Drugs can be entrapped in a polymer matrix, encapsulated in a nanoparticle core, surrounded by a shell-like polymer membrane, chemically conjugated to the polymer, or bound to the particle's surface by adsorption. The method of drug loading can be divided into two classes: (1) incorporation of the drugs during the formulation of polymeric nanoparticles and (2) absorption of the drugs into nanoparticles after the formulation of the particles. It was shown that the absorption method to load drugs after forming polymeric nanoparticles has lower loading efficiency than incorporating the drugs during the process of forming nanoparticles (Alenso et al. 1991; Ueda et al. 1998).
In the incorporation method, several factors can affect drug-loading efficiency. The concentration of monomer and the concentration of drugs can both determine the drug entrapment efficiency. For example, the drug loading of DOX into poly(butylcyanoacrylate) (PBCA) nanoparticles was shown to strongly depend on the concentration of DOX (Yang et al. 2000). Increasing the concentration of DOX from 5 to 20% resulted in a decrease in the entrapment efficiency from 24.9% to 10.5%.
In the absorption method, the surface properties of nanoparticles (such as hydro-philicity) have a strong impact on drug loading. These surface properties, in turn, are functions of the properties of the polymers themselves and the methods used during preparation.
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