Biomedical Engineering Reference
In-Depth Information
organic and the degree of covalent coupling. Importantly, release of the
polymers can be coupled to the degradation of the silica, meaning that
the materials degrade as one material, or true hybrids. The future may
well be the use of human recombinant proteins, but much development
is needed to increase their yields.
12.7 BIOACTIVE GLASSES AND TISSUE ENGINEERING
Tissue engineering is the regeneration of tissues through the combination
of engineering and biology principles. A common strategy for bone
tissue engineering is to use a scaffold as a temporary template for cells to
produce new tissue. The main difference between tissue engineering and
a synthetic bone graft that regenerates bone is that tissue engineering
is partly done in a laboratory. A scaffold can be seeded with cells in
a laboratory and encouraged to grow on the scaffold. The cells must
penetrate the scaffold, attach and produce bone matrix. A choice has to
be made as to the point at which the construct is implanted: it could
be implanted immediately after the cells are seeded; after a specified
time; or once an entire tissue has been grown and the scaffold degraded,
so only tissue is implanted. Ideal tissue engineering would be growing
entire replacement parts (organs) so that they are ready for use, using
a patient's own cells to prevent rejection. This has only so far been
achieved with skin, using polymer scaffolds, but the skin produced does
not contain many of the components of natural skin, for example,
sweat glands, pores, pigment cells and hair follicles. A big advantage of
growing a large piece of tissue and allowing the scaffold to degrade is
that the surgeon will not be implanting any synthetic material, so some
of the regulatory issues for medical devices can be avoided.
In reality, though, bone is such a complicated structure that needs a
combination of different cells, growth factors and mechanical stimuli.
The process of bone production is so complicated that it seems the only
way to achieve it would be to use the body as its own bioreactor. That
is not to say that tissue engineering does not have potential. When large
porous constructs are implanted, a limiting factor can be that blood
vessels do not penetrate, even if the scaffold is bioactive and stimulating
bone growth. Bone needs blood vessels if it is to survive, but also blood
vessels will not grow into a porous construct if there is no living tissue
inside it. This creates a bit of a 'chicken and egg' or 'Catch 22' situation,
to which tissue engineering may be the answer. Stem cells could be grown
in the pores of the scaffold in the laboratory prior to implantation so
