Table I: Composition of different hydrogel formulations used in the present study.
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Synthesis of polyethylene glycol diacrylate.
PEGDA was prepared by using an esterification reaction, involving acrylic acid and PEG of various molecular weights (see Table
I). Weighed amounts of PEG were dissolved in dichloromethane and mixed with a twice-molar solution of dicyclohexyl carbodiimide
in dichloromethane. Acrylic acid then was added (in five times molar excess) into this solution with stirring. The stirring
was continued for 2 h at room temperature followed by 2 h of stirring on an ice bath. The precipitate of dicyclohexyl urea
byproduct was filtered out by vacuum filtration, and the volume of the filtrate was reduced using a rotary vacuum evaporator
(Superfit, Mumbai, India). The product (PEGDA) was then precipitated by adding this concentrated filtrate to an excess of
diethyl ether with continuous stirring. Vacuum filtration again was performed to separate the precipitated product from the
solvent. PEGDA then was purified by dissolving in dichloromethane and reprecipitating with diethyl ether. Finally, the purified
product was dried under vacuum in a vacuum oven (Jyoti Scientific, Gwalior, India) and then stored in a vacuum desiccator
until use. PEGs of various molecular weights were used for the preparation of the copolymers.
IR spectroscopy was performed to confirm the formation of an ester linkage between PEG and acrylic acid. The IR spectra of
the reactants (acrylic acid and PEG) and the product (PEGDA) were compared for any change in vital peaks. The samples were
compressed with potassium bromide in the form of pellets and analyzed using a Fourier transform IR spectrophotometer (PE 1600,
Perkin Elmer, Waltham, MA).
Preparation of hydrogels.
Polymerization of an acrylic acid monomer was carried out in the presence of PEGDA by radiation polymerization using a photochemical
reactor. Solution polymerization was carried out in double distilled water. In Petri plates, weighed amounts of both PEGDA
and acrylic acid up to a concentration range of total polymers of about 50% weight/volume (w/v) were dissolved in water and
then exposed to ultraviolet radiations at wavelength (λ) of 365 nm for polymerization using a photochemical reactor (Jain
Scientific Glasswares, Ambala, India). The hydrogel wafers then were dried and stored in a well-closed container until further
use. To study the effect of concentration of such PEGDA cross-linkers on the properties of the hydrogels, hydrogel formulations
were prepared with various ratios of PEGDA to the acrylic acid monomer (Table 1).
The prepared hydrogels were purified by soaking the hydrogels in an excess amount of ethanol or ethanol-and-water mixture
12–24 h to remove the soluble impurities. Spectral analysis of the washing medium confirmed the completion of washing.
For easy handling and storage, the purified hydrogels were dried before drug loading. Drying was performed under a hot-air
stream flowing over the hydrogel surface using a hot- air sterilizer oven (YSI-431, Yorco, New Delhi, India) at 60 °C for
2 h followed by drying in a vacuum oven (Jyoti Scientific, Gwalior, India) at 45 °C for 2 h.
Characterization of hydrogels.
The physical appearance and texture of the hydrogels were visually evaluated, and their inner morphology was studied using
scanning electron microscopy (SEM). The samples were gold-coated and observed under an electron microscope (Leica, Ernst-Leitz-Strasse,
Germany) under various magnifications. Glass-transition temperatures of the hydrogels were measured using a differential transition
calorimeter (822E, Mettler Toledo, Columbus, Ohio). An aluminum crucible of 40-L capacity held the samples. The samples were
heated from 35 °C to 350 °C at a rate of 10 °C/min. Nitrogen gas was used as cooling (at 200 L/min) as well as the purging
medium (at 80 L/min).
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