Biomedical Engineering Reference
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microspheres was densely coated with the HAp nanoparticles. From the
transmission electron microscope (TEM) image of an ultra-thin cross sec-
tion, it was clarifi ed that poorly-crystalized HAp nanoparticles are pres-
ent only on the PLLA microsphere surfaces and a shell having a thickness
of 40-120 nm (corresponding to the thickness of one to three layers of HAp
nanoparticles) is formed. These core-shell nanocomposite microspheres
showed dispersion stability in an aqueous medium for more than 1 year
of storage at 4
C. Furthermore, it was possible to fabricate core-shell
microspheres with a particle size of a few micrometers to a few hundred
micrometers by controlling the HAp concentration in aqueous media, the
biodegradable polymer concentration in CH 2 Cl 2 , and the homogeniza-
tion rate during emulsion preparation. In addition, we succeeded in the
fabrication of fl uorescent microspheres by the introduction of oil-soluble
fl uorescent chemicals into the oil phase during the Pickering emulsion
formation.
°
9.3.3
Fabrication of Multihollow HAp-Biodegradable Polymer
Nanocomposite Microspheres
Fabrication of nanocomposite microspheres having a multihollow struc-
ture is also possible by the Pickering emulsion method, in which the oil is
evaporated from a multiple emulsion (Figure 9.8) [55]. In the formation of
a multiple emulsion using conventional molecular-level emulsifi ers, two
or more kinds of emulsifi ers having different HLB values are necessary.
Likewise, in the case of a Pickering emulsion, two or more kinds of parti-
cles having different surface hydrophilicity/hydrophobicity are necessary
to obtain multiple emulsions. Binks et al. prepared water-in-oil-in-water
(W/O/W) type Pickering emulsion using two kinds of silica particles
whose surface hydrophilicity/hydrophobicity is controlled by surface
Water
HAp nanoparticle
HAp nanoparticle
Evaporation
of oil
Water
domain
Water
domain
CH 2 Cl 2 solution of polymer
(oil phase)
Polymer
HAp nanoparticle-stabilized
water-in-oil-in-water multiple emulsion
(Pickering emulsion)
HAp nanoparticle-coated
multihollow polymer
microsphere
Figure 9.8 Fabrication of multihollow nanocomposite microspheres by
evaporation of oil from water-in-oil-in-water Pickering emulsion. Reprinted with
permission from [55]. Copyright © 2010 American Chemical Society.
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