**2.4. Comparison of AFM with QCM-D results**

The two sensitive techniques, AFM and QCM-D, described in this chapter probe two different regions of cells in a monolayer on a solid surface. The nature of the AFM limits it to probing the top surface and immediately underlying volume of individual cells. The nature of the QCM-D limits it to probing the basal areas of the cells in a monolayer. The AFM technique revealed an increase in Young's modulus, energy dissipation, and hysteresivity in response to EGF, while the QCM-D technique revealed a decrease in energy dissipation factor. The fluorescence studies of the changes in actin in the top and bottom surfaces upon exposure to EGF corroborate the differences observed in mechanical properties.

Because both hysteresivity (*η*) and the change in energy dissipation factor (∆*D*) represent the ratio between the dissipated energy and the elastic energy, these two quantities are analogous to each other (Collinsworth et al., 2002). The fact that the opposite trends in them were obtained upon exposure of the cells to EGF is perhaps not unexpected (Figure 5 and 7A). For a monolayer of cells on a solid surface, the bottoms of the cells are restricted in shape by contact with a solid surface, while the tops of the cells have no such restrictions. The bottom surfaces of the cells may therefore exhibit unique responses to the presence of the solid surface and the tops of the cells may not. Thus, contrasts in mechanical properties and in the number, size, and form of actin filaments signify that the mechanical responses of cells to EGFR signaling are regionally specific.

Interestingly, both hysteresivity (*η*) and the change in energy dissipation factor (∆*D*) required about approximately the same amount of time (30 to 40 min) to reach each respective maxima or minimum after the addition of 10 nM of EGF. Such a similarity suggests that both of these regional mechanical responses (∆*D* and *η*) were mediated by closely coupled cell signaling pathway(s) of EGFR.
