Bacterial Adhesion and Interaction with Biomaterial Surfaces - Biointerfaces: Where Material Meets Biology - page 385

13.5.3 Atomic Force Microscopy
Atomic force microscopy (AFM) has been a powerful tool in research of
microbiology. 155,168 It is not only the fact that AFM is useful in imaging the
morphology of individual microbial cells and biofilms on biomaterial sur-
faces both in dried or hydrated states, but also it is used increasingly for
mapping interaction forces on microbial surfaces 169-171 and probing the
local mechanical properties of bacterial cell surfaces or biofilms. 172 Thus, a
major advantage of the AFM over other microscopy techniques is that AFM
provides information about the nanoscale surface properties of (living) cells
and about

d n 3 r 4 n g | 3

the localization and interactions of their individual

constituents. 173
The AFM uses a very sharp probe to measure and map the morphology of
nearly any surface in ambient air or liquid environments. As the central
component of AFM, the AFM probe is important in obtaining high reso-
lution of images. The standard topographic imaging AFM probe consists of a
sharp tip attached at the end of a flexible cantilever. The appropriate choice
of cantilever and probe is dependent on the imaging mode and the char-
acteristics of surface features of interest. The parameters for selection of
probes include tip material, tip geometry, sharpness, length, stiffness, etc.
The stiffness of the cantilever should be as low as possible when the AFM is
operated in contact mode to avoid damage to the sample, e.g.,Si 3 N 4 probes,
whereas in dynamic imaging modes, the higher spring constant values of the
cantilevers may help reduce noise and instabilities, e.g., Si probes. The
second important criterion for choosing a probe is minimization or avoid-
ance of the most common artifacts in imaging generated from the tip
geometry. For example, the ultra-sharp probe with high aspect ratio, such as
a carbon nanotube probe, may be considered for imaging the surface with
high aspect ratio features such as holes or pillars. The third important
parameter of the AFM probe to be considered is the spring constant of the
cantilever, particularly for making measurements of interaction forces.
Typical spring constants for commercial AFM cantilevers range from 0.001 to
100 N m l , enabling force sensitivity down to the pico-newton level. Al-
though manufacturers provide the nominal spring constants, actual spring
constants should be calibrated by the appropriate method. 174
AFM can directly visualize the 3D topography of biomaterial and bacterial
cell surfaces at the nanometer scale, and even the structures underneath of
the surfaces. This has been extensively studied with polymers, for example,
polyurethane co-polymer is the primary material used in a variety of blood-
contacting medical devices. It consists of hard and soft segments forming
the microphase separation structure. AFM phase imaging technique can be
used to visualize the microphase separation structure of polymers and in situ
dynamic phase restructuring process during hydration. 175 A typical example
for the probe and operation parameters for imaging microphase separation
structure in polyurethane surface in solution is described in Appendix A.1.
Since AFM can be operated in an aqueous environment, it is of great interest

.