Bioactive Conducting Polymers for Optimising the Neural Interface - Biointerfaces: Where Material Meets Biology - page 217

(but not in contact with) the surface of the working electrode. The cell is then
filled with an electrolyte solution (physiological saline, DPBS or serum
containing media). The voltage limits for the scan must remain inside the
water window, a typical range is 0.6 to 0.8 V (versus Ag/AgCl). Voltage scan
rates of 50-150 mV s 1 are used, with higher scan rates producing larger,
more distinct redox peaks.

d n 3 r 4 n g | 2

A.2.2 Electrochemical Impedance Spectroscopy
The physical setup for EIS is the same as for CV; however, the stimulation
technique differs. A sucient stimulation amplitude ( 100 mV AC) is re-

quired to elicit a measurable response. It may be necessary to use a Faraday
cage to electrically isolate the sample from environmental interference.

B

A.2.3 Charge Injection Limit/Biphasic Stimulation
It is necessary to assess not only the material properties of conducting
polymers, using electrochemistry characterisation techniques, but also ap-
propriate application specific properties. Most devices use biphasic stimu-
lation to activate tissue and as such the authors adopt a current-controlled
charge balanced biphasic stimulus to examine the voltage transients which
result from active stimulation. These are square waveforms (refer to
Figure 8.8) which can be applied between a pair of stimulation sites, usually
comprised of microelectrodes within an array. It is important to stimulate
between electrodes fabricated from the same material to prevent bias of the
measured waveform from dissimilar materials. Additionally, the electrolyte
should be an adequate representation of the fluid in which the device is
intended to operate following implantation. The authors use artificial peri-
lymph or cell culture media. Saline is not suitable as it does not contain
appropriate protein content to adequately reflect the neural interface.

.

A.2.4 Film Delamination
The authors use a modified ASTM tape adhesion test (D3359-02) in order to
evaluate coating adhesion and friability. This method involves using a
scalpel to make an X-cut in the surface of the coating to reveal the underlying
substrate. Standard 3M 'Low-medium adhesion masking tape' is then ap-
plied to the coated surface and allowed to sit for 5 min. The tape is slowly
pulled off the electrode surface. An optical microscope is used to image the
electrode coating before the application of the tape and after its removal.
Analysis of images involves converting to greyscale and thresholding to
isolate the coating from the substrate. A pixel count allows for the de-
termination of the percentage of coating that was entirely removed as a
measure of delamination. Optical imaging of the coating left on the surface
of the tape accounts for partial removal of the coating and gives an indi-
cation of the friability of the coatings.