Biomedical Engineering Reference
In-Depth Information
hydroxyl, etc.). Second, hydrophilic surfaces interact
with water through both dispersion forces and Lewis
acid-base interactions. This is to be contrasted to hy-
drophobic surfaces that interact with water only through
dispersion forces (dispersion forces being a class of in-
termolecular interactions between the momentary di-
poles in matter that arise from rapid fluctuations of
electron density within molecular orbitals). Third, as
a direct result of these two features, the number of
possible water interactions and configurations is very
large, especially if the hydrophilic surface is heteroge-
neous on a microscopic scale. These features make the
problem of water behavior at hydrophilic surfaces both
computationally and experimentally challenging.
In spite of this complexity, the reasoning and rationale
applied to large polyelectrolytes in the preceding section
should apply in an approximate way to extended hydro-
philic surfaces, especially the more water-wettable types
where acid-base interactions with water predominate
over weaker dispersion interactions. This would suggest,
then, that water near hydrophilic surfaces is more dense
than bulk water, with a correspondingly less extensive
self-associated water network (row 2,
Table 3.1.5-1
).
There is some support for this general conclusion from
simplified molecular models (
Besseling, 1997; Silverstein
et al.
, 1998
).
Thickness of this putative denser-water layer must
depend in some way on the surface concentration
(number) of Lewis acid/base sites and on whether the
surface is predominately acid or predominately basic, but
these relationships are far fromworked out in detail. One
set of experimental results suggesting that hydration layers
near water-wettable surfaces can be quite thick comes
from the rather startling finding by
Pashley and Kitchener
(1979)
of 150-nm-thick, free-standing water films formed
on fully water-wettable quartz surfaces from water vapor.
These so-called condensate water films would comprise
some 600 water molecules organized in a layer through
unknownmechanisms. Perhaps these condensate films are
formed from water-molecule layers with alternating ori-
enteddipoles similar to thewater layers around ions briefly
discussed in the previous section. Note that this hypo-
thetical arrangement defeats water self-association
throughout the condensate-film layer in a manner consis-
tent with the inferred less self-associated, high-density
nature of water near hydrophilic surfaces.
Stepping back and viewing the full range of surface
wetting behaviors discussed herein, it is apparent that
water solvent properties (structure) near surfaces can be
thought of as a sort of continuum or spectrum. At one end
of the spectrum lie perfectly hydrophobic surfaces with
no surface-resident Lewis acid or base sites. Water in-
teracts with these hydrophobic surfaces only through
dispersion forces mentioned above. At the other end of
the
concentration of Lewis sites to completely disrupt bulk
water structure through a competition for hydrogen
bonds, leading to complete water wetting (0
contact
angle). Structure and solvent properties of water in con-
tact with surfaces between these extremes must then
exhibit some kind of graded properties associated with
the gradedwettability observedwith contact angles. If the
surface region is composed of molecules that hydrate to
a significant degree, as in the case of hydrogel materials,
then the surface can adsorb water and swell or dissolve. At
the extreme of water-surface interactions, surface acid or
base groups can abstract hydroxyls or protons fromwater,
respectively, leading to water ionization at the surface.
Finally, in closing this section on water properties
near surfaces, it is worthwhile to note that whereas in-
sights gained from computational models employing
hypothetical surfaces and experimental systems using
atomically smooth mica and highly polished semi-
conductor-grade silicon wafers provide very important
scientific insights, these results have limited direct bio-
medical relevance because practical biomaterial surfaces
are generally quite rough relative to the dimensions of
water (
Fig. 3.1.5-1
C). At the 0.25-nm scale, water
structure near a hydrophobic polymer such as poly-
ethylene, for example, might better be envisioned as
a result of hydrating molecular-scale domains where
methyl- and methylene-group protrusions from a ''frac-
tal'' surface solvate in water rather than a sea of close-
packed groups disposed erectly on an infinitely flat plane
that one might construct in molecular modeling. Surfaces
of functionalized polymers such as poly(ethylene tere-
phthalate) (PET) would be even more complex. Both
surface topography and composition will play a role in
determining water structure near surfaces.
Water and the biological response
to materials
It has long been assumed that the observed biological
response to materials is initiated or catalyzed by in-
teractions with material residing in the same thin surface
region that affects water wettability, arguably no thicker
than about 1 nm. In particular, it is frequently assumed
that biological responses begin with protein adsorption.
These assumptions are based on the observations that
cells and proteins interact only at the aqueous interface
of a material, that this interaction seemingly does not
depend on the macroscopic thickness of a rigid material,
and that water does not penetrate deeply into the bulk of
many materials (excluding those that absorb water).
Thus, one may conclude that biology does not ''sense'' or
''see'' bulk properties of a contacting material, only the
outermost molecular groups protruding from a surface.
Over
spectrum,
surfaces bear
a
sufficient
surface
the past decade or
so,
the validity of
this
