Biomedical Engineering Reference
In-Depth Information
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materials to which they can be applied, are listed in
Table
3.2.14-2
. Methods to modify or create surface texture or
roughness will not be explicitly covered here, though
chemical patterning of surfaces will be addressed.
3.2.14 Physicochemical surface
modification of materials used
in medicine
General principles
Buddy D. Ratner and Allan S. Hoffman
Surface modifications fall into two categories: (1) chem-
ically or physically altering the atoms, compounds, or
molecules in the existing surface (chemical modification,
etching, mechanically roughening), or (2) overcoating the
existing surface with a material having a different com-
position (coating, grafting, thin film deposition)
(
Fig. 3.2.14-1
). A few general principles provide guid-
ance when undertaking surface modification:
Introduction
Much effort goes into the design, synthesis, and fabri-
cation of biomaterials and devices to ensure that they
have the appropriate mechanical properties, durability,
and functionality. To cite a few examples, a hip joint
should withstand high stresses, a hemodialyzer should
have the requisite permeability characteristics, and the
pumping bladder in an artificial heart should flex for
millions of cycles without failure. The bulk structure of
the materials governs these properties.
The biological response to biomaterials and devices,
on the other hand, is controlled largely by their surface
chemistry and structure (see Section 3.1.4). The ratio-
nale for the surface modification of biomaterials is
therefore straightforward: to retain the key physical
properties of a biomaterial while modifying only the
outermost surface to influence the biointeraction. If such
surface modification is properly effected, the mechanical
properties and functionality of the device will be un-
affected, but the bioresponse related to the tissue-device
interface will be improved or modulated.
Materials can be surface-modified by using biological,
mechanical, or physicochemical methods. Many bi-
ological surface modification schemes are covered in
Section 3.2.16. Generalized examples of physicochemi-
cal surface modifications, the focus of this section, are
illustrated schematically in
Fig. 3.2.14-1
. Surface modi-
fication with Langmuir-Blodgett (LB) films has elements
of both biological modification and physicochemical
modification. LB films will be discussed later in this
section. Some applications for surface modified bio-
materials are listed in
Table 3.2.14-1
. Physical and
chemical surface modification methods, and the types of
Thin surface modifications
Thin surface modifications are desirable. The modified
zone at the surface of the material should be as thin as
possible. Modified surface layers that are too thick can
change the mechanical and functional properties of the
material. Thick coatings are also more subject to de-
lamination and cracking. How thin should a surface
modification be? Ideally, alteration of only the outermost
molecular layer (3-10
˚
) should be sufficient. In prac-
tice, thicker films than this will be necessary since it is
difficult to ensure that the original surface is uniformly
covered when coatings and treatments are so thin. Also,
extremely thin layers may be more subject to surface
reversal (see later discussion) and mechanical erosion.
Some coatings intrinsically have a specific thickness. For
example, the thickness of LB films is related to the length
of the amphiphilic molecules that form them (25-50
˚
).
Other coatings, such as PEG protein-resistant layers,
may require a minimum thickness (a dimension related
to the molecular weight of chains) to function. In general,
surface modifications should be the minimum thickness
needed for uniformity, durability, and functionality, but
no thicker. This is often experimentally defined for each
system.
