INTRODUCTION - Nanoindentation with Biological Applications - page 18

materials. Biological materials can be functionally graded, where the
mechanical properties vary from point to point for reasons to do with the
biological function. 22 These property variations result from local
variations in tissue composition and microstructure, and the variations
are frequently associated with the length-scales of ECM ( i.e. length
scales of tens of nanometers to micrometers) and of cell activity ( i.e.
length scales of micrometers to tens of micrometers). There is no
question that elastic modulus is a useful parameter to measure in
biological materials, and in many cases the viscoelastic response may
also be relevant for interpretation of the elastic response. The “hardness”
may still be a useful measure, although with the caveats as noted above.
The conditions in which Oliver-Pharr 15 analysis is appropriate, and the
alternatives for analysis of indentation data, will form a common theme
throughout the remainder of this work, with detailed consideration of
different contact mechanics models for data analysis including tip-
sample adhesion in Chapter 4 and in the context of viscous deformation
in Chapter 5 . Because of the complexity of both the material response
and the geometric nonlinearities that arise in contact problems, much
recent work in this field has used computational models of the
contact problem instead of relying on closed-form analytical solutions
for simpler contact problems, and this will be considered further in
Chapter 6.
Another challenge arising in the context of nanoindentation of
biological materials is the large elastic modulus range found, especially
compared with the glass, metal and ceramic materials for which
nanoindentation testing was originally designed ( Fig. 1-11 ) .
Nanoindentation instrumentation was optimized for the narrow range of
engineering materials in the GPa to hundreds of GPa range, where
typical experiments are conducted in mN and nm ranges. In contrast, for
soft tissues, with kPa-MPa modulus values, the corresponding working
ranges might be N-m or even N-mm. There are thus significant
instrumentation challenges associated with the adaptation of indentation
instrumentation to the full range of elastic modulus values that arise
when considering biological materials; these issues will be examined in
detail in Chapters 2 and 3 of this volume.