Biomedical Engineering Reference
In-Depth Information
7.2
Chapter 7.2
Scope of tissue engineering
Yoshito Ikada
In tissue engineering, a neotissue generally is regenerated
from the cells seeded onto a bioabsorbable scaffold, oc-
casionally incorporating growth factors:
cells þ scaffold þ
growth factors
/
neotissue.
In theory, any tissue could be
created using this basic principle of tissue engineering.
However, in order to achieve successful regeneration of
tissues or organs based on the tissue engineering concept,
several critical elements should be deliberately consid-
ered including biomaterial scaffolds that serve as a me-
chanical support for cell growth, progenitor cells that can
be differentiated into specific cell types, and inductive
growth factors that can modulate cellular activities. The
fundamentals of tissue engineering will be presented in
this chapter.
which functions as a template required for infiltration
and proliferation of cells into the targeted functional
tissue or organ. If any assistance by scaffold is not re-
quired for cells, we call it ''cell (or cellular) therapy'' or
''cell transplantation.'' Cell therapy avoids the compli-
cations of surgery, but allows replacement of only those
cells that perform the biological functions including
hormone secretion and enzyme synthesis. It would be
therefore convenient to divide regenerative medicine
into two subgroups, as shown in
Fig. 7.2-1
, depending on
the scaffold requirement.
The primary function of scaffolds is to provide
structure for organizing dissociated cells into appropriate
tissue construction by creating an environment that en-
ables 3-D cell growth and neotissue formation. When
cells attached to a scaffold are implanted, they will be
incorporated into the body. Cell attachment is the first
critical element in initiating cell growth and neotissue
development. Natural or synthetic biomaterials utilized
for scaffold fabrication are mostly selected on the basis of
their biocompatibility, bioabsorbability, and mechanical
properties. Much of the secondary scaffold processing is
performed to make the scaffold more porous for en-
hancement of cell infiltration and neotissue ingrowth. To
promote cell attachment various cell adhesion molecules
such as laminin (LN) have been used to coat the scaffold
before cell seeding. The traditional method of seeding
polymer scaffolds with cells has employed static cell
culture techniques. For instance, a concentrated cell
suspension is pipetted onto a collagen-coated polymer
scaffold and left to incubate for variable periods of time
for cells to adhere to the polymer. Dynamic cell seeding
employs a method in which either the medium or the
medium and scaffold are in constant motion during the
incubation period.
7.2.1 Functions of scaffold
When a tissue is severely damaged or lost, not only large
numbers of functional cells but also the matrix in tissue,
generally called
extracellular matrix
(ECM), are lost. It is
difficult to imagine how small-molecule drugs or even
recombinant proteins would be able to restore the lost
tissue and reverse the function. Because tissue represents
a highly organized interplay of cells and matrices, the
fabrication of replacement tissue may be facilitated by
mimicking the spatial organization in tissue. To this end,
we should provide an artificial or biologically derived
ECM for cells to create a neotissue. Isolated cells have
the capacity to form a tissue structure only to a limited
degree when placed as a suspension on tissue, because
they need a template that guides cell organization. In
tissue engineering we designate the substitute of native
ECM as ''scaffold'', ''template'', or ''artificial matrix''.
Scaffold provides a three-dimensional (3-D) ECM analog
