ABSTRACT
All hydrogels have the same fundamental property of swelling in the presence of water and deswelling (or shrinking) in the absence of water. Some hydrogels have additional properties, such as swelling or shrinking, in response to changes in environmental conditions. The presence of thermodynamically active functional groups on polymer chains makes the hydrogels sensitive to certain stimulants, or environmental factors (8). These hydrogels with additional functions are collectively called ‘‘environment-sensitive’’ hydrogels or ‘‘stimuli-responsive’’ hydrogels. Compared with ordinary hydrogels without such additional properties, environment-sensitive hydrogels are more advanced, and for this reason they are also called ‘‘smart’’ or ‘‘intelligent’’ hydrogels. One of the unique properties of these types of hydrogels is that they change their swelling ratio rather abruptly upon small changes in environmental factors. This abrupt volume change is known as volume transition. If the volume transition occurs to make the volume smaller, it is called ‘‘volume collapse.’’ The volume transition of smart hydrogels can occur by only a small change in environmental conditions, such as pH, temperature, ionic strength, electric field, magnetic field, ultrasound, solvents, electrolytes, external stress, light, pressure, specific molecules, or enzymes (812). It is this unique property of smart hydrogels that
makes them useful in various areas, such as pharmaceutical, biomedical, and biotechnological applications.
