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In Vivo Electrochemical Analysis of a PEDOT/MWCNT Neural Electrode Coating.

Alba NA, Du ZJ, Catt KA, Kozai TD, Cui XT - Biosensors (Basel) (2015)

Bottom Line: Equivalent circuit analysis showed that the impedance increase is the result of surface capacitance reduction, likely due to protein and cellular processes encapsulating the porous coating.Some coated electrodes exhibited steady impedance while others exhibiting large increases associated with large decreases in charge storage capacity suggesting delamination in PBS.Despite the impedance increase, coated electrodes successfully recorded neural activity throughout the implantation period.

View Article: PubMed Central - PubMed

Affiliation: Department of Bioengineering, University of Pittsburgh, 5056 Biomedical Science Tower 3, 3501 Fifth Avenue, Pittsburgh, PA 15213, USA. nicolasaalba@gmail.com.

ABSTRACT
Neural electrodes hold tremendous potential for improving understanding of brain function and restoring lost neurological functions. Multi-walled carbon nanotube (MWCNT) and dexamethasone (Dex)-doped poly(3,4-ethylenedioxythiophene) (PEDOT) coatings have shown promise to improve chronic neural electrode performance. Here, we employ electrochemical techniques to characterize the coating in vivo. Coated and uncoated electrode arrays were implanted into rat visual cortex and subjected to daily cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) for 11 days. Coated electrodes experienced a significant decrease in 1 kHz impedance within the first two days of implantation followed by an increase between days 4 and 7. Equivalent circuit analysis showed that the impedance increase is the result of surface capacitance reduction, likely due to protein and cellular processes encapsulating the porous coating. Coating's charge storage capacity remained consistently higher than uncoated electrodes, demonstrating its in vivo electrochemical stability. To decouple the PEDOT/MWCNT material property changes from the tissue response, in vitro characterization was conducted by soaking the coated electrodes in PBS for 11 days. Some coated electrodes exhibited steady impedance while others exhibiting large increases associated with large decreases in charge storage capacity suggesting delamination in PBS. This was not observed in vivo, as scanning electron microscopy of explants verified the integrity of the coating with no sign of delamination or cracking. Despite the impedance increase, coated electrodes successfully recorded neural activity throughout the implantation period.

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(a,b) Equivalent circuit models for uncoated electrodes in PBS or initial days post-implantation (model A) and uncoated electrodes at later days post-implantation as well as PEDOT/MWCNT/Dex-coated electrodes (model B), with distributed element ZD representing the polymer coating as well as tissue encapsulation; (c) Schematic of the dual-channel distributed diffusion impedance element ZD; (d–i) Representative Nyquist impedance plots of uncoated (d–f) and coated (g–i) electrodes at three time points post-implantation, with all plots exhibiting an identical frequency range (100 Hz to 33 kHz). Note contrast between uncoated and coated Nyquist plots at days 7 and 10 despite statistically identical 1 kHz impedance. The growing semi-circular features at higher frequencies within (g–i) suggest the development of encapsulation over coated electrodes not present in uncoated electrodes; (j–l) Average fitted values of modeling parameters CCPE, Q1, and β. Q1 were not fitted for days 0–2 for uncoated probes due to use of model A. Comparatively large values of CCPE during the initial four days indicates a coating benefit to electrode capacitance that diminished at later points. Reducing values of Q1 indicate a reduction in coating/electrolyte capacitance, possibly due to changing surface area. β exhibited low values during first four days, but later increased to values equivalent to uncoated electrodes, suggesting a change to the nature of the coating/electrode interface. N varied from session to session. All data presented as mean ± SD. *p < 0.01.
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biosensors-05-00618-f003: (a,b) Equivalent circuit models for uncoated electrodes in PBS or initial days post-implantation (model A) and uncoated electrodes at later days post-implantation as well as PEDOT/MWCNT/Dex-coated electrodes (model B), with distributed element ZD representing the polymer coating as well as tissue encapsulation; (c) Schematic of the dual-channel distributed diffusion impedance element ZD; (d–i) Representative Nyquist impedance plots of uncoated (d–f) and coated (g–i) electrodes at three time points post-implantation, with all plots exhibiting an identical frequency range (100 Hz to 33 kHz). Note contrast between uncoated and coated Nyquist plots at days 7 and 10 despite statistically identical 1 kHz impedance. The growing semi-circular features at higher frequencies within (g–i) suggest the development of encapsulation over coated electrodes not present in uncoated electrodes; (j–l) Average fitted values of modeling parameters CCPE, Q1, and β. Q1 were not fitted for days 0–2 for uncoated probes due to use of model A. Comparatively large values of CCPE during the initial four days indicates a coating benefit to electrode capacitance that diminished at later points. Reducing values of Q1 indicate a reduction in coating/electrolyte capacitance, possibly due to changing surface area. β exhibited low values during first four days, but later increased to values equivalent to uncoated electrodes, suggesting a change to the nature of the coating/electrode interface. N varied from session to session. All data presented as mean ± SD. *p < 0.01.

