Hydrogels for medical applications fabricated by oxidative-hydrolytic modification of cellulose

Abstract

Use of nitrate oxidants (HNO 3 or N 2 O 4 ), each of which causes both oxidation and hydrolysis of cellulose, is proposed for one-or two-stage transformation of native cellulose into microcrystalline cellulose topochemically modified with carboxyl groups. Conversion of the acid form of the preparations into the salt forms (sodium, lithium) makes it easy to obtain stable thixotropic hydrogels, revealed to be effective enterosorbents and drug vehicles in clinical trials.The products of oxidative-hydrolytic modification of cellulose are of interest for medicine not only as effective hemostatic and bactericidal agents, but also as enterosorbents. The widely distributed carbon and aluminum-containing enterosorbents cannot completely replace enterosorbents of polysaccharide nature food fibres, which have adapted to the human gastrointestinal system over the entire course of evolution. The traditional use of natural polyuronyls (pectins, polysaccharides from marine algae) in enterosorption is complicated by the possible accumulation of toxic substances from the biosphere in the vegetation period. Microcrystalline cellulose (MCC) merits special attention from the point of view of the high degree of chemical purity of polysaccharides not digestible in the human body [1]. During its preparation, extraneous contaminants primarily accumulated in the structurally unordered constituents of the fibrous precursor are removed together with this constituent during hydrolytic splitting of the fibre to the so-called limiting degree of polymerization. In addition, MCC has the unique ability to form stable thixotropic hydrogels [1], which significantly expands the possibilities of using it in the pharmaceutical and food industries. However, the efficacy of MCC as an enterosorbent is limited by the absence of the ion-exchange groups contained in natural polyuronides (pectins, laminarin, carrageenan).Fabrication of material that combines ion-exchange properties with the properties of MCC, primarily the gel-forming power, is probably only possible in conditions of intercrystallite interaction of the cellulose with the oxidant, where oxidation of crystallites is limited to their surface. This situation predetermines the relatively high degree of oxidation of cellulose, which excludes the reaction from turning into an intracrystallite reaction [2]. We propose two variants of conversion of native cellulose into carboxylated microcrystalline cellulose (CMCC) using one reagent capable of executing both oxidative and hydrolytic functions from the beginning.In one variant, the air-dried cellulose was saturated with nitrogen(IV) oxide vapors in a specially manufactured reactor and held at room temperature for the time required to attain the necessary degree of oxidation. Then a calculated amount of water was fed into the reactor and the contents were heated to the boiling point. Hydrolytic degradation of the polysaccharide, catalyzed by the mixture of nitric and nitrous acids formed, takes place in these conditions. Their concentration in terms...

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