Biomedical Engineering Reference
In-Depth Information
Using a constant cantilever defl ection maintained by feedback, contractile activity
and the change in contractile activity using perturbations in the buffer environment
(Fig.
9
, middle panel) can be interrogated while cell stiffness was examined (Fig.
9
,
right panel) using a sinusoidal scanner movement. Different cantilever defl ections
were observed under different buffer conditions for the same imaging force. This
demonstrates the possibility to quantify the coupling between subcellular substrates
to cellular functions, such as contraction, migration, growth, and differentiation.
4.3
Nanofl ow and Viscosity
Liquid viscosity is hard to measure with high precision and in small volumes.
Traditionally, ultrasonic devices are used to measure viscosity (Jensenius et al.
2000
). They operate at MHz frequency at which the measured viscosity of non-
Newtonian fl uids can differ from values obtained using low-frequency measure-
ments (Wee et al.
2005
). Flexural-mode resonance devices, such as microfabricated
cantilevers, may be more reliable since they allow for measurement at lower fre-
quencies. Using an optical detection in standard AFM equipment, viscous drag was
measured using a piezoelectric actuator to vibrate an AFM silicon cantilever
(Hauptmann et al.
1998
). Other ways of using AFM to measure liquid viscosity
include measuring the torsion in an AFM cantilever while scanning a whisker tip
inside the liquid (Mechler et al.
2004
) .
The last technique allows viscosity measurement in a thin fl uid fi lm requiring
extremely low volumes. The lateral force imaging technique is used, in which the
cantilever is scanned sideways and the torsion in the lever due to friction is mea-
sured ( Fig.
10
). From the torsion, the drag forces can be determined as a function of
the penetration depth of the AFM tip in the liquid. Theoretical calculation of the
drag coeffi cient corresponds well with the experimental values obtained for water-
glycerol mixtures. With the increased use of microfl uidics in screening platforms
and parallel assays and the ability to integrate microcantilevers into the fl uidic chip
could be used not only to measure viscosity of sample fl uids, but also to assess the
presence of various proteins/antibodies in the sample.
5
AFM in Biomaterials
5.1
Nanoelasticity and Nanopatterning in Biomaterials
Biological fi bers have nanomechanical properties that depend on their morphology as
well as the chemical heterogeneity of their constituent subunits. The correlation
between the mechanical properties and the heterogeneous subunits on a nanoscale has
been limited. Using AFM, such correlation studies are possible (Parbhu et al.
1999
) .
