Microwave-assisted synthesis of crosslinked ureido chitosan for hemostatic applications.

In this study, we present a facile method for introducing hydrophilic ureido groups (NH2-CO-NH-) into chitosan using a microwave-assisted reaction with molten urea, with the aim of enhancing chitosan's interaction with blood components for improved hemostasis. The formation of the ureido groups through nucleophilic addition reaction between the amine groups in chitosan and in situ generated isocyanic acid was confirmed by FTIR, CP/TOSS 13C NMR, and CP/MAS 15N NMR spectroscopic techniques. However, in stark contrast to the glucans, the said modification introduced extensive crosslinking in chitosan. Spectroscopic studies identified these crosslinks as carbamate bridges (-NH-COO-), which were likely formed by the reaction between the ureido groups and hydroxyl groups of adjacent chains through an isocyanate intermediate. These carbamate bridges improved ureido chitosan's environmental stability, making it particularly resistant to changes in pH and temperature. In comparison to chitosan, the crosslinked ureido chitosan synthesized here exhibited good biocompatibility and cell adhesion, rapidly arrested the bleeding in a punctured artery with minimal hemolysis, and induced early activation and aggregation of platelets. These properties render it an invaluable material for applications in hemostasis, particularly in scenarios that necessitate stability against pH variations and degradation.

Solvent-less carboxymethylation-induced electrostatic crosslinking of chitosan.

The successful N-carboxymethylation and concomitant crosslinking of solid chitosan upon heating its mixture with solid monochloroacetic acid, without the use of solvents or catalysts, is reported. The N-carboxymethylation was confirmed through the analysis of the partially depolymerized product using NMR spectroscopy, as well as a control reaction with lysine. This transformation was facilitated by the nucleophilic nature of the free amine group in the repeating unit of chitosan, which possesses lone pair of electrons capable of attacking the carbon center bearing the leaving group and displacing the leaving group in a concerted manner. The crosslinking, on the other hand, was established by the observed insolubility in aqueous acidic solutions, even when subjected to prolonged heating at 60 °C. This crosslinking occurs due to the electrostatic interactions between the carboxylate groups and the adjacent ammonium groups, as supported by evidence from FTIR spectroscopy and a control reaction involving ethyl chloroacetate. The resulting crosslinked carboxymethyl chitosan demonstrated its usefulness in the adsorption of methyl orange and fluorescein, as well as functioning as an organic catalyst for aza-Michael addition, Hantzsch reaction, and substituted perimidine synthesis.

Fractional order fast terminal sliding mode control scheme for tracking control of robot manipulators.

In this study, a fractional order fast terminal sliding mode control strategy is developed to address the trajectory tracking problem that arises when robot manipulators are subjected to uncertainties and external disturbances. A novel fractional order fast terminal sliding surface is proposed to achieve rapid finite time convergence and the explicit expression for the settling time is also formulated. To manage uncertainties, chattering phenomenon, singularities, large control gains, etc., a new fractional order fast terminal sliding mode control scheme is developed based on the proposed sliding surface. The radial basis function neural network is used in the proposed control strategy to approximate the nonlinearities and modeling errors of the robot dynamics in real time. The reconstruction error of neural network and upper bound on disturbances are handled by the adaptive compensator. The Lyapunov technique is used to examine the stability of the proposed control strategy. The proposed control technique improves the efficiency of the controller and allows for the asymptotic error convergence to occur in a finite amount of time. To compare the effectiveness of the proposed scheme to various existing control approaches, numerical simulation studies are also conducted.