Biomedical Engineering Reference
In-Depth Information
reactivity also leads to surface oxidation and other surface
chemical reactions. Second, the surface of a material is
inevitably different from the bulk. The traditional tech-
niques used to analyze the bulk structure of materials are
not suitable for surface determination because they typi-
cally do not have the sensitivity to observe the small
amount of material comprising the unique surface chem-
istry/structure. Third, there is not much total mass of
material at a surface. An examplemay help us to appreciate
thisdon a 1 cm 3 cube of titanium, the 100 ˚ oxide sur-
rounding the cube is in the same proportion as a 5-m wide
beach on each coast of the United States is to the roughly
5,000,000 m distance from coast to coast. Fourth, surfaces
readily contaminate with components from the vapor
phase (some common examples are hydrocarbons, sili-
cones, thiols, iodine). Under ultrahigh vacuum conditions
(pressures < 10 7 Pa) we can retard this contamination.
However, in view of the atmospheric pressure conditions
under which all biomedical devices are used, wemust learn
to live with some contamination. The key questions here
are whether we can make devices with controlled and ac-
ceptable levels of contamination and also avoid undesirable
contaminants. This is critical so that a laboratory experi-
ment on a biomaterial generates the same results when
repeated after 1 day, 1 week, or 1 year, and so that the
biomedical device is dependable and has a reasonable shelf
life. Finally, the surface structure of a material is often
mobile. A modern view of what might be seen at the sur-
face of a real-world material is illustrated in Fig. 3.1.4-1 B.
The movement of atoms and molecules near the sur-
face in response to the outside environment is often highly
significant. In response to a hydrophobic environment
(e.g., air), more hydrophobic (lower energy) components
may migrate to the surface of a material
a process that
reduces interfacial energy ( Fig. 3.1.4-1 B). Responding to
an aqueous environment, the surface may reverse its
structure and point polar (hydrophilic) groups outward to
interact with the polar water molecules. Again, energy
minimization drives this process. An example of this is
schematically illustrated in Fig. 3.1.4-2 . For metal alloys,
one metal tends to dominate the surface, for example,
silver in a silver-gold alloy or chromium in stainless steel.
The nature of surfaces is complex and the subject of
much independent investigation. The reader is referred to
one of many excellent monographs on this important sub-
ject for a complete and rigorous introduction (see
Somorjai, 1981, 1994; Adamson andGast, 1997; Andrade,
1985 ). For overviews of the relationship between surface
science, biology and biomaterials, see Castner and Ratner
(2002), Tirrell et al. (2000), Ratner (1988) .
When we say ''surface,'' a question that immediately
comes to mind is, ''how deep into the material does it
extend?'' Although formal definitions are available, for all
practical purposes, the surface is the zone where the
structure and composition, influenced by the interface,
differs from the average (bulk) composition and struc-
ture. This value often scales with the size of the molecules
making up the surface. For an ''atomic'' material, for ex-
ample gold, after penetration of about five atomic layers
(0.5-1 nm), the composition becomes uniform from layer
to layer (i.e., you are seeing the bulk structure). At the
outermost atomic layer, the organization of the gold atoms
at the surface (and their reactivity) can be substantially
different from the organization in the averaged bulk. The
gold, in air, will always have a contaminant overlayer,
largely hydrocarbon, that may be roughly 2 nm thick. This
d
CH 3
Poly (2-hydroxyethyl methacrylate)
pHEMA
CH 2
C
C=O
CH 2
CH 2
OH 2
In air
Under water
Fig. 3.1.4-2 Many materials can undergo a reversal of surface structure when transferred from air into a water environment. In this
schematic illustration, a hydroxylated polymer (for example, a pHEMA contact lens) exhibits a surface rich in methyl groups (from the
polymer chain backbone) in air, and a surface rich in hydroxyl groups under water. This has been observed experimentally (Ratner et al.,
1978, J. Appl. Polym. Sci. 22: 643; Chen et al., 1999, J. Am. Chem. Soc. 121(2): 446).
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