Control the Properties of Polymeric Nanoparticles

Along with various methods for forming polymeric nanoparticles is the ability to control the properties of the nanoparticles themselves. First, the polymer and its concentration used in forming nanoparticles strongly affect the particle properties. Besides different hydrophilicity and biocompatibility properties, polymers usually degrade at different rates leading to different drug release characteristics. The molecular weights of polymers have an inverted relation with the particle size, i.e., the smaller nanoparticles can be formulated from lower molecular weight polymers, but also have lower drug encapsulation efficiency in general (Hans and Lowman 2002). However, higher polymer concentrations and higher polymer molecular weights lead to higher encapsulation efficiency and larger sizes of the nanoparticles (Blanco and Alonso 1997; Song et al. 1997; Kwon et al. 2001). For example, Table 2 demonstrates the relationship between molecular weights and concentrations of PLGA on bovine serum albumin (BSA) capture efficiency where PLGA nanoparticles were prepared by emulsion solvent evaporation.

As seen in Table 2, the nanoparticles made from lower molecular weight PLGA or lower PLGA concentrations showed much lower BSA entrapment than those from higher molecular weight PLGA. Figure 10 demonstrates the effect of PLGA concentrations on the size of PLGA nanoparticles prepared by the emulsification-diffusion method using poly(vinyl alcohol) (PVA) as a stabilizer (Kwon et al. 2001). Larger size PLGA nanoparticles were formed with higher PLGA concentrations.

Second, the type and amount of surfactant/stabilizer are also factors that affect properties of nanoparticles as well as drug encapsulation characteristics. For example, phospholipids, a natural emulsifier, have been shown to improve flow and phagocytal properties of dipalmitoyl-phosphatidylcholine (DPPC) due to a more dense packing of DPPC molecules on the surface of the nanoparticles resulting in a smoother surface than particles made with the synthetic polymer, PVA (126). DPPC also improved the encapsulation efficiency compared to PVA using the emulsification solvent evaporation method. Another example is for the case of PLGA nanoparticles. Studies have shown that the PLGA nanoparticles were smaller when formulated using didodecyl dimethyl ammonium bromide (DMAB) than those prepared with PVA (Kwon et al. 2001). In another study, PLGA nanoparticles were formulated using PVA, chitosan, or PVA-chitosan blends as different stabilizers (Ravi et al. 2004). It was shown (Fig. 11) in this study that the sizes and morphologies of PLGA nanoparticles were different for different stabilizers.

Table 2 Effect of PLGA molecular weight (MW) and concentration on bovine serum albumin (BSA) incorporation (adapted with permission from (Song et al. 1997))

Concentration

Concentration

Efficiency of BSA

MW of PLGA

of PLGA (%)

of BSA (%)

capture (%)

58,000

3.0

10.0

24.8

58,000

6.0

14.0

36.8

10,2000

3.0

10.0

68.0

10,2000

6.0

14.0

74.8

PLGA concentration {% w/v)

Fig. 10 Effect of PLGA concentration on the mean particle size of PLGA nanoparticles (at the concentration of 2.5 (weigh/volume %) of poly(vinyl alcohol) (PVA)) (Reprinted with permission from (Kwon et al. 2001))

Fig. 11 Atomic force microscopy images of PLGA nanoparticles: (a) PLGA nanoparticles with PVA alone as a stabilizer, (b) with chitosan alone, and (c) with a PVA-chitosan blend. Bars represent 150 nm (Adapted with permission from (Ravi et al. 2004)

Third, the zeta potential of polymeric nanoparticles can be controlled. Zeta potential is a measure of the charge of the particle. The larger the absolute value of the zeta potential of the particle, the larger the amount of charge present on its surface. Zeta potential has a strong influence on the stability of nanoparticles as repulsive forces induced between high zeta potential particles lead to the formation of more stable particles with a more uniform size distribution. This stability is important in preventing aggregation. When a surface modification is added (like PEG), the negative zeta potential is lowered, increasing the nanoparticle stability (Vila et al. 2002).

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