Preparation of Silk Fibroin - based Hydrogels and Their Application on Controlling Plant Nutrients Release

Abstract

Nowadays, natural polymers, of which being biodegradable matrices and environmentally friendly, are widely utilized for improving efficacy of the materials on controlling fertilizer release. In this thesis work, it was carried out in three parts: in Part (1), the hydrogels were prepared by blending silk fibroin (SF) with gelatin at various weight ratios, i.e., 100:0, 75:25, 50:50 and 25:75 and converting the SF conformational structure to β-sheet by methanol. The secondary structure and crystallinity of the blended hydrogels were investigated using Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction analysis (XRD), respectively. Moreover, the swelling ratio of the hydrogels and also their kinetics of N-P-K release were also studied. Results from the FT-IR spectra confirmed the β-sheet transformation from α helix conformation of the SF after soaking in methanol and there were no differences between the spectra of SF/gelatin with different weight ratios indicating that no interactions had taken place between SF and gelatin. In addition, the increase of gelatin content in the blended hydrogels caused the decrease of the SF crystallinity as shown by the XRD spectra which corresponded to the swelling behavior of the hydrogel. Whereas the release rate of N-P-K was found to be dependent on the compositions of SF and gelatin of which its value of diffusion exponent characteristics (n) determined from the Korsmeyer-Peppas model for all of the hydrogels were smaller than 0.5. This indicates that the release mechanism of N-P-K from the hydrogels is a quasi-Fickian diffusion. For the release rate constant (k) and diffusion coefficient (D) of the SF/gelatin hydrogels, their values are lower than those of the SF itself and slightly increase with the increase of gelatin content. The release exponent (n) and release rate constant (k) of N-P-K could be compared and ranked in the following descending order N>P>K and K>P>N, respectively. After the addition of urea, there was no difference between the FT-IR spectra. The swelling ratio and the nitrogen release of SF/gelatin/urea are dependent on the temperature and pH of the medium. The results revealed that the SF/gelatin hydrogel at a weight ratio of 75:25 of SF/gelatin showed the slowest rate of urea release which became steady within a week. In order to extend the urea release rate, chitosan (CS), a natural polysaccharide, was selected for slow release formulation of the SF/gelatin hydrogel. In Part (2), the SF/gelatin hydrogels with CS composition ranging from 20 to 100 php (part per hundred of polymer) were prepared by solvent casting method. The intermolecular interaction, crystallinity and surface hydrophobicity of the blended hydrogels were then investigated using FT-IR, XRD and contact angle analyzer, respectively. Moreover, the percent porosity, degree of swelling, water solubility of the hydrogels and also their kinetics of urea release were also studied. Results from the FT-IR confirmed that no intermolecular interactions had taken place after the addition of CS into the SF/gelatin. Furthermore, the increase of CS content in the SF/gelatin blended hydrogels caused the decrease in their porosity that affected the increase in their crystallinity, degree of swelling, water solubility and surface hydrophobicity. The rate of urea release from the hydrogels also depended on the content of CS of which its value of diffusion exponent characteristics (n) determined from the Korsmeyer-Peppas model for SF/gelatin/CS hydrogels were greater than 1.0. This indicates that the urea release from the SF/gelatin hydrogels with CS is a super case 2 transport type. Moreover, the urea release rate constant (k) of the SF/gelatin/CS hydrogels is lower than that of the SF/gelatin hydrogel itself which indicates an extension of the urea release from the SF/gelatin hydrogels by CS. In Part (3), field evaluation on urea release of the SF-based hydrogel for its possibility in agricultural uses was performed in planting using Chinese kale as a model plant. It is clearly seen that the growth of Chinese kale cultivated in the soil treated with the SF-based hydrogel is greater than those of Chinese kale cultivated in hydrogel-free soil (control). The SF/gelatin/CS hydrogel is the greatest on promoting growth of Chinese kale. Therefore, the SF-based hydrogel has successfully acted as a biodegradable matrix for controlling urea release.