Biomedical Engineering Reference
In-Depth Information
(A)
(B)
FIGURE 13.19
SEM image (A) shows the lapped and polished surface of alumina with top ends of PPy
nanowires even with the surface; image (B) shows a similar sample after electrochemical reduction cycle in
NaDBS electrolyte, with nanowires visibly expanding with the infl ux of sodium ions. Scale bars represent a
distance of 2 µm.
CE and an Ag/AgCl REF to complete the three-electrode electrochemical cell. A voltage of
1 V
versus the REF was then applied between WE and CE to induce reduction of PPy accompanied by
the movement of sodium ions, which in turn resulted in the expansion of nanowires. The membrane
was then taken out of solution, dried, and imaged with SEM (Figure 13.19B). The PPy nanowires
were clearly protruding from alumina surface as the result of ion infl ux and polymer swelling.
Unfortunately, it was very hard to quantitatively evaluate the actuation with this method, since the
sample had to be taken out of the electrochemical cell and dried to image the surface. However, it
provided clear qualitative evidence of nanowire actuation.
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13.4.2.3
Real-Time Optical Microscopy of Nanowire Actuation
Two real-time experiments were carried out using an optical microscope to detect reversible actua-
tion of nanowires in both lateral and normal directions relative to electrode surface.
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First, we
dissolved the alumina membrane, leaving a 5 mm
5 mm gold fi lm covered with PPy nanowires.
The fi lm was attached at one end to a substrate and mounted so that the cross-section could be
observed with an inverted optical microscope (Figure 13.18, sample 2). The assembly was then
immersed in electrolyte solution, and the electrodes were connected as before. Upon cycling the
WE voltage between 0 and
×
1 V relative to the REF, the composite fi lm bent away and toward the
substrate, respectively (Figure 13.20). Since PPy(DBS) contracts when oxidized (0 V) and expands
when reduced (
−
1 V), the fi lm's behavior is consistent with a bimorph interaction between PPy
nanowires and gold. This effect is somewhat surprising since nanowires are discrete; however, it
seems that there is enough interaction between individual nanowires to behave similar to homoge-
neous PPy fi lms.
To detect whether the nanowires could also expand or contract in the direction normal to the
gold fi lm (along the nanowire length), the gold/PPy nanowire fi lm was completely attached to the
substrate (Figure 13.18, sample 3). A thin layer of gold was then sputtered on top of the PPy nanow-
ires to make optical measurement more precise. The substrate was subsequently sectioned and
mounted in such a manner that the cross-section was visible in the objective of the microscope.
The ends of the nanowires were clearly marked by the original gold seed layer on one side and the
sputtered thin gold layer on the other side (Figure 13.21 inset).
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