Biomedical Engineering Reference
In-Depth Information
core. Mevalonolactone (ML) is a bifunctional monomer having a cyclic lactone
structure and hydroxyl groups, and can act as both monomer and initiating group.
The author tried to synthesize hyperbranched PLA by copolymerization of ML with
LA (Fig.
6c
)[
175
]. Some branched PLA could be obtained, but the molecular
weights of the polymers were not so high because of low activity of ML.
3 Microspheres and Nanospheres
Micro- or nanosized polymer particles are generally called microspheres (MSs) or
nanospheres (NSs), respectively, and have been used for DDS. The term “nanoparti-
cle” 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.
MSs and NSs of aliphatic polyesters are generally prepared by emulsion-solvent
evaporation methods. MSs containing lipophilic drugs are prepared by oil in water
(o/w) emulsion methods. The polymer solution containing lipophilic drugs in
organic solvent (which is not miscible with water and has relatively low boiling
point, typically dichloromethane) was poured into a larger amount of water or buffer
solution with vigorous stirring and/or sonication. The obtained emulsion was
evaporated to remove organic solvent. After centrifugation, washing and drying,
the MSs containing drugs would be obtained. MSs containing hydrophilic (water-
soluble) drugs can be prepared by a water in oil in water (w/o/w) double emulsion
method (Fig.
7
). A small amount of aqueous solution containing water-soluble drugs
was added to organic solvent containing polymer with vigorous stirring and/or
sonication to give the primary w/o emulsion. The obtained primary emulsion was
D
rug aqueous
solution
Stirring or
sonication
Polymer solution
(organic solvent)
Primary emulsion
Water or buffer
surfactant
evaporation
Centrifuge
washing
Stirring or
sonication
Fig. 7 Preparation of water-soluble drug-loaded microspheres using w/o/w emulsion methods
