Biomedical Engineering Reference
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depending on the AFM design) over a user-specified time; unloading
occurs at a user-specified maximum base displacement or cantilever
deflection. As the feedback loop operates on an optoelectronic signal
based in mechanical contact, even the inventors of the first contact-mode
AFM recognized the potential to image biological surfaces under
ambient and near-physiological conditions
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: full immersion of the probe
and sample in aqueous media (ranging in complexity from a drop of
water into which the probe is submerged to a gasketed fluid cell for
continuous media exchange) does not degrade imaging resolution. Such
fluid cells can be engineered to maintain hydrogels, tissues, and cells at
stable hydration, pH, and temperature comparable to
in vitro
culture
conditions.
Figure 3-2. Schematic of scanning probe or atomic force microscope feedback required
for both imaging and indentation. After Silva and Van Vliet.
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3.1.2.
Cantilever as force transducer
Instrumented indenters are designed to minimize sample compliance and,
in many cases, acquire load and displacement signals through
independent transducers. In contrast, AFMs rely on cantilevered probes
that employ and report contact loads via cantilever deflection.
Cantilevers are typically fabricated from silicon or silicon nitride via
standard microelectronic fabrication mask lithography, and the cantilever
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