Bioelectrodes, Neural Prostheses, and Biosensors

Bioelectrode

conducts current across the interface between the body and the electronic measuring circuit

serves as a transducer to change ionic current <→ electronic current and vice

Transducer

changes an ionic current ←> electronic current

bioelectrode acts as one

Recording Electrodes

monitor/measure electrical events in the body (reads)

ex. ECG, EEG, EMG, extracellular neural recording

ECG

electrocardiogram (heart??)

type of recording electrode

EEG

electroencephalogram

type of recording electrode

EMG

electromyogram (muscle??)

type of recording electrode

Extracellular Neural Recording

used for neural prosthesis such as “brain machine interface”

high resolution from neurons

type of recording electrode

Stimulating Electrodes

transmit electrical current into the body to influence specific physiological processes (writing)

Stimulating Electrodes Examples

cardiac pacemakers and defibrillators

neural prosthesis (deep brain stimulators, cochlear implants, and functional electrical stimulations) → activate or inhibit neurons

transcutaneous drug delivery - across the skin layer (used to open up barrier) (used with spinal cord inuries to get some function back)

stimulators for wound healing/regeneration (using body’s own bioelectricity)

Bioelectronic Medicine

instead of using chemical compounds, tap into neural system and use electronics to control it (neural, digestive system. etc.)

Electrochemical Biosensors

modulation close loop so don’t just turn things on a let it go

allows for a read out of the body’s response to a stimulator (helps to know when to shut it off etc.)

Faradaic Process

happens at the electrode-electrode interface

charge transfer governed by Faraday’s law

at interface, charge can be exchanged through redox chemical reactions bc electrons can’t just jump across

Faraday’s Law

the amount of chemical change occurring at an electrode-electrode interface is directly proportional to the current that flows through the interface

Electordes undergo mainly faradaic processes called what?

charge transfer electrodes

or

non-polarizable electrodes

Polarization at Electrode-Electrode Interface

half cell potential of electrode results in electrical double layer formation (considered as a capacitor)

another type of charge transfer

direct current can’t just cross but alternating current can cross through non-faradaic process

Non-faradaic Process

current flow can be passed by charging/discharging a capacitor

polarization at electrode-electrode interface

Perfectly Polarizable Electrodes

do not undergo faradaic process

Polarizable Electrodes

noble metal Pt, Au, or stainless steel

most bioelectrodes are

MIxed Situation

no 100% Faradaic or non-Faradaic (always a mixture but one is more dominant)

Equivalent Circuit Model - Biopoential Electrode in Contact withan Electrolyte

Ehc - half-cell potential

Rd and Cd - make up impendance of electrode-electrode interface and polarization effects

Rs - series resistance (from interface effects and resistance of the electrolyte)

circuit with half cell potentiall in series with Rd and Cd in parallel and then R

Impedance Spectroscopy

impedance is a fcn of frequency

dropping is capacitor

how to measure mixed interface

Non-polarizable Electrode

can be made by electrolytic process or sintering process

ex. Ag/AgCl → reversible rcns; silver wire/disk, put in a Cl soln., oxidize? → silver ion + chlorine togetehr to form electrode

preferred by bio stuff bc isn’t based on movement bc not conducting current through it???

Ag/AgCl

impedance increases as AgCl layer increases = more resistance (contributes to its own resistance)

Ag (silver) - Faradaic/polarizable but AgCl non-Faradaic/non-polarizable

Skin Surface Electrode

need to overcome the stratum corneum layer (top layer of skin/epidermis → layer of dead cells that makes teh surface very hydrophobic so hard for ions to pass through) → gel essentially hydrates it so ions can get through and signal is measureable (sweat too) and lowers impedance

Biopotential Electrode Varieties

used fo EEGs

Myoelectric Prosthesis

for amputees

nerves are still there and tryong to find missing body part and if doesn’t it casues pain

so redirect neurons to chest instead then use EMG?? to measure what neurnon wants to do (code to control prosthesis)

Brain Machine Interface I

another application of myoelectric??

using motor neurons in the motor cortex in the brain (usually for paralyzed people)

use thoughts to control robotic arm to get food

Brain Machine Interface II

same implant as other brain one (motor neurons) just in a person

Various MEAs for Neural Recording

Utah array (sold by Blackrock) (silicon w/ doped needles??)

microwire array (manually made)

Michigan Proves

have gold pads contacting w/ off chip instrumentation

polycrystalline silicon thin-film conductors

gold or iridium electrode sites

uses same process that is used to make computer chips

Challenges at Neural Interface

1. unreliable single unit yield

2. signal fades over time

3. signla drift (stability of recording = not good → calibrate systems often)

4. stimulation threshold increase

essentially very tiny signals so electrode needs to be close to neuron to be able to pick up the signal (within 100 microns)

Problem - Unreliable Chronic Recording

?????????

Cellular Response around the Inserted Neural Electrode

2 contributing factors (biological response and mechanical stuff)

inserting an electrode will break blood vessls and/or the blood-brain barrier

Microglia

can be resting/ramified or activated/amoeboid

Resting or Ramified Microglia

no Fc receptors or macrophage specific ones

large, dense chromatin masses in small flattened nucleus

5-10% of glial pop.

many long, thin processes

essentially sruvery environment for anything wrong

Activated or Amoeboid Microglia

upregulation of lytic enzymes

retraction of cytoplasmic procssses to stouter, thicker pseudopodia

accumulation of phagocytosed material and lipid droplets

ameboid/motile abilities

release of inflammatory factors MHC I, II expression

active proliferation

similar to monocyte → macrophage

Astrocytes

normal or reactive

Normal Astrocyte

8-10 nm diamter GFAP filaments

round nucleus

10-40 nm glucogen particles

large mitochondira

gap junctions w/ other astrocytes

maintain proper neuronal environment (structural and function support)

30-65% of glial pop.

Reactive Astrocyte

large increase in GFAP filaments, glycogen deposits

irregular nucleus

increase in mitochondria and multiple golgi complexes

hypertrophy

phagoctosis

proliferation - produce ECMs to wrap around neurons (like fibroblasts)

migration

extracellular matrix population

production of neurotrophic and inflammatory factors

GFAP

an intermediate filament that has a unique stain

Surface Chemistry Matters

antifouling zwitterionic polymers carry large amounts of positive/negative ions, are highly hydrophilic, resist protein adsorption, and cell attachment

Mechanics Matters

stimulating device inserted into materials with different mechanical moduli → how does this change the strain the tissue is experiencing

Size Matters

large device - morve invasive = more inflammation

small - electrodes the size of neuron stromas = seamles integration w/o glial response (future direction??)