High-strength cellulose-polyacrylamide hydrogels: Mechanical behavior and structure depending on the type of cellulose.

Two types of stiff and high-strength composite hydrogels possessing the structure of interpenetrating polymer networks were synthesized via free-radical polymerization of acrylamide carried out straight within the previously formed physical network of regenerated plant cellulose or bacterial cellulose (PC and BC respectively) that was swollen in the reactive solution. The mechanical behavior of synthesized hydrogels subjected to the action of compressive deformations with different amplitude values was studied. The analysis of the stress-strain curves of compression tests of the hydrogels of both types obtained in different test conditions demonstrates the substantial difference in their mechanical behavior. Both the PC- and BC-based hydrogels withstand successfully the one-shot compression with the amplitude up to 80%, but in the conditions of the multiple compression tests (cyclic compressions) during the subsequent compression acts the dramatic increase in the stiffness of the BC-based hydrogels was observed at the deformation region beyond 60%. This effect can be explained by the deep reorganization of the intermolecular structure of the material with the stress-induced reorientation of BC micro-fibrils. Submicron- and micron-scale specific features of structures of composite hydrogels of both types were studied by cryo-scanning electron microscopy to explain the peculiarities of the mechanical effects observed.

Unusual effect evidenced at the investigations of the mechanical behavior of composite hydrogels under cyclic compression.

An unusual type of mechanical behavior was registered while studying the swollen hydrogel compositions "cellulose-polyacrylamide" in the conditions of multiple cyclic compression tests with the broad variation of the deformation speed. While increasing the deformation rate the clearly seen inversion of the positions of compression and decompression parts of the cyclic stress-strain curves was detected. While carrying out the cyclic compression tests with relatively low deformation speed (about 100-200% of the initial sample's height per minute) the well defined hysteresis of the stress-strain curve can de seen and in these conditions the decompression part of the curve is situated inferior the part corresponding to compression. But while increasing the speed of the deformation the tendency to the progressive approach of the compression and decompression curves to each other is clearly seen. This effect results in the full disappearance of the hysteresis at some value of the deformation speed: the decompression curve coincides with the compression curve. Along with the further increase of the deformation speed the hysteresis appears again but the curve corresponding to compression process is situated inferior the curve corresponding to decompression: the "inversion" of the hysteresis was detected. The precise character of this process depends upon the stiffness of the hydrogel under study. Up to date the convincing explanation o this effect can not be put forward. The authors can only present some hypotheses to explain this phenomenon.

[PERIFOCAL TISSUE REACTIONS TO IMPLANTATION OF THE SAMPLES OF HYDROGEL MATERIAL BASED ON POLYACRYLAMIDE WITH THE ADDITION OF THE CELLULOSE (AN EXPERIMENTAL STUDY)].

Tissue reactions to the grafting of samples of composite hydrogel based on polyacrylamide and cellulose of different origin (plant and bacterial) were studied in 48 laboratory outbred rats and 24 rabbits of chinchilla breed. The observations were carried out on Days 5, 14, 45 and 90 after placement of implants in the muscle, joint cavity, deep defects of the articular cartilage and subchondral bone. The study has revealed no migration and degradation of the samples. On Day 90, the signs of their biointegration (regardless of their nature) were detected in the muscles and in one case (the sample containing bacterial cellulose) in the cavity of the knee joint of the rat. The materials had good biocompatibility with muscle, cartilage and bone tissues, did not cause perifocal inflammation and effectively functioned as a prosthetic articular cartilage until the end of the study period.

Anisotropic swelling and mechanical behavior of composite bacterial cellulose-poly(acrylamide or acrylamide-sodium acrylate) hydrogels.

Bacterial cellulose-polyacrylamide (BC-PAAm) composite hydrogels are prepared by synthesis of PAAm networks inside the BC matrices. The behavior of these gels and of the ionic ones obtained via partial hydrolysis of BC-PAAm gels is studied under swelling and compressive deformation conditions. The dependences of the hydrogels' properties on the BC matrix preparation conditions, gel synthesis conditions and the BC content in the hydrogel compositions are studied. Two types of BC gel pellicle are used in the hydrogel synthesis, namely matrix pellicles subjected to pre-pressing (samples of series A) and those not subjected to any mechanical actions before synthesis (series B samples) containing about 99% water. The effect of anisotropic swelling of type A hydrogels is detected. The type B specimens swell isotropically. Both types of hydrogel exhibit substantial anisotropy of their mechanical properties, apparent in different shapes of compression stress-strain curves of samples cut out from the gel plates in various directions. Composite hydrogels show superb mechanical properties, including compression strength up to 10 MPa and the ability to withstand long-term cyclic stresses (up to 2000-6000 cycles) without substantial reduction of mechanical properties.