Studying Cell–Surface Interactions In Vitro: A Survey of Experimental Approaches and Techniques - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 49

4.3 Electrochemical Approaches for Studying Cell-Surface
Interactions
Experimental approaches based on electrochemical impedance analysis—also
referred to as impedimetric approaches—are emerging and very versatile research
tools for studying cell-substrate interactions in real time. The basic principle of
this technique was introduced by Giaever and Keese in 1984 and has been
continuously optimized ever since. In the initial publication the technique was
named electric cell-substrate impedance sensing or short ECIS and it has paved
the way for several modifications that are all based on the ECIS principles [ 45 ]. In
ECIS, adherent cells are grown on the surface of planar gold film electrodes, which
are pre-deposited on the bottom of a cell culture dish by thin-film technology. The
gold films serve as growth substrate for the cells and, at the same time, as elec-
trodes for the electrochemical measurement. With the cells adhering essentially to
the electrode surface, there is only a gap of 20-200 nm between the cell bodies and
the measurement probe. By virtue of this arrangement the measurement is
particularly sensitive to changes that occur within the cell-surface junction in
unprecedented detail.
In the most commonly used configuration the measurement system contains
two electrodes: a small working electrode (5 9 10 -4 cm 2 ) and a substantially
larger (*500-fold) counter electrode (Fig. 9 ). The electric circuit is completed
by the cell culture medium on top of the cell layer. ECIS is based on measuring
changes in the electrochemical impedance of the gold film electrodes at different
AC frequencies (alternating current). As the cells behave essentially like insu-
lating particles, they force the current to flow through the cells or around the cell
bodies. Both situations result in an increase of the measured impedance due to
the presence of the cell bodies on the electrode surface compared to a cell-free
electrode.
When initially suspended cells are seeded on an ECIS electrode, it is possible to
follow attachment and spreading of the cells upon the electrode surface from time-
resolved impedance readings due to the gradual constriction of current flow. ECIS
recordings are therefore particularly well-suited to follow the kinetics of cell
spreading. Once the cells form a confluent monolayer the impedance becomes
stationary as long as the cells do not change their shape. In this situation ECIS is
capable of monitoring all experimental challenges that are mirrored by a change in
cell shape. As many chemical, physical or biological stimuli result in minute
changes of cell morphology, the technique is widely applicable in many research
areas that will not be addressed here, such as cytotoxicity screening, GPCR-
mediated signal transduction or stem cell differentiation. ECIS readings rely only
on small amplitude currents and voltages such that the cells are not affected in any
way by the electric field used for the measurement. It is considered to be a
non-invasive approach even if the experiment spans several days or even weeks.
The method was originally developed using gold as the electrode material, and
gold is by far the most widely used material due to its inertness, its biocompatibility,