Studying Cell–Surface Interactions In Vitro: A Survey of Experimental Approaches and Techniques - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 57

Fig. 14 a Shift in resonance frequency during attachment and spreading of initially suspended
MDCK cells. From the upper to the lower curve seeding densities (in 10 5 cm -2 ) increase (open
circle 0; filled circle 1.3; triangle 1.8; filled down triangle 3.7; filled square 7.7; open square 15).
Df max indicates the maximum frequency shift observed for a given seeding density. b Maximum
frequency shift Df max as a function of the cell density seeded on the resonator surface at time
point zero. T = 37 C
continuous progress in cell attachment and spreading. The total shift in resonance
frequency (Df max ) increases with increasing seeding density up to a threshold
value. This becomes obvious when the total frequency shift is plotted against the
number of seeded cells, providing a saturation curve (Fig. 14 b).
For low seeding densities the frequency shift is proportional to the fractional
surface coverage with cells. However, when all adhesion sites on the surface are
occupied by cells, the frequency shift does not further increase even though the
number of seeded cells is further raised. This observation confirms that only
those cells in direct contact with the resonator surface contribute to the QCM
signal.
Wegener et al. [ 58 ], who studied the adhesion of different mammalian cell
types, have additionally found that confluent monolayers of different cell types
produce individual shifts in resonance frequency, possibly reflecting individual
molecular architectures of their cell-substrate contacts. With a more detailed
understanding of this cell-type-specific QCM readout, unprecedented information
about the interactions of cells with in vitro surfaces will become available. This
next step requires methodological improvements with more observables than just
the resonance frequency. The resonance frequency of the quartz resonator is an
integral parameter sensitive to both mass deposition and changes in the density or
viscosity of the material in contact to the resonator surface [ 59 ]. Discrimination
between these two contributions is not possible from readings of the resonance
frequency alone. This, however, becomes important when the resonator is loaded
with a complex material that neither behaves like a rigid mass nor has uniform
contact with the surface. Cells are viscoelastic bodies for which the linear rela-
tionship between adsorbed mass and change in frequency is not valid [ 60 , 61 ].
Rodahl et al. [ 61 , 62 ] developed an extension of the traditional QCM technique,