand, is anionic. It is often evaluated as a calcium-salt for drug delivery applications(International journal of biological macromolecules 75