Biomedical Engineering Reference
In-Depth Information
Figure 15.4.
AFM/Optical microscope image series of a mechanical test
of a junction of electrospun fibrinogen fibers. The junction is “unzipping”
under the applied load (courtesy of Martin Guthold).
and iii) the density and strength of junctions between fibers. All
threeneedtobeknownsothatastructurewithpredictablemechan-
icalpropertiescanbedesigned.However,directmeasurementofthe
mechanicalpropertiesofnanoscopicfibersisdi
cult.Nevertheless,
this knowledge is needed to construct and test mechanical models
of fiber networks and thus permit rational design of scaffold struc-
tures.
The recently developed technique of combined atomic force
microscopy and fluorescence microscopy (AFM/FM) allows deter-
mination of the mechanical properties of individual nanoscopic
fibers in buffer or ambient conditions
67
and has been applied to
the characterization of native fibrin fibers, electrospun fibrinogen
fibers, and electrospun collagen fibers (Fig. 15.4). This technique is
ideally suited to investigate numerous natural and synthetic elec-
trospun fibers as it can apply forces from 10
-
2
nN to 10
4
nN
to fibers of a few to several hundred nanometers in radius and
report elastic moduli from 10
6
Pa to 10
9
Pa. Carlisle
et al.
used this
approach to extract a host of mechanical properties of the fibers,
including extensibility, elastic limit, breaking strength of fibers,
breaking strength of junctions, elastic (Young's modulus) and vis-
cous component of the stretch modulus (stiffness), stress relax-
ation times, speed-dependent (frequency-dependent) stress-strain
behavior,creepbehavior,energylossandenergystorageperstretch
cycle, toughness (absorbed energy), and strain-hardening/strain-
softening behavior.
The nanolevel and microlevel strains in an electrospun matrix
are key to understanding how tissue-level physiologic forces influ-
encecell/matrixinteraction.However,giventhedi
cultyofdirectly
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