Biomedical Engineering Reference
In-Depth Information
in spite of this lack of knowledge and because of
the great socioeconomic value of biomaterials,
design-directed and trial-and-error approaches
have been applied to engineer biocompatibility.
The remainder of this chapter presents applica-
tions of surface modification of biomaterials,
pointing out, to the extent currently possible,
how surface modification affects biocompatibil-
ity in reference to the concepts presented in
Fig-
ures 8.3 and 8.4
.
BOX 8.8
THE BIOLOGICAL
RESPONSE TO
MATERIALS
Surface hydration is the first step in the
interaction of a biomaterial with a biological
milieu that controls protein adsorption and
all other subsequent steps in the biological
response to materials.
8.2.5 Limitations of Trial and Error
Before discussing specific applications of surface
modification, it is of interest to contemplate why
design-directed and trial-and-error approaches
to engineering biocompatibility are not entirely
adequate for biomaterials development. That is
to ask, why is it necessary to understand the
fundamentals summarized by the descriptive
chemical reaction of
Figure 8.4
? After all, design-
directed engineering with trial-and-error experi-
mentation was how all of the biomaterials used
in the medical devices represented in the health-
care pyramid of
Figure 8.1
were developed.
What value is a fundamental understanding of
biocompatibility?
One answer to this probing question is that
more of the same approach can be expected to
invent more of the same sort of biomaterials. But
we need much better biocompatibility for
advanced medical devices; more of the same is
simply not adequate to meet evolving medical
needs. There is, of course, the possibility of a
happenstance breakthrough discovery by trial
and error, and this would be highly desirable.
But other than functionality for the particular
purpose under investigation, how would we
know how to leverage this breakthrough in a
general way? At issue here is that discovery
without understanding the science underlying
discovery provides no basis for prospective opti-
mization or generalization. If a material just hap-
pens to be biocompatible for a specific application,
improve biocompatibility because it is the prop-
erties of water at the surface resulting from the
initial hydration reaction that control all down-
stream events. The only step that is subject to
manipulation is the first one—the interaction of
water with the biomaterial surface (
Box 8.8
).
This then is the target of engineering biocom-
patibility—learning to manipulate surface
chemistry so that the dynamic interphase region
created upon immersion of a biomaterial into a
biological milieu leads to “control of interac-
tions with components of living systems,” as
discussed in the Introduction to this chapter.
This engineering must, of course, be specific to
the nature of the milieu in which it is immersed.
That is to say, engineering biocompatibility has
to be tailored to the biomedical application, as
discussed in Section
8.1.2
.
Both
Figures 8.3 and 8.4
may be broadly
descriptive of the biological response to materi-
als, but neither provides the detail required to
prospectively design surface-engineered solu-
tions to biocompatibility issues. At this stage in
the development of biomaterials science, these
details remain well out of reach and are part of
cutting-edge research in biomaterials surface
science
[16]
. Stated in the language of materials
science, structure-property relationships linking
material properties to utility in biomedical appli-
cations are effectively unknown. Nevertheless,
