Biomedical Engineering Reference
In-Depth Information
surface. However, as seen in Figure 13.24B, one end of the nanowires remains attached to the
gold conductive layer underneath and thus providing the electrical connection for electrochemical
experiments.
13.4.4.3 Time Response
Apart from reducing the density of nanowires per unit area, the fabrication methodology outlined in
Section 13.4.4.2 has an important advantage of coating the conducting seed layer with an insulating fi lm
of SiO
2
in the areas with no nanowire growth. In subsequent electrochemical experiments, the double-
layer capacitance is, therefore, limited only to the nanowires. If the remaining areas of Au conductive
layer were left exposed to the solution, the current needed to charge up this large capacitor would have
been far larger than the current due to the electrochemical response of the nanowires, and this expense
would also making precise measurements of nanowire electrochemical performance diffi cult.
The electrochemical characterization of isolated PPy nanowires is shown in Figure 13.25. Well-
defi ned characteristic oxidation and reduction peaks can be seen on the cyclic voltammetry graph
(Figure 13.25B), verifying that the current is indeed due to PPy nanowires undergoing electro-
chemical redox reaction. The chronoamperometric curve (Figure 13.25A) shows the time response
of isolated PPy nanowires. The current reaches 90% of its fi nal value in 0.5 s or less, showing a
signifi cantly faster electrochemically controlled actuation compared with PPy fi lms or high-density
nanowires. In fact, this response time begins to approach the speed of biological nanoactuators such
as skeletal muscle protein bundles.
13.4.4.4
Theory and Discussion
As discussed in Section 13.2, there are several factors that determine the speed of actuation in PPy
devices. Two of the most important contributors to time delay seen between the application of the
voltage step and the completion of the electrochemically induced redox reactions in PPy matrix are
polymer chain conformational changes and diffusion of dopant ions and water molecules.
8
The con-
formational changes play a role in opening up the polymer structure for subsequent diffusion and are
responsible for the current peak seen in chronoamperograms of PPy fi lms doped with small ions. In
the case of PPy doped with DBS, this peak is weak, visible only as an infl ection in the doping chro-
noamperogram of PPy(DBS) thick fi lms (Figure 13.6) but largely absent in chronoamperograms of
both thin fi lms and nanowires. Therefore, it seems reasonable to assume that diffusion, including the
parallel diffusion of electronic charge, counterions, and water,
28
is the major factor affecting the time
response of small-scale PPy(DBS) devices. Diffusion time depends heavily on the device geometry
(shape and dimensions), and analytical expressions can be derived from Fick's second law of mass
2.5
V
=
−
1 V
100
2.0
50
1.5
1.0
0
0.5
V
=
0
V
0.0
−
50
−
0.5
1.0
−
−
100
V
= 0 V
−
1.5
−
150
−
2.0
0.0
0.2
−
1.2
−
1.0
−
0.8
−
0.6
−
0.4
−
0.2
−
1.0
−
0.5
0.0
0.5
1.0
1.5
2.0
(A)
Time (s)
(B)
Voltage (V)
FIGURE 13.25
Electrochemical experiments on isolated nanowires. (A) Chronoamperometry or current
response versus time to voltage steps, indicated by arrows, is shown. (B) Cyclic voltammetry or current
response to a linear voltage ramp at 200 mV/s is shown.
