ABSTRACT
This chapter describes time-controlled systems, especially external signal-responsive drug release. Drugs reputedly insoluble in water can be dissolved and temporarily trapped in the lipophilic microphase. Because of the pH-dependent destabilization occurring through an all-or-none cooperative mechanism, instantaneous release can be obtained within a very narrow pH range. Liposomes have been applied as safe and effective carriers for drug delivery purposes, responsiveness toward exogenous stimuli being a fundamental characteristic. Redox-sensitive or electrochemical control of drug delivery has been investigated. Ion movement through membranes containing fixed ionic sites is dependent on the nature and number of such charged sites; changing these factors will affect the membrane ionic resistance. Drug permeation through thermosensitive hydrogels was also investigated by Bae et al. They considered that there are two classes of thermosensitive hydrogels, classified according to the origin of thermosensitivity in aqueous swelling.
