Page 50 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN002G-62 May 19, 2001 19:27

190 Biomaterials, Synthetic Synthesis, Fabrication, and Applications

biostability via modifications to the synthesis procedure. Once a cell-seeded scaffold is implanted into host tissue
They are being explored for the encapsulation of liver and it must allow blood vessels to grow into the new tissue.
dopamine-secreting cells for the treatment of liver disor- This can be promoted by the addition of a slow-release
dersandnervoussystemdisordersaswellastheprevention angiogenic growth factor which stimulates growth of new
of scar-tissue formation after surgery. blood vessels. Although a polymer scaffold-cell-growth
factor complex as described earlier was thought neces-
sary for effective regeneration of tissue, scaffolds alone,
B. Directing Tissue Formation
cells alone, and diffusible bioactive factors alone may also
Tissue engineering is causing significant advances to be servetoallowregenerationoftissueunderspecificcircum-
made in “guided tissue formation.” Isolated cells within a stances. Tissues which are being prepared or are proposed
larger body of tissue exhibit little ability to organize them- for preparation via this route include cartilage, skin sub-
selves and form tissues. However, if cells are in fairly close stitutes, and dental/orthopedic materials.
proximity to one another they can grow and exhibit order
in the formation of tissue with particular chemical and bio-
C. Large-Scale Culture of Therapeutic Cells
logical properties. The goal in tissue engineering has been
to create an in vitro environment that would enable cells Conventional methods of cell culture have not been suc-
to organize themselves to form functioning tissues. The cessful in the in vitro culture of cells for transplantation.
intention would then be to implant the artifically created Novel methods of culture are being devised in order to
cell structure to create new tissue or organs. In order to satisfy the demand for cultured cells. Special bioreactors
do this degradable polymer scaffolds have to be synthe- and optimal, precisely controlled culture conditions are
sized. Approaches to this have used copolymers of lactic necessary to generate large quantities of therapeutic cells
acid and glycolic acid. Controlled porosity can be gen- such as bone marrow cells for transplant to cancer pa-
erated by use of salts and other additives that are later tients undergoing chemotherapy. A goal for the future is
washed out prior to seeding the scaffold with cells. An the culture of stem cells from marrow which have been
alternative approach to the generation of porosity is to use genetically modified to counter disease.
carbon dioxide to dissolve the polymer phases and then
allow them to reform around the gas bubbles on placement
D. Future Directions
of the carbon dioxide dissolved polymer in an atmosphere
of air. Tissue engineering has had successes in the blocking of
Living structural supports hold cells close together us- unwanted rejection reactions between implants and host
ing adhesive proteins such as fibronectin and vitronectin tissues, in the synthesis of polymer or polymer–cell com-
which bind reversibly to cell surfaces via a specific amino posites for tissue repair without scarring, in the develop-
acid sequence-arginine–glycine–aspartic acid. This en- ment of tissue cell culture for therapeutic cells and in the
ables the cells to adhere to and interact with each other and growth of simple tissues in the laboratory.
with collagen and other constituents of the extracellular Future goals include the development of synthetic
matrix. A modification of a lactic acid–lysine copolymer strategies to materials for implantation which overcome
has been made with the tripeptide attached to the polymer the bodies natural immune response and to generate “uni-
via an amino group on the lysine. This approach to the syn- versal” donor cells that could be given to all as their im-
thesis of synthetic polymers with the essential components mune characteristics would not be recognized by the host
of natural proteins is being widely investigated. It com- as “foreign.” Learning how to stimulate the regeneration of
bines the advantages of synthetic polymers with their de- complex multicellular structures in vivo is important and
sired materials characteristics such as strength, controlled would allow the regeneration of (potentially) all tissues in
degradation and processibility together with the essential situ. An important component of these advances will be
features of biological polymers such as cell recognition the identification of specific cell signalling pathways and
and the capacity to control cell differentiation. A goal is their spatial and time based involvement in the generation
to modify the specificity of the interaction between the of organs built up from many tissue types. Practically all
polymer support and cells of choice. Specific advances in tissues are capable of being repaired by tissue engineering
this area have been made for substrates used in bone repair principles. Engineers, scientists, and clinicians need to use
which have been altered by binding peptides comprising their understanding of synthetic and natural materials and
lysine–arginine–serine–arginine to the surface. This al- the way in which the human body functions at the cel-
lows specific interaction with osteoblasts rather than en- lular level to develop the next generation of biomaterials
dothelial cells and fibroblasts that are also present in an and cellular transplants for use in the human body. The
implant area thus encouraging the formation of new bone. field is wide open for innovation in the development of a