Biomedical Engineering Reference
In-Depth Information
amorphous polymers, glass, metal, and other biological materials, for
sustained immersion times.
3. AFM-EnabledIndentation
As compared to industrial, structural materials for which instrumented
indentation is now ubiquitous, biomaterials exhibit high compliance and
significant structural heterogeneity. For these reasons, there is an
increasing number of studies that employ scanning probe or atomic force
microscopy to indent and infer mechanical properties of biomaterials.
Although both instrumented indenters and AFMs are capable of applying
load and acquiring load-displacement traces, there are several important
differences and caveats to the use of AFMs as indenters. This section
outlines the distinct operating principles and experimental considerations
unique to AFM-enabled indentation.
3.1
. Instrumentation
3.1.1.
Acquisition of calibrated load and displacement signals
The operating principles of scanning probe microscopes based on optical
detection of cantilever deflection, often termed atomic force microscopes
due to the picoNewton-scale forces achievable, are responsible for the
advantages and limitations of this mode of indentation on biological
materials.
Figure 3-2
shows a schematic of this instrument, comprising a
silicon-based cantilever of calibrated stiffness
k
that is typically
displaced normal to the sample surface via a calibrated piezoactuator.
Deflection of this cantilever upon contact is detected via a Class II (red)
laser, which reflects from the free end of the cantilever into a position
sensitive detector such as a quadrant photodiode. Signal feedback relates
the cantilever base position required to induce or maintain a user-
specified probe deflection; this feedback is implemented during scanning
(imaging) of a sample surface in AFM contact mode, as well as during
indentation. Indentation loading of the sample surface proceeds via user-
specified displacement of the cantilever base (or the sample stage,
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