Mentions: Curve fitting and equivalent circuit analysis was applied to the measured data using a method developed by Bisquert [82,83] that has since been employed to model the electrical characteristics of both in vivo inflammatory tissue encapsulation [84] as well as PPy/CNT films on intracortical electrodes in vitro [63,85]. Using this method, data is fitted to one of two models. Model A (Figure 3a) is a simple Randles circuit employing a constant phase element (CPE) in parallel with a resistor, and has been commonly used to model bare microelectrodes in electrolyte [86,87]. In this model, the CPE is representative of the double layer capacitance of the metal recording surface, while the parallel resistance RCT is representative of the charge transfer resistance, or the resistance of the material to the transfer of faradaic current. The CPE is modeled using two terms: CCPE, the coefficient of CPE capacitance per unit length (F·sα−1·cm−1), and β, a parameter defined by the phase angle of the CPE. β has a value between 0 and 1, where β = 1 represents an ideal capacitor with phase angle 90° and β = 0.5 represents a CPE with phase angle 45° (also known as a Warburg impedance). The physical correlate of β is not well understood, and is thought to be related to surface roughness, charge uniformity, coating bulk properties, or varying reaction rates along the electrode surface [87]. A second resistive element RSER represents the solution resistance of the bulk saline/tissue environment. For typical microelectrodes composed of a blocking material such as platinum, RCT is expected to be extremely high, leading to the electrode behavior being dominated by the probe capacitance [85]. In this study, model A was used to fit data collected from uncoated electrodes in vitro or at very early time points in vivo, in plots where multiple time-constant behavior was not observed.


In Vivo Electrochemical Analysis of a PEDOT/MWCNT Neural Electrode Coating.

Alba NA, Du ZJ, Catt KA, Kozai TD, Cui XT - Biosensors (Basel) (2015)

(a,b) Equivalent circuit models for uncoated electrodes in PBS or initial days post-implantation (model A) and uncoated electrodes at later days post-implantation as well as PEDOT/MWCNT/Dex-coated electrodes (model B), with distributed element ZD representing the polymer coating as well as tissue encapsulation; (c) Schematic of the dual-channel distributed diffusion impedance element ZD; (d–i) Representative Nyquist impedance plots of uncoated (d–f) and coated (g–i) electrodes at three time points post-implantation, with all plots exhibiting an identical frequency range (100 Hz to 33 kHz). Note contrast between uncoated and coated Nyquist plots at days 7 and 10 despite statistically identical 1 kHz impedance. The growing semi-circular features at higher frequencies within (g–i) suggest the development of encapsulation over coated electrodes not present in uncoated electrodes; (j–l) Average fitted values of modeling parameters CCPE, Q1, and β. Q1 were not fitted for days 0–2 for uncoated probes due to use of model A. Comparatively large values of CCPE during the initial four days indicates a coating benefit to electrode capacitance that diminished at later points. Reducing values of Q1 indicate a reduction in coating/electrolyte capacitance, possibly due to changing surface area. β exhibited low values during first four days, but later increased to values equivalent to uncoated electrodes, suggesting a change to the nature of the coating/electrode interface. N varied from session to session. All data presented as mean ± SD. *p < 0.01.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4697137&req=5

