Biomedical Engineering Reference
In-Depth Information
including wood,
30,38,39
bamboo,
40
and spider silk,
9
but anisotropy methods
than account for the influence of the moduli along other axes were not
used in the analyses. Further development of anisotropic models and
experimental methods specifically for soft tissues would greatly enhance
the field of nanoindentation of biological materials by allowing
researchers to more fully capture the complex mechanical behavior of
these materials, rather than relying on simplifying assumptions.
4.4
. Data interpretation
A final area of concern in applying nanoindentation to biological
materials is how to interpret the resulting data. There is often a desire
to compare mechanical parameters measured by nanoindentation with
the same properties measured using bulk testing. However, due to the
complexity in structure of many biological materials, and the local
heterogeneities associated with their complex and hierarchical
microstructure, it may not be realistic to assume that parameters
measured at the nano- or micro-scale should correlate with bulk
properties.
In fact, a key advantage of nanoindentation is the ability to
map local variations in mechanical properties within a tissue, which will
not translate to bulk characterization. For validation of experimental
methods, a homogeneous compliant material can (and should) be
characterized, and results correlated with macroscale testing
techniques.
21
However, care should be taken in extrapolating the results
from nanoindentation studies of heterogeneous biological materials to
macroscale properties.
An additional concern with data interpretation is whether modulus
values or other parameters measured through indentation testing have
quantitative meaning or only qualitative meaning. For example, cartilage
studies demonstrated that the modulus measured by nanoindentation
was dependent upon tip size,
22
and the authors suggested that only
experimental data collected under indentical experimental conditions be
compared. Considering all the sources of error in measuring mechanical
properties of soft tissues using current commercial instrumentation
and experimental techniques (
e.g.
, sink-in, adhesion), the lack of suitable
control materials to validate experimental methods, and the expectation
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