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, 6 (3), 421-9

Conducting-polymer Nanotubes Improve Electrical Properties, Mechanical Adhesion, Neural Attachment, and Neurite Outgrowth of Neural Electrodes

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Conducting-polymer Nanotubes Improve Electrical Properties, Mechanical Adhesion, Neural Attachment, and Neurite Outgrowth of Neural Electrodes

Mohammad Reza Abidian et al. Small.

Abstract

An in vitro comparison of conducting-polymer nanotubes of poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(pyrrole) (PPy) and to their film counterparts is reported. Impedance, charge-capacity density (CCD), tendency towards delamination, and neurite outgrowth are compared. For the same deposition charge density, PPy films and nanotubes grow relatively faster vertically, while PEDOT films and nanotubes grow more laterally. For the same deposition charge density (1.44 C cm(-2)), PPy nanotubes and PEDOT nanotubes have lower impedance (19.5 +/- 2.1 kOmega for PPy nanotubes and 2.5 +/- 1.4 kOmega for PEDOT nanotubes at 1 kHz) and higher CCD (184 +/- 5.3 mC cm(-2) for PPy nanotubes and 392 +/- 6.2 mC cm(-2) for PEDOT nanotubes) compared to their film counterparts. However, PEDOT nanotubes decrease the impedance of neural-electrode sites by about two orders of magnitude (bare iridium 468.8 +/- 13.3 kOmega at 1 kHz) and increase capacity of charge density by about three orders of magnitude (bare iridium 0.1 +/- 0.5 mC cm(-2)). During cyclic voltammetry measurements, both PPy and PEDOT nanotubes remain adherent on the surface of the silicon dioxide while PPy and PEDOT films delaminate. In experiments of primary neurons with conducting-polymer nanotubes, cultured dorsal root ganglion explants remain more intact and exhibit longer neurites (1400 +/- 95 microm for PPy nanotubes and 2100 +/- 150 microm for PEDOT nanotubes) than their film counterparts. These findings suggest that conducting-polymer nanotubes may improve the long-term function of neural microelectrodes.

