Biomedical Engineering Reference
In-Depth Information
aspect ratio with diameters as small as 1 nm and length of several micrometers. CNTs made from a
single graphene sheet result in single-walled carbon nanotubes (SWCNT), while several graphene
sheets make up multiwalled carbon nanotubes (MWCNTs). Ever since their discovery in 1991 by
Iijima
[2]
, there has been intense interest in these allotropes of carbon due to their unique physical
and chemical properties and potential applications in a wide range of fields, from electronic devices
and sensors to nanocomposite materials of high strength and low weight. On the other hand, CNFs
are long, thin strands of material with diameters about 10
1000 nm, which are also mostly com-
posed of carbon atoms but bonded together in microscopic crystals and aligned parallel to the long
axis of the fiber. The crystal alignment makes the CNFs exceptionally strong for their size. These
two kinds of three-dimensional carbon nanomaterials are also viewed as a class of biomaterials
with high potential for biomedical applications
[3,4]
(
Figure 18.1
).
CNTs are well-ordered, hollow nanostructures consisting of carbon atoms bonded to each other
via sp
2
(hybrid orbital comes from 1 S and 2 P orbital) bonds, which are stronger than sp (hybrid
orbital comes from 1 S and 1 P orbital) and sp
3
(hybrid orbital comes from 1 S and 3 P orbital)
bonds. These features are exactly the key factors rendering CNTs excellent mechanical strength
and high electrical and thermal conductivity.
In a biomimetic viewpoint, the three-dimensional CNTs/CNFs resemble the nanofibrous net-
work of natural ECM. SWCNTs have an average diameter of 1.5 nm, and their length varies from
several hundred nanometers to several micrometers. In contrast, the diameter of MWCNTs typically
ranges between 10 and 100 nm
[8]
. These were envisioned to be good candidates for bone tissue
engineering. Studies have demonstrated that CNTs/CNFs were superior for bone regeneration due
to their osteoconductivity
[9]
. One of the underlying mechanisms might be the electrochemical
interactions between CNTs/CNFs and cells. A study had reported that electroconductive MWCNTs
were less cytotoxic as compared to MWCNTs
[10]
. Moreover, conductive MWCNTs affected signifi-
cantly the mitochondrial membrane polarity, the intracellular pH, and the reorganization of cytoskele-
ton actin filaments, and cell functions were strictly dependent on electrochemical interactions
[10]
.
Another study stated that CNTs/CNFs with cell favorable surface properties might promote
(A)
(B)
(C)
5 nm
500 nm
10 nm
FIGURE 18.1
(A) High-resolution TEM micrograph of a SWNT bundle, (B) TEM image of MWCNT, and (C) SEM image of CNFs.
Part (A) reproduced with permission from Ref.
[5]
, Part (B) reproduced with permission
from Ref.
[6]
, and Part (C) reproduced with permission from Ref.
[7]
