Biomedical Engineering Reference
In-Depth Information
Table 3.1.4-1 Common methods to characterize biomaterial surfaces
Cost
c
Method
Principle
Depth
analyzed
Spatial
resolution
Analytical
sensitivity
3-20
˚
Contact angles
Liquid wetting of surfaces
is used to estimate the
energy of surfaces
1 mm
Low or high
depending on
the chemistry
$
10-250
˚
ESCA (XPS)
X-rays induce the emission
of electrons of characteristic
energy
10-150 mm
0.1 at%
$$$
50-100
˚
100
˚
Auger electron
spectroscopy
a
A focused electron beam
stimulates the emission
of Auger electrons
0.1 atom%
$$$
10
˚
-1 mm
b
100
˚
SIMS
Ion bombardment sputters secondary ions
from the
surface
Very high
$$$
FTIR-ATR
IR radiation is adsorbed
and excites molecular
vibrations
1-5 mm
10 mm
1 mol%
$$
5
˚
1
˚
STM
Measurement of the
quantum tunneling current
between a metal tip and a conductive
surface
Single atoms
$$
5
˚
40
˚
, typically
SEM
Secondary electron
emission induced by a
focused electron beam
is spatially imaged
High, but
not quantitative
$$
a
Auger electron spectroscopy is damaging to organic materials and is best used for inorganics.
b
Static SIMS
z
10
˚
, dynamic SIMS to 1 mm
c
$, up to $5000; $$, $5000-$100,000; $$$, >$100,000.
whereas if that car has not been polished in a long time, the
liquid will flow evenly over the surface. This observation,
with some understanding of the method, tells us that the
highly polished car probably has silicones or hydrocarbons
at its surface, while the unpolished car surface consists of
oxidizedmaterial. This type of observation, backed upwith
a quantitative measurement of the drop angle with the
surface, has beenused inbiomaterials science to predict the
performance of vascular grafts and the adhesion of cells to
surfaces.
The phenomenon of the contact angle can be explained
as a balance between the forcewithwhich themolecules of
the liquid (in the drop) are being attracted to each other (a
cohesive force) and the attraction of the liquid molecules
for the molecules that make up the surface (an adhesive
force). An equilibrium is established between these forces,
the energy minimum. The force balance between the
liquid-vapor surface tension (g
1v
) of a liquid drop and the
interfacial tension between a solid and the drop (g
sl
),
manifested through the contact angle (q) of the drop with
the surface, can be used to quantitatively characterize the
energy of the surface (g
sv
). The basic relationship de-
scribing this force balance is:
g
sv
¼
g
sl
þ
g
lv
cos q
The energy of the surface, which is directly related to its
wettability, is a useful parameter that has often correlated
strongly with biological interaction. Unfortunately, g
s
v
cannot be directly obtained since this equation contains
two unknowns, g
sl
and g
sv
. Therefore, the g
sv
is usually
approximated by the Zisman method for obtaining the
critical surface tension (
Fig. 3.1.4-4
), or calculated by
solving simultaneous equations with data from liquids of
different surface tensions. Some critical surface tensions
for common materials are listed in
Table 3.1.4-2
.
Experimentally, there are a number of ways to measure
the contact angle, and some of these are illustrated in
Fig. 3.1.4-5
. Contact angle methods are inexpensive, and,
with some practice, easy to perform. They provide a ''first
line'' characterization of materials and can be performed in
any laboratory. Contact angle measurements provide
