Synthesis and characterization of oxidized cellulose

Samples of oxidized cellulose (OC) with various carboxyl contents and degrees of crystallinity were obtained by the oxidation of native and mercerized cellulose with a solution of nitrogen(IV) oxide in CCl₄. A detailed characterization of these OC samples was performed. The effect of oxidation conditions (concentration of N₂O₄ in the solution and oxidation time) and starting cellulose material on OC characteristics (carboxyl, carbonyl and nitrogen content, degree of crystallinity and polymerization, surface area and swelling, and acidic properties) was investigated. Reactivity in the oxidation process was higher in mercerized cellulose than in native cellulose. The action of dilute solutions (10–15%) of N₂O₄ did not affect the degree of crystallinity of cellulose samples. Under these conditions, the oxidation took place mainly in amorphous regions and on the surface of crystallites. Oxidation in a concentrated (40%) N₂O₄ solution led to the destruction of crystallites, which increased the surface area and swelling of cellulose in water. The surface area and the swelling of OC samples increased with a decrease in the index of crystallinity. The acidic properties of OC were shown to increase with an increase of swelling in water. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4785–4791, 2004     

Characterization of H3PO4/HNO3–NANO2 oxidized bacterial cellulose and its usage as a carrier for the controlled release of cephalexin

Biocompatibility, biodegradation, good sorption characteristics, and unique structure of highly oxidized bacterial cellulose (OBC) are of great interest for the development of new drug delivery systems. In this study, OBC with 9.6, 13.0 and 19.5% carboxyl groups for 5, 20, and 48 h of synthesis, respectively, was successfully obtained using the HNO₃/H₃PO₄–NaNO₂. The results of morphological analysis showed that with an increase in the number of carboxyl groups, OBC fibers become thicker and rougher. Fourier-transform infrared spectroscopy showed the formation of carboxyl groups in the OBC after the oxidation reaction. The crystallinity of the samples according to X-Ray diffraction analysis decreased with increasing reaction time. The immobilization of cephalexin in the polymer matrix was studied in detail, it took 120 min to achieve balance in the solution with a concentration of 1 mg/ml, and the maximum amount of a sorbed antibiotic reached 43 mg/g. The drug release in vitro at 37 °C in PBS with pH 7.4 and 2.0 was prolonged. Various models were used to describe the release mechanism, the best of which was Ritger-Peppas with a diffusion exponent value ranging from 0.743 to 0.830, which explains the drug release mainly through non-Fickian diffusional release. The cephalexin-loaded OBC showed high antimicrobial activity against Gram-negative bacteria Escherichia coli and Gram-positive bacteria Staphylococcus aureus. The structure and properties of the resulting highly oxidized cellulose make it an excellent candidate as a drug delivery carrier with prolonged antimicrobial drug release characteristics.

     

Multilayer adsorption of amino acids on oxidized cellulose

The adsorption of amino acids (AA) (glycine, L-alanine, L-proline) on oxidized cellulose (OC) with various carboxyl contents and degrees of crystallinity from aqueous and water/ethanol solutions was studied. It was found that multilayer adsorption occurs in concentrated solutions of AA. It proceeds according to successive mechanisms via adsorption of AA zwitterions onto carboxyls of already adsorbed AA. This leads to formation of chain AA associates in the OC phase. A sharp increase in swelling accompanies multilayer adsorption. It was established that structural characteristics and degree of polymerization of OC are the main factors that affect multilayer adsorption. The distribution of carboxyls in the OC phase also plays an important role. Multilayer adsorption does not proceed in water/ethanol solutions and in the case of the cationic form of AA.     

Adsorption of zwitterionic drugs onto oxidized cellulose

Adsorption of zwitterionic drugs (beta-lactam antibiotics and amino acids) onto samples of oxidized cellulose (OC) with various carboxyl contents and structural characteristics from aqueous and water/alcohol solutions was investigated. The adsorption process can be described according to the theory of localized stoichiometric adsorption and represented by Langmuir isotherms. It was established that the constants of interfacial distribution mainly increase with increased relative sorbate hydrophobicity. The dependencies of adsorption on pH of equilibrium drug solution have a maximum at pH 3-3.5, which is caused by peculiarities of dissociation of OC and sorbates. The drug uptake is shown to increase with an increase of alcohol mole fraction in the solution and transfer to the binary water/isopropanol from water/ethanol solutions. The dominant contribution to the increase of uptake is the desolvation of ionic groups of zwitterions in the solution, which increases with increased alcohol content. The degree of crystallinity of the sorbent has no considerable effect on drug adsorption from aqueous solutions. In water/alcohol solutions the adsorption of drugs by OC samples with similar exchange capacity increases with reducted uniformity of carboxylic group distribution in the volume of the polymer, which is connected with increased accessibility of carboxylic groups for sorbate molecules.