CONCLUSIONS AND OUTLOOK - Nanoindentation with Biological Applications - page 344

Finally, and not unrelated to the other two applications, there is a great
need in the medical device industry to make implants with mechanical
properties which are not incompatible with the natural tissues. This
field—man-made biomaterials and medical implants—has not been
considered extensively in this volume to this point, but represents an
economic faction of interest in the discussions to follow.
The earliest applications of nanoindentation to biological tissues
over a decade ago examined bones and tooth-tissues—materials that
were comparable in mechanical stiffness to the engineering materials
and systems for which commercial nanoindentation instruments were
originally developed. The stiffness of bones and tooth tissues ranges
from around 10 GPa to over 100 GPa, while the elastic modulus of two
common calibration materials—fused silica (sometimes “fused quartz”)
and single crystal aluminum—is around 70 GPa for both, such that
the working range is a mN-nm load-displacement ( P - h ) framework for
either mineralized tissues or engineering metals and ceramics. Metals
and ceramics exhibit little to no time-dependent deformation (with the
exception of extremely soft metals such as single crystal indium or soft
ionic crystals such as single crystal sodium chloride). Bones and tooth
tissues exhibit minimal time-dependence when dried, although they
can exhibit a greater degree of time-dependent deformation when
in a physiologically hydrated state. Regardless, the stiffness and the
dominance of elastic or elastic-plastic deformation in bones and teeth
meant that the existing nanoindentation instrumentation and data analysis
techniques were directly applicable to mineralized tissues.
There were significantly greater challenges when the first experiments
were performed on soft and hydrated biological materials, such as
articular cartilage. Cartilage is approximately 75% water, which gives
rise to dramatic viscoelastic and poroelastic deformation. The base
elastic modulus ranges from less than 1 MPa to several MPa. Instead of
working in a mN-nm load-displacement ( P - h ) framework, we move to a
mN- m framework. For transducers with a maximum displacement

range of several micrometers, the forces developed in soft tissue
indentation may challenge the load resolution of transducer. Of course,
softer materials can be indented using an atomic force microscope
instead of an instrumented indenter. In terms of environmental control,

μ