Insulated Conducting Polymer
Nanowires
Bryan Ruddy
Objective:
Produce insulated conducting polymer wires under 1
micrometer in diameter and over 1 millimeter in length. Use such wires to fabricate intravascular
neural recording electrodes.
Background:
Effective, minimally
invasive brain-machine interfaces (BMIs) will serve as an enabling technology
for advances in medical diagnosis, prostheses, and even consumer
electronics. Currently, however, the
only effective, high-bandwidth BMIs are highly invasive microelectrode arrays
implanted directly into brain tissue.
While such arrays are already seeing medical use in cochlear and retinal
implants, and are used in animal research on prosthesis control, their
installation is a major surgical procedure with accompanying risks of infection
and/or brain damage. Conversely,
electrodes worn on the head are completely non-invasive, but can only detect
low-bandwidth brain-wave patterns, and cannot stimulate the brain.
Recently, Dr. Llinas proposed a BMI scheme in which very
fine electrodes are brought into the capillaries in the brain by blood
flow. These intravascular electrodes can
be inserted via a catheter, eliminating the risks of surgery, but are small
enough and close enough to the individual neurons to provide high-fidelity
signals. In order to provide good
signals, the bulk of the electrode must be insulated, with only the tip
exposed, and the tip must have a high surface area. The electrode must also be under 1 micrometer
in diameter in order not to interfere with blood flow.
Early experiments by Dr. Llinas’ group validated the
intravascular electrode concept using glass-insulated platinum wires 600
nanometers in diameter, with platinum black tips. However, these metal nanowires were too stiff
to navigate easily the bends in the capillary bed, and posed long-term
biocompatibility questions. Therefore,
we proposed to replace the inorganic nanowire electrodes with organic
materials: conducting polymers for the wire and its tip, and conventional
plastics for the insulation.
Results:
Our work towards fabricating all-polymer nanowires
electrodes has progressed in tow stages.
Initially, we focused on producing a proof-of-concept system to
demonstrate the validity of the polymer approach with a larger-scale
system. Wires were fabricated by slicing
thin films of the conducting polymer polypyrrole into square-cross-section
strips 10 millimeters long, and insulated with polyimide. The resulting electrodes were inserted into
frog sciatic arteries, and used to obtain high-fidelity signals from the
sciatic nerve.
We are currently focused on scaling this system down to the
sub-micrometer dimensions required for capillary insertion. We have successfully produced polyaniline
(another conducting polymer) wires insulated with poly(ethylene oxide) 3
micrometers in diameter and tens of millimeters in continuous length. Continuing work includes the development of
improved wire fabrication equipment and the design of instrumentation to
characterize the nanowires electrodes produced.
Figure 1: A. Electron micrographs of conducting polymer
microwires. The wire has a 15 um square cross-section with a total length (not
shown) of 20 mm. B. Assembled polymer
wire e. C. Relative locations of frog
sciatic artery, nerve, and conventional surface electrode. D. Recordings made from frog sciatic nerve in
vitro.
Figure 2: Transmission electron micrograph of conducting
polymer nanowires, with insulation removed to show the wire. The polyaniline wire is 650 nm in diameter,
with a 650 nm thick layer of poly(ethylene oxide) insulation covering it.