Biomedical Engineering Reference
In-Depth Information
resulting in a degradation of film morphology as well as electrical and
mechanical properties.
3,11
Physical adsorption and covalent tethering of molecules to the surface of a
conducting polymer film have the advantage of leaving the bulk properties of
the conducting polymer intact.
26,34
However, this comes at the expense of
limited capacity for incorporation. Adsorption to the surface may better re-
tain biofunctionality in the short term, due to the absence of solvents or
electrochemical stimulation which may act to denature these molecules.
However, the biomolecules are only held in place by weak physical inter-
actions and will likely be displaced by molecules with stronger anities
upon implantation, greatly
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limiting the duration of
the
coatings
biofunctionality.
Covalent tethering of biomolecules to the conducting polymer surface
provides a more permanent means of attachment although may require the
use of solvents increasing the risk of biomolecular denaturation. Covalent
tethering can also give control over the spatial presentation of bioactive
molecules. Both physical adsorption and covalent tethering require post-
fabrication processing that may not be practical for application within
neuroprosthetic devices; this is reflected in the literature with the majority of
biomolecular incorporation utilising either bioactive dopants or physical
entrapment.
Depending upon the method of incorporation and the specific molecule in
question, the presentation of biologically active molecules will fall into one
of two categories. In the case of covalent tethering and/or the use of large,
entrapped molecules the presentation will be surface only; alternatively,
conducting polymers can act as drug delivery systems when utilising smal-
ler, more mobile, biologically active molecules.
.
8.3.2 Conducting Polymers as Bioactive Surfaces
Under certain circumstances incorporated bioactive molecules may be im-
mobilised within or on the surface of the conducting polymer matrix, thus
limiting their biological interaction to presentation of molecules on the
electrode surface. Immobilisation of bioactive molecules is dependent upon
either the method of incorporation or on the properties of the molecule in
question. Covalent tethering and physical adsorption of biomolecules will
obviously lead to a surface-only presentation, however, bioactive dopants
and entrapped molecules may also become immobile within the polymer
matrix due to their size or specific polymer interactions. Because the volume
of interaction is limited to only the surface, bioactive molecules incorpor-
ated in this fashion tend to have bioactivity focused on neural cell attach-
ment or presentation of bioactive molecules that are effective at very low
concentrations.
Several studies have examined incorporating laminin peptide fragments
as dopants within conducting polymer films. Stauffer and Cui individually
cultured primary cortical rat neurons and cortical murine astrocytes on PPy
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