Biomedical Engineering Reference
In-Depth Information
monomer can be bought commercially from different providers. However, these solutions
do not always contain the full-length collagen molecule, which can affect its properties.
Preparation of a collagen gel from these solutions is quite simple, the collagen solution, pro‐
vided at pH 2, self-assemble at 37°C and at a neutral pH. Cells can be added to the solution
after neutralisation and before casting of the gel, allowing for a uniform distribution of cells
in the construct. As cells migrate and elongate into the gel, they will try to anchor them‐
selves to the collagen fibers, but since the collagen gels are soft, cells will deform the fibril
network causing the gel to compact [35]. If this contraction is controlled by applying a static
constraint in one direction, the gel will contract differentially in the constrained and uncon‐
strained axis. This will result in an anisotropic construct because collagen fibers and cells
will become aligned in the constrained axis [34, 36, 37].
Thomopoulos et al. [28] used fibroblast populated collagen gels that were constrained either
uniaxially or biaxially to evaluate the anisotropy generated in the gel. Uniaxial static strain re‐
sulted in gel contraction in the unconstrained axis and lead to a structural and mechanical ani‐
sotropy. They found no difference between tendon fibroblasts and cardiac fibroblasts in
anisotropy generated in the construct. They also demonstrated that active remodeling of the
gel by cells is not necessary for the development of anisotropy in collagen gel. Indeed, uniax‐
ially constrained collagen gel without cells also become anisotropic. This surprising result
could be explained by the force generated by collagen polymerisation [38]. They also devel‐
oped a mathematical model to predict anisotropy in fibroblast-populated collagen gels [39].
They found that mechanical anisotropy could not be explained solely by collagen fibers align‐
ment, but also take into account the redistribution of collagen fibers upon remodeling, nonaf‐
fine fiber kinematics [40, 41] and fiber lengths. Costa et al. [42] also investigated the
mechanism of cell and matrix alignment in constrained collagen gels. They constrained fibro‐
blast-containing collagen gels with different shapes (square, triangle and circle) and liberated
one or more of the edges to create anisotropy into the gel. Contrasting with a previous report
of Klebe et al. [43], they showed that fully constrained gels present random cell and matrix ori‐
entation. Nevertheless, on the basis of the results obtained in their study, Costa et al. pro‐
posed that rather than aligning along the local direction of greatest tension, cells orient
parallel to the local free boundary. An interesting result was obtained with the round shape
constrain. As expected, no alignment were present when gel was uniformly constrained, but
when they cut a central hole in the construct, the gel contracted away from the central hole
and cells aligned in the circumferential orientation. This result is consistent with previous re‐
sults obtained in blood vessel reconstruction in which a collagen gel is contracting around a
central mandrel causing circumferential cell alignment [34]. Grinnell and Lamke [44] cul‐
tured fibroblasts on hydrated collagen lattices. They found that cells reorganized the network
and aligned the collagen fibrils in the plane of cell spreading, becoming more densely packed.
They noted that the lattice has thinned to one-tenth of its original thickness.
Weinberg and bell [45] used collagen gel seeded with bovine cells to produce the first tissue-
engineered blood vessel. This weak construct was supported by a Dacron mesh in order to
sustain physiological pressure. This method was improved later by other groups to enhance
mechanical properties of the collagen gel to get rid of the synthetic material, but those con‐
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