Biomedical Engineering Reference
In-Depth Information
onto the remaining resist structure by galvanization.
After removal of the remaining resist either a metal
structure or mold for subsequent cost-effective replica-
tion processes is achieved.
Further, it has to be emphasized that for a correct
assessment of surface parameters various requirements
have to be met. A first condition is the provision of
a reference line to which measurements can be related.
Also, surface parameters have to be determined with
a clear separation between roughness and waviness
components. This separation has to be achieved by an
electronic filtering procedure. In view of this, perhaps
the most important measurement requirements are the
parameters measuring length over the substrate surface
and cutoff wavelength of the filter used. Measuring
or tracing length has to be described in terms of
real evaluation length (lm) and pre- and overtravel (lv and
ln). The function of the electronic filter is to eliminate
waviness and roughness frequencies out of the surface
profile. As surface features differ in both their wave-
length and surface profile depths, various filters are
available. The filter type to be selected for a specific
surface profile is defined in DIN standards. Use of the
wrong
mCP
The
m
CP method, developed in the laboratory of
George Whitesides, provides a simple method to create
patterns over large surface areas at the micro and
even nanoscale (
Kumar
et al.
, 1994
). A master silicon
template or mold is formed by conventional photo-
lithographic and etching methods generating the
micron-scale pattern of interest. Onto that template,
a curable silicone elastomer is poured. When the sili-
cone polymer cures, it is peeled off and then serves as
a rubber stamp. The stamp can be ''inked'' in thiols,
silanes, proteins or other polymers (see Section 3.2.14).
Flat and curved surfaces can be patterned with these
m
CP stamps.
filter
will
result
in
incorrect
measurements
(
Sander, 1991
).
Parameters for the assessment
of surface microtexture
Characterization of surface
topography
Since the final biological performance of a microtextured
surface is determined by the size and dimensions of
the surface features, specific surface parameters have
to
Various methods are available to describe surface fea-
tures. SEM can be used to obtain a qualitative image of
the surface geometry. Contact and noncontact profilom-
etry are methods to quantify the surface roughness.
be
provided
to
describe
and
define
the
surface
structure.
The definition of surface parameters is mostly based
on a two-dimensional profile section, Occasionally, three-
dimensional
Contact profilometry
profiles
are
created
(see
the
next
two
sections).
In general, for the quantitative description of surface
microtexture, three parameters can be used:
1.
Amplitude parameters, to obtain information about
height variations
2.
Spacing parameters, to describe the spacing between
features
3.
Hybrid parameters, a combination of height and
spacing parameters
These parameters are presented as Ra, Rq, Rt, Rz, Rsk,
Rku (amplitude parameters), Scx, Scy, Sti (spacing pa-
rameters), and
Dq
and l
q
(hybrid parameters). The R-
parameters are denominations for a two-dimensional
description. The S-parameters stand for a three-
dimensional evaluation. These S-denominations are
generally accepted since the work of
Stout
et al.
(1993)
.
For a detailed description of available surface para-
meters, reference can be made to
Sander (1991)
and
Wennerberg
et al.
(1992)
. A brief summary is given in
Table 3.2.15-1
.
The principle of contact profilometry is that a finely
pointed stylus moves over the detected area. The vertical
movements of the stylus are switched into numerical
information. This method results in a two-dimensional
description of the surface. The advantage of contact
profilometry is that the method is inexpensive, direct,
and reproducible. Contact profilometry can be applied
on a wide variety of materials. The major disadvantage is
that the diameter of the pointed stylus limits its use to
surface features larger than the stylus point diameter.
Another problem is that, because of the physical contact
between the stylus and substrate surface, distortion of
the surface profile can occur.
Noncontact profilometry
In this method, the pointed stylus is replaced by a light or
laser spot. This spot never touches the substrate surface.
The light or laser beam is focused on the surface and the
light is reflected and finally converted to an electrical
signal. In this way both two- and three-dimensional
