Below this temperature, a hydrophilic state changes to a hydrophobic state. The most important thermoresponsive polymer is poly( N-isopropylacrylamide) (PNIPAM), which undergoes a sharp coil-to-globule transition in water at 32 ☌. The value of the transition temperature may vary slightly depending on the experimental method used, as different stages of the gelation process may be measured by each technique.

Various methods to determine sol-gel transition or phase change have been known, including cloud point measurement, differential scanning calorimetry (DSC) and rheology. Phase or reversible conformational changes take place for thermosensitive polymers over small variations of temperature. The LCST is mainly dependent on the hydrogen bonding between water molecules and the structure of functional monomer units of polymers. This unique property is characteristic of polymers that dissolve when cooled and phase separate when heated above a certain temperature, known as a lower critical solution temperature (LCST). Most polymers are soluble in aqueous media when heated, but some water soluble polymers may precipitate from the solution upon heating. Importance of Thermoresponsive Poly(Vinyl Caprolactam)(PNVCL) The present review focuses mainly on the recent progress of the thermoresponsive behavior of PNVCL and their stimuli responsive gels for biomedical applications. The incorporation of thermoresponsive polymers with other responsive moieties can exhibit dual responsive properties. Temperature-responsive hydrogels especially in micro or nano forms show a volume phase transition at a specific temperature that causes a sudden change in the solvent state. The recent interest in these systems has focused on hydrogels in the form of macro-, micro-, and nanogels because of their promising applications in drug delivery systems, cell encapsulation and enzyme immobilization.

The drug can be released via a range of mechanisms triggered by various stimuli. Bioactive molecules can be trapped easily by an equilibrium swelling method or by simply mixing before the formation of hydrogels. In these systems, a polymeric matrix can protect a drug from hostile environments (such as low pH and enzymes), while controlling drug release when the gel structure is changing in response to environmental stimuli. Hydrogels have been used extensively in the development of smart delivery systems.

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