Biomimetic Self-Organization and Self-Healing - Engineered Biomimicry - page 340

normally not be combined with a hard surface,
and the mussel byssus has therefore attracted
scientific interest.
The byssus threads are silky filaments com-
posed of a fibrous core and a rough cuticle
coating [45, 47] . The core exhibits remarkable
toughness and self-healing that is explained by
sacrificial bonds [45] . Instead of the normal
cross-links seen in collagen structures, the
byssus core uses ligand-metal complexes as
bonds. These bonds are only half as strong as
covalent bonds but are reversibly breakable
many times. The collagen core is coated with
a 5- μ m-thin composite structure consisting of
equal amounts of matrix and dispersed gran-
ules [45, 47] . The cuticle is about five times
harder than the core of the byssus. Within the
cuticle, a large number of small granules with
a size in the order of 200-800 nm (depending
on the mussel species) are evenly distributed
in a matrix material. The granules play a cen-
tral role in preventing crack propagation.
Both the granules and the matrix material con-
sist mainly of the curly shaped mussel foot pro-
tein (mfp-1), but the concentration of mfp-1 is
higher within the granules than in the matrix
material. Part of the mfp-1 protein is a ligand
called dopa that forms ligand-metal complexes
with very small amounts of metals like iron and
calcium (metal content less than 1%). These com-
plexes function as bonds between the protein
chains. Byssus extension up to 30% is explained
by the straightening out of the curly protein
chains.
At higher extensions, the bonds within the
matrix will break, but in a distributed way to
form many small micro cracks in between the
granules. The granules prevent the cracks from
extending further. A simplified sketch illustrates
the principle in Figure 13.14 .
The potential for self-healing polymers is sig-
nificant. Self-healing paint, in which small
scratches repair themselves, will have an obvi-
ous advantage on cars and on many industrial
products and consumer goods. Not only will it

vertebrae is activated by contraction of tail mus-
cles. An external pull of the tail causes the
remaining tissue to break along weak planes.
The principles of autotomy could be used in
many places—for example, in fire protection of
buildings and in flow control of sewer systems
to avoid massive central overflows during heavy
rains. Disassembly of industrial products such
as cars and washing machines can also benefit
from these principles. When the products are
disposed at the end of the use period, it is impor-
tant that the different material fractions can be
separated easily. One way to do this is to build
in fracture lines at strategic places in the product
so that it is easy and fast to remove panels, win-
dows, and other parts.

13.7 SELF-HEALING

Mussels such as Mytilus californianus or Mytilus
edulis can attach to rocks and other surfaces
using thin byssus threads, as shown in Figure
13.13 . The mussels use 50-100 individual bys-
sus threads to fasten themselves to accessible
surfaces of the rocky seashore [45] . The byssus
thread has remarkable properties: It can repeat-
edly extend up to 100% strain and has good anti-
deterioration properties against microbial attack
and abrasion from suspended sand particles,
thanks to a hard surface [46] . Extensibility can

FIGURE 13.13 Anchor thread byssus from the blue mussel

Mytilus edulis .