Biomedical Engineering Reference
In-Depth Information
amine with surfactant-protected molecules that results in the cross-linking of the
polymer chains around the NMs that renders them water soluble.
19
In another
process, biomolecules and phospholipid micelles have been covalently linked
with semiconductor QDs for application in ultrasensitive biological detection.
Polyethylene glycol (PEG) is a polymer widely used for various applications
including encapsulation of NMs (
Figure 3.3
) due to its hydrophilicity and low
antigenicity. In addition to steric stabilization of superparamagnetic iron oxide
nanoparticles (SPIONs), PEG prevents plasma opsonization as well as uptake by
macrophages, thereby increasing SPION circulation time in vivo. PEG-coated SPI-
ONs are efficiently internalized by cells via fluid-phase endocytosis and through
amphiphilic affinity to lipid bilayers on plasma membranes.
94
. These qualities,
however, increase their potential to overload cells with iron and become toxic.
95
Polyvinyl alcohol (PVA) is another organic polymer used as a coating for
SPIONs. It has excellent film-forming, emulsifying, and adhesive properties. The
use of PVA for intravenous (i.v.) delivery is limited due to poor persistence as well
as agglomeration in a ferrofluid. However, when cross-linked to form a magnetic
gel, PVA can be used as a vitreous eye substitute because it is a highly biocompat-
ible material.
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Potentially, when used with SPIONs, this biocompatibility can be
utilized in targeted drug delivery, tissue engineering, and biosensor technology.
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Thioctic acid and PEGs have been used in simple esterification schemes,
followed by reduction of the 1,2-dithiolane to synthesize a series of PEG-
terminated dihydrolipoic acid (DHLA-PEG) as capping substrates.
24,97
The cap
exchange reaction of TOP/TOPO-capped QDs with these substrates produced
water-soluble nanocrystals that are stable over extended periods of time. These
were also stable over a broad pH range, from weakly acidic to basic conditions
(pH 5-12) but these ligands lacked specific functional end groups for easy imple-
mentation of simple conjugation techniques, such as avidin-biotin binding. Uyeda's
FIGURE 3.3
TEM of PEG-encapsulated 30-nm diameter IOMNPs.
Courtesy of Ocean NanoTech.
