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Biocomposite hydrogels with carboxymethylated, nanofibrillated cellulose (c-NFC) powder were prepared by UV polymerization of N-vinyl-2-pyrrolidone with Tween 20 trimethacrylate as a cross-linking agent for replacement of the native, human nucleus pulposus (NP) in intervertebral disks. The swelling ratios and the moduli of elasticity in compression of neat and biocomposite hydrogels were evaluated in dependence of c-NFC concentration (ranging from 0 to 1.6% v/v) and degree of substitution (DS, ranging from 0 to 0.23). The viscoelastic properties in shear and the material relaxation behavior in compression were measured for neat and biocomposite hydrogels containing 0.4% v/v of fibrils (DS ranging from 0 to 0.23), and their morphologies were characterized by cryo-scanning electron microscopy (cryo-SEM). The obtained results show that the biocomposite hydrogels can successfully mimic the mechanical and swelling behavior of the NP. In addition, the presence of the c-NFC shows lower strain values after cyclic compression tests and consequently creates improved material relaxation properties compared with neat hydrogels. Among the tested samples, the biocomposite hydrogel containing 0.4% v/v of c-NFC with a DS of 0.17 shows the closest behavior to native NP. Further investigation should focus on evaluation and improvement of the long-term relaxation behavior.

a b s t r a c tThe swelling and compressive mechanical behavior as well as the morphology and biocompatibility of composite hydrogels based on Tween Ò 20 trimethacrylate (T3), N-vinyl-2-pyrrolidone (NVP) and nanofibrillated cellulose (NFC) were assessed in the present study. The chemical structure of T3 was verified by Fourier transform infrared spectroscopy and proton nuclear magnetic resonance, and the degree of substitution was found to be around 3. Swelling ratios of neat hydrogels composed of different concentrations of T3 and NVP were found to range from 1.5 to 5.7 with decreasing concentration of T3. Various concentrations of cellulose nanofibrils (0.2-1.6 wt.%) were then used to produce composite hydrogels that showed lower swelling ratios than neat ones for a given T3 concentration. Neat and composite hydrogels exhibited a typical nonlinear response under compression. All composite hydrogels showed an increase in elastic modulus compared to neat hydrogel of about 3-to 8-fold, reaching 18 kPa at 0% strain and 62 kPa at 20% strain for the hydrogel with the highest NFC content. All hydrogels presented a porous and homogeneous structure, with interconnected pore cells of around 100 nm in diameter. The hydrogels are biocompatible. The results of this study demonstrate that composite hydrogels reinforced with NFC may be viable as nucleus pulposus implants due to their adequate swelling ratio, which may restore the annulus fibrosus loading, and their increased mechanical properties, which could possibly restore the height of the intervertebral discs.

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The mechanical and damping properties were measured for unidirectional, 0/90 and twill 2/2 flax fibre (FF) composites containing 40 vol% of fibres in an epoxy matrix. All composites were processed by resin transfer moulding. The influence of the impregnation quality, the fibre/matrix adhesion, the quality of the fibres, the twist angle of the FF yarns and the crimp in the FF fabrics on these properties were quantified. Twisted yarns in an unidirectional FF fabric impacted negatively on the stiffness while increasing the damping of about 57% through enhancement of the intra-yarn friction. In twill 2/2 fabrics, twisted yarns decreased Young's modulus owing to the induced crimp but increased the damping through enhanced inter-yarn friction. With improved fibre/matrix adhesion, a damping increase up to 109% was obtained through better activation of the intrinsic damping mechanisms of an elementary FF. The best compromise between damping and stiffness was obtained with FF having less dislocations.

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