Biomedical Engineering Reference
In-Depth Information
the structural unit that can exist as a physicochemically
stable entity in solution, namely, the triple helical colla-
gen molecule.
In type I collagen, two of the three polypeptide chains
have identical amino acid composition, consisting of
1056 residues and are termed a1(I) chains, while the
third has a different composition, it consists of 1038
residues and is termed a2(I). The three polypeptide
chains fold to produce a left-handed helix, whereas the
three-chain supercoil is actually right-handed with an
estimated pitch of about 100 nm (30-40 residues). The
helical structure extends over 1014 of the residues in
each of the three chains, leaving the remaining residues at
the ends (telopeptides) in a nonhelical configuration. The
residue spacing is 0.286 nm and the length of the helical
portion of the molecule is, therefore, 1014
0.286 or
290 nm long.
The fourth-order or quaternary structure denotes the
repeating supermolecular unit structure, comprising sev-
eral molecules packed in a specific lattice, which consti-
tutes the basic element of the solid state (microfibril).
Collagen molecules are packed in a quasi-hexagonal lat-
tice at an interchain distance of about 1.3 nm, which
shrinks considerably when the microfibril is dehydrated.
Adjacent molecules in the microfibril are approximately
parallel to the fibril axis; they all point in the same di-
rection along the fibril and are staggered regularly, giving
rise to the well-known D-period of collagen, about 64 nm,
which is visible in the electron microscope. Higher levels
of order, eventually leading to gross anatomical features
that can be readily seen with the naked eye, have been
proposed, but there is no general agreement on their
definition.
collagen is commonly used as an implant. Since
implanted gelatin is much more rapidly degradable than
collagen, a characteristic that can seriously affect im-
plant performance, these assays are essential tools for
quality control of collagen-based biomaterials. Fre-
quently, such biomaterials have been processed under
manufacturing conditions that may threaten the in-
tegrity of the triple helix.
Collagen fibers also exhibit a characteristic banding
pattern with a period of about 65 nm (quaternary
structure). This pattern is lost reversibly when the pH of
a suspension of collagen fibers in acetic acid is lowered
below 4.25
0.30. Transmission electron microscopy or
small-angle X-ray diffraction can be used to determine
the fraction of fibrils that possess banding as the pH of
the system is altered. During this transformation, which
appears to be a first-order thermodynamic transition, the
triple helical structure remains unchanged. Changes in
pH can, therefore, be used to selectively abolish the
quaternary
structure
while
maintaining
the
tertiary
structure intact.
This experimental strategy has made it possible to
show that the well-known phenomenon of blood platelet
aggregation by collagen fibers (the reason for use of col-
lagen sponges as hemostatic devices) is a specific prop-
erty of the quaternary rather than of the tertiary
structure. Thus collagen that is thrombo-resistant
in vitro
has been prepared by selectively ''melting out'' the
packing order of helices while preserving the triple he-
lices themselves.
Figure 3.2.8-2
illustrates the banding
pattern of such collagen fibers. Notice that short seg-
ments of banded fibrils persist even after very long
treatment at low pH, occasionally interrupting long seg-
ments of nonbanded fibrils (
Fig. 3.2.8-2
, inset).
The porosity of a collagenous implant normally makes
an indispensable contribution to its performance. A
porous structure provides an implant with two critical
functions. First, pore channels are ports of entry for cells
migrating from adjacent tissues into the bulk of the im-
plant for tissue serum (exudate) that enters via capillary
suction or of blood from a hemorrhaging blood vessel
nearby. Second, pores endow a material with a frequently
enormous specific surface that is made available either
for specific interactions with invading cells (e.g., myofi-
broblasts bind extensively on the surface of porous
collagen-glycosaminoglycan copolymer structures that
induce regeneration of skin in burned patients) or for
interaction with coagulation factors in blood flowing into
the device (e.g., hemostatic sponges).
Pores can be incorporated by first freezing a dilute
suspension of collagen fibers and then inducing sub-
limation of the ice crystals by exposing the suspension to
low-temperature vacuum. The resulting pore structure is
a negative replica of the network of ice crystals (primarily
dendrites). It follows that control of the conditions for
Biological effects of physical
modifications of the native
structure of collagen
Crystallinity in collagen can be detected at two discrete
levels of structural order: the tertiary (triple helix)
(
Fig. 3.2.8-1C
) and the quaternary (lattice of triple
helices) (
Fig. 3.2.8-1D
). Each of these levels of order
corresponds, interestingly enough, to a separate melting
transformation. A solution of collagen triple helices is
thus converted to the randomly coiled gelatin by
heating above the helix-coil transition temperature,
which is approximately 37
C for bovine collagen, or by
exceeding a critical concentration of certain highly po-
larizable anions, e.g., bromide or thiocyanate, in the
solution of collagen molecules. IR spectroscopic pro-
cedures, based on helical marker bands in the mid- and
far IR, have been developed to assay the gelatin content
of collagen in the solid or semisolid states in which
