Ryan, Eimear M.
The Incorporation and Controlled
Release of Anionic Drugs from a
Polypyrrole Membrane Film.
PhD thesis, National University of Ireland Maynooth.
In this thesis, results are presented and discussed on the synthesis and
characterisation of polypyrrole (PPy) doped with various anionic drugs. The drugs,
which were small to medium‐large in size, included the aqueous soluble
dexamethasone 21‐phosphate disodium (NaDex) and valproic acid sodium
(NaVPA) salts, and the less soluble diclofenac sodium (NaDF) salt. Two other
drugs, indomethacin sodium (NaIndo) and sulindac sodium (NaSul) with very
imited solubility in aqueous solution, were chosen.
The incorporation of dexamethasone (Dex2‐) and diclofenac anions (DF‐) within the
PPy membrane was achieved by a potentiostatic mode of growth from an aqueous
solution of pyrrole and the drug under investigation. For the PPy doped with Dex2‐,
characterisation and release studies found the doping level of the polymer to be
about 0.30, and approximately 31 μmol cm‐2 of Dex2‐ was incorporated into the
polymer upon polymerisation. Furthermore, it was observed that the rate of
release could be controlled by the potential applied with approximately 89 % of
the Dex2‐ released within 60 min at an applied potential of – 0.900 V vs SCE.
For the PPy doped with DF‐, unusual patterns in growth and morphology were
observed. During the deposition of the polymer, the rate of polymerisation
decreased with increasing time and higher applied potentials. The polymer had
features of an insulating film, as evident from electrochemical impedance
measurements, while SEM confirmed the presence of crystal‐like shards on the
surface of the polymer. These findings suggest that insoluble drug crystals are
formed during polymerisation. DF‐ displays a limited solubility in aqueous
solutions and during the oxidation of the monomer and drop of pH at the surface,
the equilibrium is shifted from the soluble DF‐ towards the insoluble HDF causing
insoluble crystals of the drug to deposit on the surface of the polymer, which
hinders further polymerisation.
The incorporation of two insoluble medium sized drugs, NaIndo and NaSul, into
the PPy film was also investigated. Deposition of PPy doped with either of the
drugs in question was carried out in ethanol and tetrabutylammonium perchlorate
(TBAP) was added to increase the conductivity. It was found that the PPy films
formed in this organic medium were not as conducting as those formed in the
aqueous solution, and doping levels were considerably lower than those
previously reported for PPy. The addition of the TBAP introduces the small and
mobile, ClO4‐, anion which is well known to dope PPy. UV‐visible spectroscopy was
used to calculate that approximately 2.19 x 10‐6 mol cm‐2 of ClO4‐ was present in
the polymer. However, this is quite minute compared to the estimated amount of
drug doped within the polymer; 260 μmol cm‐2 and 60 μmol cm‐2 of Indo‐ and Sul‐,
respectively. As seen with the other polymers the rate of release was controlled by
the applied potential.
Finally, the formation of PPy doped with a small soluble anionic drug, VPA‐, was
studied. Although this was the smallest of all the drugs studied, it was not possible
to incorporate this drug into the PPy membrane electrochemically. This was
explained in terms of the solubility of the anion at low pH values. At pH values
below 5.6 the equilibrium of the VPA‐ is shifted towards the insoluble HVPA. As the
monomer is oxidised, there is a decrease in the local pH in the vicinity of the
electrode and this causes the HVPA to precipitate from solution. This, in turn,
prevents any PPy from being deposited at the electrode. Vapour phase
polymerisation is offered as an alternative approach to immobilise this drug into
the PPy film.
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