NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 287

Based on their size effect results, the authors further suggested
several implications of the study for interpretation of indentation testing
of cartilage and design of future experiments. 22 First, the mechanical
properties measured by different investigations may vary substantially if
different indenter sizes are used, so care must be taken in comparing the
results of different studies. Second, to minimize the influence of size
effects in an experiment, a single size tip should be used for the entire
study; consistent cartilage thickness may also be important. The authors
further suggest that a tip should be selected to ensure a large enough
contact area to measure tissue level properties rather than cell-level
properties, and a penetration depth should be selected to simulate
the 20% compression typically experienced during cartilage loading
in vivo . 22 Similar constraints with regards to sampling tissue-level
properties and simulating physiologically relevant loading conditions
could be considered for all soft tissues.
The indenter size effect observed in this study 22 and the depth
dependent contact stiffness observed by Franke et al. 19 both highlight a
potential limitation in comparing data between experiments. Both
authors postulate that these effects are due to the inhomogenous cartilage
structure as a function of depth. 19,22 Even if this is the source of variation,
it makes it difficult to compare mechanical properties between different
experiments and even different tissue regions, since tissue thickness
and/or structural organization may vary from region to region. This
concern could translate to other inhomogeneous tissues as well. Another
possible contributor to both the observed trends is substrate effect. The
cartilage regions tested in these studies have finite thicknesses, typically
on the order of 0.5 to 2 mm. Below the cartilage layer is a much stiffer
bone layer. As penetration depths increase relative to cartilage thickness,
the modulus measured by indentation could become increasingly
influenced by the underlying bone layer. In addition, the aspect ratio (the
ratio of tip radius to sample thickness) has been shown to influence
the mechanical properties measured by indentation, leading to the
development of correction factors as a function of aspect ratio for
macroscale cartilage indentation. 12,14,17 The substrate effect and its role in
influencing tip selection will be discussed in more detail in Section 4.