Biomedical Engineering Reference
In-Depth Information
Tissue engineering also covers joint replacements, including connective tissue recreation
and bone grafts. Artificial heart valves implement bovine and porcine tissues along with
bioartificial substances. Organ failure is treated with innovations in the field as well, with
treatment for everything from liver cancer to breast reconstruction. Blood transfusions
and dental surgery advancements are just two more examples of the wide range of applica-
tions of tissue engineering technologies.
Bone marrow transplantation works to regenerate the most prolific organ of the body.
Marrow is responsible for the production of blood cells and is often damaged by mye-
loablative treatment regimens, such as chemotherapy and radiation. Modern methods
involve harvesting patient samples of marrow prior to the therapy regimen and reinject-
ing them following treatment. The body regenerates its marrow supply, causing a tempo-
rary immunodeficiency.
In the case of pancreatic and liver tissue development, a bioreactor model is used. Bioreac-
tors are systems consisting of a large number of cells that take in an input of reactants and
output a set of products. Bioreactors have also been implemented for blood cell production
from hematopoietic tissue. The two types of bioreactor systems are hollow fiber and
microcarrier-based systems. In the hollow fiber system, a large number of small-diameter,
hollow tubes are bundled together by a larger shell tube. The small tubes are injected with
organ-specific cells that are suspended in a collagen-based matrix. The matrix will contract,
leaving space within the small tubes. The patient's own blood or plasma is injected into the
larger, encompassing tube and is allowed to nourish the hepatopoietic cells by flowing
through the newly emptied space in the smaller tubes. In microcarrier-based systems, small
beads (less than 500
m) with surfaces specially treated for cell attachment are either posi-
tioned in a packed or fluidized bed or incorporated in hollow fiber cartridges. In the packed
bed method, a column is filled with the beads and capped at each end with porous plates to
allow perfusion. Success rates rely on fluid flow rate through the column, as well as the
density of packed beads and dimension ratios of the column.
Biomaterials play a significant role in tissue engineering. In each of the previous exam-
ples, biomaterials prove an integral component of tissue regeneration and reconstruction.
From the obvious application of artificial valve design to the less apparent role of injection
needle design in bone marrow transplantation, biomaterial development is a necessary
step in the advancement of tissue engineering. Devices must provide mechanical support,
prevent undesirable tissue interactions, and potentially allow for timely biodegradation.
Biomaterial devices can be broken down into two types, each existing on a scale as small
as a few hundred microns. Immunoprotective devices contain semipermeable membranes
that prevent specific host immune system elements from entering the device. Open devices,
in contrast, are designed for systems to be fully integrated with the host and have large
pores (greater than 10
m
m), allowing for free transport of cells and molecules.
Pore sizes within a biomaterial directly correlate to the functions of the device. The struc-
ture of a pore is determined by the continuity of individual pores in the device, as well as
the size and size distribution. The three classifications of porous materials are microporous,
mesoporous, and macroporous. Microporous materials have pores with a diameter less
than 2 nm and allow for transport of small molecules, including gases. Mesoporous materi-
als allow for transport of small proteins and have pores with diameters ranging from 2 to
50 nm. Macroporous materials have pores with diameters greater than 50 nm and allow
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