CHApter 2 An evolutionary perspective
28
cells (osteocytes) associated with the bone tissue first detect material that has been subjected to large deformations and damaged.
Subsequently, these sensors send information to a group of cells
that will remodel the old bone and form a new one. However, by
far the most striking property of bone is its capacity to self-heal a
fracture. Typically, a broken bone starts by experiencing a cut in the
blood supply, and cells die. This process cleans up the area of dead
material before reinitiating the blood supply and sending stem
cells. These cells are a precursor to cells that eventually will produce
cartilage, fibrous tissue, and new bone (Figure 2.14).
The skin is also an outstanding self-healing system. How many
times through life have you had your wounds completely regenerated? The wound-healing process starts with the formation of a
blood clot on the wound to avoid unwanted chemicals from
penetrating healthy tissue. Subsequently, a network of capillaries
forms, with diameters around 8–10 µm, to supply nutrients through
the blood and accomplish cell division and cellular growth.
Inspired by these ideas, researchers at the University of Illinois have
recently developed self-healing materials that consist of a microencapsulated healing agent and a catalyst distributed throughout a
composite matrix. If the material cracks, microcapsules will break
and release a healing agent. The healing agent will react with the
catalyst to repair the damage. This is similar to the case in which a
cut in the skin triggers blood flow to promote healing.
To create these materials, the researchers begin by building a scaffold, which consists of polymeric ink in the form of continuous
filaments in three dimensions. Once the scaffold has been produced, the ink filaments are embedded with a resin. After curing,
the resin is heated, turning the ink into a liquid. The ink is then
removed, leaving behind a substrate with a network of microchannels. Finally, an epoxy coating is deposited on the substrate
while the network of microchannels is filled with a liquid healing
agent. Under these conditions, when a crack forms, it propagates
until it finds one of the fluid-filled microchannels. At this point,
the healing agent moves from the capillary into the crack, where it
interacts with the catalyst particles and repairs the damage.
Last but not least, we ought to discuss the human brain. It is definitely the most incredible piece of machinery we have encountered
so far. It allows us to think and even dream. We have tried to
emulate or surpass the human brain with the development of
computers. However, we now know that computers are nothing like
our brains. Brains are essentially carbon; computers are mainly
Figure 2.12
Schematic three-dimensional view of a white
rhinoceros horn, where the various components of
the composite material forming one horn lamina
are indicated. Each horn tubule is composed of the
cortex and medullary cavity, which is surrounded
by melanized and non-melanized intertubular
matrix. (Courtesy of Tobin Hieronymus et al. Ohio
University.)
Horn tubule
Cortex Medullary
cavity
Mellanized
intertubular matrix
Single horn
lamina
Inter tubular
matrix
Tissue strain
~0.1 – 10 mm
~100 nm
~2 – 4 nm
~1 – 2 nm
Fibril strain
Mineral strain
extrafibrillar matrix
fibril
extrafibrillar
mineral
particles
mineral platelet
collagen matrix
Figure 2.13
Hierarchical structure of bone. The yellow
regions are the collagen fibrils and the red plates
are the apatite crystals. The multidimensional
configuration of the various bone components
is crucial for efficient load transfer. Typically,
the strain decreases from the tissue level to the
mineral particle level by a factor of 6. (Courtesy
of Himadri Gupta et al. Max Planck Institute of
Colloids and Interfaces.)
28
cells (osteocytes) associated with the bone tissue first detect material that has been subjected to large deformations and damaged.
Subsequently, these sensors send information to a group of cells
that will remodel the old bone and form a new one. However, by
far the most striking property of bone is its capacity to self-heal a
fracture. Typically, a broken bone starts by experiencing a cut in the
blood supply, and cells die. This process cleans up the area of dead
material before reinitiating the blood supply and sending stem
cells. These cells are a precursor to cells that eventually will produce
cartilage, fibrous tissue, and new bone (Figure 2.14).
The skin is also an outstanding self-healing system. How many
times through life have you had your wounds completely regenerated? The wound-healing process starts with the formation of a
blood clot on the wound to avoid unwanted chemicals from
penetrating healthy tissue. Subsequently, a network of capillaries
forms, with diameters around 8–10 µm, to supply nutrients through
the blood and accomplish cell division and cellular growth.
Inspired by these ideas, researchers at the University of Illinois have
recently developed self-healing materials that consist of a microencapsulated healing agent and a catalyst distributed throughout a
composite matrix. If the material cracks, microcapsules will break
and release a healing agent. The healing agent will react with the
catalyst to repair the damage. This is similar to the case in which a
cut in the skin triggers blood flow to promote healing.
To create these materials, the researchers begin by building a scaffold, which consists of polymeric ink in the form of continuous
filaments in three dimensions. Once the scaffold has been produced, the ink filaments are embedded with a resin. After curing,
the resin is heated, turning the ink into a liquid. The ink is then
removed, leaving behind a substrate with a network of microchannels. Finally, an epoxy coating is deposited on the substrate
while the network of microchannels is filled with a liquid healing
agent. Under these conditions, when a crack forms, it propagates
until it finds one of the fluid-filled microchannels. At this point,
the healing agent moves from the capillary into the crack, where it
interacts with the catalyst particles and repairs the damage.
Last but not least, we ought to discuss the human brain. It is definitely the most incredible piece of machinery we have encountered
so far. It allows us to think and even dream. We have tried to
emulate or surpass the human brain with the development of
computers. However, we now know that computers are nothing like
our brains. Brains are essentially carbon; computers are mainly
Figure 2.12
Schematic three-dimensional view of a white
rhinoceros horn, where the various components of
the composite material forming one horn lamina
are indicated. Each horn tubule is composed of the
cortex and medullary cavity, which is surrounded
by melanized and non-melanized intertubular
matrix. (Courtesy of Tobin Hieronymus et al. Ohio
University.)
Horn tubule
Cortex Medullary
cavity
Mellanized
intertubular matrix
Single horn
lamina
Inter tubular
matrix
Tissue strain
~0.1 – 10 mm
~100 nm
~2 – 4 nm
~1 – 2 nm
Fibril strain
Mineral strain
extrafibrillar matrix
fibril
extrafibrillar
mineral
particles
mineral platelet
collagen matrix
Figure 2.13
Hierarchical structure of bone. The yellow
regions are the collagen fibrils and the red plates
are the apatite crystals. The multidimensional
configuration of the various bone components
is crucial for efficient load transfer. Typically,
the strain decreases from the tissue level to the
mineral particle level by a factor of 6. (Courtesy
of Himadri Gupta et al. Max Planck Institute of
Colloids and Interfaces.)