|
A key
design feature shared by many 100 year-old barns and
some modern skyscrapers is that the external shell
carries the building's load with a minimal use of
internal columns for support. Internal floors and walls,
if any, function in a structural way to stiffen the
building. This resourceful design allows for a very
strong structure with a maximum of interior space
available for other purposes.
The bones in the human body capitalized on this design
feature long before farmers and architects did. In fact,
studying bone construction and function provides a
mini-course illustrating important engineering
principles.
Sophisticated Engineering Properties
A quality product begins with materials that have
superior engineering properties. Bone is constructed
much like reinforced concrete, in which a cage of steel
reinforcing bars ("rebar") is embedded. The reinforcing
"rods" in bone are made from minute strands of collagen
fibers, 360 of which could be put end-to-end in the
width of a human hair.
Each fiber is composed of three substrands wound in
rope-like fashion around each other so tightly that
along the area of contact only the smallest amino acid
would fit in the space between the strands. In order to
work, this particular amino acid would need to be
designated for every third position in each
strand--which is exactly what is specified in the DNA
code. Collagen fibers are linked so strongly that their
resistance to being pulled apart in tension is actually
greater than the resistance present in an equal amount
of steel rebar.
The bone's equivalent of the cement/aggregate part of
concrete is composed of apatite. Apatite is a
medium-hard mineral with properties similar to marble
and is found widely distributed in rocks. A microscopic
view shows that individual apatite crystals are bound to
the collagen fibers and linked as a continuous mesh--but
at full size the structure appears solid. Compared to
reinforced concrete, bone is more flexible and has more
strength to resist crushing compressive loads. This is
vitally important, since a man lifting a 70-pound box
actually exerts a normal compressive load of over 500
pounds on one of his vertebrae--just imagine the loads
of Olympic weightlifters.
An important area of engineering research is designing
materials that are fatigue resistant. Fatigue is a
progressive failure due to the localized and cumulative
damage that occurs when material is subjected to cyclic
loading. Counteracting this is imperative, since in one
year each hip bone for the average person will sustain
about 1.8 million cyclic loads. Bone is one of the most
fatigue resistant materials known due to its unique
blend of strength, stiffness, and flexibility.
The actions of bending (compression/tension) and torsion
(twisting) on bone are at their highest within the
external shell. Dense-compact bone, able to resist these
actions, is built into the shell. Inside, a
three-dimensional network of small boney material
resembling a porous sponge, called spongy bone, is found
throughout small bones and at the ends of long bones.
Spongy bone absorbs shocks and also contributes inner
bracing or stiffness. The thin bony inner bracing
elements do not grow randomly, but look and function
like the support struts in the Eiffel Tower. Some
studies demonstrate that if engineers apply a
stress-strain analysis to a cross section of bone, it
reveals that the bony braces are built along lines of
maximum stress relative to the mechanical forces applied
to them.
Fundamental Engineering Principles
Engineering efficiency strives for designs that
completely fulfil an intended purpose while using
minimal resources. Engineers can only dream of highways
so efficient that they automatically expand from two to
four lanes with population growth and contract with
declines. In contrast, bone size does constantly change
in response to demand throughout a person's life.
This highly efficient process called remodelling ensures
that more bone is built in specific locations when it is
subjected to heavy-repetitive loads and less is built
when it carries lighter loads. In infants, 100 percent
of the calcium is exchanged in their bones every year.
For people in their 20s, the equivalent of 20 percent of
the skeleton is replaced yearly--though high stress
areas like inside the head of the upper leg bone may be
replaced up to three times per year.
Remodelling also functions as a non-stop maintenance
program for bone by tearing out old bone and replacing
it with new. Concrete or block walls would last for ages
if they had an outer covering that could continuously
replace weak spots, repair cracks, or swap out rusty
rebar. Remarkably, bones do possess such a covering.
The periosteum is composed of two important layers. The
thin, lightweight outer layer consists of very flexible
but extremely tough high-tensile-strength fibers akin to
high-performance membranes that are now being utilized
to wrap new buildings. The inner layer is composed
mostly of two different types of cells kept in delicate
balance--one type destroys bone and the other builds
bone. These crucial cells are the workhorses for
remodeling. The entire layer adheres tenaciously to the
bone by means of strong, perforating fibers that embed
in the collagen-apatite matrix. The concentration of
these fibers varies and is appropriately very dense at
spots where tendons connect to the bone.
A robust object withstands a lot of harm but continues
functioning as intended. Sometimes the best response to
a destructive force is to flex rather than to offer
direct resistance. Automobile makers design "crumple
zones" of materials intended to fold up or shear apart
so crash forces are absorbed rather than transmitted to
occupants.
Bones resist fractures in similar fashion. At the
smallest level of collagen fibers, not all of the bonds
are fixed solid. Some, called sacrificial bonds, are
weaker bonds intended to break upon impact. Their exact
arrangements in bone absorb and then disperse many
forces that could rapidly reach the fracture threshold.
But unlike a car's crumple zone, a bone's sacrificial
bonds can repair themselves after the trauma, making
them ready for another strike.
Damage Repair
Bones do have structural limits and can succumb to
fractures that range from hairline to fully displaced.
The cleanup and repair of bone exemplifies a
thoughtfully engineered construction plan. A major
fracture tears blood vessels, causing extensive bleeding
and tissue swelling (pain results from torn or
compressed nerves). Fortunately, blood eventually clots
around the fracture, starting the healing. Within 48
hours, cells invade the blood clot and use it as a
template to build a microfiber meshwork that acts as the
"scaffolding" supporting the rest of the repair work.
Other prerequisites to proper healing include broken
bone ends being brought close together, aligned
properly, and immobilized, with a sustained blood supply
and the area kept free of infection.
The fracture zone is full of bone fragments and dead
cells. Cells specialized in tissue demolition dismantle
unusable bone fragments into their component parts.
Other cells engulf and digest tissue debris. Valuable
recyclable materials are saved and actual wastes carried
off in the bloodstream to be discarded.
Man-made splints support fractures to prevent large
damaging movements, but the broken ends still need
further stabilization. Certain cells, called
fibroblasts, work off of the "scaffold," laying down
collagen fibers to span the break. Once some of the
collagen bridge is made, new cartilage can be placed
concurrently around the fibers. Fibroblasts will
transform themselves into chondroblasts to produce this
cartilage. Once built, the collagen-cartilage unit
functions as new inner rebar, forming material
(controlling the shape and location of the new bone),
and the temporary bracing--all in one package.
Bones are living tissue and need to be nourished. Inside
bone is an ingenious system of microscopic canals that
comprise a thoroughfare to shuttle nutrients.
Bone-building cells have multiple slender arms that
radiate out from the cell body. When new bone is made,
hundreds of these cells join their arms together to form
a three-dimensional network that will become the basis
of the canal system.
These cells will actually build new bone all around
themselves and thus become entrapped within the bone. In
essence, the cells not only make boney "concrete," but
amazingly become their own forming material for the
interior canals. With its job making bone now complete,
this cell transforms itself into a
nourishing/pressure-sensor cell called an osteocyte.
Repair and remodeling processes make bone so resilient
that in time a repaired bone may look almost identical
to the original.
Conclusion
Bone structure is an engineering marvel. For its
stress environment, it achieves nearly maximum
mechanical efficiency with minimum mass, which designers
call an optimized structure. Thus, bones remain a
testimony to the genius of their Creator, the Lord Jesus
Christ. In fact, bones are such an important feature in
human design that they will remain with us for all
eternity in our resurrected bodies, as Jesus
demonstrated for His disciples (Luke 24:39).
|
