Biomedical Engineering Reference
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to deliver molecules to a surface via a solvent meniscus, which naturally forms in the
ambient atmosphere. This direct-write technique offers high-resolution patterning capabilities
for a number of organic [17,18], semiconducting [19], or metallic materials [19,20] in nanoscale
dimensions. Lim and Mirkin [21] have reported the preparation of self-doped polyaniline
(PANI) and doped polypyrrole (PPY) lines down to 310 and 290 nm widths, respectively, at
writing speeds of about 0.8 µm/s. The ionically charged CPs were used as the “ink” for writ-
ing on oppositely charged substrates. Electrostatic interactions between the water-soluble ink
materials and charged substrates provided a significant driving force for the generation of sta-
ble DPN patterns on the silicon substrates. Liu and coworkers [22] have reported the writing
of poly 3,4-ethylenedioxythiophene down to 30 nm by polymerization of 3,4-ethylene-
dioxythiophene at the AFM tip/substrate interface in what they term as electrochemical-DPN.
4.2.2
Mechanical Stretching
In this procedure, CP NWs are fabricated by electrochemical polymerization of the corre-
sponding monomer onto a sharp scanning tunneling microscope (STM) tip that is held at
a small distance (between 20 and 100 nm) from an electrode [13]. The STM tip is coated
with an insulation layer except a few nm 2 at the end so that the growth of PANI is local-
ized at the tip end. After forming a CP NW between the STM tip and the electrode, the
diameter of the nanowire is further reduced by stretching it with the STM tip. Highly con-
ductive PANI with diameter of about 20 nm has been reported using this method [13].
4.2.3
Electrospinning
This method uses a microfabricated scanned tip as an electrospinning source. The tip is
dipped in a polymer solution to gather a droplet as a source material. A voltage applied to
the tip causes the formation of a Taylor cone, and at sufficiently high voltages, a polymer
jet is extracted from the droplet. By moving the source relative to a surface, acting as a
counter-electrode, oriented nanofibers can be deposited and integrated with microfabri-
cated surface structures. In addition to the uniform fiber deposition, the scanning tip elec-
trospinning source can produce self-assembled composite fibers of micro- and
nanoparticles aligned in a polymeric fiber [23-26].
4.2.4
Template-Directed Methods
Like other 1-D nanostructures, 1-D CP nanostructrures can be synthesized by template-
directed methods. Porous anodic alumina oxide (AAO), track-etched porous polymer
membranes, and mica are the three types of templates that are commonly used and elec-
trochemical deposition is usually the technique of choice. Electrochemical deposition is
accomplished by coating one face of the template with an inert conducting film (e.g., gold
and platinum) and using this inert conducting film as the anode. The polymer is then elec-
trochemically synthesized within the pores of the membrane. The length of the nanowires
is determined by the current density and deposition time. The diameter of nanowire is
determined by the pore diameter of template. CPs show preferential deposition along the
walls of the polycarbonate membrane resulting in nanotubule structures due to solvopho-
bic interactions. These tubules close up as the deposition time is increased and eventually
results in nanowires [27,28]. Chemical template synthesis can be similarly accomplished by
simply immersing the membrane into a solution of the desired monomer and its oxidizing
agent. Following nanowire electrodeposition, the conducting film used for electrochemical
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