Biomedical Engineering Reference
In-Depth Information
BOX 8.4
BOX 8.5
PROTEINS
THE BIOMATERIAL
INTERFACE
Biological macromolecules comprising
amino acids that serve as antibodies,
enzymes, hormones, and structural compo-
nents of cells and tissues. The
proteome
is the
entire set of proteins made by an organism
that subsumes thousands of different kinds
of proteins. Proteins are Mother Nature's
tools for accomplishing nearly all physiolog-
ical functions; proteins are Mother Nature's
agents of change.
A pseudo two-dimensional region separat-
ing the physical surface of a biomaterial
from a contacting aqueous biological milieu
that instantly arises because of the interac-
tion with water that can cause the redistribu-
tion of ions near the surface.
create a thin pseudo two-dimensional zone of
water directly adjacent to the surface, referred to
as
interfacial
or sometimes
vicinal
(near to) water
[20]
. Vicinal-water chemistry is different from
that of water within the bulk solution due to the
physicochemical interactions with the surface
that constitute hydration reactions. The exact
thickness of the vicinal-water region no doubt
depends on surface chemistry of the biomate-
rial. The structure/reactivity of vicinal water is
both an important and a controversial subject in
materials science
[21, 22]
. But the evidence seems
to be that the vicinal-water region is probably no
thicker than a few layers of water molecules.
The reason that vicinal-water chemistry is
thought to be influential in the biological
response to biomaterials is that water solvent
properties correlate with the extent of hydrogen
bonding
[21, 22]
. Water is a relatively poor sol-
vent at low temperatures near the density maxi-
mum (at 3.98 °C) because nearly all hydrogen
bonds are used up in a self-associated network
of water molecules. At 0 °C, water ice is entirely
self-associated by hydrogen bonds that form the
familiar ice crystal structure. By contrast, water
steam (100 °C) is quite corrosive because nearly
all hydrogen bonds are available to participate
in solvation reactions. Thus it may be anticipated
that changes in hydrogen bonding induced by
contact with surfaces at ambient temperatures
8.2.3 Why Surfaces are Important in
Biomaterials
Water is the biological solvent system and the
majority molecule in all biological environments
[15]
. Biology is dissolved or suspended in an
aqueous solution of various ions (such as Na
+
,
Mg
+
2
, and Cl
−
) and many different biological
macromolecules, such as proteins and sugars. For
example, human blood is approximately 45% by
volume water and salts and about 10% by volume
proteins
[16]
. There are more than 1,000 differ-
ent types of proteins in blood, varying some 10
decades in concentration
[17, 18]
. Such mixtures
are not typically chemically defined in a precise
way and are frequently referred to as a
biological
milieu
or simply a
milieu
. This milieu may also con-
tain cells of various kinds. Human blood is about
45% by volume “formed elements” that include
platelets, red and white blood cells (
Box 8.5
).
8.2.3.1 The Interface
An immediate consequence of bringing a bioma-
terial into contact with a biological milieu is inter-
action with water. Water is very small, only about
the size of atomic oxygen, and hydrates surfaces
to varying degrees depending on surface chem-
istry/energy
[19]
. These hydration reactions
