Biomedical Engineering Reference
In-Depth Information
anisotropy, as the sodium ions and water molecules that enter the polymer matrix will tend to follow
the layered structure of the fi lm.
From the data shown in Figure 13.22 for PPy(DBS) nanowires, it can be readily seen that
the small angle peak is absent while the broad diffraction due to pyrrole-pyrrole spacing is still
present. Therefore, it must be concluded that the nanowires do not possess the layered plane
structure in the
z
-direction (along the nanowire axis, or growth direction) that the PPy(DBS)
fi lms possess. The reason for this conclusion was not readily apparent, since the electropoly-
merization conditions were identical for both the fi lms and the nanowires but must be related to
the nanoscale geometry. A possible cause involves surface interactions during polymerization
between the precipitating PPy chains and the walls of the alumina pore, which disrupt the for-
mation of the layered plane structure. Some evidence for this mechanism can be gathered from
Figure 13.17B, where the mechanical lapping of the top portion of the nanowires gives some
idea of the nanowire cross-section. It can be seen that the cross-section is not homogeneous
but consists of two phases—the circumference and the center regions of the nanowire, support-
ing the assumption that the surface interactions with the alumina pore walls affect nanowire
morphology.
The internal morphology of the PPy(DBS) material has a strong effect on the anisotropy of
the volume change and therefore on the magnitude of the volume change in a given direction.
The reversible expansion of the PPy fi lms in lateral direction, where there is no alternating plane
structure, is approximately 3% of the original size. This expansion is closely mirrored by 3% revers-
ible length change of PPy(DBS) nanowires, which similarly lack layered plane structure in this
direction as evidenced by x-ray diffraction experiments. It can be inferred from this data that the
smaller, than expected, actuation magnitude of PPy nanowires is due to the morphology change of
PPy when synthesized in nanowire geometry.
13.4.4 T
IME
R
ESPONSE
OF
I
SOLATED
N
ANOWIRES
13.4.4.1
Actuation Speed in High-Density Nanowires
The speed of nanowire actuation, as observed with optical microscopy, varied depending on the
position of the nanowires in the sample. Specifi cally, the nanowires at the edges of the sample
responded signifi cantly faster (2 s) than those closer to the middle of the sample (30-40 s).
The likely reason for the slower actuation is the impeded ion diffusion in the nanowires with close
neighbors. As seen in the SEM images, the number of nanowires per unit area or nanowire density
is quite high, and for the nanowire in the middle of the sample, the ion diffusion proceeds from top
to bottom rather than from the nanowire wall to the center. Thus, the characteristic diffusion length
is not 100 nm radius of the nanowire, but rather 43 µm length of the nanowire. The signifi cantly
longer ion diffusion slows down the actuation speed (and current response) of the nanowires and
makes the determination of true time constant of single nanowire diffi cult.
13.4.4.2
Fabrication of Isolated (Low-Density) Nanowires
To measure the time response of the nanowires unimpeded by negative effects of high density,
a novel fabrication method was employed to increase the space between the nanowires. An
alumina membrane with 20 nm pore size was used as a template for Au electrode electroplating
(Figure 13.23).
As the fi rst step in the fabrication of electrodes, a suspension of 0.5 µm polystyrene beads in
water was deposited onto the alumina membrane. The membrane was then air-dried, and a 50 nm
fi lm of SiO
2
was sputtered onto the side of the membrane with the microbeads. The beads were then
removed by ultrasonication followed by sputter deposition of 10 nm Ti and 200 nm Au Ti lms to form
a conductive seed layer for electroplating. The SiO
2
layer completely covered the 20 nm pores, pre-
venting access of the electroplating solution to the seed layer below. On the other hand, the removal
