Biomedical Engineering Reference
In-Depth Information
elongation (Kater et al. 1988 ), it is likely that 4-HNE would also exhibit such action.
Indeed, neurons grown on MWCNTs coated with 4-HNE exhibited increased
neurite number, length, and branching when compared to neurons grown on bare
MWCNTs. This effect appeared to be mediated by 4-HNE, since increased growth
was not evident when neurons were grown on MWCNTs in which adsorption of
4-HNE was carried out in an excess of histidine that binds to 4-HNE (Uchida and
Stadtman 1992 ). Taken together, this study was the fi rst to show that both bare and
functionalized CNTs are biocompatible with neuronal survival and growth.
Furthermore, noncovalent modifi cations of CNTs with a biomolecule can be used to
modulate neurite outgrowth and neurite branching.
Coating CNTs with biological molecules using adsorption may not be a viable
option for generation of long-term scaffolds. Namely, while MWCNTs are inert
materials and are readily available for long-term exposure to neural tissue/cells,
biological compounds could degrade. Furthermore, a biologically active compound
applied by physical adsorption could simply dissociate and undergo diffusion, thus
effectively lowering its local concentration. Consequently, a more permanent mod-
ifi cation of CNTs by covalent linking of various stable chemical groups and mol-
ecules would be a desirable alternative functionalization strategy. Hu et al. ( 2004 )
provided evidence that neuronal growth can be modulated by covalent modifi ca-
tions made predominantly at the ends of MWCNTs to systematically vary their
surface charge. At the onset of the study, Hu et al. used the so called as-prepared
MWCNTs (AP-MWCNTs) that were produced by chemical vapor deposition
(CVD). These AP-MWCNTs were deposited onto glass coverslips precoated with
PEI and used as scaffolds/substrates for neuronal growth. Initially, neuronal cul-
tures, prepared from hippocampi of 0- to 2-day-old Sprague-Dawley rats, were
fi xed and examined using SEM. As expected, both PEI and AP-MWCNTs were
permissive substrates for neuronal growth, albeit neurite outgrowth was parsimoni-
ous on MWCNTs when compared to that of neurons grown on PEI (Fig. 1a and b ).
Neurons on CNTs grew at least 1 week in culture. These fi ndings are consistent
with previous work utilizing fi xed neuronal cultures (Mattson et al. 2000 ) . Similar
data was obtained when living neurons were studied using the vital stain calcein
and fl uorescence microscopy (Fig. 1c and d ). Again, neurons grown on positively
charged PEI outperformed neurons grown on presumably neutrally charged
AP-MWCNTs. However, since PEI and AP-MWCNTs also assume different topol-
ogy, any comparison in regard to the sole role of charge on neuronal growth could
not be made. To further investigate whether surface charge could be exploited as a
modulator of neuronal growth and neurite outgrowth, various functionalizations of
the MWCNT backbone were prepared. Here, AP-MWCNTs were refl uxed in nitric
acid to remove the metal catalyst residues; this procedure additionally led to uncap-
ping of CNTs and termination of their open ends with carboxyl groups resulting in
functionalized MWCNT-COOH. Further reaction of MWCNT-COOH with oxalyl
chloride resulted in the acyl chloride intermediate MWCNT-COCl, which was then
used for covalent attachment of either ethylenediamine (EN) or poly- m -aminoben-
zene-sulfonic acid (PABS) to produce two additional fi nal products: MWCNT-EN
and MWCNT-PABS. Thus, using this method, Hu et al. generated MWCNTs with
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