NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 305

levels of compression, as has been pointed out for cartilage studies. 22
Some commercial indentation systems have displacement limits as low
as 5 micrometers, which is often insufficient to accurately observe the
mechanical response of the soft tissue under physiologically relevant
conditions, or in the presence of substantial adhesion. High load
indenter systems have higher displacement limits (often exceeding 100
micrometers) which can solve this problem, but the tradeoff is decreased
load resolution. Combining a high stroke length with high load resolution
is an issue to be considered in the development of improved
nanoindentation systems for soft tissue applications.
4.2.3 . Open-loop vs. closed-loop control
Many indentation systems have both open- and closed-loop modes
of operation. When operating a nanoindenter, a certain load (or
displacement) profile is defined by the user, and that profile is then
transferred to the transducer. In open-loop mode, there is no feedback
from the actual load or displacement at the tip back to the system for
corrections. Instead, it is assumed that the desired profile is transferred to
the tip-sample interface without any losses. While this is a reasonable
assumption for stiff engineering materials that exhibit only small
amounts of displacement (< 500 nm) and no creep, for soft tissues which
exhibit large displacements (typically > 1 micrometer) and substantial
amounts of creep there is substantial loss of the applied load or
displacement in the instrumentation before reaching the tip-sample
interface. For example, in some commercial nanoindentation systems
the tip is supported by leaf springs. When operated in open-loop load
control, the amount of load that goes into deflecting the springs
increases with the amount of displacement of the tip. This results in
the characteristic linear sloped region of the load-displacement curve
( Fig. 9-4 ) during the hold period of the trapezoidal load function, and
upon removal of load.
To correct for these problems, most nanoindenters now offer closed-
loop load- and displacement-control options. These methods provide
feedback from the actual load or displacement of the tip back to the