1. Electrochemical Tunability in Glassy Carbon
Microelectrodes for
Neural Stimulation and Recording
Master Thesis Defense - Roberto Gavuglio
Advisor: Dr. Sam Kassegne

2. Publication
2
Kassegne, Vomero, Gavuglio et al., “Electrical Impedance, Electrochemistry,
Mechanical Stiffness, and Hardness Tunability in Glassy Carbon MEMS μECoG
Electrodes”, J. Microelectronic Engineering, dx.doi.org/10.1016/j.mee.2014.11.013
Gavuglio, Kassegne, et al., ”Long-Term In-vivo and In-vitro Characterizations of
GCMEMS μECoG Electrodes”, under preparation, 2014.

3. Motivation of this study
Glassy Carbon: New material for neural probes?
• Motivation 1 - Decreasing damages and risks associated
with neuroprosthetic implants (Stiffness & Impedance
Matching)
• Motivation 2 - Increase the long-term reliability (In-Vitro
Corrosion Experiment)
Glassy Carbon VS Metal Electrodes

4. Overview of Neural Stimulation and Recording

5. Neural Stimulation and Recording – Clinical
1. Cortical Stimulation: cortical
neuronal networks involved in
pathophysiology processes -
disturbances of motion and epilepsy).
2. Deep Brain Stimulation: subcortical
neuronal networks in the thalamus
involved in pathophysiology processes
(Parkinson’s Disease, Depression and
Dystonia)
Disease Target Technique
Parkinson’s
Disease
Subthalamic
nucleus
DBS
Epilepsy Cortical Cortical
Obsessive-
compulsive
disorder
Nucleus
accumbens
DBS
Depression Subgenual
cingulate cortex
DBS
Addiction Nucleus
accumbens
DBS
Alzheimer’s
Disease
Nucleus basalis DBS
Chronic Pain Spinal cord SCS

6. Solid-State
Electronics Charge Injection
Neuro-Physiology
Neuro-Chemistry
Ohmic World Faradaic & Capacitive World
• Flow of ionic charges in
the biological tissue that
cause stimulation of
surrounding neurons
• Faradaic Injection: redox
reactions
• Capacitive injection:
electrochemical double-
layer (EDL)
Neural Stimulation and Recording – Interface Challenge

7. FARADAIC
INJECTION
Neural Stimulation and Recording – Mechanism
Positive Ions
Negative Ions
ELECTRODE

8. CAPACITIVE
INJECTION
Neural Stimulation and Recording – Mechanism
Positive Ions
Negative Ions
DoubleLayer
ELECTRODE

9. • Miniaturization of Neuroprosthetics
• Diminishing invasivity and oxidative stress
• Increasing accuracy in stimulation and recording
Material Charge Injection Mechanism Maximum CIC
(mC/cm2)
Pro/Con
Platinum Faradaic/Capacitive
Double-Layer Charging, oxide
formation and reduction
0.1 – 0.35 Standard of care / Long-Term
release of metallic ions –
Corrosion in highly oxidative
environment
Titanium Nitride Capacitive – Double-Layer
Charging
1 Capacitive Charge Injection /
Long-Term toxicity
Iridium Oxide Faradaic – Oxide valency
changes
0.9 - 4 High Charge Injection / Difficult
Microfabrication
Unstable in long-term
applications
CNT Capacitive – Double Layer
Charging
1 Capacitive Charge Injection /
Long-term Stability problems –
dispersion of CNT
Neural Stimulation and Recording – Microarrays

10. Addressing Tissue Damage and Long-Term
Stability of Neural Probes
Introducing Glassy Carbon
SU8-100 Pyrolyzed Carbon

11. Glassy Carbon – Tunable Material
• Biocompatible
• Conductive
• Capacitive Charge Injection
• Maximum Temperature of
Pyrolysis Change in ribbon
dimensions La, Lc
• Ramping rate of heating
Roughness

12. BCI ‘Design Space’ for Impedance, Stiffness, and Hardness
brain
Glassy carbon
Glassy Carbon – Tunable Material

13. MAXIMUM
TEMPERATURE
OF PYROLYSIS
(C°)
TOTAL PYROLYSIS TIME
RAMPING RATE OF HEATING
(C°/min)
Glassy Carbon – Lithography & Pyrolysis

14. Impedance, Stiffness, and Electrochemical Tuning

15. E = 1.5KPa intercortical
1.5MPa spinal cord
160GPa Utah mArray
H = 13 GPa Utah mArray
3 GPa Parylene-C
10x Tunability in E
Glassy Carbon – Stiffness Tuning

