Biomedical Engineering Reference
In-Depth Information
6.4.2 Surface Adhesion
In general, when a material is used in biomedical applications, the surface of that
material is exposed to a mixture of small solute molecules, proteins, and cells.
Plasma contains more than 100 identified proteins with specific functions and
varying biologic properties. The most abundant proteins in blood are albumin, fi-
brinogen, and IgG. These get adsorbed to the surface of the implant instantaneously
following exposure to the systemic circulation. The continuous interaction of blood
with artificial contact surfaces can lead to a substantial damage of blood cells and
plasma factors. Routinely used blood-contacting devices such as needles, cannulae,
and blood containers all have very different blood compatibility (termed as hemo-
compatibility) requirements. For example, a needle may reside in the bloodstream
for only a short time and the primary concern for a needle would be hemolysis, the
destruction of red blood cells as a result of chemical interaction with the needle
material. However, a cannula may be implanted for a much longer time and the
primary concern is thrombogenicity, or clotting, which can be caused not only by
chemical interaction, but also by the flow rate of the blood. Localization of the
formed blood clot near the brain could potentially cause a stroke with the blockade
of blood flow.
Surface properties of the biomaterial could be different from the bulk material
properties, uniquely reactive, readily contaminated, and mobile (i.e., can change
depending on environment). The character of the adsorbed protein layer that medi-
ates subsequent events is thought to be dependent on the properties of the substrate
surface such as roughness, chemistry of molecules, inhomogenous surfaces, crys-
tallinity or disorder, and hydrophobicity (wettability). All affect how the material
would interact with the biological component.
Understanding biomaterial surface structure and its relationship to biological
performance is important in utilizing the biomaterial for any biomedical appli-
cation and to develop surface modification strategies for biomaterials, if needed.
Water spreads on or wets some solids and does not others. Surface energy and
work function determine the wettability and ultimately blood compatibility. Figure
6.7(a) shows some of the possible wetting behaviors of a drop of liquid placed on
a horizontal, solid surface (the remainder of the surface is covered with air, so two
fluids are present). Case A represents a liquid, which wets a solid surface well (e.g.,
water on a very clean copper). The angle
shown is the angle between the edge of
the liquid surface and the solid surface, measured inside the liquid. This angle is
called the
contact angle
and is a measure of the quality of wetting. The contact an-
gle describes the shape of the drop at the liquid-solid-vapor three-phase line while
in thermodynamic equilibrium. For perfect wetting, in which the liquid spreads as
a thin film over the surface of the solid,
θ
θ
is zero. Case C represents the case of no
wetting. If there were zero wetting,
θ
would be 180
°
. However, the gravity force
on the drop flattens the drop, so that 180
angle is never observed. If gravity is not
present, the drop shape is spherical assuming. In general, a liquid is said to wet
a surface if
°
θ
is less than 90
°
and does not wet if
θ
is more than 90
°
. Values of
θ
less than 20
are
strong nonwetting. This might represent water on teflon or mercury on clean glass.
°
are considered strong wetting, and values of
θ
greater than 140
°
