Biomedical Engineering Reference
In-Depth Information
unsuccessfully, as the implant is too large
[3]
.
Fibroblasts are therefore signaled to generate
a collagen capsule around the implant, which can
lead to implant loosening
[50]
. To try to avoid this
type of reaction, many potential solutions have been
investigated, such as preadsorbing protein layers,
coating with materials such as calcium phosphates or
incorporating peptide chains onto the surface, with
the aim of using an outer layer that may be more
recognizable to the repair system of the biological
environment
[52
e
58]
. Therefore, the ultimate goal of
surface modification of biomaterials, such as PEEK,
is to manipulate surface chemistry and structure to
control protein adsorption and thereby control the
interaction of cells with the materials.
deposition must be controlled, efficient, and within
reasonable costs
[49]
.
10.2.3 Surface Analytical
Assessment
The success of a new surface is governed by the
biological and chemical criteria, and to assess
whether these criteria are being met, surface analyt-
ical techniques are used. During surface modifica-
tion, only a thin layer (less than 20
m
m) is actually
being modified, and contaminants are easily intro-
duced during these treatments. To ensure that the
intended surface is being produced, sensitive
analytical techniques are used
[60]
. Surface analyt-
ical techniques allow the chemistry and structure of
the surfaces to be analyzed to identify and understand
the extent of the surface modification, and the
resulting effect on the surface properties.
10.2.2 Chemical Criteria
The surface-modified layer should be as thin as
possible, because a thicker modified layer could affect
the mechanical and functional properties of the bulk
material. Thicker modified layers are also disadvan-
tageous as they may delaminate due to the mismatch
in properties between the modified layer and the bulk
of the material. The surface modification should
ideally incorporate only the outermost 0.3
e
1nm,
although in practice, modified regions and deposited
films are thicker as a molecularly thin layer may lead
to uneven coverage
[47]
. Molecularly thin layers may
also be susceptible to erosion or surface reversal/
reorientation. Surface reorientation occurs as a result
of gradual alteration of the surface chemistry due to
diffusion or translation of surface atoms or molecules
as a result of their external environment. The thickness
of some surface coatings is dictated by the chain
length of the molecule being deposited, such as
Langmuir
e
Blodgett films or polymeric coatings. The
surface modification reactions alter constrained
functional groups that are anchored in the bulk of the
material on one side and facing a solution or gas on
the other, without weakening the bonds with the bulk
material. The surface-modified layer must be resistant
to delamination in solutions such as biological fluids.
The surface functional groups produced by different
surface modification treatments must be character-
ized, as many techniques produce an array of groups,
and it is important to identify what is being incorpo-
rated into the surface
[59]
. For biomaterial applica-
tions, the treatment must produce a uniform layer
independent of shape and geometry. The surface
treatment must be reproducible, uniform, stable, and
10.3 Surface Modification
Techniques
10.3.1 DirectdPhysical/Chemical
Physical modification techniques are divided into
two distinct categories: the first involves chemical
alteration of surface structures, while the second
involves deposition of an additional external layer to
the surface of the substrate. In this section, the
various physical modification techniques will be
described.
High-energy species are often required to modify
polymer surfaces, as they are characteristically
nonreactive materials. These physical treatments
cause matter such as atoms and atomic clusters to be
generated and deposited on the substrate surface.
10.3.1.1 Flame
Flame treatments are frequently used to introduce
oxygen-containing species to the surface to improve
the adhesion of paints and printing media to the
surface. The species formed by high-temperature
flame treatment include ions, radicals, and molecules
in excited states. The process involves burning one or
more flames held at a fixed distance from the sample
or moving the sample through the treatment area for
a fixed time
[61,62]
. The effects of the treatment are
varied through flame composition, flame tempera-
ture, sample movement, and distance between the
