CELLS AND MEMBRANES - Nanoindentation with Biological Applications - page 336

interfaces), DLVO double layer interaction. The specific forces involve
ligand-receptor interactions and are mainly responsible for cell signaling.
The actual surface force law or potential is difficult to measure
experimentally. Most biomedical and engineering literature assumes
adhesion to be effective only for adherends in intimate contact with zero
intersurface separation, though this is not true in general since the force
range usually spans the nm-mm range, yet the cell dimension is in
micrometers and intercellular separation is in nanometers.
Notwithstanding the many qualitative techniques for thin film
adhesion, a number of quantitative methods are documented in the
literature, e.g. statistical distribution of cells remaining in adhesion
contact when subject to a liquid flow, micro-pipette aspiration, and
reflection interference contrast microscopy (RICM). Some of these
techniques are also applicable in macroscopic tissue level.
In this chapter, we focus on a modified version of nanoindentation
similar to the shaft-loaded blister test where the deformation is confined
to a local area comparable to the probe dimension. A general force
potential is devised, which takes the form of a step function or a
Heaviside function with a finite magnitude, F s , and range, w s ,
corresponding to finite adhesion energy, . We will consider adhesion of

isolated thin films in general before proceeding to single cells.

γ

4.1. Membrane adhesion

Figure 10-7 shows a sketch of the adhesion test configuration. Rather
than applying a compressive force on the sample membrane via a ball
bearing, a tensile force is applied to a flat punch ( e.g. AFM tip with a
planar or blunt tip) which is in adhesion contact with the membrane. A
circular delamination front is driven into the probe-membrane interface.
Simultaneous measurement of the force, probe displacement and contact
radius yields the adhesion energy. The governing elastic equation is the
same as Eq. 10-15 and the delamination trajectory can be formulated
using a thermodynamic energy balance between the energy stored in the
overhanging elastic annulus and energy expenditure on making new
surfaces.