biosensors-05-00618-f003: (a,b) Equivalent circuit models for uncoated electrodes in PBS or initial days post-implantation (model A) and uncoated electrodes at later days post-implantation as well as PEDOT/MWCNT/Dex-coated electrodes (model B), with distributed element ZD representing the polymer coating as well as tissue encapsulation; (c) Schematic of the dual-channel distributed diffusion impedance element ZD; (d–i) Representative Nyquist impedance plots of uncoated (d–f) and coated (g–i) electrodes at three time points post-implantation, with all plots exhibiting an identical frequency range (100 Hz to 33 kHz). Note contrast between uncoated and coated Nyquist plots at days 7 and 10 despite statistically identical 1 kHz impedance. The growing semi-circular features at higher frequencies within (g–i) suggest the development of encapsulation over coated electrodes not present in uncoated electrodes; (j–l) Average fitted values of modeling parameters CCPE, Q1, and β. Q1 were not fitted for days 0–2 for uncoated probes due to use of model A. Comparatively large values of CCPE during the initial four days indicates a coating benefit to electrode capacitance that diminished at later points. Reducing values of Q1 indicate a reduction in coating/electrolyte capacitance, possibly due to changing surface area. β exhibited low values during first four days, but later increased to values equivalent to uncoated electrodes, suggesting a change to the nature of the coating/electrode interface. N varied from session to session. All data presented as mean ± SD. *p < 0.01.
Mentions: Curve fitting and equivalent circuit analysis was applied to the measured data using a method developed by Bisquert [82,83] that has since been employed to model the electrical characteristics of both in vivo inflammatory tissue encapsulation [84] as well as PPy/CNT films on intracortical electrodes in vitro [63,85]. Using this method, data is fitted to one of two models. Model A (Figure 3a) is a simple Randles circuit employing a constant phase element (CPE) in parallel with a resistor, and has been commonly used to model bare microelectrodes in electrolyte [86,87]. In this model, the CPE is representative of the double layer capacitance of the metal recording surface, while the parallel resistance RCT is representative of the charge transfer resistance, or the resistance of the material to the transfer of faradaic current. The CPE is modeled using two terms: CCPE, the coefficient of CPE capacitance per unit length (F·sα−1·cm−1), and β, a parameter defined by the phase angle of the CPE. β has a value between 0 and 1, where β = 1 represents an ideal capacitor with phase angle 90° and β = 0.5 represents a CPE with phase angle 45° (also known as a Warburg impedance). The physical correlate of β is not well understood, and is thought to be related to surface roughness, charge uniformity, coating bulk properties, or varying reaction rates along the electrode surface [87]. A second resistive element RSER represents the solution resistance of the bulk saline/tissue environment. For typical microelectrodes composed of a blocking material such as platinum, RCT is expected to be extremely high, leading to the electrode behavior being dominated by the probe capacitance [85]. In this study, model A was used to fit data collected from uncoated electrodes in vitro or at very early time points in vivo, in plots where multiple time-constant behavior was not observed.

Bottom Line: Equivalent circuit analysis showed that the impedance increase is the result of surface capacitance reduction, likely due to protein and cellular processes encapsulating the porous coating.Some coated electrodes exhibited steady impedance while others exhibiting large increases associated with large decreases in charge storage capacity suggesting delamination in PBS.Despite the impedance increase, coated electrodes successfully recorded neural activity throughout the implantation period.

View Article: PubMed Central - PubMed

Affiliation: Department of Bioengineering, University of Pittsburgh, 5056 Biomedical Science Tower 3, 3501 Fifth Avenue, Pittsburgh, PA 15213, USA. nicolasaalba@gmail.com.

ABSTRACT
Neural electrodes hold tremendous potential for improving understanding of brain function and restoring lost neurological functions. Multi-walled carbon nanotube (MWCNT) and dexamethasone (Dex)-doped poly(3,4-ethylenedioxythiophene) (PEDOT) coatings have shown promise to improve chronic neural electrode performance. Here, we employ electrochemical techniques to characterize the coating in vivo. Coated and uncoated electrode arrays were implanted into rat visual cortex and subjected to daily cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) for 11 days. Coated electrodes experienced a significant decrease in 1 kHz impedance within the first two days of implantation followed by an increase between days 4 and 7. Equivalent circuit analysis showed that the impedance increase is the result of surface capacitance reduction, likely due to protein and cellular processes encapsulating the porous coating. Coating's charge storage capacity remained consistently higher than uncoated electrodes, demonstrating its in vivo electrochemical stability. To decouple the PEDOT/MWCNT material property changes from the tissue response, in vitro characterization was conducted by soaking the coated electrodes in PBS for 11 days. Some coated electrodes exhibited steady impedance while others exhibiting large increases associated with large decreases in charge storage capacity suggesting delamination in PBS. This was not observed in vivo, as scanning electron microscopy of explants verified the integrity of the coating with no sign of delamination or cracking. Despite the impedance increase, coated electrodes successfully recorded neural activity throughout the implantation period.

Show MeSH
Related in: MedlinePlus