1. Biopotential Electrodes (Ch. 5)

2. Electrode – Electrolyte Interface
Electrode Electrolyte (neutral charge)
C+, A- in solution C
Current flow C C+ e- C A- C+ e- A-
C+ : Cation A- : Anion e- : electron
Fairly common electrode materials: Pt, Carbon, …, Au, Ag,…
Electrode metal is use in conjunction with salt, e.g. Ag-AgCl, Pt-Pt black, or polymer coats (e.g. Nafion, to improve selectivity)

3. Electrode – Electrolyte Interface
General Ionic Equations a) b)
a) If electrode has same material as cation, then this material gets oxidized and enters the electrolyte as a cation and electrons remain at the electrode and flow in the external circuit.
b) If anion can be oxidized at the electrode to form a neutral atom, one or two electrons are given to the electrode.
The dominating reaction can be inferred from the following :
Current flow from electrode to electrolyte : Oxidation (Loss of e-)
Current flow from electrolyte to electrode : Reduction (Gain of e-)

4. Half Cell Potential
A characteristic potential difference established by the electrode and its surrounding electrolyte which depends on the metal, concentration of ions in solution and temperature (and some second order factors) .
Half cell potential cannot be measured without a second electrode.
The half cell potential of the standard hydrogen electrode has been arbitrarily set to zero. Other half cell potentials are expressed as a potential difference with this electrode.
Reason for Half Cell Potential : Charge Separation at Interface
Oxidation or reduction reactions at the electrode-electrolyte interface lead to a double-charge layer, similar to that which exists along electrically active biological cell membranes.

5. Measuring Half Cell Potential
Note: Electrode material is metal + salt or polymer selective membrane

6. Some half cell potentials
Standard Hydrogen electrode
Note: Ag-AgCl has low junction potential & it is also very stable -> hence used in ECG electrodes!

7. Polarization
If there is a current between the electrode and electrolyte, the observed half cell potential is often altered due to polarization.
Note: Polarization and impedance of the electrode are two of the most important electrode properties to consider.

8. Nernst Equation
When two aqueous ionic solutions of different concentration are separated by an ion-selective semi-permeable membrane, an electric potential exists across the membrane.
For the general oxidation-reduction reaction
Note: interested in ionic activity at the electrode
(but note temp dependence
The Nernst equation for half cell potential is
where E0 : Standard Half Cell Potential E : Half Cell Potential
a : Ionic Activity (generally same as concentration)
n : Number of valence electrons involved

9. Polarizable and Non-Polarizable Electrodes
Use for recording
Perfectly Polarizable Electrodes
These are electrodes in which no actual charge crosses the electrode-electrolyte interface when a current is applied. The current across the interface is a displacement current and the electrode behaves like a capacitor. Example : Ag/AgCl Electrode
Perfectly Non-Polarizable Electrode
These are electrodes where current passes freely across the electrode-electrolyte interface, requiring no energy to make the transition. These electrodes see no overpotentials. Example : Platinum electrode
Use for stimulation
Example: Ag-AgCl is used in recording while Pt is use in stimulation

10. Ag/AgCl Electrode Relevant ionic equations Governing Nernst Equation
Solubility product of AgCl
Ag+Cl-
Fabrication of Ag/AgCl electrodes
Electrolytic deposition of AgCl
Sintering process forming pellet electrodes

11. Equivalent Circuit Rd+Rs Rs Frequency Response
Cd : capacitance of electrode-eletrolyte interface
Rd : resistance of electrode-eletrolyte interface
Rs : resistance of electrode lead wire
Ecell : cell potential for electrode
Corner frequency
Rd+Rs Rs
Frequency Response

12. Electrode Skin Interface
Ehe
Alter skin transport (or deliver drugs) by:
Pores produced by laser, ultrasound or by iontophoresis
Electrode Cd Rd
Sweat glands Rs
and ducts
Gel
100 m Re
Ese EP RP CP Ce
Stratum Corneum
Epidermis
100 m
Dermis and
subcutaneous layer Ru
Nerve endings
Skin impedance for 1cm2 patch:
200 1MHz
Capillary

