Biomedical Engineering Reference
In-Depth Information
BOX 8.6
BOX 8.7
THE BIOMATERIAL
INTERPHASE
PROTEIN ADSORPTION
Adsorption can concentrate Mother Nature's
agents of change (
Box 8.4
) within the
biomaterial interphase, conferring biological
activity to a synthetic material that affects
biocompatibility.
A three-dimensional region separating the
physical surface of a biomaterial from a con-
tacting aqueous biological milieu wherein
important physicochemical reactions that
catalyze, mediate, or moderate the biological
response to the biomaterial occur.
limits
[16, 21]
! Water concentration within the
interphase is commensurately lower than bulk
solution since two objects, water and protein
molecules, cannot occupy the same place at the
same time. The viscosity of the interphase is also
quite different from that of bulk solution
[25]
.
Interphase chemistry is very different from bulk
solution chemistry in nearly every way.
Not all biomaterial surfaces adsorb protein,
however, at least in the early phase of material
contact with a biological milieu. As hydrophilicity
increases, protein adsorption decreases to vanish-
ing quantities near water wetting, characterized
by a water contact angle
θ
→
65
◦
for most types
of materials. (See Ref.
20
for a discussion of water
wetting relevant to biomaterial applications.) Min-
erals and other surfaces with ion-exchange prop-
erties are an exception to this general rule
[16]
.
Water molecules are so strongly bound to
hydrophilic materials
(θ <
65
◦
)
that protein can-
not displace water from the interphase and enter
the adsorbed state. The biological response to
hydrophilic materials is observed to be quite
different than that to hydrophobic materials
(θ >
65
◦
)
, presumably because of the influence
of adsorbed proteins
[16, 21, 26, 27]
. Examples
of this hydrophilic/hydrophobic contrast in the
biological response to materials
[21, 22, 27]
include contact activation of blood coagulation
[28]
and mammalian cell adhesion
[26]
. Long-
term contact of a material with a biological
milieu might invite considerable biological
will have a significant effect on vicinal-water sol-
vent properties
[19]
that, in turn, will influence
the distribution of ions near the water-contacting
surface
[21-23]
and possibly affect pH within the
vicinal-water region. A biological entity such as
a protein or a cell entering the vicinal-water
region can encounter significantly different
water chemistry than experienced in bulk solu-
tion, depending on the extent to which self-asso-
ciation of vicinal water has been affected by the
presence of the surface (
Box 8.6
).
8.2.3.2 The Dynamic Interphase
Subsequent to the initial hydration reactions
mentioned previously, macromolecules such
as proteins might adsorb to the hydrated sur-
face, creating a complex and truly three-dimen-
sional region referred to as an
interphase
[20, 22]
.
The term
interphase
, rather than
interface
, is used
to emphasize that this region can be significantly
thicker than the vicinal-water region (interface)
created by surface hydration. Adsorbed proteins
can form multilayers depending on biomate-
rial surface properties
[16]
, and with molecular
diameters in the 5-10 nm range, multiple layers
of proteins might constitute an interphase that is
tens of nm thick
[24]
(
Box 8.7
).
Protein concentration within the interphase
can be very high, much higher than bulk solution
and, in fact, even higher than protein solubility
