Biomedical Engineering Reference
In-Depth Information
processing, as in, for example, attachment of
cells that excrete and deposit proteins directly
onto a surface through a process quite different
from adsorption from solution.
Presumably, then, at least in the immediate or
acute phase of interaction of a biomaterial with
a biological milieu, interphase thickness depends
on biomaterial water-wetting characteristics,
with a thick proteinaceous interphase near
hydrophobic materials and a thinner interphase
(interface) near hydrophilic materials. Regard-
less of biomaterial hydrophilicity, we can antici-
pate that the interphase is a dynamic region
with chemistry quite different from that of bulk
solution. The term
dynamic
has been applied
here because proteins might both adsorb and
desorb, causing a flux of water, proteins, and
associated ions into, and out of, the interphase.
The overall biological response to materials is
really a response to interphase chemistry.
of a blood clot on the surface of a catheter (an
embolus) that detaches from the catheter due to
shear stress under blood flow, travels in the
bloodstream (embolization), and becomes lodged
in a distal part of the circulatory system (an arm
or a leg), thereby causing swelling and pain
(thrombophlebitis) or sometimes death if the clot
lodges in a critical organ such as the brain (stroke).
8.2.4.1 Williams' Four Components of
Biocompatibility
The foregoing pattern of events is summarized
by
Williams' Four Components of Biocompatibil-
ity,
diagrammed in
Figure 8.3
to represent the
essence of David Williams' original descrip-
tion
[30]
, here modified to emphasize a cascade
of causes and effects that spans both time and
space. The time coordinate ranges from the
nanoseconds involved in biomaterial surface
hydration reactions, through the milliseconds
involved in protein adsorption (
acute
or short
term), to full service life of the biomaterial last-
ing hours, days, or years (
chronic,
or long term).
The spatial coordinate ranges from a few tenths
8.2.4 The Biological Response to the
Dynamic Interphase
The observed macroscopic biological response to
biomaterials can, and most usually does, involve
many complex biological and biochemical reac-
tions, even in the relatively simple application
of sterile disposables that are not implanted into
a body. These reactions can include triggering of
linked enzyme reactions that can amplify the bio-
logical response to a biomaterial, such as occurs
in blood coagulation
[29]
. Cells can participate in
biological response to material by becoming acti-
vated or by adhering to the surface. The initial sur-
face hydration and adsorption reactions occurring
when the biomaterial surface is first immersed
into a biological milieu are thus obscured in whole
or part by these secondary reactions.
In general, the biological environment is quite
corrosive and the biomaterial can degrade with
time. Also, in general, biological and biochemical
reactions propagate outward from the surface of
the biomaterial and can cause systemic effects
distant from the surface. An example is formation
FIGURE 8.3
Diagrammatic representation of Williams'
Four Components of Biocompatibility concept. A cascade of
cause-and-effect reactions is caused by bringing a biomaterial
into contact with a biological milieu, propagating along both
spatial and temporal coordinates. Both coordinates span
several orders of magnitude, making the biological response
to materials a very complex biophysical phenomenon. See
also
Figure 8.4
.
