Studying Cell–Surface Interactions In Vitro: A Survey of Experimental Approaches and Techniques - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 59

excited sequentially along a narrow frequency band around its fundamental
resonance and the electrical impedance of the system is recorded. Figure 15 a
shows a typical impedance spectrum for a medium-loaded resonator compared to
the same resonator covered with a confluent monolayer of NRK cells.
The presence of cells on the resonator surface is most obviously expressed by a
significant damping of the shear oscillation as indicated by shifts of the minimal
(|Z min |) and the maximal impedance (|Z max |). Attachment of the cells to the reso-
nator induces only a minor shift of the impedance spectrum along the frequency
axis towards lower frequencies. This confirms that the cell monolayer induces
primarily dissipation of motional energy and only a negligible storage of elastic
energy.
Attachment and spreading of initially suspended cells to the resonator surface is
followed over time by continuously recording impedance spectra such as the one
shown in Fig. 15 a. The gradual coverage of the surface is mirrored in the change of
the minimal impedance magnitude D|Z min |, which is directly extracted from the
raw data (blue horizontal lines in Fig. 15 a), as a function of time (Fig. 15 b).
Figure 15 b shows the time course of the minimal impedance D|Z min | during the
attachment and spreading of initially suspended NRK cells seeded to confluence
upon the resonator surface, expressed relative to the value for the medium-loaded
resonator. Immediately after cell inoculation (t = 0), D|Z min | shows a characteristic
steep increase, reaching a stationary value once spreading is complete. Since the
cells were seeded to confluence into the measuring chamber, i.e. they already
cover the entire quartz surface after sedimentation and adhesion without any need
for further cell proliferation, the final change in D|Z min | corresponds to a confluent
monolayer of cells on the surface. The initial increase of D|Z min | with time
describes the kinetics of cell attachment and spreading, which is quantified by two
parameters, t 1/2 and s. The quantity t 1/2 describes the time that is needed to reach
the half-maximal change of D|Z min |, which corresponds to half-maximal surface
coverage. The slope s of the attachment curve quantifies the spreading rate and is
directly proportional to the adhesion energy of the cells.
With the help of these different operational modes it is possible to unravel
several key features of any QCM-based analysis of cell-surface junctions:
1. QCM readings only report on specific, integrin-mediated cell adhesion to the
resonator surface. Sedimentation and loose attachment of cells to the reso-
nator surface via nonspecific interactions do not influence the QCM readout
[ 58 , 66 ].
2. QCM readings are only sensitive to those parts of the cellular body that are
involved in making cell-substrate contacts and that are close to the resonator
surface [ 66 ]. Thus, sensitivity is confined to the cell-surface junction.
3. The presence of a confluent cell layer on top of the resonator surface leads to a
significant increase in viscous energy dissipation, usually many times (2- to
10-fold) larger than the increase in the stored energy [ 67 ]. The impact of cells
on energy dissipation was shown to be cell-type-dependent reflecting individual
acoustic properties.