Stimuli-responsive polymers are a special class of smart polymeric materials that can undergo a reversible or irreversible change in their physical, chemical, or structural properties when exposed to a small change in an external stimulus. These polymers are also called stimulus-responsive polymers, intelligent polymers, or smart polymers. The stimulus may be a change in temperature, pH, light, electric field, magnetic field, ionic strength, solvent composition, pressure, or the presence of specific chemical or biological molecules. Their ability to sense environmental changes and respond accordingly makes them important materials in modern chemistry, medicine, biotechnology, separation science, sensors, coatings, and controlled-release systems. Ordinary polymers generally retain their properties under a wide range of environmental conditions, whereas stimuli-responsive polymers are designed so that their polymer chains interact strongly with changes in their surroundings. A stimulus can alter the interactions between polymer chains and the surrounding medium. As a result, the polymer may undergo changes such as swelling, shrinking, dissolution, precipitation, aggregation, changes in permeability, changes in optical properties, or changes in mechanical strength. The response may occur because of changes in hydrogen bonding, electrostatic interactions, hydrophobic interactions, or other molecular forces. The response of a stimuli-responsive polymer can be classified according to the type of stimulus. Temperature-responsive polymers respond to changes in temperature. Their solubility or hydration changes at a particular temperature, causing the polymer to expand, contract, dissolve, or precipitate. pH-responsive polymers contain acidic or basic groups that can gain or lose protons when the pH changes. This changes the charge density of the polymer and consequently its swelling and solubility. Light-responsive polymers contain photoactive groups that undergo structural or chemical changes when exposed to a particular wavelength of light. Electric- and magnetic-field-responsive polymers can change their structure or dimensions when subjected to electrical or magnetic fields. Some polymers are also responsive to specific ions, enzymes, glucose, or other biomolecules. An important characteristic of stimuli-responsive polymers is that their response is usually highly sensitive to a relatively small environmental change. For example, a temperature-sensitive polymer may remain soluble