This review article describes synthetic polymers used as drug carriers with a clear focus on the consideration "what are important factors in the appropriate choice of polymers for specified purposes of drug targeting". First, the review explains classifications, structures, and syntheses of polymers from the viewpoint of their applications for drug carriers. Second, the review describes four methodologies for polymeric drug-carrier designs, followed by brief introduction of three examples of the polymeric drug carriers.

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Biodegradable polymers, which can be degraded into non-toxic elements, metabolized, absorbed and excreted, are utilized as drug carriers and depots in drug delivery systems. They are frequently used in injectable formulations such as soluble nanoparticles, microspheres, and gelling system. In this chapter, we focus on polylactide and the copolymers as popular biodegradable polymers, and the developments of drug delivery system by hybridization of polylactides with some hydrophilic polymers are introduced.

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Thermoresponsive polymers are known to exhibit switchable soluble/insoluble properties across their phase transition temperatures in aqueous media. In the past quarter century, intelligent biomaterials have been molecularly designed by introduction of thermoresponsive polymers on/into material interfaces and bioactive molecules. In drug delivery fields, the thermoresponsive polymeric materials are also actively studied to enhance drug activity and reduce side effects of drugs. In this review, the author summarizes properties of various thermoresponsive polymers, and introduces the thermoresponsive polymeric materials for drug modulation and site-specific targeting, for example, intelligent hydrogels with thermally modulated drug release and nano-scaled drug carriers for cancer chemotherapy.

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Photoresponsive polymeric materials are promising not only for cell-targeting in vivo, but also for intracellular navigations such as the internalization, the endosomal escape, and the nuclear import. Furthermore, the controlled release as well as the activation of drugs is caused by the photoresponsive delivery. Finally, such photoresponsive polymeric system can pave the way for double targeting of drugs resulting in temporal and spatial delivery. The photoresponsive synchronization of biocompatible materials and novel photo-irradiating apparatus might bring a various happiness to the refractory patients in near future.

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To increase usefulness of liposomes as drug delivery systems, it is crucial to provide various functions and high performances to liposomes. Approach from polymer chemistry is an efficient method to develop liposomes with high functionalities and high performances. In fact, various types of functional liposomes have been constructed by introduction of polymers to liposomes. This review describes mainly our research on development functional liposomes using polymers with temperature- and pH-sensitivities and shows importance of polymer chemistry for development of functional liposomes.

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