Figures

Figure 1
Figure 1
Schematic illustration and optical micrographs of fabrication process of conducting-polymer nanotubes on the surface of neural microelectrodes. a,b) Before surface modification, c,d) electrospinning of PLLA nanofiber templates on the neural microelectrode, e,f) electrochemical deposition of conducting polymer (PEDOT) on the electrode sites and around electrospun PLLA nanofiber templates as a function of deposition time (deposition charge density from 0.24 C cm–2 to 2.88 C cm–2), g,h) dissolving away of electrospun PLLA nanofiber templates and formation of conducting-polymer nanotubes.
Figure 2
Figure 2
SEM image of electropolymerized nanostructured conducting polymers on the electrode sites with deposition charge density of 1.44 C cm–2. 3D view: a) PPy film, d) PEDOT film, g) PPy nanotubes, j) PEDOT nanotubes. Top view: b) PPy film, e) PEDOT film, h) PPy nanotubes, k) PEDOT nanotubes. High-magnification top view: c) PPy film, f) PEDOT film, i) PPy nanotubes, l) PEDOT nanotubes.
Figure 3
Figure 3
a) Conducting-polymer outgrowth diameter as a function of deposition charge density, PPy film (stars), PPy nanotubes (triangles), PEDOT film (diamonds), and PEDOT nanotubes (circles).b) Thicknessof PPy nanotubes (triangles)and PEDOTnanotubes (circles)as a function of deposition charge density. It can be seen that the thickness increased from 2.2 ± 1.2 μm to 30 ± 2.5 μm for PPy nanotubes and from 2.5 ± 1.4 μm to 18 ± 2.1 μm for PEDOT nanotubes by increasing deposition charge density from 0.24 C cm–2 to 2.88 C cm–2. Data are shown for ± standard deviation (n = 10). c) SEM image of PEDOT nanotube outgrowth on silicon dioxide showing diameter outgrowth of 100 ± 5.3 μm. d) SEM image of PPy nanotube outgrowth on silicon dioxide showing diameter outgrowth of 60 ± 3.5 μm. PEDOT and PPy nanotube were electropolymerized on electrode sites with a deposition charge density of 1.44 C cm–2.
Figure 4
Figure 4
SEM images of conducting polymers after CV measurement on neural electrode: a–d) PEDOT film, e-f PPy film on neural electrode showing delamination on the edge of polymer film. c) Higher-magnification image of (a). d) Higher-magnification image of (b). f) Higher-magnification image of (e). g) PEDOT nanotubes. h) PPy naotubes. PPy nanotubes and PEDOT nanotubes remained firmly attached to the neural electrode after CV measurement. The delamination height was measured as shown in (c), (d), and (f). The delamination height was 3.7 ± 1.3 μm for PPy film and 13.4 ± 2.5 μm for PEDOT film (±Stdv, n = 8). More delamination of PEDOT film was observed than for PPy film on the edges of electrode sites (p < 0.0001). PEDOT and PPy films and nanotubes were electropolymerized on the electrode site with deposition charge density of 1.44 C cm–2. CV measuerement was carried out by applying scanning voltage from –0.9 V to 0.5 V with scan rate 100 mV s–1 for 5 cycles.
Figure 5
Figure 5
SEM images of electropolymerized PPy nanotubes on neural microelectrode sites as a function of deposition charge density. a) 0.24 C cm–2, b) 0.72 C cm–2, c) 0.96 C cm–2, d) 1.44 C cm–2, e) 1.92 C cm–2, and f) 2.88 C cm–2. It can be seen that the thickness of the PPy nanotube layer increased from 2.2 ± 1.2 μm to 30 ± 2.5 μm. These SEM images demonstrate that PPy nanotubes tend to grow more in the vertical direction.
Figure 6
Figure 6
Electrical properties of neural microelectrodes modified with nanostructured conducting polymers. a) Bode plot of electrochemical impedance spectroscopy over a frequency range of 1–105 Hz; the initial impedance of bare iridium was 468.8 ± 13.3 kΩ at 1 kHz, which decreased to 28.3 ± 2.6 kΩ for PPy film, 19.5 ± 2.1 kΩ for PPy nanotubes, 10.8 ± 1.8 kΩ for PEDOT film, and 2.5 ± 1.4 kΩ for PEDOT nanotubes. b) Phase plot of electrochemical impedance spectroscopy over a frequency range of 1–105 Hzshowing that both the uncoated and coated electrodes were capacitive in the low-frequency range (<10 Hz). PEDOT nanotubes were almost purely resistive at 1 kHz (≈0°) in comparison with bare iridium electrode (uncoated), which was much more capacitive (≈55°). c) CV; the potential was swept from –0.9 to 0.5 V at a scan rate of 100 mV s–1. The CCD increased from 0.1 ± 0.5 mC cm–2 (bare iridium) to 160 ± 8.3 mC cm2 for PPy film, 184 ± 5.3 mC cm–2 for PPy nanotubes, 240 ± 9.4 mC cm–2 for PEDOT film, and 392 ± 6.2 mC cm–2 for PEDOT nanotubes. Bare iridium (squares), PPy film (stars), PPy nanotubes (triangles), PEDOT film (diamonds), and PEDOT nanotubes (circles). Conducting polymers were deposited with an applied charge density of 1.44 C cm–2. d) CV for PPy nanotubes electropolymerized with a deposition charge density from 0.24 C cm–2 to 2.88 C cm–2. CCD of PPy nanotubes increased from 125 ± 3.2 mC cm–2 to 625 ± 6.5 mC cm–2 with increasing deposition charge density from 0.24 C cm–2 to 2.88 C cm–2.
Figure 7
Figure 7
DRG explants cultured on conducting-polymer films and nanotubes. Ganglia on PPy film (a) and PPy nanotubes (c) degraded and had shorter and more branched neurites than PEDOT film (b) and PEDOT nanotubes (d), respectively. The extent of branching is best observed at higher magnification of PPy nanotubes (e) and PEDOT nanotubes (f). Conducting-polymer nanotubes produced longer neurites than their corresponding films, with PEDOT nanotubes producing the longest neurites overall (g). Column height represents the mean while error bars reflect the standard error of the mean for 10 neurites per condition (n = 10).

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