BME 732 - Electrode Types, Use, and Biocompatibility

medicineman's version from 2015-10-05 18:11

General Information

Question Answer
Electrode requirementsInterface must be safe and reliable, degree of invasiveness must not exceed what is necessary, and the neural signals must have sufficient information to support BCI use
Electrode reliabilitySignal-to-noise ratio (SNR) should not decrease over time
Functional componentsArray of electrodes for recording, lead or trace that electronically connects the electrodes to the separate electronics, the dielectric, the substrate, and surface coatings
Array of electrodesSite material and the area, roughness, and shape of the contact surface determine the electrical characteristics
LeadMust have low resistance to minimize signal loss and sufficient flexibility and robustness to avoid breakage and mechanical stress on the electrode contact
DielectricThe material, or composite material, that electrically insulates each individual lead from the surrounding tissue. Generally characterized by its dielectric constant, leakage resistance, shunt capacitance, and the degree to which it degrades over time in tissue
SubstrateProvides structural integrity of of each shank of the electrode array
Surface coatingModify electrical, mechanical, or biological characteristics of microelectrode array. They can lower impedance, sense neurochemicals, or deliver small doses of drugs
Impedance and resistanceResistance is linear while impedance has a capacitive component

Electrode - Electrolyte Interface

Question Answer
Electrode acts as a transducerChemical to electrical potentials (ionic current to electrical current)
Current in the bodyCarried by ions
Current in electronics Carried by electrons
Electrode-Electrolyte InterfaceDraw diagram
Current flow at the Electrode-Electrolyte (Electrode-Tissue) Interface Electrons move in the opposite direction of the current flow. Cations move in same direction, anions move in opposite direction. No current at equilibrium
OxidationLoss of electrons
ReductionGain of electrons
Redox reactionsDependent on how corrosive the metal is
Half-Cell PotentialElectric potential difference between the electrolyte surrounding the metal and the rest of the solution
OverpotentialDifference between non-zero current and zero-current half-cell potentials


Question Answer
Perfectly polarizable electrodesNo charge across the electrode when current is applied. Current does not cross, but changes the concentration of ions at the interface. Behave like capacitors. Noble metals (platinum/gold) are the closest to perfect; difficult to oxidize and dissolve
Perfectly non-polarizable electrodes Charges freely cross the interface when current is applied. No overpotential generated. Behave like resistors. Silvers/silver-chloride good example. Commonly used in stimulation applications
Equation for currentI=1/2*pi*f*C
Electrode circuit modelDraw the circuit model and label the components/describe what they represent
Motion artifactsGel is disturbed which perturbs the charge distribution, changing the half-cell potential. Minimized by using non-polarizable electrodes and mechanical abrasion of the skin
Electrode geometriesMetal-plate, metal-disk, foam pad, metal suction, percutaneous, and glass micropipet
Michigan-arrayA fixed silicon-based array. Position fluctuates with movement and blood pressure due to small diameter of microwires
Utah-arrayFloating array. Each shank contains a platinum insulated electrode which allows for higher resolution at each recording site. Increased resolution allows recording from various layers of the cortex. Rigidity can cause problems in vivo

Nernst Equation

Question Answer
EHalf-cell potential
E0Standard half-cell potential
RUniversal gas constant (8.31 J/mol-K)
TAbsolute temperature in Kelvin
nvalence of the electrode material
FFaraday constant (96,500 C/(mol/valence))
aCn+Ionic activity of cation Cn+ (availability to enter into a reaction)

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