Biomedical Engineering Reference
In-Depth Information
MWCNTs are the strongest and stiffest materials, which have been discovered in term
of tensile, elastic modulus, inertness to react with other chemical material, as well as bio-
compatibility. These characteristics endow CNTs a candidate for cells (Section 4.5.1), DNA
(Section 4.5.2), and enzyme (Section 4.5.3) detection. Another critical property of CNTs is
the surface/volume ratio and the ability to fabricate on other polymer and alloy as a cata-
lyst (Section 4.5.4). Meanwhile, graphene, which has similar characters as CNTs, has been
considered as a proper material in fabricating electrode in lithium ion batteries (Section
4.5.5) and supercapacitor (Section 4.5.6). The family of amorphous carbon-based films
include a wide range of films having different properties, such as DLC films, which are
very hard and, under certain environmental conditions, might have friction coefficients in
the 10
−2
range that makes them excellent candidates for coating (Section 4.5.7) the moving
parts of orthopedic implants.
Cells Detection
The electrical conductivity and impedance of biological materials have been investigated
over the past century using conventional electrodes [152]. Recently, with the advance of
nanotechnology, nanoelectrodes based on electrochemistry method can offer advan-
tages such as reduced double-layer capacitance, fast convergence to a steady-state signal,
enhanced current density arising from increased mass transport at the working elec-
trode interface, low detection limits, and improved signal-to-noise ratios. Electrochemical
impedance has been utilized for human body measurement to cells detection. In 2007,
Yun et al. [153] reported on nanoelectrode array embedded into a polydimethylsioxane
channel to distinguish LNCaP (lymph node carcinoma of the prostate) prostate cancer
cells. In this report, 8-mm-tall high-density MWCNTs arrays were synthesized by water-
assisted CVD. The gold-coated MWCNTs were then fabricated on 5-mm-square Fe/Al
2
O
3
/
SiO
2
/Si substrate, and the substrates were embedded into a polydimethylsioxane channel.
Each tower (1 × 1 mm) of the patterned array contained about 50 million nanotubes. The
nanotubes' average diameter was 20 nm, and the aspect ratio was 200,000. The surface
area of each tower (average diameter: 20 nm and length: 8 mm MWCNTs) was 2500 mm
2
.
Electrochemical impedance characterization of nanotubes electrodes was measured under
a fluidic channel with different solutions and LNCaP prostate cancer cells.
From the phase and magnitude plot, the phase angle is different between the HBSS and
the LNCaP prostate cells, which is more capacitive with the increase in incubation time. In
Figure 4.29, the Nyquist plot shows that the diameter of the semicircle at high-frequency
increases obviously with incubation time. The EIS equivalent circuit was given by Randles
model with a CPE. As the HBSS is only a one-layer capacitance, the equivalent circuit of
LNCaP prostate cell was given by [
R
u
(
C
cl
R
cl
)
Z
CPE
], where
C
cl
and
R
cl
are the cell capacitance
and resistance. Extracted parameters were CPE = 0
.
134
μ
S,
α
=
0
.
83, and
R
u
= 950
Ω
. A
higher value of
α
was indicative of the closeness of the CPE to an ideal capacitor (
α
= 1). The
author assumed that the reason was because the LNCaP cells formed a tight contact and
a cover to the electrodes, and then blocked the ion movement. Extracted parameters show
that both
C
cl
and
R
cl
increased after incubation time increased. Since the solution resis-
tance represents the bulk properties of the electrolyte solution and the LNCaP cells settled
on the electrode's surface, solution resistance does not change much. On the other hand,
the cell layer capacitance
C
cl
and cell layer resistance
R
cl
affect the interface property of
the electrode-electrolyte due to insulating or coating the surface by LNCaP cells. The EIS
results showed that the MWCNTs electrode could characterize different solutions, which
suggested that its applications as a cell-based biosensor and gold functionalized nanotube
