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. 2008 Mar;29(9):1273-83.
doi: 10.1016/j.biomaterials.2007.11.022.

Experimental and theoretical characterization of implantable neural microelectrodes modified with conducting polymer nanotubes

Mohammad Reza Abidian  1 , David C Martin
Affiliations

Experimental and theoretical characterization of implantable neural microelectrodes modified with conducting polymer nanotubes

Mohammad Reza Abidian et al. Biomaterials. 2008 Mar.

Abstract

Neural prostheses transduce bioelectric signals to electronic signals at the interface between neural tissue and neural microelectrodes. A low impedance electrode-tissue interface is important for the quality of signal during recording as well as quantity of applied charge density during stimulation. However, neural microelectrode sites exhibit high impedance because of their small geometric surface area. Here we analyze nanostructured-conducting polymers that can be used to significantly decrease the impedance of microelectrode typically by about two orders of magnitude and increase the charge transfer capacity of microelectrodes by three orders of magnitude. In this study poly(pyrrole) (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) nanotubes were electrochemically polymerized on the surface of neural microelectrode sites (1250 microm(2)). An equivalent circuit model comprising a coating capacitance in parallel with a pore resistance and interface impedance in series was developed and fitted to experimental results to characterize the physical and electrical properties of the interface. To confirm that the fitting parameters correlate with physical quantities of interface, theoretical equations were used to calculate the parameter values thereby validating the proposed model. Finally, an apparent diffusion coefficient was calculated for PPy film (29.2+/-1.1 x 10(-6) cm(2)/s), PPy nanotubes (PPy NTs) (72.4+/-3.3 x 10(-6) cm(2)/s), PEDOT film (7.4+/-2.1 x 10(-6) cm(2)/s), and PEDOT nanotubes (PEDOT NTs) (13.0+/-1.8 x 10(-6) cm(2)/s). The apparent diffusion coefficient of conducting polymer nanotubes was larger than the corresponding conducting polymer films.

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Figures

Figure 1
Figure 1
Optical micrographs of stimulating/recording microelectrodes arrays that were fabricated in CNCT and assembled in our laboratory. The probe tip will be inserted in motor cortex or auditory cortex and the rest of the probe will be tethered to the skull: (A) 8 channel silicon substrate acute probe, (B) High magnification image of acute tip probe demonstrates the silicon substrate and gold electrode sites with surface area of 1250 μm2.
Figure 2
Figure 2
Procedure of surface modification of neural microelectrodes in order to create CP nanotubes: (A) Neural microelectrode before surface modification. (B) Electrospinning of biodegradable polymer nanofibers templates (PLLA) on the neural microelectrode. (C) Electrochemical polymerization of CPs around the electrospun nanofibers (D) Removing of electrospun core fibers to create nanotubular-CPs.
Figure 3
Figure 3
Equivalent circuit model of electrode-CP-electrolyte interface. The circuit elements are: solution resistance (RS), coating capacitance (CC), pore resistance (Rpore), double layer interface impedance ZCPE, charge transfer resistance Rt, and finite diffusion impedance ZT (including CT and RT).
Figure 4
Figure 4
Scanning electron micrographs of electropolymerized PPy and PEDOT nanotubes on neural microelectrode sites. (A) Top view of PPy nanotubes, (B) three-dimensional view of PPy nanotubes, (C) Top view PEDOT nanotubes, and (D) three-dimensional view of PEDOT nanotubes. PPy nanotubes with deposition charge density 1.44 C/cm2.
Figure 5
Figure 5
Scanning electron micrographs of electropolymerized PPy nanotubes on neural microelectrode sites as a function of deposition charge. (A) 6 μC, (B) 9 μC, (G) 12 μC, (H) 18 μC, (I) 24 μC, and (J) 36 μC.
Figure 6
Figure 6
Electrical properties of neural microelectrodes modified with conducting polymer nanotubes. (A) Impedance spectroscopy over a frequency range of 1-105 Hz: bare gold (black squares), PPy nanotubes (green triangles), and PEDOT nanotubes (red circles). (B) Cyclic voltammetry: bare gold (black squares), PPy nanotubes (green triangles), and PEDOT nanotubes (red circles). Deposition charge density = 1.44 C/cm2, Scan rate = 0.1V/s.
Figure 7
Figure 7
Impedance data of measured and calculated PEDOT nanotubes with applied charge density of 1.44 C/cm2 (C) Bode plot, (D) Nyquist plot. The calculated data were obtained by fitting of the experimental.
Figure 8
Figure 8
(A) Diffusional pseudocapacitance (CT) and diffusional resistance (RT) for PPy NTs as a function of applied charge density. (B) Diffusion coefficient of PPy NTs as a function of applied charge density.
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