Biomedical Engineering Reference
In-Depth Information
1.2
Staying Alive Despite the Second Law
A living body is not a closed but an open system and through this the body deceives
physics. It puts a brake on an uncontrolled increase in entropy S , i.e., the increase
of heat content Q per degree Kelvin, heat being the least noble form of energy.
A body is a complex biological system that by a self-regulating process main-
tains a dynamic stability for all vital parameters (temperature, ionic concentration,
metabolicrate,energyconversion...).Theprocessiscalled
homeostasis
or, as some
authors prefer,
homeodynamics
because of the dynamic character of the process.
Self-regulating
may be substituted by
negative feedback
which is the basic principle
of a branch of science named
cybernetics
. Feedback is a mechanism that reacts to
disturbance of conditions through modifications of equal size and opposite direc-
tion. The disturbance may concern the global body, for example change of external
temperature or humidity of the environment, or local parts, for example a change
in ionic concentration at an implant interface. Feedback tries to maintain the inter-
nal balance. A sustained disturbance may go beyond the capability of the feedback
mechanism to reestablish equilibrium and at that point, the mechanism itself may
become destructive for the system it is intended to protect. The overencapsulation
by fibrous tissue of the stem of a hip prosthesis, the inhibition of apposition of new
bone and the subsequent failure of stability may be understood in these physical
terms.
The foregoing paragraph seems not to learn us anything how implants should be
improved or manufactured. The intention was to introduce homeostasis as the iron
consequence of the physics of complex open systems. Although at first sight it is
far away from the prosaic reality of biomaterial business, it is not. In testing the
biological performance of a material or a device
in vitro
, the compelling dynamic
character of the body site, in which it is intended to function, is often ignored in the
concept of experiments.
1.3
Scaling of Plants and Animals
Since the time of Vesalius, the machine concept of the human (or animal) body has
continued to develop till today. Seventeenth century masterpieces and milestones in
this respect are the studies of blood circulation by William Harvey (1578-1657)
De
Motu Cordis et sanguinis in animalibus, Anatomica Exercitatio
[
8
] and the motion
of animals by Giovanni Alfonso Borelli (1608-1679) in
De Motu animalium
[
9
].
These are descriptions in qualitative mechanical terms of those magnificent run-
ning, swimming or flying
machines
. The interplay between geometry, physics and
mechanics, be it of rigid bodies, fluids or gases, and biology is obvious, and is
translated in a number of mathematical laws.
Leonardo da Vinci was already fully aware of this interplay. He was dreaming
that, one day, it would be possible for man to sustain himself in the air by modifying
