Biomedical Engineering Reference
In-Depth Information
for measuring force curves, which may also allow imaging in contact mode. Some
instruments even allow a kind of hybrid IC-AFM/force spectroscopy, where imaging
can be performed in IC-AFM, and when the area of interest is located, the instrument
locates the surface using amplitude modulation, and only stops the tip oscillation during
acquisition of a curve. This can reduce tip damage before tip acquisition. Attempting to
measure force curves in selected regions of a sample with nanometre resolution can be
challenging, partly due to sample drift, but also due to positioning difficulties and
linearized scanners can help greatly. An alternative to carrying out force spectroscopy in
one location is to perform the experiment in a grid pattern over the sample surface, thus
enabling a grid of force curves which can be processed into a map of adhesion forces or
sample stiffness. It's worth noting that at 1 Hz per force curve acquisition of 1 Hz, a
256
256 pixel map would take many hours to acquire, so such maps are usually obtained
at low resolutions.
Regardless of the manner in which such a curve is recorded, the result is a plot of
deflection versus distance, which the user usually wants to convert to force versus
distance. The first step is to convert the deflection (V) into the actual distance the tip
moved (m), then using the spring constant (N/m), and this can be converted to force (N).
The normal deflection sensitivity is easily obtained by measuring the slope of a deflection
signal versus vertical piezo displacement plot on a stiff, hard surface [142]. The surface
chosen is often an extremely stiff one such as sapphire or stainless steel, but it is only
important that it is considerably stiffer than the tip; measurements with flexible cantilevers
could use any reasonably stiff surface for this. The user must obtain such a deflection-
calibration curve to accompany each set of data without realignment of the laser on the
cantilever; the exact alignment of the optical system directly affects this calibration [351].
Once this is obtained, the curve may be converted to force-distance with the normal spring
constant. See Section 2.5 for procedures for calibration of normal spring constants.