13. Motion Artifact
Why
When the electrode moves with respect to the electrolyte, the distribution of the double layer of charge on polarizable electrode interface changes. This changes the half cell potential temporarily.
What
If a pair of electrodes is in an electrolyte and one moves with respect to the other, a potential difference appears across the electrodes known as the motion artifact. This is a source of noise and interference in biopotential measurements
Motion artifact is minimal for non-polarizable electrodes

14. Body Surface Recording Electrodes
Electrode metal
Electrolyte
Think of the construction of electrosurgical electrode
And, how does electro-surgery work?
Metal Plate Electrodes (historic)
Suction Electrodes
(historic interest)
Floating Electrodes
Flexible Electrodes

15. Commonly Used Biopotential Electrodes
Metal plate electrodes
Large surface: Ancient, therefore still used, ECG
Metal disk with stainless steel; platinum or gold coated
EMG, EEG
smaller diameters
motion artifacts
Disposable foam-pad: Cheap!
(a) Metal-plate electrode used for application to limbs. (b) Metal-disk electrode applied with surgical tape. (c)Disposable foam-pad electrodes, often used with ECG

16. Commonly Used Biopotential Electrodes
Suction electrodes
No straps or adhesives required
precordial (chest) ECG
can only be used for short periods
Floating electrodes
metal disk is recessed
swimming in the electrolyte gel
not in contact with the skin
reduces motion artifact
Suction Electrode

17. Commonly Used Biopotential Electrodes
Metal disk
Insulating
package
Double-sided
Adhesive-tape
ring
Electrolyte gel
in recess
(a)
(b)
Reusable
Snap coated with Ag-AgCl
External snap
Gel-coated sponge
Disposable
Plastic cup
Plastic disk
Tack
Dead cellular material
Foam pad
Capillary loops
Germinating layer
(c)
Floating Electrodes

18. Commonly Used Biopotential Electrodes
Flexible electrodes
Body contours are often irregular
Regularly shaped rigid electrodes
may not always work.
Special case : infants
Material :
- Polymer or nylon with silver
- Carbon filled silicon rubber
(Mylar film)
(a) Carbon-filled silicone rubber electrode. (b) Flexible thin-film neonatal electrode. (c) Cross-sectional view of the thin-film electrode in (b).

19. Internal Electrodes
Needle and wire electrodes for percutaneous measurement of biopotentials
(a) Insulated needle electrode. (b) Coaxial needle electrode. (c) Bipolar coaxial electrode. (d) Fine-wire electrode connected to hypodermic needle, before being inserted. (e) Cross-sectional view of skin and muscle, showing coiled fine-wire electrode in place.
The latest: BION – implanted electrode for muscle recording/stimulation
Alfred E. Mann Foundation

20. Fetal ECG Electrodes
Electrodes for detecting fetal electrocardiogram during labor, by means of intracutaneous needles (a) Suction electrode. (b) Cross-sectional view of suction electrode in place, showing penetration of probe through epidermis. (c) Helical electrode, which is attached to fetal skin by corkscrew type action.

21. Electrode Arrays
Contacts
Insulated leads
(b)
Base
Ag/AgCl electrodes
Ag/AgCl electrodes
Base
Insulated leads
(a)
Contacts
(c)
Tines
Base
Exposed tip
Examples of microfabricated electrode arrays. (a) One-dimensional plunge electrode array, (b) Two-dimensional array, and (c) Three-dimensional array

22. Microelectrodes Why Measure potential difference across cell membrane
Requirements
Small enough to be placed into cell
Strong enough to penetrate cell membrane
Typical tip diameter: 0.05 – 10 microns
Types
Solid metal -> Tungsten microelectrodes
Supported metal (metal contained within/outside glass needle)
Glass micropipette -> with Ag-AgCl electrode metal
Intracellular
Extracellular

23. Metal Microelectrodes
Microns! R
Extracellular recording – typically in brain where you are interested in recording the firing of neurons (spikes).
Use metal electrode+insulation -> goes to high impedance amplifier…negative capacitance amplifier!

24. Metal Supported Microelectrodes
(a) Metal inside glass (b) Glass inside metal

25. Glass Micropipette heat pull
Ag-AgCl wire+3M KCl has very low junction potential and hence very accurate for dc measurements (e.g. action potential)
heat
pull
A glass micropipet electrode filled with an electrolytic solution (a) Section of fine-bore glass capillary. (b) Capillary narrowed through heating and stretching. (c) Final structure of glass-pipet microelectrode.
Fill with intracellular fluid or 3M KCl
Intracellular recording – typically for recording from cells, such as cardiac myocyte
Need high impedance amplifier…negative capacitance amplifier!

