INDENTATION AND ADHESION AT SMALL LENGTH SCALES - Nanoindentation with Biological Applications - page 37

indentation deformation. These values are much more amenable to
measurement with AFM and nanoindenters designed explicitly for such
small load testing; in both cases the probe spring is much more compliant
than that of a typical nanoindenter and the loads required are comparable
to those exerted by surface forces. Adhesion effects are thus expected to
be significant for many indentation measurements of mechanical
properties of biological materials. If 100 kPa is taken as a representative
modulus value, measurement by AFM is clearly required and adhesion
effects will be almost inescapable; very stiff cantilevers may be used to
minimize deflection from surface force effects, but such cantilevers will
also exhibit very little deflection during indentation contact with the very
compliant material. Hence a framework is required to incorporate
adhesion effects into indentation measurements of biological materials. It
is clear from the discussion above that such a framework should focus
heavily on the stiffness of the measurement probe relative to the stiffness
of the surface force and bulk deformation fields. Such a framework will
be developed in Chapter 4 , but first Chapter 3 will provide a survey of
the experimental issues and challenges associated with measurements of
contact responses of biological materials.