Biomedical Engineering Reference
In-Depth Information
mechanical properties in a material and correlate changes in mechanical
behavior with changes in morphology or composition. Mechanical
testing techniques with high spatial resolution will allow for more refined
mapping of material properties in biological systems.
Although tools have long existed to characterize the structure of
materials on small length scales, such as optical, X-ray and scanning
electron microscopy (SEM), only in recent years have tools become
available to measure mechanical properties on similar length scales. Two
of the most versatile commercial approaches to characterizing
mechanical properties of biological materials at small length scales are
the scanning probe techniques of atomic force microscopy (AFM) and
nanoindentation.
As discussed in detail in
Chapter 2
, the atomic force microscope
(AFM) is a tool traditionally used for submicrometer testing. While
the AFM was originally developed as a tool for mapping surface
topography,
2
it has since been adapted to measure both tribological
properties (such as adhesion and friction) and elastic and plastic material
properties.
3
A typical AFM probe consists of a sharp tip attached to the
end of a cantilever beam. For force measurements, the cantilever tip is
pressed into the surface and the deflection is monitored and converted
to force values based on contact mechanics theory and a cantilever
mechanics model.
4
Due to the low loads applied in AFM and its
high sensitivity, atomic force microscopy is most commonly applied
to the measurement of atomic and molecular forces. In biological
applications, AFM has been effectively used to measure intermolecular
forces and mechanical properties of biomolecules and living cells.
4,5
Characterization of tissue-level properties using an AFM is challenging
due to the very small displacement and load ranges of the probes, and
the use of the cantilever to apply load, and hence AFM has seen limited
use in the area of tissue characterization. Instead, nanoindentation has
served as the submicrometer resolution mechanical testing tool for
tissue-level studies.
In comparison to AFMs, nanoindenter systems apply perfectly
normal loads to the sample and can accurately measure loads and depths
of higher magnitude. With a typical working force range of 100 nN to
Search WWH ::
Custom Search