Page 37 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

P1: GLM Revised Pages
Encyclopedia of Physical Science and Technology EN002G-62 May 19, 2001 19:27

Biomaterials, Synthetic Synthesis, Fabrication, and Applications 177

force with a minimum of loss. This is achieved through TABLE III Polymers Used in Medical Devices
the parallel arrangement of collagen fibers to form rope- Polymer Medical device applications
like structures with a high modulus of elasticity and high
tensile strength. Tendons, such as the Achilles tendon, that Polyethylene Hip, tendon/ligament implants and
are under a lot of stress probably contain collagen fibers facial implants
that are more highly cross-linked to reduce the rate of Polyethylene terphthalate Aortic, tendon/ligament and
facial implants
stress–relaxation to an insignificant level.
Polymethylmethacrylate Intraocular lens, contact lenses and
Skin is a complex tissue made up of a thick collage-
bone cement
nous layer (the dermis), a basement membrane and an
Polydimethylsiloxane Breast, facial and tendon implants
overlying keratinized epidermal layer. The mechanical
Polyurethane Breast, vascular and skin implants
properties of skin arise principally from the dermis which
is a three-dimensional feltwork of continuous collagen
fibers embedded in a protein–polysaccharide matrix rich
in both dermatan sulfate and hyaluronic acid, the latter an ion reservoir for both cations and anions. In material
being used to reduce frictional wear between the collagen terms bone is a three-phase material; the organic fibers
fibers. Elastin fibers are distributed throughout the tissue (collagen) can be compared to steel cables in reinforced
or concentrated in the lower layers of the dermis depend- cement, the inorganic crystalline phase (carbonated hy-
ing on the precise location of the skin within the body. The droxyapatite) to a heat-treated ceramic and the bone ma-
arrangement of elastin fibers within a collagen framework trix to a base substance which performs various cellular
results in a material showing rubber-elastic properties at functions. The unique physical and mechanical properties
small extensions but is limited at longer extensions by the of bone are a direct result of the atomic and molecular
dimensions of the collagen framework. interactions intrinsic to this unusual composite material.
Cartilage acts as a material that maintains the shape Bone comprises collagen fibrils intimately associated
of ears, the nose, and the invertebral disc. Cartilage con- in an orderly fashion with small calcium phosphate
tains collagen fibers, a proteoglycan matrix phase rich in crystals. The crystals are of carbonated-hydroxyapatite
chondroitin 4- and 6-sulfate and sometimes elastin fibers more correctly described as the mineral dahllite. The
and yet the material must be able to resist compression formula Ca 8.3 (PO 4 ) 4.3 (CO 3 ) x (HPO 4 ) y (OH) 0.3 represents
and bending forces. Cartilage can be thought of as a hy- bone mineral with the values of X and Y changing with age
drostatic system in which the fluid element is provided (Y decreases and X increases with increasing age, whereas
by the hydration water of the proteoglycan gel and the X+Y remains constant with age equal to 1.7!). Traces
container provided by the collagen fiber meshwork which of other elements such as silicon may also be associated
immobilizes the molecules of this gel. Thus, the rigid- with deposition of the mineral phase. The individual crys-
ity of the system arises from the osmotic swelling of the tals have an average length of 50 nm (range 20–150 nm),
proteoglycan gel against the constraints imposed by the width 25 nm (range 10–80 nm) and thickness of 2–5 nm.
collagen fiber system. Cartilage may additionally be min- In addition to collagen at 85–90% of the detectable protein
eralized and will be discussed below in conjunction with there are more than 200 noncollagenous proteins (NCPs)
other mineralized tissues. present. The three major classes of NCP’s are acidic gly-
coproteins, proteoglycans and Gla- (γ -carboxyglutamic
acid) proteins. The acidic glycoproteins contain consid-
C. Natural Mineralized Tissues,
erable amounts of the amino acids phosphoserine, phos-
Bone, Cartilage, and Enamel
phothreonine, and γ -carboxyglutamic acid. The phospho-
Vertebrates construct their skeletal and dental hard parts poteins are intimately associated with the initiation and
from calcium phosphates with calcium carbonates being regulation of crystal growth and may serve as a source
used for balance organs and egg shells. Bone, dentin, of inorganic phosphate on enzymatic release by phos-
enamel, and mineralized cartilage all contain crystalline phatases.Theproteoglycanshaveoneormore(negatively)
calcium apatite phases but the crystals exhibit differ- charged glycosaminoglycan chains attached to the main
ent sizes, compositions, and ultrastructural organization. protein chain and may be present to inhibit crystal growth
Apart from enamel they all contain collagen fibers, and due to their negative charge and to reserve the extracellu-
additional inorganic salts and biomolecules. lar space for future calcium phosphate crystal growth due
Bone has unusual physical and mechanical properties to their ability to structure water. Both these classes of
in that it is able to support its own weight, withstand acute proteins together with alkaline phosphatase are found in
forces, bend without shattering and can be flexible with- a range of mineralized tissues and their wide distribution
out breaking within predefined limits. Bone also acts as suggests that they have a basic role to play in controlling