Biomedical Engineering Reference
In-Depth Information
Figure 9-5.
Schematic illustrating tip selection considerations for indentation of a tissue
sample. Comparison of contact sizes during indentation using sharp tips (
e.g.
, AFM tip
(A) or Berkovich tip (B)), small diameter blunt tips (
e.g.
, 10
μ
m diameter flat-ended
conical tip (C) or 20
μ
m diameter spherical tip (D)), and large diameter blunt tips (
e.g.
,
100
m diameter cylindrical flat punch (F)). Note
that projected contact areas will depend both on tip geometry and depth of penetration
(except for flat-ended conical tips and flat punch tips, which have a constant contact area
independent of penetration depth). Also, flat-ended conical tips, spherical tips, and flat
punch tips come in a range of diameters ranging from micrometers to millimeters. The
tips shown here are merely representative of geometries used for soft tissue indentation
testing.
μ
m diameter spherical tip (E) or 100
μ
spherical tips than for flat punches (constant contact area) or Berkovich
tips (self-similar shape as a function of depth).
Another concern that can affect tip selection and data interpretation
is the possibility of substrate effect, or a stiff substrate underlying a
soft material influencing the mechanical properties measured by
nanoindentation. When indenting in soft tissues of finite thickness, such
as cartilage on subchondral bone, the thickness of the sample can limit
both the contact radius of the tip and the depth of penetration that should
be used in the study. Studies of thin films of relatively homogeneous
engineering materials have led to the rule of thumb that the indentation
depth should not exceed 10% of the sample thickness.
68
Similar
constraints also apply to the aspect ratio, the ratio of contact radius
to sample thickness. For example, larger scale indentation studies
in cartilage have also demonstrated that the aspect ratio influences
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