Biomedical Engineering Reference
In-Depth Information
positive ions move into the polymer matrix from the surrounding electrolyte to maintain charge
neutrality, causing the polymer to swell. However, the geometrical changes are not linear with
voltage and can be explained by the cyclic voltammetry data. The reduction peaks occur after the
voltage reaches
0.8 V,
the expansion reaches its near maximum value, corresponding to a drop in current in the cyclic
voltammetry curve as reduction of the nanowires is nearly complete.
It was observed that the time response of the nanoactuators depended on their position in the
sample. The nanowires at the edge tended to respond much faster than those in the middle of the
sample, with response times (oxidation to reduction and back to oxidized state) ranging from 2 s for
the edge to more than 40 s for the middle of the sample (determined from measurements of current
versus time). Since the response time is diffusion limited and defi ned by the position of a nanowire
on the sample, we can conclude that the mass transport of sodium ions from the solution to the
nanowires (as opposed to diffusion of ions inside the PPy) and the current density achieved at an
individual nanowire are the limiting factors in our experiments. The switching speed of an isolated
nanowire with no close neighbors is much faster than 2 s as the concentration of ions and the current
density are maximized, which is examined further in this section.
The PPy(DBS) nanowires retain actuation behavior of fi lms prepared from the same mate-
rial; however, some important changes can be seen. The vertical expansion (normal to electrode
surface) of the fi lms can be as high as 30% of the thickness of the fi lm, but the same mode of
expansion in nanowires produces displacements of only
−
0.6 V with the attendant increase in expansion at the same voltage. After
−
3% of nanowire length. It is not likely
that this change is due to the damage the nanowires sustained during dissolution of alumina, since
the nanowires display very good electrochemical redox behavior. A more likely reason for this
difference is that the PPy(DBS) nanowires do not retain the same internal morphology as the
PPy(DBS) fi lms. As reported in the literature, PPy fi lms synthesized with surfactant dopants such
as DBS
−
display a columnar structure oriented perpendicular to the conducting seed substrate
where the nucleation and subsequent growth of the polymer fi lm starts.
24
In this case, a template
of nanosized pores causes the polymerization process to occur in a somewhat different manner.
It has been mentioned earlier that the polymer nanowires appear as nanotubes when viewed from
above with a SEM. Then the top portion of these 40 µm long nanowires is removed by mechanical
lapping to investigate whether the hollow center region extends to the conducting seed layer. It was
found that only the top 1-2 µm portion is, in fact, a nanotube while the remaining length is a solid
nanowire. This result suggests that the polymer growth starts both on the metal seed layer and on
the pore walls, which is quite different from the case of the fi lm, which grows in only one direction.
The difference in polymerization conditions induced by these very different geometries is likely the
reason behind the different actuation performance of PPy(DBS) fi lms and nanowires. This conclu-
sion is further supported by the difference in redox-induced geometric changes of PPy(DBS)
microactuators when compared with fi lms of similar thickness as described in Section 13.3.
Together, the evidence from fabrication of micro- and nano-PPy structures led to undertake a
closer examination of the morphological changes accompanying different synthesis conditions
of PPy(DBS) devices.
∼
13.4.3 P
OLYPYRROLE
N
ANOWIRE
M
ORPHOLOGY
PPy fi lms doped with DBS undergo anisotropic volume change when undergoing electrochemical
redox reactions. The anisotropy is signifi cant: PPy(DBS) fi lms undergo lateral length change (along
the plane of the fi lm) of 2-3%, while the thickness change (normal to the plane of the fi lm) can be as
high as 30%.
2,16
The authors hypothesized that the large relative difference in expansion modes can
be attributed to an anisotropic morphology of the PPy(DBS) fi lms. X-ray diffraction (XRD) data
in the literature supports this hypothesis: PPy fi lms doped with
n
-alkyl sulfonate ions have a strong
scattering peak at low-diffraction angles (2°
<
2
θ
<
5°) in addition to the broader diffraction peak
at larger angles (15
°
<
2
θ
<
25
°
) that is characteristic of the distance between pyrrole units in the
