A small burst release initially was observed from all the formulations, approximately 3–4% of the loaded drug was released during the first hour, which may have resulted from the adsorption of some of the drug molecules on the surface of the hydrogels. In the later phase, the release of the drug in the hydrogel matrix was assisted by the diffusion of water through the matrix. The release rate, therefore, decreased after the initial burst release because diffusion through the cross-linked polymer matrices is a time-consuming phenomenon. In each case, however, the release rate was faster for hydrogels made of higher molecular weight PEG, which might be as a result of the decrease in cross-linking density with the increase in molecular weight as the molar composition of PEGDA decreases for the same weight composition. The release rate is slower for higher molar percentages of PEGDA. As with the increase in hydrophilicity, the ease of drug release decreases because the drug diffuses out along with water as solution. For example, after 96 h, 80.38% of the loaded drug was released from B₆₀ whereas 33.23% diclofenac was released from formulation D₆₀ .

Table IV (a): Cumulative percentage of drug released by various hydrogel formulations.

Table IV (b): Cumulative percentage of drug released by various hydrogel formulations.

Drug release from matrix systems generally takes place by diffusion. As the diffusion of water in the present case increased with an increase in molecular weight of PEGDA (as found by the diffusion coefficient studies), the drug-release profile also followed similar trends (see Tables IV[a]) and IV [b]) as the drug was released from the hydrogels in the form of an aqueous solution. In the case of the same weight percentage of PEGDA, an increase in the molecular weight of PEGDA led to an increase in the release rate. For example, after 96 h, 35.24% of the loaded drug was released from B₁₀ , whereas 59.35% diclofenac was released from formulation B₄₀ within the same time period.

Table V: Results of data-fitting analysis of release profiles of the hydrogel formulations.