Biomedical Engineering Reference
In-Depth Information
ice nucleation and growth can lead to a large variety of
pore structures (
Fig. 3.2.8-3
).
In practice, the average pore diameter decreases with
decreasing temperature of freezing while the orientation
of pore channel axes depends on the magnitude of the
major heat flux vector during freezing. In experimental
implants the mean pore diameter has ranged between
about 1 and 800
m
m; pore volume fractions have ranged
up to 0.995; the specific surface has been varied
between about 0.01 and 100 m
2
/g dry matrix; and
the orientation of axes of pore channels has ranged
from strongly uniaxial to almost random. The ability of
collagen-glycosaminoglycan copolymers to induce re-
generation of tissues such as skin, the conjunctiva and
peripheral nerves depends critically, among other fac-
tors, on the adjustment of the pore structure to desired
levels, e.g., a pore size range of about 20-125
m
mfor
skin regeneration and less than 10
m
m for sciatic nerve
regeneration appear to be mandatory. Determination
of pore structure is based on principles of stereology,
the discipline which allows the quantitative statistical
properties of three-dimensional structures of implants
to be related to those of two-dimensional projections,
e.g., sections used for histological analysis.
a given reagent has actually led to modification of a sub-
stantial fraction of the residues of an amino acid in the
molecule. This is equivalent to proof that a reaction has
proceeded to a desired ''yield.'' Furthermore, proof that
a given reagent has attacked only a specific type of amino
acid, rather than all amino acid residue types carrying the
same functional group, also requires chemical analysis.
Historically, the chemical modification of collagen has
been practiced in the leather industry (since about 50%
of the protein content of cowhide is collagen) and in the
photographic gelatin industry. Today, the increasing use
of collagen in biomaterials applications has provided
renewed incentive for novel chemical modification, pri-
marily in two areas. First, implanted collagen is subject to
degradative attack by collagenases, and chemical cross-
linking is a well-known means of decelerating the deg-
radation rate. Second, collagen extracted from an animal
source elicits production of antibodies (immunogenicity)
and chemical modification of antigenic sites may poten-
tially be a useful way to control the immunogenic re-
sponse. Although it is widely accepted that implanted
collagen elicits synthesis of antibodies at a far smaller
concentration than is true of most other implanted pro-
teins, treatment with specific reagents, including enzy-
matic treatment, or cross-linking, is occasionally used to
reduce significantly the immunogenicity of collagen.
Collagen-based implants are normally degraded by
mammalian collagenases, naturally occurring enzymes
that attack the triple helical molecule at a specific loca-
tion. Two characteristic products result, namely, the
N-terminal fragment, which amounts to about two-thirds
of the molecule, and the C-terminal fragment. Both of
these fragments become spontaneously transformed
(denatured) to gelatin at physiological temperatures via
the helix-coil transition and the gelatinized fragments
are then cleaved to oligopeptides by naturally occurring
enzymes
Chemical modification of collagen
and its biological consequences
The primary structure of collagen is made up of long
sequences of some 20 different amino acids. Since each
amino acid has its own chemical identity, there are
20 types of pendant side groups, each with its own
chemical reactivity, attached to the polypeptide chain
backbone. As examples, there are carboxylic side groups
(from glutamic acid and aspartic acid residues), primary
amino groups (lysine, hydroxylysine, and arginine resi-
dues), and hydroxylic groups (tyrosine and hydrox-
ylysine). The collagen molecule is therefore subject to
modification by a large variety of chemical reagents. Such
versatility comes with a price: Even though the choice of
reagents is large, it is important to ascertain that use of
that
degrade
several
other
tissue
proteins
(nonspecific proteases).
Collagenases are naturally present in healing wounds
and are credited with a major role in the degradation of
collagen fibers at the site of trauma. At about the same
time that degradation of collagen and of other ECM
triplet sequence Gly-Pro-Hyp illustrates elements of collagen triple-helix stabilization. The numbers identify peptide backbone atoms. The
conformation is determined by trans peptide bonds (3-4, 6-7, and 9-1); fixed rotation angle of bond in proline ring (4-5); limited rotation of
proline past the C¼O group (bond 5-6); interchain hydrogen bonds (dots) involving the NH hydrogen at position 1 and the C¼O at position
6 in adjacent chains; and the hydroxy group of hydroxyproline, possibly through water-bridged hydrogen bonds. (Reprinted from
K. A. Piez and A. H. Reddi, editors (1984). Extracellular Matrix Biochemistry. Elsevier, New York, Chap. 1, Fig 1.6. p. 7, with permission.)
(C) Tertiary structure
d
the global configuration of polypeptide chains, representing the pattern according to which the secondary
structures are packed together within the unit substructure. A schematic view of the type I collagen molecule, a triple helix 300 nm long.
(Reprinted fromK. A. Piez and A. H. Reddi, editors (1984). Extracellular Matrix Biochemistry. Elsevier, New York, Chap. 1, Fig. 1.22, p. 29,
with permission.) (D) Quaternary structure
d
the unit supermolecular structure. The most widely accepted unit is one involving five collagen
molecules (microfibril). Several microfibrils aggregate end to end and also laterally to form a collagen fiber that exhibits a regular banding
pattern in the electron microscope with a period of about 65 nm. (Reprinted from E. Nimni, editor (1988). Collagen, Vol. I, Biochemistry,
CRC Press, Boca Raton, FL Chap. 1, Fig. 10, p. 14, with permission.)
