6.4.4 Drug loading content and encapsulation effic...
The drug loading content and encapsulation efficiency are two important parameters to determine the drug delivery performance of polymer nano-drug carriers.Drug loading content refers to the mass fraction of drug in the polymer nano-drug，which can be calculated by equation 6-1：
where C represents the drug loading content，Mdrug and Mcarrier represent the mass of the drug and polymer nano-drug carrier，respectively.
Encapsulation efficiency refers to the ratio of drug loaded in the polymer nano-drug carrier to the total amount of the drug added during the preparation of the drug formulation，which can be calculated by equation 6-2：(https://www.daowen.com)
where EE is the encapsulation efficiency，Mdrug and Mtotal drug represent the mass of the drug in the polymer nano-drug carrier and the total mass of the added drug，respectively.
To measure the drug loading and encapsulation efficiency，we need to determine the mass of the polymer nano-drug carrier，loaded drug，and the total drug.Mass of polymer nano-drug should be determined since the yield of drug loading is usually less than 100％.The mass of nano-drug can be determined by a balance or thermogravimetric analysis after freeze-drying.To determine the concentration of drugs in solution，we can employ a standard curve method based on UV absorbance，fluorescence intensity，or high-performance liquid chromatography.
The amount of drugs in polymer nano-drug carriers can be determined by differential method，the complete release method，and the direct measurement method.The differential method calculates the loaded drug by the differences between the feeding and the remained drugs.For some drugs，we can utilize the fluorescence or absorption spectrum changes after being loaded in the polymer nano-drug carrier to check the loading efficiency.In more cases，methods such as high-speed or ultrafiltration centrifugation are used to separate nano-drug from free drugs and determine the unloaded drugs.However，the residual polymer nanodrug carrier in the supernatant，the absorption of the drug by the filter membrane，and the agglomeration and precipitation of the drug itself during the loading process will cause certain errors.We can also quantify the amount of drugs after completely destroying the carrier by adjusting the pH or dissolving in an organic solvent to promote drug release.The drug content can also be determined by the absorption peak directly if the drug has a characteristic absorption peak in the polymer nano-drug carrier.