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of SCKs, protecting the biopolymer from enzymatic digestion and possibly allowing
for gene therapy applications (Thurmond et al. 1999). In another study, the shells of
SCKs were functionalized with an oligomeric peptide sequence known to bind with
certain cells (Liu et al. 2001). In vitro experiments demonstrated that nanoparticles
without the conjugated peptide did not associate with the cells. In contrast, functio-
nalized particles aggregated at cell surfaces and some were taken into cells,
suggesting potential for targeted delivery applications. Further work has shown
that shell-immobilized biotin (Qi et al. 2004) and mannose (Joralemon et al. 2004)
can be used by SCKs as targeting ligands for cells. Similar approaches have shown
promise in delivering an antibacterial peptide to cells in vitro (Becker et al. 2005)
as well as in creating artificial viruses with antigen-coated SCKs (Joralemon et al.
2005). Although high levels of nanoparticles can cause cell death, lower levels are
well tolerated and do not cause death or large-scale morphological changes in
mice (Becker et al. 2004). Wooley's groups has also conjugated SCKs with known
cancer cell ligands in the hopes of targeted delivery of antitumor drugs (Pan et al.
2003), and “clickable” SCKs promise custom core and/or shell functionalization
of premade nanoparticles (O'Reilly et al. 2005). In vivo results have yet to be reported
for most of these formulations, however, making it difficult to reach conclusions
regarding the ultimate usefulness of the SCK approach at this time.
8.5. CONCLUSION
Polymeric capsule preparation is achieved by self-assembly of small molecules, poly-
mers, and particles into nanometer- and micron-sized objects. Despite the age of the
field and number of materials produced thus far, researchers have only just begun to
tap the full potential of polymeric capsules that are partially illustrated here with
examples of catalytic capsules and drug-delivery vehicles.
We predict that polymeric capsules will be produced with more precision through the
use of milli-, micro-, and nanofluidics. Next-generation capsules will be synthesized
with greater chemical complexity, leading to more effective responses to stimuli,
time, and their environments. Complex capsules will also act as microreactors, allowing
chemists to synthesize small molecules with higher efficiency than current methods
provide. To meet these expectations, more groups worldwide must dedicate themselves
to creating, assembling, and testing new polymeric capsules.
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