Biomedical Engineering Reference
In-Depth Information
assumption seems to have been confirmed through nu-
merous studies employing SAMs supported on glass,
gold, and silicon in which variation of the outermost
surface functional groups exposed to blood plasma, pu-
rified proteins, and cells indeed induces different out-
comes ( Fragneto et al. , 1995; Liebmann-Vinson et al. ,
1996; Margel et al. , 1993; Mooney et al. , 1996; Owens
et al. , 1988; Petrash et al. , 1997; Prime and Whitesides,
1993; Scotchford et al. , 1998; Singhvi et al. , 1994;
Sukenik et al., 1990; Tidwell et al. , 1997; Vogler et al. ,
1995a, b ). But exactly how surfaces influence ''bio-
compatibility'' of a material is still not well understood.
Theories attempting to explain the role of surfaces in
the biological response fall into two basic categories. One
asserts that surface energy is the primary correlating
surface property ( Akers et al. , 1977; Baier, 1972; Baier
et al. , 1969 ), the other states that water solvent prop-
erties near surfaces are the primary causative agent
( Andrade et al. , 1981; Andrade and Hlady, 1986; Vogler,
1998 ). The former attempts to correlate surface energy
factors such as critical surface energy s c or various in-
terfacial tension components while the latter attempts
correlations with water contact angle q or some variant
thereof such as water adhesion tension s ¼ s lv cos q ,
where s lv is the interfacial tension of water. Both ap-
proaches attempt to infer structure-property relation-
ships between surface energy/wetting and some measure
of the biological response. These two ideas would be
functionally equivalent if water structure and solvent
properties were directly related to surface energy in
a straightforward way (e.g., linear), but this appears not
to be the case ( Vogler, 1998 ) because of water structuring
in response to surface (adsorption) energetics, as de-
scribed in preceding sections.
Both surface energy and water theories acknowledge
that the principle interfacial events surfaces can promote
or catalyze are adsorption and adhesion. Adsorption of
proteins and/or adhesion of cells/tissues is known (or at
least strongly suspected) to be involved in the primary
interactions of biology with materials. Therefore, it is
reasonable to anticipate that surfaces induce a bio-
logical response through adsorption and/or adhesion
mechanisms. The surface energy theory acknowledges
this connection by noting that surface energy is the
engine that drives adsorption and adhesion. The water
theory recognizes the same but in a quite different way.
Instead, water theory asserts that surface energetics is
the engine that drives adsorption of water and then, in
subsequent steps, proteins and cells interact with the
resulting hydrated interface either through or by
displacing a so-called vicinal water layer that is more or
less bound to the surface, depending on the energetics of
the original water-surface interaction. Furthermore,
water theory suggests that the ionic composition of vic-
inal water may be quite different than that of bulk water,
with highly hydrated ions such as Ca 2 þ and Mg 2 þ pref-
erentially concentrating in water near hydrophilic sur-
faces and less hydrated ions such as Na þ and K þ
preferentially concentrating in water near hydrophobic
surfaces. It is possible that the ionic composition of vic-
inal water layers further accounts for differences in the
biological response to hydrophilic and hydrophobic
materials on the basis that divalent ions have allosteric
effects on enzyme reactions and participate in adhesion
through divalent ion bridging.
Water is a very small, but very special, molecule.
Properties of this universal biological solvent, this es-
sential medium of life as we understand it, remain more
mysterious in this century of science than those of the
very atoms that compose it. Self-association of water
through hydrogen bonding is the essential mechanism
behind water solvent properties, and understanding
self-association effects near surfaces is a key to un-
derstanding water properties in contact with bio-
materials. It seems safe to conclude that no theory
explaining the biological response to materials can be
complete without accounting for water properties near
surfaces and that this remains an exciting topic in bio-
materials surface science.
Acknowledgments
The author is indebted to the editors for helpful and
detailed discussion of the manuscript and to Professor
J. Kubicki for molecular models used in construction of
figures. Mr. Brian J. Mulhollem is thanked for reading the
manuscript for typographical errors.
Bibliography
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