16. Glassy Carbon
Microelectrodes
Omnetics
Connector
II) 2D Flat Wire/ElectrodesI) Pillar Electrodes
104x Tunability in |Z| . Same
range as cortex W to MW.
Glassy Carbon – ImpedanceTuning

17. Experimental Set-up
• Microelectrodes tested 2x2 array
• Back attached to a copper foil with conductive
silver paste
• Insulated with PDMS
• Top part of microelectrodes exposed
• Three-electrodes electrochemical cell – working
electrodes glassy carbon – Reference electrode
Ag/AgCl – Counter Electrode Pt
Glassy Carbon – Electrochemical Tuning

18. Experiment 1 – EIS (Electrochem Impedance Spectroscopy)
•Roughness Index - Depression of semicircle
0.8 - 1
•Double Layer Capacitance - Height of
semicircle
High-frequency semicircle – Capacitive Behavior
Glassy Carbon – Electrochemical Tuning

19. • CIC: Maximum charge that can be injected in a solution
without causing irreversible chemical reactions
• Water window of hydrolysis for glassy carbon +/- 1 V
Experiment 2 – CIC (Charge Injection Capacity)
Glassy Carbon – Electrochemical Tuning
APPLY MEASURE

20. Electrochemical Tuning Experimental Results

21. EC Characterization - Double-Layer Capacitance
• Capacitance is directly proportional to the
surface area of the electrodes. A wider area
allows a larger electrical double-layer

22. • The roughness of the surface depends on
the ramping rate and max pyrolysis
temperature
• Gas evolution of the oxygen out of the
pillar during the pyrolysis process
• Annealing effect
• Max Pyrolysis Temperature and/or
ramping rate of heating – increase RSA
EC Characterization - α – Roughness

23. • CIC increases consistently
• Correlation between CIC and α
• Max T Pyrolysis related to edge planes density
• 3x CIC of Pt – no REDOX
Mat. CIC (mC/cm^2)
Pt 0.3
TiN 1
EC Characterization – Charge Injection Capacity

24. EC Characterization – Charge Injection Capacity
0.8 < α < 1
A value of α close to 1 indicates smooth surface, a value of α of 0.8 indicates rough
surface.
Increase in the maximum pyrolysis temperature leads to increase in the surface
roughness.
Porosity of the glassy carbon electrode can be optimized by varying the pyrolysis time.
Porosity is related to surface roughness.
700.2 Protocol, Ramp Rate = 1.6C/min 700.7 Protocol, Ramp Rate = 5.8C/min
1000.2 Protocol, Ramp Rate = 2.4 C/min 1000.7 Protocol, Ramp Rate = 8.3C/min

25. Long Term Stability & Corrosion Study

26. • T1000 (ramp rate 2.4 C°/min)
• T700 (ramp rate 1.6 C°/min)
• Immersion in solution PBS 9%+ H2O2 30mM
• Oxidative solution simulating inflammatory
response
Study was conducted on two protocols :
Long-Term In-vitro Corrosion Study – Set-up

27. 7 days 14 days 21 days
1000°
700°
Long-Term In-vitro Corrosion Study

28. 100 um
Day 3 Day 8
T700T1000
side
top
side
top
Day 14 Day 21Day 1
Corrosion Experiment PBS + H2O2
Long-Term In-vitro Corrosion Study

29. • Adsorption of oxygen on carbon surface.
• Reaction with oxygen on the high reactive edge planes to form carbon dioxide.
• Partial passivation of the exposed surface. Graphite Oxide
• Major corrosion occurs in pyrolyzed carbon with fabrication temperature 700 C°.
• Open porosity T700. Closed Porosity T1000.
Long-Term In-vitro Corrosion Study

30. • The overall stability of CIC was confirmed for T1000
samples. A slight decrease is due to oxidative stress
Long-Term In-vitro Corrosion Study

31. Corrosion in GC <<
Corrosion in Pt (0.2%)
Long-Term In-vitro Corrosion Study
GC Vs Pt

32. Conclusions
• Demonstrated tunability of electrochemical
properties of glassy carbon depending on
pyrolysis process: Max Pyrolysis T° – Ramp rate
• CIC glassy carbon higher than CIC of Platinum
• Capacitive communication with biological fluid,
no irreversible redox reactions
• Excellent corrosion resistance
• Corrosion forms CO2 - no toxic metallic ions

33. Acknowledgments
• Dr. Sam Kassegne
• Dr. Karen May-Newman
• Dr. Mahasweta Sarkar
• Maria Vomero
• Alessio Cinopri
• Pieter Van Niekerk
• Mieko Hirabayashi
• Varsha Ramesh
• All MEMS Lab
members

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