In order to determine the correlation between the mechanical property changes and the degradation of hydrogels, constant frequency compression tests for different dynamic strains were performed on some selected hydrogels during degradation. Figure 11 shows the stress–strain curves of S-
CA80-2, L-CA100-2 and HC-CA200-2 hydrogels during the degradation study. Those samples were selected for this study for the reason that they have the highest swelling and a low degradation extent in their group.
Figure 11. Stress–strain curves in compression representing the correlation between mechanical property change and degradation of selected hydrogels, (a) S-CA80-2 formulation, (b) L-CA100-2 formulation and (c) HC-CA200-2
formulation
The dynamic compression modulus is the ratio of stress to strain, and it is an important parameter of compressive deformation. Figure 12 shows the compression modulus of the hydrogel at the strain of 12%. A greater value of compression modulus indicates more stiffness of the hydrogel, and a lesser value indicates that the hydrogels are more compliant and easy to compress. The compression modulus of the samples decreases with immersion time presumably due to a combination of the degradation of the gel structure and the increased swollen nature of the gel with the aqueous fluid.
Figure 12. Compression modulus, of S-CA80-2, L-CA100-2 and HC-CA200-2 hydrogels during the degradation study
4. Conclusion
This research describes the development of drug delivery hydrogels from natural polymers, starch, hemicellulose, and lignin using reactive extrusion in the presence of citric acid with and without SHP catalyst. Equilibrium swelling of the hydrogels was as high as 1380%. Based on the material formulation, as well as the pH of the medium, the produced hydrogels may show extreme differences in swelling behavior. Hydrogels swelling degree increased at high pH values due to electrostatic repulsion generated by the ionized citric acid moieties grafted to the polymer. Furthermore, the weight loss of such a system in physiological solution is highly dependent on the concentration of CA during the reactive extrusion process and also dependent on the molecular weight of the polymeric material. The combination of the properties of the developed hydrogels might allow for the application of these renewable plant based biopolymers in a wide range of biomedical applications. This study demonstrates the characteristics and potential of natural polymers as a drug release system.
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