Abstract The past half of a century has seen increasing attention focused on the development of polymer nanoparticle based drug delivery systems as being capable of improving the efficacy and obviating the systemic side effects of a wide range of antineoplastic drugs in human cancer therapy. This has led to the emergence of the multifunctional ‘smart’ nanoparticles of carrying the protective polymer capping, targeting ligand, diagnostic label, stimuli-sensitive segments, image contrast agent, and other demanding functional groups. In this article we introduce the basic knowledge and general features of polymer nanoparticles for the purpose of drug delivery. In addition, various types of polymer nanoparticles used in drug delivery systems are presented. The criteria for the development of the required polymer nanoparticles are also summarized.
Keywords Polymer nanoparticles; drug delivery; stimuli responsiveness; controlled release; biodegradable polymer; liposome.
The essence of ”nano-” science and technology is based on the finding that the properties of materials over the size range of 1-100 nm differ from those of the bulk material. The unique properties of these various types of intentionally produced nanomaterials provide them with novel electrical, catalytic, magnetic, mechanical, thermal, and imaging features that are highly desirable for applications in commercial, medical, military, and environmental sectors. These materials may also find their way into more complex nanostructures and systems. As new uses for materials with these special properties are identified, the number of products containing such nanomaterials and their possible applications continues to grow.
In the fields of molecular biology and medicine, cancer has been the leading cause of death and a serious threat to the body health of human beings. Until now, the main techniques to fight the cancer are non-targeted chemotherapy and radiation. However, it is unavoidable to prevent systemic side effects to the human body due to non-specific uptake by normal, healthy, noncancerous tissues because of the instinctive properties of the chemotherapy chemical agent featured with high toxicity and a lack of tumor specificity.
In order to overcome the limitations of free chemotherapeutic agents, targeting of tumors with nanoparticulate drug carriers has received much attention and expectance [1-2]. Nanocarriers can offer many avenues over free drugs for the following aspects : (1) protect the drug from premature degradation; (2) prevent drugs from prematurely interacting with the biological environment; (3) enhance absorption of the drugs into a selected tissue (for example, solid tumour); (4) control the pharmacokinetic and drug tissue distribution profile; and (5) improve intracellular penetration.
Let us first recall the short but rapid development history of drug delivery systems (Figure 1). The first nanotechnology drug delivery system lipid was found in the 1960s, later known as liposomes . After that, biomaterials made of a variety of organic and inorganic substances were developed for drug delivery. In 1976, the first controlled release polymer drug delivery system was reported . In 1980, pH stimuli drug delivery systems to trigger drug release  and cell specific targeting of liposomes were reported [7-8]. In 1987, the first long circulating liposome named ‘stealth …
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