Biomedical Engineering Reference
In-Depth Information
processes. Various SBFs have been developed for biomineralization study. The conventional SBF
mimics the human blood plasma in the ion concentrations of Ca
2
1
, HPO
4
2
2
,Na
1
,Cl
2
,K
1
,Mg
2
1
,
and SO
2
2
, except with a significant deficiency in its HCO
3
concentration (4.2 mM). From this
conventional SBF, the SBF-induced biomimetic process is rather slow and takes normally up to a
few weeks to induce the nucleation of CaP mineralites. It has been known that the biomineraliza-
tion induced by SBF includes two steps of nucleation and particle growth. One way to accelerate
the nucleation is to use supersaturated SBFs (e.g., 1.5, 2.5, 5, and even 10 times SBF). Another
way is to use the surface functional groups, usually negative carbonyl groups, to attract calcium
cations and induce nucleation
[91]
.
Using a revised supersaturated SBF (r-SBF), Akasaka et al.
[92]
found that needle-like apatite
crystallites directly grew starting from the surface of HCl-purified MWCNTs after immersion for 2
weeks, considering the MWCNTs might be acting as core for initial crystallization of the apatites.
However, in this condition, the reproducibility of sizes and shapes of apatites formed on MWCNTs
were poor because the r-SBF was highly supersaturated and was difficult to handle. Instead of using
supersaturated SBF, Liao et al.
[93]
applied solutions of 0.5 M CaCl
2
and 0.5 M H
3
PO
4
(Ca/
P
1.66) to coat HCl-treated MWCNTs. They gradually added the solutions to MWCNT disper-
sions through separate tube pumps. In these procedures, fibril-like nHA polycrystals were formed
and oriented at a certain angle to the long axis of the CNTs. The defects were analogous to edge
dislocations along the surface of CNTs. Having served as the nucleation sites for nHA, these
defects had been functionalized principally into carboxylic groups. Aryal et al.
[94]
immersed car-
boxylated CNTs in conventional SBF for 7
5
21 days and found that MWCNT-COOH was capable
of nucleating HA crystals from SBF within 7 days, which resulted in the formation of hierarchy
assemblies. The large surface area of nanotubes enabled the interaction of SBF and carboxyl group
and the nucleation of HA initiated through the carboxyl group.
But some researchers thought CNTs-COOH itself are not efficient for inducing the formation of
apatite. They used the carbonyls on CNTs to do some additional modification. Xiao et al.
[95]
pre-
pared two types of prefunctionalized MWNTs, which were acid-oxidized MWCNTs and covalently
modified MWCNTs with PEG. The influences of the acid-oxidization duration, prephosphorylation,
and PEGylation of MWCNTs on in situ growth of HA were further investigated in SBF with ionic
concentrations of 2, 5, and 10 times, respectively, at 37
o
C for 24 h. The results exhibited that all these
factors had positive effects on the HA crystals growth; especially the PEGylation of MWCNTs
played a key role during the deposition (
Figure 18.8
). The binding affinity between the HA crystals
and the PEG-MWCNT surface was strong enough by the ionic and hydrogen bond interactions,
which was very helpful to improve the dispersion ability and biocompatibility of nanotubes. Yan
et al.
[96]
coated CNTs with polydopamine by a simple and feasible route, and then its in vitro bioac-
tivity and cytocompatibility was assessed by immersion study in SBF. As a result, it had been demon-
strated that the introduction of polydopamine coating could greatly enhance the bioactivity, which
was attributed to the good combination of catecholamine structure of polydopamine. Tan et al.
[97]
functionalized MWCNTs with surfactant sodium dodecyl sulfate (SDS) and conducted mineralization
by alternately dispersing the SDS-MWNTs into aqueous solutions of CaCl
2
and Na
2
HPO
4
each for
30 min. SDS could provide some negative charges as nucleation sites for HA on the tube surface.
Mineralization study on CNFs is relatively sparse. Wan et al.
[7]
prepared 3-D CNFs by carbon-
ization under inert conditions with 3-D bacterial cellulose nanofibers as starting carbon sources.
The resulting CNFs showed 3-D fibrous structural features with diameter ranging from 10 nm to
