Biomedical Engineering Reference
In-Depth Information
surface, it is defl ected up or down as it interacts with morphological
details of the specimen surface. In constant force mode, the tip is
constantly adjusted to maintain a constant defl ection which keeps
the probe tip at a constant height above the surface. It is this height
adjustment that is displayed as data. In this operational mode the
tip is in hard contact with the surface. The stiffness of the cantile-
ver needs to be less than the effective spring constant holding atoms
together, which is on the order of 1-10 nN/nm. Most contact mode
cantilevers have a spring constant of <1 N/m.
Non-contact mode
- this operational mode uses an oscillating cantile-
ver. A stiff cantilever is oscillated in the attractive regime, meaning
that the tip is quite close to the sample, but not touching it. This 'non-
contact' condition gives this operational mode its name. The forces
between the tip and sample are quite low, on the order of pN (10
−12
N). The operational mode is based on measuring changes to the res-
onant frequency or amplitude of the cantilever.
1.6.4 Topographic and phase imaging modes in the AFM
The AFM can be operated in the topographic and phase imaging modes
and some instruments can obtain both images simultaneously. These are
the two most commonly used AFM imaging modes that are used to char-
acterize biomaterials. In the topographic mode the contours and undu-
lations of a surface can be visualized with the resolution of fi ne detail
down to the nanometer level. With most of the new biomaterials being
polymeric in nature and relatively soft, topographic imaging is commonly
carried out in the 'tapping mode'. In the tapping mode or 'intermittent
contact' mode, a stiff cantilever is oscillated closer to the sample than
in non-contact mode. Part of the oscillation extends into the repulsive
regime, so the tip intermittently touches or 'taps' the surface. Very stiff
cantilevers are typically used, as tips can get 'stuck' in the water contami-
nation layer that exists on most specimen surfaces. The advantage of tap-
ping the surface is improved lateral resolution and less surface distortion
on soft samples. Lateral forces such as drag, common in contact mode, are
also virtually eliminated.
In phase imaging, variations in surface properties such as hardness or
stiffness can be measured across the scanned surface. In this imaging mode,
the cantilever is oscillated as it is scanned across the specimen. The varia-
tions in a particular surface property will appear as brighter or darker areas
in the phase image depending on the measured phase shift of the oscillating
cantilever relative to the driving signal, due to varying interactions between
the probe tip and the sample. The phase shift can be used to measure other
