6.1.1　Introduction of nano-drug carriers

Nanoparticles（NPs）have been widely used as carriers for nano-drug delivery system.Drug molecules could be loaded inside or on the surface of carriers by physical adsorption，encapsulation or chemical bonding.Nano-drug delivery system is applicable to various administration routes including vein，muscles，skin，mucous membrane，oral，etc.It allows effective control of drug release rate，improving the biological membrane permeability，altering the body distribution，and elevating bioavailability，etc.Since the 1990s，with the rise and development of nanotechnology，nano-drug loading technology developed rapidly.The birth of nano-drug carrier is possibly because of the insolubility of about half of the drugs.This problem can be overcome by using nanocarriers，because most NPs can well disperse in water.Furthermore，the stability of the active drugs could be improved when the drugs are loaded inside the carriers.Moreover，it enables slow-release of drugs，which can prolong the duration of action and reduce dosing frequency of drugs.In addition，the drugloaded NPs can enter target cells through endocytosis，thus improving the utilization of drugs.

Since the first nano-drug delivery system was reported in 1995，more than 50 systems are currently under clinical investigation，and the number is increasing.Improving the efficacy and reducing side effects of drugs are particularly required in clinical applications.Therefore，developing nano-drug carriers for targeted delivery and controlled release of drugs is one of the hot spots in biomedicine fields.Targeted delivery could be achieved by modifying carriers with specific substances or functional groups，and the release rate is controllable by employing physical，chemical and biological interactions.Such nano-drug carriers are helpful for improving pharmacokinetic parameters of drugs.In addition，diagnosis reagents，detection reagents and treatment drugs could be loaded in the same carrier，which could be used as a promising therapeutic agent.(https://www.daowen.com)