*Lipid and Polymeric Nanocapsules DOI: http://dx.doi.org/10.5772/intechopen.103906*

cells of the tumoral tissue, due to the natural tumoral barrier. There are nanosystems that can be pre-programmed to alter their structure and release the encapsulated molecule more effectively. In this case, molecular sensors are incorporated which respond to physiological or biological stimuli, such as changes in pH, redox potential, or enzymes. Drug release can occur via passive systemic targeting or active receptor targeting. Plasmids of DNA, si-RNA, and other therapeutic nucleic acids can be carried [1]. The development of pH-sensitive drug-delivery systems for the selective release of anticancer drugs is promising, given that healthy tissue has a pH of 7.4, the average extracellular pH in tumoral tissue is 6.8, and the pH of intracell components such as endosomes and lysosomes varies from 4.5 to 6.5. The cause of this difference is that the high metabolic rate required for tumor growth provokes hypoxia in the tumoral region. Therefore, the specificity of delivery systems for low pH levels is a suitable strategy to improve chemotherapy effectiveness, on one side, and reduce cytotoxicity levels, on the other. Most systems proposed for this kind of strategy are organic, but they still present some inconveniences such as low biocompatibility, complex manufacture, and limited drug release indexes [53]. In the case of melanoma therapy, lipid nanocapsules prepared from N-vinylpyrrolidone and vinyl imidazole showed pH-responsive ability and improved drug entrance into the tumoral cells. The copolymers were inserted into the surface of the nanocapsules, and they particularly changed from neutral charge at physiological pH to positive charge in acid conditions [114]. Besides, polyurea/polyurethane nanocapsules displaying pH-synchronized amphoteric properties were proposed. Such properties facilitate their accumulation and their selectivity for acidic tissues, such as tumoral tissues [115]. Stimuliresponsive multi-layered nanocapsules were also prepared with Eudragit®, chitosan, sodium alginate, and poly-L-arginine. They were loaded with curcumin and delivery was studied under similar pH conditions to those of the gastrointestinal tract. These nanocapsules were observed to shield the compound from being released in the stomach and allow it to be released in the intestine [116]. A LBL nanocapsule was obtained from hyaluronic acid functionalized with anionic azobenzene co-assembled with cationic poly diallyl dimethylammonium chloride. It is a novel UV-induced (365 nm) decomposable nanocapsule. Its size enables it to cross biological barriers, permits a prolonged circulation in the blood, and improves accumulation in the tumor. Later, it can be eliminated after UV-induced dissociation. Similarly, a nanocapsule was prepared with the anionic alginate-azo and cationic chitosan, and the anticancer drug doxorubicin was loaded onto these nanocapsules [117, 118].
