The aspects of drug loading also are covered by a number of reviews [6, 7, 250]. Drugs may be bound to the nanoparticles by a number of ways: (a) incorporation into the interior of nanocapsules, (b) incorporation into monolithic nanoparticles in form of a solid solution or solid dispersion , (c) covalent binding [13, 14, 193, 195, 197],
(d) electrostatic binding [48, 50, 52], and (e) surface adsorption [35, 254-256]. In most cases, the physicochemical properties of the drug determine the method of nanoparticle preparation and, hence, also the method of drug binding. Thermolabile drugs cannot be incorporated into nanoparticles produced involving heating. Hydrophilic drugs cannot be incorporated into hydrophobic nanoparticles without difficulties and vice versa. Consequently, in many cases drug loading is not sufficient.
Surface adsorption in general is governed by a Lang-muir type of interaction [35, 254-256]. Surfactants generally reduce adsorption. In some cases, however, they improve adsorption [59, 257].
The precise determination of the drug content in certain cases may be a problem because of the colloidal nature of the carrier . The problems involved have been reviewed in detail .
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