Biomedical Engineering Reference
In-Depth Information
is also a difference in composition between bulk and sur-
face, but it is not the atomic/molecular rearrangements
we are discussing here. For a polymer, the unique surface
zone may extend from 10 nm to 100 nm (depending on
the polymeric system and the chain molecular weight).
Figure 3.1.4-1
B addresses some of these issues about
surface definitions. Two more definitions must be con-
sidered. An interface is the transition between two
phases, in principle an infinitely thin separation plane. An
interphase is the unique compositional zone between two
phases. For the example, for gold, we might say that the
interphase between gold and air is 3 nm thick (the struc-
turally rearranged gold atoms
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the contaminant layer).
ascertain its purity. Samples can be surface analyzed prior
to and after storage or shipping in containers to ensure that
the surface composition measured is not due to the con-
tainer. As a general rule, the polyethylene press-close bags
used in electron microscopy and cell culture plasticware
are clean storage containers. However, abrasive contact
must be avoided and each brand must be evaluated so that
a meticulous specimen preparation is not ruined by con-
tamination. Many brands of aluminum foil are useful for
packing specimens, but some are treated with a surface
layer of stearic acid that can surface contaminate wrapped
biomaterials, implants or medical devices. Aluminum foil
should be checked for cleanliness by surface analysis
methods before it is used to wrap important specimens.
Parameters to be measured
Many parameters describe a surface, as shown in
Fig. 3.1.4-3
. The more of these parameters we measure,
the better we can piece together a complete description of
the surface. A complete characterization requires the use of
many techniques to compile all the information needed.
Unfortunately, we cannot yet specify which parameters are
most important for understanding biological responses to
surfaces. Studies have been published on the importance of
roughness, wettability, surface mobility, chemical composi-
tion, electrical charge, crystallinity, and heterogeneity to
biological reaction. Since we cannot be certainwhich surface
factors are predominant in each situation, the controlling
variable or variables must be independently ascertained.
Surface analysis general comments
Two general principles guide sample analysis. First, all
methods used to analyze surfaces also have the potential to
alter the surface. It is essential that the analyst be aware of
the damage potential of the method used. Second, be-
cause of the potential for artifacts and the need for many
pieces of information to construct a complete picture of
the surface (
Fig. 3.1.4-3
), more than one method should
be used whenever possible. The data derived from two or
moremethods should always be corroborative.When data
are contradictory, be suspicious and question why. A third
or fourth method may then be necessary to allow confi-
dent conclusions to be drawn about the nature of a surface.
These general principles are applicable to all materials.
There are properties (only a few of which will be
presented here) that are specific to specific classes of
materials. Compared with metals, ceramics, glasses, and
carbons, organic and polymeric materials are more easily
damaged by surface analysis methods. Polymeric systems
also exhibit greater surface molecular mobility than in-
organic systems. The surfaces of inorganic materials are
contaminated more rapidly than polymeric materials
because of their higher surface energy. Electrically con-
ductive metals and carbons will often be easier to char-
acterize than insulators using the electron, X-ray, and ion
interaction methods. Insulators accumulate a surface
electrical charge that requires special methods (e.g.,
a low-energy electron beam) to neutralize. To learn about
other concerns in surface analysis that are specific to
specific classes of materials, published papers become
a valuable resource for understanding the pitfalls that can
lead to an artifact or inaccurate results.
Table 3.1.4-1
summarizes the characteristics of many
common surface analysis methods, including their depth
of analysis and their spatial resolution (spot size ana-
lyzed). A few of the more frequently used techniques are
described in the next section. However, space limitations
Surface analysis techniques
General principles
A number of general ideas can be applied to all surface
analysis. They can be divided into the categories of
sample preparation and analysis described in the following
paragraphs.
Sample preparation
A key consideration for sample preparation is that the
sample should resemble, as closely as possible, thematerial
or device being subjected to biological testing or implan-
tation. Needless to say, fingerprints on the surface of the
sample will cover up things that might be of interest. If the
sample is placed in a package for shipping or storage prior to
surface analysis, it is critical to knowwhether the packaging
material can induce surface contamination. Plain paper in
contact with most specimens will transfer material (often
metal ions) to the surface of thematerial. Many plastics are
processed with silicone oils or other additives that can be
transferred to the specimen. The packaging material used
should be examined by surface analysis methods to
