INTRODUCTION - Nanoindentation with Biological Applications - page 6

in contact mechanical testing a probe with known or calibrated shape is
brought down onto a flat surface of material. The total sample volume V 0
can be substantially larger than the volume of material mechanically
tested, V ( Fig. 1-3 ) . Thus, while a tensile test averages the mechanical
response over the whole sample, a contact test measures local behavior
over a contact diameter of 2 a . Contact mechanical testing has become
popular for examining local variations in mechanical behavior because of
this small ratio of V / V 0 . Indentation testing at very small length scales has
been frequently referred to as “nanoindentation” although in reality it
ranges from true nanoindentation to microindentation ( Fig. 1-2 ) .
Due in part to the hierarchical and multiscale organization of
natural materials, the mechanical response can vary depending on the
length-scale of observation. Especially at fundamental length-scales,
there is dramatic structural heterogeneity in natural materials due to
distributions in shape, size, composition, and mechanical properties
of the constituents. Nanoindentation is thus ideal for probing local
heterogeneities in natural materials, and can be used to map out local
heterogeneity, and great scope exists both for experiments at very small
(nm) length scales and for multi-scale investigations incorporating
nanomechanical results with higher levels of organization (length-scales
from nm to m).
In the following sections, classes of biological materials are reviewed.
Nanoindentation testing techniques are introduced, and the limitations of
traditional nanoindentation are discussed in the context of natural
materials. The “roadmap” for the remainder of this volume is presented.
2. BiologicalMaterialsTaxonomy
The natural world is broad and diverse. There are existing
mechanisms for classifying biological organisms based on an eight-level
hierarchy: domain, kingdom, phylum, class, order, family, genus,
species. The two-name “scientific” designation according to genus-
species is the standard identification mechanism for biological
organisms, for example Escherichia coli ( E. coli , a bacterium) or Homo