26. Electrical Properties of Microelectrodes
Metal Microelectrode
Metal microelectrode with tip placed within cell
Use metal electrode+insulation -> goes to high impedance amplifier…negative capacitance amplifier!
Equivalent circuits

27. Electrical Properties of Glass Intracellular Microelectrodes
Glass Micropipette Microelectrode

28. Stimulating Electrodes
Features
– Cannot be modeled as a series resistance and capacitance
(there is no single useful model)
– The body/electrode has a highly nonlinear response to
stimulation
– Large currents can cause
– Cavitation
– Cell damage
– Heating
Platinum electrodes:
Applications: neural stimulation
Modern day Pt-Ir and other exotic metal combinations to reduce polarization, improve conductance and long life/biocompatibility
Steel electrodes for pacemakers and defibrillators
Types of stimulating electrodes
Pacing
Ablation
Defibrillation

29. Intraocular Stimulation Electrodes
Reference : Lutz Hesse, Thomas Schanze, Marcus Wilms and Marcus Eger, “Implantation of retina stimulation electrodes and recording of electrical stimulation responses in the visual cortex of the cat”, Graefe’s Arch Clin Exp Ophthalmol (2000) 238:840–845

30. In vivo neural microsystems (FIBE): challenge

31. In vivo neural microsystems (FIBE): biocompatibility - variant

32. In vivo neural microsystems (FIBE): state of the art

33. Introduction: neural microsystems
Instrumentation for neurophysiology
Neural Microsystems
MEMS - Microsystems
Neural microelectrodes

34. Introduction: types of neural microsystems applications–
External electrodes
Subdural electrodes
Micro-electrodes
Microsensors
Human level
Animal level
Tissue slice level
Cellular level
In vivo applications
In vitro applications

35. Microelectronic technology for Microelectrodes
Bonding pads
Si substrate
Exposed tips
Lead via
Channels
Electrode
Silicon probe
Silicon chip
Miniature
insulating
chamber
Contact
metal film
Hole
SiO2 insulated
Au probes
Exposed
electrodes
Insulated
lead vias
(b)
(d)
(a)
(c)
Beam-lead multiple electrode.
Multielectrode silicon probe
Multiple-chamber electrode
Peripheral-nerve electrode
Different types of microelectrodes fabricated using microfabrication/MEMS technology

36. Michigan Probes for Neural Recordings

37. Neural Recording Microelectrodes
Reference :

38. In vivo neural microsystems: 3 examples
University of Michigan
Smart comb-shape microelectrode arrays for brain stimulation and recording
University of Illinois at Urbana-Champaign
High-density comb-shape metal microelectrode arrays for recording
Fraunhofer Institute of Biomedical (FIBE) Engineering
Retina implant

39. Multi-electrode Neural Recording
Reference :
Reference :

40. WPI’s Nitric Oxide Nanosensor

41. Nitric Oxide Sensor Developed at Dr.Thakor’s Lab, BME, JHU
Electrochemical detection of NO
Left: Schematic of the 16-electrode sensor array. Right: Close-up of a single site. The underlying metal is Au and appears reddish under the photoresist. The dark layer is C (300µm-x-300µm)

42. Cartoon of the fabrication sequence for the NO sensor arrayG D H
Cartoon of the fabrication sequence for the NO sensor array
A) Bare 4” Si wafer B) 5µm of photoresist was spin-coated on to the surface, followed by a pre-bake for 1min at 90°C. C) The samples were then exposed through a mask for 16s using UV light at 365nm and an intensity of 15mW/cm2. D) Patterned photoresist after development. E) 20nm of Ti, 150nm of Au and 50nm of C were evaporated on. F) The metal on the unexposed areas was removed by incubation in an acetone bath. G)A 2nd layer of photoresist, which serves as the insulation layer, was spun on and patterned. H) The windows in the second layer also defined the microelectrode sites.

43. NO Sensor Calibration

44. NO Sensor Calibration

45. Multichannel NO Recordings