Biomedical Engineering Reference
In-Depth Information
strategy associated with growth factors in tissue engineering is not to use a bolus
dose of growth factors but to maintain the growth factor concentration at an
optimal level for a certain period. For the sustained delivery of biologically active
agents, carriers or delivery vehicles are generally employed, but there are few
reports that have explored carriers effective in the sustained release of growth
factors. Much more efforts are required to enhance the benefi cial effects of growth
factors on tissue engineering.
14.3.2
Favorable Environments for Tissue Regeneration
There are two modes of tissue engineering for tissue construction. One is
in vitro
(
ex vivo
) tissue engineering and the other
in vivo
(
in situ
) tissue engineering. In
the beginning of tissue engineering research, many people attempted to construct
living tissues outside the human body, that is,
in vitro
or
ex vivo
. Although a
number of joint ventures were established to this end, most of them failed in the
in vitro
production of clinically applicable tissues on large scales. It may imply that
it is diffi cult for us to create the artifi cial environment that is effective for cells to
generate tissues outside the human body. Generally, a substrate to which cells
attach is required for cells to survive, proliferate, and differentiate. It will be not
diffi cult to prepare such substrates from biomaterials, but continuous supply of
oxygen and nutrients to cells producing tissues is a hard task, because the supply
is often disturbed by the tissues produced. The ideal route for oxygen and nutrient
supply to cells is through capillaries, but suffi cient capillary formation is impos-
sible in the
in vitro
tissue engineering. This may be the reason for very limited
applications of
in vitro
tissue engineering mostly to epidermal production. Tissue
engineering below means the
in vivo
tissue engineering unless specifi ed.
An essential requirement for tissue engineering is to provide cells with a favo-
rable environment for tissue regeneration. In the case of
in vitro
tissue engineer-
ing, we, researchers, should create the environment that is the most effective for
the cells in terms of tissue regeneration including cell proliferation, migration,
and differentiation. In contrast to the
in vitro
tissue engineering, we do not need
to create the optimal environment by ourself in the
in vivo
tissue engineering. The
patient body will produce the most effective environment for the tissue regenera-
tion by itself, if we could effectively support it.
What tissue engineers can help cells is to offer a good substrate for cell attach-
ment, an effective barrier for preventing undesirable cells from invasion into the
regeneration site, and a facility for promoting capillary formation, in other words,
neovascularization. When a permissive environment optimal for cells to regener-
ate tissues is formed expectedly by these supplies, tissue regeneration will smoothly
proceed by itself. However, a very large number of current studies on tissue engi-
neering but a very small number of clinical trials so far imply that such an envi-
ronment optimal for the
in vivo
tissue engineering can be produced only with great
diffi culty.
