Biomedical Engineering Reference
In-Depth Information
(a)
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Figure 4.4
Electron microscopy images and DLS data of NCs: (a) liposomes loaded with
monomers; (b) liposomes containing polymer NCs, after polymerization; (c) polymer NCs,
after lipid removal. (d) SEm image of polymer NCs, after lipid removal, hydrolysis, precipita-
tion, multiple washings, resuspension, and freeze-drying. (Adapted with permission from Ref.
[13]. © Wiley.)
surfactants and polymer, and perhaps from the ability of the bilayer to rearrange to
accommodate the growing polymer.
The monomer-to-surfactant and monomer-to-cross-linker ratios play a large role
in determining the structural rigidity and permeability of NCs [17]. Figure 4.5 shows
TEm and DLS data for experiments varying these ratios. Higher monomer-to-lipid
ratio resulted in stable spherical NCs (Fig. 4.5a). Lowering the amount of monomers
and cross-linkers in the bilayer resulted in capsules that appeared collapsed upon
drying (Fig. 4.5b-d). Lowering the fraction of the cross-linker produced structures
with larger diameter, consistent with the adoption of a “pita bread” structure upon
drying (Fig. 4.5e). In the absence of cross-linkers, no NCs formed (Fig. 4.5f).
Permeability studies showed expected structure/performance relationship—
lowering the fraction of the cross-linker rendered capsule shells more permeable.
Variations in the amount of monomers and/or cross-linkers in the bilayer offer a likely
explanation of previously reported variations in the structure of vesicle-templated
architectures. For example, the polymerization of hydrophobic monomers within the
bilayers of the surfactant dioctadecyldimethylammonium bromide (DODAB) was
described in a series of papers. In several previous reports, polymer product did
not resemble the vesicle structure, and usually, different hybrid morphologies were
obtained (Fig. 4.6) [11, 25, 26]. The polymerization of styrene (or styrene and butyl
methacrylate) with or without cross-linked produced the so-called parachute
architecture or “necklace” morphology, which was characterized by dense polymer
beads that are attached to a pure surfactant vesicle bilayer. The copolymerization of
styrene with a small amount of polymerizable surfactant gave a “wrapped parachute”
or “matrioshka” structure, wherein each parachute vesicle-polymer hybrid is encap-
sulated in a unilamellar vesicle. These types of architectures were formed probably
because of phase separation of the polymer matrix from the bilayers and additionally
because of insufficient amount of monomers in bilayer as discussed earlier [17].
Other methods for the synthesis of hollow NCs include the formation of polymer-
somes [27-31], LBL assembly [32-39], and cross-linking of supramolecular spherical
assemblies (Fig. 4.7) [40-46]. By using different types of synthetic amphiphiles
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