4.2.3. Characterization of Linseed-Co-AAM Graft Copolymeric Hydrogels Swelling Studies

Swelling studies were performed using the primly weighed dry hydrogel discs. The dry hydrogel discs were immersed in medium (having pH 1.2 and 4.5) and were allowed to swell until swelling equilibrium. At predetermined time intervals, swollen hydrogels were taken out of the medium; weight was noted after removing the excess water with filter paper that, ultimately, was placed back in same media. The following equation was used to calculate swelling behaviour:

$$\text{Swellingling} = \frac{\text{Ws} - \text{Wd}}{\text{Wd}} \tag{1}$$

where Ws = weight of hydrogel in swollen form, W<sup>d</sup> = weight of hydrogel in dry form [20,36,44].

Percentage of Equilibrium Swelling/Equilibrium Water Content

The swelling was continued until each gel achieved a constant weight. Percentage of equilibrium swelling (%ES) or equilibrium water content (EWC) was determined by the following equation:

$$\% \text{ ES} = \frac{\text{Meq} - \text{Wd}}{\text{Meq}} \times 100 \tag{2}$$

where Meq is the weight of swollen gel at equilibrium and W<sup>d</sup> is the weight of dried gel discs [45].

#### Drug Loading

Nicorandil was used as a model drug for the development of sustained-release formulations for the treatment of hypertension [46]. Drug loading into the hydrogel disc was performed by using the adsorption method. A 1% w/v drug solution was prepared in phosphate buffer having pH 4.5. One disc of each formulation was dipped in 100 mL of 1% drug solution until swelling equilibrium. Discs were removed from the solution and washed out with distilled water to remove an excess of the drug. After, they were allowed to air dry at room temperature first and then oven-dried at 40 ◦C. The amount of drug loaded in the discs was determined by the following formula given in equation 3.

$$\text{Total drug loaded} = \text{W}\_{\text{L}} - \text{W}\_{\text{U}} \tag{3}$$

where W<sup>L</sup> = weight of dried drug loaded disc, W<sup>U</sup> = weight of dried unloaded disc [46,47].

#### 4.2.4. Instrumental Analysis

Fourier Transform Infrared (FTIR) Analysis

The Fourier transform infrared (FTIR) spectra were recorded on an FTIR (prestige-21 Shimadzu) spectrometer for the pure model drug, unloaded hydrogel and drug-loaded hydrogel in order to identify formation of any new bond [48]. For this purpose, samples of the hydrogels and drug were mixed with KBr solution, dried, crushed and kept under hydraulic pressure (150 kg/cm<sup>2</sup> ) to make the disc; spectra were recorded at a wavelength of 4000–500 cm−<sup>1</sup> .

#### Scanning Electron Microscopy (SEM) Analysis

SEM is a widely applied technique for the evaluation of shape and surface morphology of hydrogels. Dried hydrogel discs were cut to specific sizes and were fixed on an aluminium stub. Hydrogels were freeze-dried and then coated with gold in a high vacuum evaporator. The coated samples were scanned and examined under an electron microscope to expose surface morphology [49,50].

#### 4.2.5. In-Vitro Drug Release Study

In-vitro drug release studies were performed using USP-dissolution apparatus II at 37 ± 0.5 ◦C to evaluate the release behavior of all hydrogel formulations. Every disc was placed in dissolution medium, maintained at a temperature of 37 ◦C and stirred at a rate of 50 rpm to maintain a uniform drug concentration in the medium. An aliquot of 5 mL was withdrawn at specified time points, i.e., 0.5, 1, 2, 3, 4, 5, 6, 8, 10 and 12 h and absorbance of nicorandil was measured at a wavelength of 262 nm. In order to keep the dissolution medium volume constant, samples were replaced with an equal volume of fresh buffer maintained at 37 ± 0.5 ◦C. Standard calibration curves of nicorandil were obtained and absorbance was taken at 262 nm. Release kinetics of nicorandil from hydrogels was evaluated by dissolution data modeling by using DD Solver software [51].

Percentage of drug release in hydrogels was determined by using the following equation:

$$\text{In vitro percentage drug release} = \text{F}\_{\text{t}}/\text{F}\_{\text{load}} \times 100\tag{4}$$

where F<sup>t</sup> = release of drug at time t, Fload = amount of drug loaded in disc.

#### *4.3. Mathematical Models of Drug Release Kinetics*

To evaluate the release pattern of nicorandil, and zero-order, first order, Higuchi, Hixson–Crowell, Korsmeyer–Peppas kinetic models were applied.

**Author Contributions:** Conceptualization, A.M.; methodology, S.M.; formal analysis, A.E. and U.R.T.; investigation, A.M.; resources, N.S.M.; data curation, U.R.T. and M.S.A.; writing—original draft preparation, N.S.M.; writing—review and editing, A.M.; visualization, M.S.A.; supervision, A.E.; project administration, S.M.; funding acquisition, A.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

