Biomedical Engineering Reference
In-Depth Information
13.4 POLYPYRROLE NANODEVICES
13.4.1 I
NTRODUCTION
In recent years, intensifi ed effort was launched to create nanosized devices capable of carrying
out mechanical work. The work reported in the literature has ranged from top-down fabrication
of nanoelectromechanical actuators with ultraviolet or electron-beam lithography and scanning-
probe microscopy manipulation to bottom-up chemical synthesis of molecular machines and
DNA motors and harvesting natural biomotors.
17-19
Applications of successful nanomachines are
potentially numerous and far-reaching, including advances in computation and communications
through the use of rapidly switching nanoactuators coupled to mirrors or magnetic storage ele-
ments. Nanorobotic devices capable of operating in a fl uid environment could be used in biomedical
sciences and in health care, allowing small-scale manipulation of fl ows and particles. Some of the
problems encountered in fabrication and operation of nanomechanical devices include potential
for mass production, ease of control, and restrictions on operating environment. For example, the
use of electron-beam lithography is suitable for fabricating components one at a time, in a serial
fashion, and hence creating large numbers of nanoactuators with this method could be prohibitively
expensive. On the other hand, chemical synthesis or utilization of naturally occurring nanomotors
could allow simultaneous fabrication of large numbers of these devices; however, these motors can
be diffi cult to control and require very restricted operating environments.
In this section, a different approach to nanowire fabrication is described. Template synthesis
in nanoporous membranes can be used to create PPy nanowires
20
that can be operated as electri-
cally controlled nanoactuators in an aqueous fl uid environment. A template synthesis method has
been used to shrink the dimensions of a conducting polymer (PPy) actuator to nanoscale to simul-
taneously synthesize
10
8
(pore density of an anodized alumina template can range from 10
8
to
10
12
/cm
2
) artifi cial muscle nanowires capable of electrically controlled reversible expansion and
contraction. The fabrication methodology, performance evaluation, and characterization of PPy
nanoactuators are described in this section (13.4) in detail.
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13.4.2 P
OLYPYRROLE
N
ANOWIRE
E
LECTROPOLYMERIZATION
AND
E
VALUATION
OF
THE
E
LECTROCHEMICALLY
C
ONTROLLED
V
OLUME
C
HANGE
13.4.2.1 Removal of Hollow Region of Synthesized Polypyrrole
Nanowires by Mechanical Lapping
PPy fi lms can be synthesized on a conducting surface by electropolymerization, and various
dopant ions can be incorporated into the polymer altering many of its properties. In template
synthesis of PPy(DBS) nanowires, one side of a nanoporous membrane is coated with conduct-
ing material, and then the PPy is electropolymerized through the pores. We used a 60 µm thick
alumina membrane with 200 nm diameter pores, resulting in nanowires of the same diameter
with a length controlled by polymerization time.
21
As seen in Figure 13.17A, the top portion of
the nanofi bers is hollow with very thin sidewalls (
20 nm) and probably unsuitable for mechani-
cal actuation. To remove the hollow tube portion of the nanofi ber, the following procedure was
used: PPy nanofi bers of 40 µm length were electropolymerized in the alumina template as above,
but before the alumina was dissolved, an extra step was added to the fabrication sequence. The
alumina membrane with PPy nanofi bers was subjected to mechanical lapping to remove the
top 10-15 µm of the nanofi bers (Figure 13.18, sample 1). The alumina was then dissolved in
0.2 M NaOH (aq.) solution, and the nanofi bers were imaged with a scanning electron microscope
(SEM). In Figure 13.17B, it can be seen that the remaining lower portions of the nanofi bers are
solid and more suitable for actuation, and only the top few micrometers of the PPy(DBS) nano-
fi bers were hollow.
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