Preparation And Controlled Release Of Crosslinked Imidacloprid-Loaded Carboxymethyl Chitosan Gel Beads

Carboxymethyl chitosan (CMCTS) was prepared by a two-step alkalization and refined by isoelectric-point precipitation followed by conversion to water-soluble sodium carboxymethyl chitosan (CMCNa) with sodium hydroxide and then organic-solvent precipitation. The CMCNa products were characterized for the degree of substitution (DS), intrinsic viscosity, viscosity average molecular weight and IR spectroscopy, respectively.The commercial shrimp-shell and self-made crab-shell carboxymethyl chitosan were used respectively as a carrier to formulate gel beads with the model drug imidacloprid by using a mixture of calcium chloride and glutaraldehyde as a cross-linking agent. The gel beads were characterized using FTIR and SEM. The release profiles of imidacloprid from the beads were investigated in-vitro. Swelling and erosion experiments of the beads were performed. The factors influencing the formulation of beads, drug loading, encapsulation efficiency and the release rate of imidacloprid were investigated, including the viscosity-average molecular weight and the degree of substitution (DS) of CMCTS, the concentration of CMCTS solution, cross-linking methods, the amount of cross-linking agent, cross-linking time, drying conditions, and the addition of an inorganic filler as well. The release results were evaluated using the generalized model, and related to the results of swelling and erosion of beads to understand the transport mechanism.The size of beads remarkably increased with the concentration of carboxymethylchitosan solution used for formulation, and the color of the beads darkened accordingly, and was not significantly changed by increasing the exposure time and concentrations of the cross-linking agent.The loading of imidacloprid and the encapsulation efficiency considerably decreased by increasing the concentration of glutaraldehyde, and increased by increasing the concentration, the viscosity-average molecular weight and the DS of CMCTS, and was not significantly changed by increasing the exposure time and concentrations of the cross-linking agent. The use of high viscosity-average-molecular-weight crab-shell CMCTS and the addition of an inorganic filler in formulation considerably increased the loading of imidacloprid and the encapsulation efficiency, and has the benefits of lowing the cost of formulations .The release rate of imidacloprid reduced and followed an increase with increasing concentrations of glutaraldehyde and CaCl2, and decreased with increasing the concentration, the viscosity-average molecular weight and the DS of CMCTS, the use of high viscosity-average-molecular-weight crab-shell CMCTS, the addition of an inorganic filler and pre-crosslinking in formulation, but was not significantly affected by increasing cross-linking time. The release time was prolonged from 80h to 108h with the use of self-made crab-shell CMCTS (cone: 23g/L) instead of commercial shrimp-shell CMCTS (cone: 35g/L) as a carrier for the formulation under the optimized conditions, i.e.: the concentration of glutaraldehyde of 10%, the concentration of CaCl2 of 0.25mol/L, the cross-linking time of 1h.The percentage water uptake of imidacloprid-loaded beads indicated that the swelling ratio was lower for the gel beads based on the high viscosity-average-molecular-weight crab-shell CMCTS (5.17×10~5) than that based on the low one(4.22 × 10~5). The erosion rates of the gel beads based on the crab-shell CMCTS decreased with increasing the DS of CMCTS as a carrier. The results of swelling and erosion were in good agreement with the release results.The SEM data indicated that the imidacloprid-loaded beads had spherical shape with rough surface. The ionically-covalently cross-linked network of gel formed by chelating of Ca2+ and covalent bonding of glutaraldehyde with CMCTS and the absence...