NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 273

of the tissues in health and disease, developing diagnostic tools
and treatments, and designing replacement tissues that can adequately
replace the load-bearing function of the native tissue. From the materials
development perspective, there is a growing interest in the field of
“biomimetics”, or studying nature to develop solutions to engineering
problems. For example, spider silk fibers have a combination of high
strength, high toughness, and low density unmatched by synthetic fibers
(silk is stronger by weight than steel), and bone and nacre (abalone shell)
are ceramic composites with better fracture toughness than traditional
engineering materials. Understanding structure-property relationships in
these materials could aid in the development of synthetic materials with
comparable properties.
In addressing these medical and materials problems, there is a clear
need for submicrometer or micrometer-level characterization of the
mechanical properties of biological materials. For example, tissues and
other biological materials are typically comprised of hierarchical
structures with relevant length scales down to nanometers or
micrometers. Characterization of their organization and mechanical
behavior at those small length scales is critical to understanding, and
ultimately mimicking, their behavior at the macroscale. Small sample
sizes are another motivation for the use of high spatial resolution
mechanical testing techniques for characterization of biological materials.
For example, spider silk has a diameter of ~5 micrometers, and many
tissue specimens are acquired from small animal models (such as mice,
rats, and rabbits) or biopsy specimens. These specimens may not be
suitable for traditional macroscopic testing techniques.
A third argument for mechanical testing on small length scales is the
desire to characterize materials on a size scale of interest to cells or
insects. For example, neurons have been shown to preferentially grow on
less stiff substrates, and many plants are designed to interact with the feet
of insects. Since cells are on the order of 10 micrometers, and insect feet
can end in hairs on the order of 1 micrometer, 1 the ability to probe the
mechanical properties of the substrates using contacts of similar size to
the biological entities that will be interacting with the substrate can be
quite valuable. Finally, for some applications it is desirable to map the