Biomedical Engineering Reference
In-Depth Information
substratum wettability (dashed arrows connect-
ing water/buffer wetting to the biological
response in
Figure 8.7
). No doubt, this kind of
calibration curve is used by manufacturers of
sterile-disposable tissue-culture flasks and
dishes as a rapid quality-control tool. According
to
Figure 8.4
, this calibration scheme works
because water wetting is predictive of inter-
phase characteristics that lead to bioadhesive
outcomes, at least over the short time interval of
the acute biological response to materials. A
similar strategy has been applied to blood
plasma coagulation induced by contact with
materials to infer a relationship between surface
energy and the propensity of blood plasma to
coagulate (i.e., clot)
[28, 98]
.
A limitation of this calibration approach to
biomaterials surface modification is that it does
not reveal how the observed correlation between
water wetting and the biological response occurs.
A full appreciation of the part of
Figure 8.4
labeled amenable to comprehension would min-
imally require an inventory of proteins adsorbed
to the surface, if any. Since this remains out of
our collective grasp
[16]
, it seems that our under-
standing will remain at the level of calibration
curve. Nevertheless, it is generally possible to
measure surface wetting by the biological milieu
understudy (plasma, serum, cell-culture fluids,
etc.) that includes the effect of protein adsorp-
tion. Compared to wetting in pure water or
buffer (dashed arrows on the right side of
Figure 8.7
), the effect of adsorption on interfacial
energetics is obtained in a phenomenological
way that does not require an inventory of
adsorbed proteins. A surface energy-biological
response calibration made at this level is that
much closer to the cause and effects suggested
by
Figure 8.4
, and a reproducible correlation
provides some clues about the nature of the
dynamic interphase that guides the observed
response. For example, a comparison of relation-
ships between mammalian cell adhesion and
water wettability in the presence and absence of
proteins provides evidence that initial stages of
bioadhesion are controlled by surface energetics
and that adsorbed proteins affects bioadhesion
by altering surface energetics of the adhesive
process
[79, 80, 135]
. The role of protein adsorp-
tion in controlling surface-contact-induced
coagulation of blood plasma has been clarified
using a similar approach
[28, 98]
.
Surface-analytical methods measuring chem-
ical composition are at least one step removed
from the cause-and-effect approach suggested
by
Figure 8.4
, because surface chemistry is not
in and of itself predictive of water/buffer
wettability or the surface energetics that drive
interfacial phenomena of adsorption and
adhesion. A two-stage calibration scheme
suggested by
Figure 8.7
can be used in this
instance
[77]
. Surface engineering or modification
(Section
8.3
) creates surface chemistry that can
be characterized by any one of many surface
analytical tools, such as electron spectroscopy
(ESCA) or secondary ion mass spectroscopy
(SIMS); see, for examples, Refs.
95 and 130
.
Tensiometric measurement of water wettability
of these surfaces enables a primary calibration
that relates surface chemistry to surface energy
(for example, oxygen composition measured by
ESCA to water contact angles
[20]
). Measurement
of surface wetting by the authentic biological
milieu under study expands this primary
calibration by including the effects of protein
adsorption as mentioned earlier.
When surfaces bear immobilized biomimetic
functional groups, such as functional biological
molecules or templates of biological motifs, data
interpretation almost assuredly will be at the
level of a calibration. In this situation, dose-
response relationships are vastly more complex
because of the inclusion of biological activity.
The surface does not acquire biological activity
only through processes outlined in
Figure 8.4
but rather is biologically active before imposi-
tion into the biological milieu. The reactivity of
the surface is thus not directly related to physics
and chemistry but involves a level of biological
activity that physics and chemistry do not
