Biomedical Engineering Reference
In-Depth Information
cavity, Cicchetti et al.
[80]
exposed human gingival fibroblasts to 50, 75, 100, 125, 150
g/ml
SWCNTs for 24 h and investigated genotoxicity, cytotoxicity, oxidative stress, and stress response.
They found that SWCNTs produced genotoxic effects at all doses, but the two highest doses
induced a strong decrease of the cell proliferation and cell survival, causing apoptosis.
μ
18.6
Fabrication of carbon nanomaterials with improved
osteogenic bioactivity
Although the toxicity of carbon nanomaterials remains a controversial topic, researchers are still very
much interested in using them for bone regeneration due to their unique mechanical properties and
special enhancement on bone-related cells' response. How further to reduce their toxicity and increase
their osteogenic activity has long been an important issue for bone tissue engineering
[81]
.The
as-produced CNTs tend to entangle and bundle up. They are insoluble in most types of solvents; par-
ticularly, their poor dispersion in aqueous environment makes them difficult to be used in biological
systems
[82]
. As stated above, the CNTs aggregation might cause cytotoxicity. Besides, the residual
metal catalysts in pristine CNTs are another source of cytotoxicity. For applications as scaffolds for
bone regeneration, CNTs/CNFs should further support new bone growth and initiate apatite formation,
while pristine CNTs/CNFs have little functional groups and are hard to meet these demands. To inte-
grate CNTs/CNFs into biological systems, therefore, functionalization of CNTs/CNFs is necessary.
The most common functionalization strategy of CNTs is through oxidation. In this method,
CNTs are refluxed in strong acids (usually sulfuric acid and nitric acid) to bring some carboxylic
groups to the caps and sidewalls of CNTs through an oxidation process. The introduction of hydro-
philic carboxylic groups not only helps the stable dispersion of CNTs in aqueous solutions but also
provides bonding sites for other suitable chemicals
[14,17,18,83]
. Meanwhile, it allows better
capacity to attract calcium cations to enhance nucleation and formation of apatite in vitro and
in vivo, making CNTs bioactive and osteoconductive. CNFs can be chemically modified similarly
to CNTs, except that the reactions can only take place on fiber surface
[84]
. Calcium phosphate
(CaP) compounds are key structural materials of natural bones and teeth
[85]
. Among different
forms of CaP compound, HA and
-TCP) are the two most important and
well-known biomaterials used for bone substitution and reconstruction. They demonstrate high bio-
affinity owing to their chemical composition, crystal structure, and Ca/P ratio similar to apatite
found in the human skeletal system. Integrating CaP compounds into or onto CNT/CNF scaffolds,
possessing good mechanical properties, and excellent osteogenicity can be valuable for bone regen-
erative applications
[86]
.
β
-tricalcium phosphate (
β
18.6.1
Biomineralization
By different ways, HA/CNT composites have been prepared, such as electrophoretic deposition
[87]
, in situ CVD
[88]
, inclusion of CNTs in a CaP sol-gel matrix
[89]
, and biomineralization
[90]
.
Among them, biomineralization by immersing CNTs/CNFs in SBFs or applying an alternative
soaking process are simple, quick, and efficient, and by which apatite layers form on CNT/CNF
surface. The surface chemistry of CNTs/CNFs plays a vital role in these biomineralization
