Electrochemical Impedance Spectroscopy (EIS)
procedure
classical technique: 3 electrodes
- apply sinosoidal potential to electrochemical cell
- -> measure response signal (voltage or current)
- calculate impedance Z
- evaluate amplitude and phase shift of response signal
- repeat measurement for a frequency spectrum
Resistance - Real part vs. imaginary part
Real part: measure for energy consumption of the system (heat)
Imaginary part: phase-shifted, measure for electrical energy, which is stored in form of an electric field
size of stimulation voltage
stimulation voltage must be small
Methods to measure passive electronic properties of cells
1) Transepithelial electrical resistance (TEER)
2) Electric Cell-substrate impedance sensing (ECIS) on metal electrodes
3) FET Cell-substrate Impedance Sensing (FETCIS)
TEER
measurement technique for in vitro barrier model systems
- 2 electrodes, layer of cells on a semipermeable membrane in the middle
- -> extract impedance
ECIS
working principle
- impedance measurement between metal microelectrodes
- small gold electrode with cells
- large gold counter electrode
ECIS - extractable parameters
Resistance between cells
membrane capacitance
impedance from cell-adhesion sites
Cytometer
for exact single cell analysis individualized cell flow is needed
–> hydrodynamic focussing
- measurement of size and volumina of cells by impedance spectroscopy
Cell adhesion probing with FETCIS
current pathways
- stimulation voltage applied at reference electrode –> two current pathways to the sensor
1st: capacitive current through cell
2nd: resistive path through cleft
FETCIS
advantages
advantages:
- lower impedance –> good for single cell
- large bandwidth
- cell migration can be measured
Cell migration mechanism
repetitive sequence: membrane protrusion and adhesion, concentration of cytoskeleton and detachment of the cell’s rear
–> gradual dislocation of the adhesion strength is generating the movement
Liquid junction potential
- interface between two solutions with different components or concentrations
- -> small potentials may develop at junctions
- a large concentration difference between the liquid phases results in a high liquid junction potential
- rate of diffusion of each ion is proportional to its speed in an electric field
Sharp microelectrode recordings
- technique needs tiny flexible glass needles with openings below 1 um
- -> special, high-impedance amplifiers are needed
only for membrane potential recordings
passive electronic properties of cells
1) Barrier functions of cells and tissues
- cell growth
- cell integrity
- carrier function
- -> TEER, ECIS
2) Cell Substrate adhesion
- cell growth
- proliferation
- migration
- invasion
- -> ECIS, FETCIS
