Release of the drug in response to those systems results from stimuli-induced changes in the gels or in the micelles, which may deswell, swell or erode SB203580 HCC in response to the respective stimuli. In these systems, the drug is released after stimulation by any biological factor, like temperature or any other chemical stimuli.25,26 The mechanisms of drug release include ejection of the drug from the gel as the fluid phase syneresis out, drug diffusion along a concentration gradient, electrophoresis of charged drugs toward an oppositely charged electrode and liberation of the entrapped drug as the gel or micelle complex erodes. There has been much interest in the development of a stimuli-sensitive delivery system that releases therapeutic agents in the presence of a specific enzyme or protein.
These systems are considered excellent delivery candidates, since they can be modified according to the task to be achieved. They are further classified into: Thermoresponsive pulsatile release Hydrogels that undergo reversible volume changes in response to changes in temperature are known as thermosensitive gels. Thermosensitive hydrogels have been investigated as possible drug delivery carriers for stimuli-responsive drug delivery. Hydrogels are crosslinked networks of biological, synthetic or semi-synthetic polymers. These gels shrink at a transition temperature that is related to the lower critical solution temperature (LCST) of the linear polymer from which the gel is made. One of the common characteristics of temperature-sensitive polymers is the presence of hydrophobic groups, such as methyl, ethyl and propyl groups.
From the many temperature-sensitive polymers, poly(N-isopropylacrylamide) (PINPAm) is probably the most extensively used. PINPA cross-linked gels have shown thermoresponsive, discontinuous swelling/deswelling phases, swelling, for example, at temperatures below 32��C and shrinking above this temperature. Krezanoski et al. describe the use of the reversed thermal gelation (RTG) system, consisting of a polyol polymer, such as Pluronic?. Gels of this type of polymer display low viscosity at ambient temperature, and exhibit a sharp increase in viscosity as the temperature rises. Yuk et al. developed temperature-sensitive drug delivery systems utilizing an admixture of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (F-68) and poly vinyl alcohol (PVA).
28 The pulsatile release of acetaminophen occurred due to pulsatile change in temperature between 35��C and 40��C.29 Chemical stimuli-induced pulsatile GSK-3 release The development of stimuli-sensitive delivery systems has been the latest topic of interest. These systems release therapeutic agents in the presence of any biological factor, like enzyme, pH or any other chemical stimuli. One prominent application of this technology has been the development of a system that can automatically release insulin in response to elevated blood glucose levels. Kazunori et al.