Nanospheres hydrophobicity

Micro- or nanosized polymer particles are generally called microspheres (MSs) or nanospheres (NSs), respectively, and have been used for DDS. The term nanoparticle is more general and includes polymer micelles and nanogels, which are described in Sects. 4-6. Although polymer micelles and nanogels have sufficient surface hydrated layers for dispersion or solubilizaton in aqueous media, MSs and NSs are basically spherical particles of hydrophobic polymers without enough hydrated layers. 

Nanospheres of PLGA were prepared by emulsion-evaporation by employing homogenization and a solvent system consisting of acetone in which both the drug and polymer were dissolved [95], The encapsulation of 5FU in these nanospheres was low compared to the more hydrophobic drug, indomethacin. It was reported that the hydrophilic 5FU leaked to the aqueous phase. PGL nanoparticles were prepared by aldol condensation loading 5FU with an efficiency of 14.32% [50]. 

Because the forces of attraction prevail when molecules are brought into sufficiently dose proximity under normal conditions, release is best effected if both the strength of the interaction and the degree of contact are minimized. Aliphatic hydrocarbons and fluorocarbons achieve the former effect, finely divided solids the latter. Materials such as microcrystalline wax [64742 42-3] and hydrophobic silica [7631-86-9] combine both effects. Some authors refer to this combined effect as the ball bearing mechanism. A perfluoroalkylated fullerene nanosphere would perhaps be the ultimate example of this combined effect (17). These very general mechanistic remarks can be supplemented by publications on the mechanism of specific classes of release agents such as metallic stearates (18), fatty acids and fluorinated compounds (19), and silicone-coated rdease papers (20,21). The mechanism of release of certain problem adherents, eg, polyurethanes, has also been addressed (22,23). 

Block Copolymers are macromolecules which are composed of blocks usually in linear as it shown in Fig. 3.20, where it is illustrated a classical block copolymer. Main block copolymers are amphiphilic block copolymers having united hydrophilic blocks to hydrophobic blocks. Amphiphilic block copolymer have surfactant properties and form different kinds of associations, such as micelles, nanospheres, nanocapsules and polymersomes This tipe of association can act like excellent vehicles of several active principles. The composition, aggregate formation and the different applications of these materials have been reviewed [112], Figure 3.20 also illustrates the nanoparticulate drug delivery systems formed by amphiphilic block copolymers and their general characteristics. 

Rouzes, C., R. Gref, et al. (2000). Surface modification of poly(lactic acid) nanospheres using hydrophobically modified dextrans as stabilizers in an o/w emulsion/evaporation technique. J Biomed Mater Res 50(4) 557-65. 

Solvent precipitation techniques have been generally applied for hydrophobic polymers, except for dextran nanospheres. Several techniques described in the literature are based on the mechanism of polymer precipitation. 

In this technique, a hydrophobic polymer is dissolved in an organic solvent, such as chloroform, ethyl acetate, or methylene chloride and is emulsified in an aqueous phase containing a stabilizer (e.g., PVA). Just after formation of the nanoemulsion, the solvent diffuses to the external phase until saturation. The solvent molecules that reach the water-air interphase evaporate, which leads to continuous diffusion of the solvent molecules from the inner droplets of the emulsion to the external phase simultaneously, the precipitation of the polymer leads to the formation of nanospheres. The extraction of solvent from the nanodroplets to the external aqueous medium can be induced by adding an alcohol (e.g. isopropanol), thereby increasing the solubility of the organic solvent in the external phase. A purification step is required to assure the elimination of the surfactant in the preparation. This technique is most suitable for the encapsulation of lipophilic drugs, which can be dissolved in the polymer solution. 

Biocompatible and biodegradable PLG nanoparticles (80-150 nm) have been prepared by following the nanoprecipitation technique [33]. The nanoparticles were coated with a 5-10 nm thick layer of PPO-PEO block copolymer or with tetrafunctional (PEO-PPO)2N-CH2-CH2-N(PPO-PEO)2 [33]. Such coats are bound to the core of the nanosphere by hydrophobic interactions of the PPO chains, while the PEO chains protrude into the surrounding medium and form... 

Their hydrophobic/hydrophilic content seems to be just right for applications in cancer and gene therapies. Such nanospheres are prepared by dispersing the methylene chloride solution of the copolymer in water and allowing the solvent to evaporate [38]. By attaching biotin to the free hydroxy groups and complexa-tion with avidin, cell-specific delivery may be attained.NMR studies of such systems [39] revealed that the flexibility and mobility of the thus attached PEG chains is similar to that of the unattached PEG molecules dissolved in water. Re-... 

Lemos-Senna, E., Wouessidjewe, D., Lesieur, S., Puisieux,F., Couarrazze, G., and Duchene, D. (1998), Evaluation of the hydrophobic drug loading characteristics in nanoprecipitated amphiphilic cyclodextrins nanospheres, Pharm. Dev. Technol, 3,1-10. 

Phospholipid polymers having a 2-methacryloylox-yethyl phosphorylcholine (MPC) were investigated as a solubilizer for paclitaxel. The paclitaxel solubility was observed to increase up to 5.0mg/ml in the presence of a copolymer of MPC and Ai-butyl methacrylate (BMA), poly(MPC-co-BMA), with 70mol% of the BMA unit. The MPC polymer forms a polymer aggregate with the diameter of 23 nm, called a polymeric lipid nanosphere, in aqueous media by hydrophobic interaction, which may solubilize hydrophobic drugs. 

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