Studying Cell–Surface Interactions In Vitro: A Survey of Experimental Approaches and Techniques - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 56

(QCM) that has a long track record as a mass-sensitive tool to study adsorption
reactions at the solid-liquid interface. It operates non-invasively and with a superb
time resolution that is much better than necessary for most cell-related studies. The
core component of this technique is a thin, disk-shaped piezoelectric (AT-cut)
quartz crystal sandwiched between two gold film electrodes. When an oscillating
potential difference is applied between the surface electrodes, the piezoelectric
resonator is excited to perform mechanical shear oscillations parallel to the crystal
faces at the resonator's resonance frequency (Fig. 13 ). This mechanical oscillation
is highly sensitive to changes that occur at the resonator surface, so that adsorption
or desorption processes can be followed by readings of the resonance frequency
f [ 51 ] or by analyzing the shear oscillation of the resonator using principles of
impedance analysis [ 52 - 54 ].
For many years the QCM technique was used as an established and accepted
tool for studying deposition processes of thin material films in the gas phase or in
vacuum. As long as the adlayer film is rigid and homogeneous, the resonance
frequency decreases in proportion to the amount of deposited mass [ 55 ], providing
a balance with nanogram sensitivity. Recent progress in designing better oscillator
circuits to determine the resonance frequency or alternative readout approaches
has paved the way to monitor adsorption processes even in an aqueous environ-
ment—a prerequisite for the detection of protein adsorption or cell adhesion
processes under physiological conditions.

4.4.1 Monitoring Attachment and Spreading on Protein-Coated
Resonators

The most sensitive operational mode for a quartz resonator is the active oscillator
mode. Here, the quartz resonator is integrated as the frequency-controlling element
in an oscillator circuit and the resonance frequency of the crystal is recorded with
high sensitivity and a time resolution of less than 1 s. The oscillator circuit only
compensates for energy losses and maintains the quartz resonator at its resonance
frequency. The general applicability of the QCM technique in the active oscillator
mode for studying cell adhesion has been demonstrated by various authors
addressing a variety of bioanalytical issues. Gryte et al. [ 56 ], Redepenning et al.
[ 57 ] and Wegener et al. [ 58 ] monitored the attachment and spreading of initially
suspended mammalian cells on the resonator surface in real time by readings of the
resonance frequency. They showed that the attachment and spreading of
mammalian cells upon the resonator surface induced a decrease in the resonance
frequency that was proportional to the fraction of the surface area covered with
cells (Fig. 14 ). Thus, time-resolved measurements of the resonance frequency
mirror the kinetics of cell attachment and spreading on the resonator surface.
To illustrate the quality of the data, Fig. 14 a shows the time course of the
resonance frequency shift when increasing amounts of MDCK cells are seeded on
the resonator surface at time zero. After a transient slight increase of the resonance
frequency due to warm-up of the medium, Df decreases, reporting on the