Biomedical Engineering Reference
In-Depth Information
(PANI) have lower conductivities than PPy and PEDOT. As a
consequence, PT is mainly used as biosensors. PANI can be used in
various applications since it is semi-flexible, may exist as bulk films
or dispersions and requires simple doping chemistry [RAV 10].
3.4.6.
Polymer surfaces
Polymeric biomaterials possess surface characteristics that may
considerably differ from the bulk characteristics. Like metals and
ceramics, contamination of polymeric surfaces may occur and it is
generally attributed to the migration of impurities, or additives used
for the polymer processing; polymeric surface may also oxidize when
processed by melt techniques. Furthermore, the most striking feature
is the dynamic surface of polymer materials since surface polymeric
segments can reorient themselves into various conformations and
rearrange their chemical groups according to the surrounding
environment. Indeed, polymeric surfaces are in a thermodynamically
metastable state and the conformation change occurs because
polymers tend to minimize their surface energies. Surface molecules
can have little up to almost liquid-like mobility, depending mainly on
the polymer glass transition temperature. For instance,
poly(dimethylsiloxane) possesses a very low glass transition
temperature (−50°C) and a liquid-like mobility as it is in a rubbery
state. In contrast, poly(methyl methacrylate), with a glass transition
temperature of 110°C, has a much lower mobility since it is in a glassy
state. The change in conformation is of great importance since
it influences the surface chemical structure, its hydrophilicity/
hydrophobicity, ionic groups, as well as the domain structure of multi-
component systems. Indeed, microphase heterogeneous surfaces may
arise in block copolymers. Moreover, anisotropy of polymer surfaces
may be found depending on the polymer process. Finally, all these
factors and the possible conformation alteration of the surface after
implantation may dramatically impact the biological reactions at the
interface, and as a consequence, surfaces of polymeric biomaterials
need to be appropriately and intensively characterized [LYM 02].
