*3.5.4 Drug stability*

The stability of nanoparticles can be measured via visual inspection in closed tubes at both room temperature and elevated temperatures. Each month, the nanoparticles can be evaluated based on their phase separation, % transmittance, and globule size [7, 41]. Examples include PLGA nanoparticle dispersions [6] compared to PLGA microparticle dispersions [7] in phosphate-buffered saline (pH 7.4 at 37°C) to mimic the physiological environment. The nanoparticle suspension presented no phase separation, suggesting that these nanoparticles did not precipitate (**Figure 7a**). The microparticles could have a higher gravitational force relative to the nanoparticles, leading to their easy precipitation, which can be observed from the phase separation of the microparticle solution (**Figure 7b**). Therefore, drug administration of these microparticles needs to carry a "*shake well before use*" label [22].

## *3.5.5 Drug solubility*

Excess drug was added to the nanoparticles to optimize the formulation, similar to the optimization of the other ingredients. The nanoparticles were continuously stirred

#### **Figure 7.**

*SEM micrographs and digital photographs of PLGA nanoparticles (a) and PLGA microparticles (b) prepared via an emulsion process [6, 7].*

for 24 h at room temperature, and samples were removed and centrifuged at 600 rpm for 10 min at predetermined time points. The quantity of soluble drugs in the optimized nanoparticles was determined by calculating the amount of drug present in the residue and comparing it with the total amount of added drug. The individual ingredients were used for comparison of the solubility of the drug in the nanoparticles [7, 42].
