Biomedical Engineering Reference
In-Depth Information
tissue regeneration by the supporting materials. Such a trouble would not arise if
small experimental animal models like rat are used for tissue engineering studies.
Only the use of bioabsorbable materials that will be resorbed, matching with the
neo tissue formation, would circumvent this problem.
14.4
Structure of Scaffolds
When a scaffold is defi ned as any biomaterials used to encourage tissue regenera-
tion, it may include the substrate for cell attachment, the barrier for cells to retain
the site for tissue regeneration, the guide for cells to create a tissue giving the
contour of the regenerated tissue, the carrier for the sustained release of growth
factors, and the mechanical support until to tissue regeneration. It is unlikely that
a single biomaterial can address all these requirements, although such a simple
case is often seen in studies using rats as animal model. It should be emphasized
that the scaffold that is clinically applicable is practically different from that for
small animals, mostly because of difference in mechanical strength. The material
property necessary and common to all scaffolds is temporally controlled
biodegradability.
14.4.1
Surface Structure
The most important role of scaffolds in tissue engineering is to provide an attach-
ment site for the cells responsible to the tissue regeneration. Similar to embryonic
development, multiple cells should assemble to a specifi ed form for tissue forma-
tion. To this end, cells would bind each other through the cell-cell interactions
and, in addition, cells attach to a substrate for their survival, proliferation, and
differentiation. If tissue regenerates by the help of growth factors alone, a carrier,
not a substrate, will be required for their sustained delivery.
Fibronectin is well known as a cell-adhesive protein and has been very often
attempted to immobilize on the scaffold surface. However, chemical modifi cation
of synthetic polymer materials with entire ECM molecules or relevant peptide frag-
ments is not always necessary for scaffolds used in tissue engineering, because
fi bronectin molecules are more or less present in both serum and body liquids and
adsorb to the scaffold surface unless it is too hydrophilic like nonionic hydrogels or
too hydrophobic like fl uorinated polymers. As the fi bronectin adsorption needs a
certain period, scaffolds lacking immobilized fi bronectin would take a longer time
for cell attachment than those with immobilized fi bronectin. Collagen is also cell-
adhesive and hence frequently employed for the enhancement of cell attachment.
Because of poor cell adhesion, hydrogel scaffolds need surface modifi cation,
when applied in tissue engineering, but they are basically not appropriate for scaf-
folds, since their mechanical strength is too low to retain the environment neces-
sary for tissue regeneration.
