Biomedical Engineering Reference
In-Depth Information
on flexible surfaces showed increased mobility and reduced spreading. This
result was ascribed to mechanical effects on the cell cytoskeleton, possibly
through influences on receptor association.
A similar strategy of variation of polymer properties was employed in order
to examine the effect of substrate mechanical behavior on neurite extension of
dorsal route ganglion in 3D culture experiments.
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Gels with different
concentrations of agarose were prepared and characterized in the terms of their
mechanical properties by standard rheometry. The rate of neurite extension for
the various gel substrates was performed using time-lapse microscopy and it
was shown that the higher the gel concentration, the lower the rate of neurite
extension. A mathematical model was applied to the results in an attempt to
derive a relationship between polymer mechanical properties and ganglion
behavior. A significant conclusion was reached that scaffolds for potential
neural growth should involve an appreciation of scaffold elasticity.
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Collagen-coated polyacrylamide gels were used in a study of the spreading
and other properties of aortic smooth muscle cells in a similar approach to that
mentioned above.
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In this case the elasticity of the polymer substrates were
determined suing a cantilever-based atomic force microscopy (AFM) method.
The elastic modulus could be calculated from a literature procedure. In a
second appraisal of gel stiffness a macroscopic tension method was used for
comparison with the AFM results. Cell spreading, cell shape and cytoskeletal
or focal adhesion assembly and fluorescent intensity was studied by fluor-
escence microscopy following the usual cell fixing and staining protocols. The
SMCs spread significantly higher on 'rigid' collagen-coated glass compared
with the gels in agreement with the work outlined above on epithelial and
fibroblast cells. (Any role played by the collagen layer in terms of stiffness was
ignored.) Cell spreading properties found in this work are reproduced in
Figure 2.16. Interestingly, the authors ascribe the peaks observed in the plots to
SMC crawling capability associated with cell ligand density. The biphasic
phenomena exhibited in the plots is attributed to cell crawling being initially
limited at low ligand densities whereby a cell cannot form adequate
attachments to pull itself forward or spread. But at high ligand densities, a cell
cannot detach from enough ligand to bring its rear forward. In addition to cell
spreading the authors also investigated the role of collagen much as was the
case of studies described in section 2.5.4.
Finally, it is noteworthy that research involving substrate stiffness has led to
direct attempts to compare the compliance of natural tissue with synthesized
chemistries. An example of this sort of study is that of Georges et al.
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on
neuron and glial cell growth in mixed cortical cultures. In this case, two types of
polymer matrix, fibrin and the more usual polyacrylamide, were investigated.
The dynamic shear modulii of both gels were measured via spectrometric
rheometry and cell interactions with the matrices were examined by standard
immunocytochemistry. It was found that, on 'soft' gels, astrocytes do not
spread and have disorganized F-actin compared with the cytoskeletons of
astrocytes on 'hard' surfaces. Neurons, however, extend long neurites and
polymerize actin filaments on both soft and hard gels. Laminin-coated soft gels
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