Biomedical Engineering Reference
In-Depth Information
Table 7.1. RMS roughness values of
the different
treatment procedures
represented by the images shown in Figure 7.2.
R
q
(nm)
Sample (Figure 7.2)
Treatment
A
Mechanical polishing
8
+
4
B
Mechanical polishing then acid etching
270
+
27
C
Sandblasting and plasma treatment
900
+
300
D
Grinding on SiC paper
160
+
110
considerably flatter overall than the pit-free sample shown as Figure 7.2C. However, the
use of the roughness values allows a more quantative approach to surface texture
characterization. As an example, the
R
q
(also known as rms roughness) values of the
different treatment procedures represented by the images shown in Figure 7.2 are shown
in Table 7.1. It is clear that the processing of these titanium implant surfaces has a huge
impact on their roughness, and this is very likely to be an important factor on their
performance. For example, roughness values of the surfaces on implants as measured by
AFM can be correlated to the degree of bone contact and bone formation of the implant
[366, 414]. Rougher surfaces appear to enhance bone contact.
Measuring roughness is a simple and quantative way to compare surfaces. However,
some practical issues must be borne in mind in order to make accurate comparisons of
sample roughness. The general rule of thumb is that all scanning, processing and analysis
must be identical in order to be able to compare roughness values. The main parameter that
affects the value of roughness obtained is the size of the scan measured, so this must
always be the same when comparing roughness of different surfaces. On the other hand,
the pixel resolution has much less effect on the data obtained, varying only for extreme
values [415]. See Figure 7.3 for illustrative plots that demonstrate the relationship between
image size (spatially, and in terms of pixel density), and roughness.
Processing of the data, in particular levelling, will affect the value of roughness calcu-
lated. For example, with polynomial line-by-line levelling, as is shown in Figure 7.3, higher
orders reduce the roughness value obtained [361]. The plot at the right of Figure 7.3 shows a
line scan along a femoral hip replacement implant, subjected to different polynomial
flattening treatments. The line profiles are clearly quite different, and the values of
R
a
show that this is dramatically reflected in the roughness parameters. Depending on the
reason for measuring the roughness, some levelling should always be applied, so it must
simply be applied consistently. Filters should also be used with caution, as they can
drastically affect the roughness results obtained.
In order for roughness results to be useful, measurement parameters including the size
of the area measured and image treatment should be specified with the results [408, 416].
It has also been shown that the measured roughness depends on the probe used and its
condition [408]. This is because less sharp probes will tend to smooth out the data as they
cannot image fine features on the sample surface.
7.1.2 Hardness measurement of polymer films
Stiffness or hardness measurement using AFM is one of the more common non-topographic
experiments. Although there are a number of other techniques able to make hardness
