68
3 Mechanical Aspects of Biosystems
A major distinction with non-living materials is that living systems are adaptive
by having active control over the behavior of their structures, and have repair
mechanisms at the nanoscale. For example, bone under stress can be made stronger
in the direction of the stress (to a limit), and purposefully yield (as in jaw bone under
the side pressure of teeth). Our bridges and buildings do not yet have nanoscale
repair, nor do they grow stronger at locations of greatest stress.
The strongest structures made from atoms have those atoms locked in atomic
bonds. However, most ordinary solids have inhomogeneities due to being a mixture
of differing materials or irregular arrays of crystals. Even single crystals grown from
a single solute in solution tend to have nanoscale defects and atomic dislocations.
As a carbon atom can bond strongly to other carbons sharing one to four valence
electrons with its neighbors, atomic structures built by linking many carbon atoms,
such as in fullerenes and bucky tubes, are the basis of many of the strongest materials
known. 40
3.10.1 Stress and Strain in Bone
Bone is a composite material, made from collagen fibers embedded in an organic
matrix containing crystals of calcium hydroxyapatite, 41 making bone reach a
stiffness of Y = 5 to 30 giganewton per square meter, which is about 10 to
30 times the stiffness of collagen (See Table 3.4). Bone density is about 2 g per
cubic centimeter. Increasing the mineral content of bone by 20% can increase
its stiffness by 230%. A bone’s strength roughly depends on its mineral density
squared. However, this also makes the bone more vulnerable to breakage, because
microfractures can more easily grow when mineral density is high. The petrous
temporal surrounding the cochlea and middle ear ossicles is stiff in order to reflect
and transmit sound without absorption, but is not as strong as leg bone.
Vertebrae fracture under compression near 200 meganewton per square meter;
bones fail under tension at about 140 meganewton per square meter, and fracture
under shear near 70 meganewton per square meter. Live bone, given sufficient time,
can repair microfractures, such as those which might be produced in the lower limbs
from vigorous running or jumping. Ground reaction forces on bones can reach 12
times body weight during jumping for hard ground. Live bone is an adaptive tissue
capable of repair, regeneration, and reconfiguration according to its history of stress.
With sufficient time, microdamage is repaired by osteoclasts (cells which reabsorb
bone) and osteoblasts (cells which regenerate bone). After repeated stress, the bone
material grows stronger, within limits, in the direction of the stress.
40 Buckminster Fuller (1895–1983) was an architect, inventor, and philosopher who made innovative structures, including a geodesic dome built from triangles.
41 3Ca 3 (PO 4 ) 2 .Ca(OH) 2 .
3 Mechanical Aspects of Biosystems
A major distinction with non-living materials is that living systems are adaptive
by having active control over the behavior of their structures, and have repair
mechanisms at the nanoscale. For example, bone under stress can be made stronger
in the direction of the stress (to a limit), and purposefully yield (as in jaw bone under
the side pressure of teeth). Our bridges and buildings do not yet have nanoscale
repair, nor do they grow stronger at locations of greatest stress.
The strongest structures made from atoms have those atoms locked in atomic
bonds. However, most ordinary solids have inhomogeneities due to being a mixture
of differing materials or irregular arrays of crystals. Even single crystals grown from
a single solute in solution tend to have nanoscale defects and atomic dislocations.
As a carbon atom can bond strongly to other carbons sharing one to four valence
electrons with its neighbors, atomic structures built by linking many carbon atoms,
such as in fullerenes and bucky tubes, are the basis of many of the strongest materials
known. 40
3.10.1 Stress and Strain in Bone
Bone is a composite material, made from collagen fibers embedded in an organic
matrix containing crystals of calcium hydroxyapatite, 41 making bone reach a
stiffness of Y = 5 to 30 giganewton per square meter, which is about 10 to
30 times the stiffness of collagen (See Table 3.4). Bone density is about 2 g per
cubic centimeter. Increasing the mineral content of bone by 20% can increase
its stiffness by 230%. A bone’s strength roughly depends on its mineral density
squared. However, this also makes the bone more vulnerable to breakage, because
microfractures can more easily grow when mineral density is high. The petrous
temporal surrounding the cochlea and middle ear ossicles is stiff in order to reflect
and transmit sound without absorption, but is not as strong as leg bone.
Vertebrae fracture under compression near 200 meganewton per square meter;
bones fail under tension at about 140 meganewton per square meter, and fracture
under shear near 70 meganewton per square meter. Live bone, given sufficient time,
can repair microfractures, such as those which might be produced in the lower limbs
from vigorous running or jumping. Ground reaction forces on bones can reach 12
times body weight during jumping for hard ground. Live bone is an adaptive tissue
capable of repair, regeneration, and reconfiguration according to its history of stress.
With sufficient time, microdamage is repaired by osteoclasts (cells which reabsorb
bone) and osteoblasts (cells which regenerate bone). After repeated stress, the bone
material grows stronger, within limits, in the direction of the stress.
40 Buckminster Fuller (1895–1983) was an architect, inventor, and philosopher who made innovative structures, including a geodesic dome built from triangles.
41 3Ca 3 (PO 4 ) 2 .Ca(OH) 2 .