Biomedical Engineering Reference
In-Depth Information
possible, are built of long chains of only 20 different amino acids that are held together by
peptide bonds. Proteins have a myriad of functions in the human body. They can function
as enzymes that catalyze thousands of important chemical reactions that are essential to
life. Cell signaling molecules responsible for cell migration and proliferation are made
of proteins. Proteins are the building blocks of the supporting extracellular matrix of
many tissues. Changes in the levels of proteins or the structure of proteins lead to altered
function and are responsible for many diseases. Implanting a natural product made of
proteins is usually desired over synthetic polymers because of this ability of the proteins
to communicate with cells.
The directional bonds within proteins give rise to the high mechanical properties of nat-
ural polymers. For example, the ultimate tensile strength of silk is higher than that of drawn
nylon, one of the strongest synthetic polymers. Furthermore, the elastic modulus of silk is
nearly 13 times that of the elastic modulus of nylon. As with synthetic polymers, the
amount of cross-linking greatly affects the mechanical properties. For example, elastin
found in our skin is made of coiled proteins with few cross-links, which makes it much
more flexible than Type I collagen found in our bones that is made of rod-like molecules
assembled into a repeating crystalline structure and are highly cross-linked.
There are also natural ceramic materials used in biomedical applications. Natural ceramics
are typically calcium-based, such as purified calcium phosphate bone crystals or calcium car-
bonate coral and are both used in orthopedic applications as bone substitutes. Intact bone is
a composite of both a natural ceramic and a polymer, and this makes bone much tougher
(resistant to fracture) than synthetic ceramics due to their multilayered structure that prevents
crack propagation. Small ceramic crystals are precisely arranged and aligned and are sepa-
rated by thin sheets of organic matrix material that provides an interface. A crack in the mate-
rial is forced to follow this tortuous organic matrix path.
Natural materials exhibit a lower incidence of toxicity and inflammation as compared to
synthetic materials, particularly if they are patient-derived autografts; however, it is often
expensive to produce or isolate natural materials. There is also variability between lots of
natural materials when they are obtained from a variety of patients (allografts) or plant
or animal sources, which makes it difficult to maintain consistency and sometimes prevents
widespread commercial use. If the natural tissue comes from a different species than
human, such as bovine or porcine sources, it is called a xenograft. The isolation or purifica-
tion steps typically involve the use of solvents to extract the desired component from the
rest of the tissue or the use of solvents to remove the undesired components such as cells
from the tissue and leave the desired natural material intact. Collagen can be prepared by
either method. If it is labeled as soluble collagen, it has been removed by pepsin enzymatic
treatment from natural tissues such as porcine skin. Fibrillar collagen is prepared from nat-
ural tissue, such as tendon, by salt and lipid, and acid extraction steps to remove the non-
collagenous proteins and molecules, leaving the collagen fibers intact.
Biopolymers also may be produced by bacteria. Production of polyhydroxybutyrate
(PHB) is carried out through a fermentation procedure. The bacteria produce the polymer
in granules within their cytoplasm when they are fed a precise combination of glucose
and propionic acid. The cells are then disrupted, and the granules are washed and collected
by centrifugation and then dried. This polymer has properties similar to polypropylene and
polyethylene, but it degrades into natural components found in the body. Because of the
