Biomedical Engineering Reference
In-Depth Information
oxidant, citrate ferric, was applied to the adherents, and the strips were combined
together, fixed for 30 min in air, and then placed into water for 12 h. The conditions
gave an adhesive strength of 0.16 MPa for the bone specimen, while the skin
specimen failed to attach. In the experiment, because the bone specimen has a
porous surface, the mechanical locking by the permeating and setting of the
co-polypeptide via the covalent cross-linkage, rather than the chemisorption to
the surface, might occur. Excess formation of covalent cross-linking via DOPA
might lose another side of function in the free catechol group. Application of simple
DOPA-peptides via covalent cross-linkage might depend on controlled amounts of
the counterpart; although the free catechol group remains, control using tyrosinase
or oxidant is difficult. This might be the reason for the limited success in studies on
humid attachment. In air, a co-polypeptide of Tyr and Lys with oxidant or tyrosi-
nase gave the tensile strength of a maximal 4 MPa on steel [
69
]. Much lower
adhesive strengths of simplified peptides in humid conditions than those in the air
imply that other amino acids may be significant for composition/sequence in
practical attachment under humid conditions or in water.
Protein extracts from the mussel foot were applied for attachment of porcine skin
without adding an oxidant or enzyme [
71
]. A paste of the protein extracts with
water was applied onto the specimens, and then they were combined together in air
and placed in 80% relative humidity for 24 h for curing. This resulted in 1 MPa for
the failure stress, which is comparable to that of fibrin measured under the same
conditions. The material is actually a mixture of several proteins and is assumed to
mainly contain the bulk protein in the disk and the coating protein of whole byssus,
both of which are probably posttranslationally modified to DOPA. The proteins are
actually denatured in the preparation, thus they act as a random polymer. Adhesive
shear strength of mussel protein extracts for porcine intestinal submucosa was also
measured with the addition of metal ions as a curing agent [
87
]. In a similar manner
as the sample preparation mentioned above, 0.2-0.5 MPa of adhesive strength
resulted from 1 h-curing with metal ions in the air. The values were, however,
much lower than those of a commercial ethyl cyanoacrylate.
Combining natural processing with DOPA-functionality is a recent challenge.
A simple polymer mimic of the tubeworm cement system was attempted for
bonding materials in wet conditions with a preparation of the coacervation complex
[
88
]. The complex coacervate was prepared from two acrylic copolymer mimics:
one with a phosphate group and catechol group and another with primary amine
[
89
]. An oxidant in water was first placed on the surfaces of bovine bone specimens,
and the coacervate was subsequently added to the surfaces. The two specimens
were joined together, were covered with wet gauze, and were placed for more than
24 h into a container containing a wet sponge to maintain humidity. This produced
about 0.1 MPa for the failure stress, which is one-third the value of commercial
cyanoacrylate glue under the same conditions. A hybrid recombinant protein with
modification by tyrosinase was also applied to the formation of complex coacerva-
tion, and it seemed to have increased the adhesive strength by nearly twofold [
90
].
Though this research is in its initial steps, the approaches cited combined with
natural processing are essential to develop practical materials.
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