Biomedical Engineering Reference
In-Depth Information
copy, provide little information regarding the surface of, for example, biomolecules
or living cells. Atomic force microscopy (AFM) was developed during mid 1980s
(Binnig et al.
1986
). Shortly after its invention, imaging in liquids was demon-
strated (Drake et al.
1989
), showing the potential of the instrument for biological
applications. The power of AFM comes with the fact that it can combine other
imaging modalities as well as physical/chemical measurements to examine both
structure and function of biological materials. The second benefi t of AFM is that it
can routinely be operated in aqueous and physiologically relevant solutions.
2
Atomic Force Microscopy
2.1
Operating Principle
AFM technology is based on raster scanning a cantilevered tip in the
x
-
y
plane over
a sample surface, either by attaching the sample or the cantilevered tip itself to a
scanner. The
z
(vertical) position of the sample (or tip) is monitored simultaneously.
The scanner consists of piezoelectric ceramic elements, most often used in a tube-
like arrangement, giving the ability to have a hardware resolution of 0.1 nm laterally
and 0.01 nm vertically. This is achieved by making use of the property of piezoelec-
tric materials to change size proportional to an applied electric fi eld ( Fig.
1
).
The tube scanner uses a thin-walled, radially polarized piezoelectric ceramic
with one inner electrode and segmented outer electrodes that can provide motion in
Fig. 1
Operating principle of AFM. A cantilevered tip is scanned over the sample, maintaining
constant force using feedback control to keep cantilever defl ection of amplitude (measured using
an optical beam defl ection system) constant (
top graph
). Topography is recorded, and simultane-
ous additional channels can show phase, defl ection, amplitude, or current fl ow between tip and
sample (
bottom graph
)
