27
The horn is a composite made of two types of keratin. One type is
in the form of tubules that are densely packed (see Figures 2.11 and
2.12). These tubules, which range from 300 to 500 µm, comprise
around 40 layers of cells. Surrounding these tubules is another type
of keratin that is continuous and acts as a matrix. Because the
matrix and the tubules are the same material, the interfacial strength
is very good, which leads to a rigid material that is very tough to
break. Following this concept, there is now great interest in emulating these horn structures at the nanoscale by designing materials
with nanotubules embedded in a matrix. In other words, this
approach is not very different from dispersing nanowires or nanotubes in a polymer matrix.
Another remarkable property of natural materials is their ability to
self-repair and adapt to the environment. This is crucial for
efficiency and, ultimately, survival of the fittest. By contrast synthetic materials rarely exhibit this property. Therefore, producing
materials capable of repairing cracks, restoring functions, or selfproducing is still a challenge that nanotechnology can help meet.
Two interesting examples of natural materials that exhibit selfhealing properties are bone and skin.
Bone is essentially a composite material formed by an organic matrix
(collagen) and reinforced by mineral particles (apatite). The collagen
is in the form of fibrils of about 100 nm in diameter and 5–10 µm
long that consist of an assembly of 300 nm long and around 1.5 nm
thick. The apatite are platelike crystals with a thickness around 1.5–
4.5 nm that fill the collagen fibrils. The collagen fibrils are separated
by extrafibrillar material, which consists of extrafibrillar collagen and
apatite that coats the fibrils (see Figure 2.13).
The remarkable result of this hierarchical configuration is that the
bone can hold weight and toughness to absorb energy and not
break into small fragments (see Figure 2.14). When subjected to a
load, the whole bone deforms to different degrees. In fact, the
tissue, fibrils, and mineral particles absorb successively lower levels
of strain, in a ratio of 12 : 5 : 2. Although the mechanisms of deformation are not fully understood, it seems that the interface between
the extrafibrillar matrix and the collagen fibrils breaks and reforms
under load, providing a way for damage repair at the molecular
scale. On the other hand, the hard nanoparticles of apatite are
shielded from excessive loads, although they can achieve strains of
0.15–0.2%, which is twice the fracture strain of bulk apatite.
Bone is also capable of constant remodeling, eliminating
damaged bone and replacing it with new material. Strain sensor
Nanomaterials and Nanostructures in Nature
Figure 2.10
Spider web.
Figure 2.11
Microstructure of a white rhinoceros horn. The
horn tubules, surrounded by the intertubular
matrix, act as a composite material. (Courtesy of
Tobin Hieronymus et al., Ohio University.)
Intertubular matrix
Horn tubules
Intertubular matrix
Horn tubules
2 mm
The horn is a composite made of two types of keratin. One type is
in the form of tubules that are densely packed (see Figures 2.11 and
2.12). These tubules, which range from 300 to 500 µm, comprise
around 40 layers of cells. Surrounding these tubules is another type
of keratin that is continuous and acts as a matrix. Because the
matrix and the tubules are the same material, the interfacial strength
is very good, which leads to a rigid material that is very tough to
break. Following this concept, there is now great interest in emulating these horn structures at the nanoscale by designing materials
with nanotubules embedded in a matrix. In other words, this
approach is not very different from dispersing nanowires or nanotubes in a polymer matrix.
Another remarkable property of natural materials is their ability to
self-repair and adapt to the environment. This is crucial for
efficiency and, ultimately, survival of the fittest. By contrast synthetic materials rarely exhibit this property. Therefore, producing
materials capable of repairing cracks, restoring functions, or selfproducing is still a challenge that nanotechnology can help meet.
Two interesting examples of natural materials that exhibit selfhealing properties are bone and skin.
Bone is essentially a composite material formed by an organic matrix
(collagen) and reinforced by mineral particles (apatite). The collagen
is in the form of fibrils of about 100 nm in diameter and 5–10 µm
long that consist of an assembly of 300 nm long and around 1.5 nm
thick. The apatite are platelike crystals with a thickness around 1.5–
4.5 nm that fill the collagen fibrils. The collagen fibrils are separated
by extrafibrillar material, which consists of extrafibrillar collagen and
apatite that coats the fibrils (see Figure 2.13).
The remarkable result of this hierarchical configuration is that the
bone can hold weight and toughness to absorb energy and not
break into small fragments (see Figure 2.14). When subjected to a
load, the whole bone deforms to different degrees. In fact, the
tissue, fibrils, and mineral particles absorb successively lower levels
of strain, in a ratio of 12 : 5 : 2. Although the mechanisms of deformation are not fully understood, it seems that the interface between
the extrafibrillar matrix and the collagen fibrils breaks and reforms
under load, providing a way for damage repair at the molecular
scale. On the other hand, the hard nanoparticles of apatite are
shielded from excessive loads, although they can achieve strains of
0.15–0.2%, which is twice the fracture strain of bulk apatite.
Bone is also capable of constant remodeling, eliminating
damaged bone and replacing it with new material. Strain sensor
Nanomaterials and Nanostructures in Nature
Figure 2.10
Spider web.
Figure 2.11
Microstructure of a white rhinoceros horn. The
horn tubules, surrounded by the intertubular
matrix, act as a composite material. (Courtesy of
Tobin Hieronymus et al., Ohio University.)
Intertubular matrix
Horn tubules
Intertubular matrix
Horn tubules
2 mm