Nano Fibrillated Cellulose Based Composite Hydrogels for an Injectable Intervertebral Disc Implant

Replacing the gel-like central part of the intervertebral disc (IVD), so-called nucleus pulposus (NP), via a minimally invasive surgery is one of the most recent approaches in the lower back pain treatments. An appropriate biomaterial for NP replacement (NPR) preferably possesses the mechanical and swelling properties similar to those of the native tissue. Furthermore, it should be injectable, biocompatible and stable in the IVD environment. From the surgical point of view, employing a photopolymerizable hydrogel for NPR is beneficial as it can be injected in the liquid form to the IVD and subsequently cured in situ. Such an approach provides a better control for the surgeon over the operation and minimizes the invasion of the surrounding tissue. The application of hydrogels as load-bearing biomedical components is, however, limited by their mechanical properties. Conventional approaches to address such an issue often favors one mechanical property while compromising the others. The present study aims to develop a novel injectable, photopolymerizable hydrogel for NPR with concurrently enhanced mechanical properties and optimal processability. Hydrogels based on photopolymerization of a monomer, i.e. N-vinyl-2-pyrrolidone (NVP), and cross-linking of a preformed functionalized polymer, i.e. poly(ethylene glycol)dimethacrylate (PEGDM), are synthesized. The hydrogels are thoroughly characterized as a function of cross-link density and initial water content in order to examine their applicability for NPR. While a wide range of properties is obtained by variation of the aforementioned parameters, an enhancement of stiffness is associated with brittleness or loss of water content. In addition, homogenous volumetric photopolymerization of such hydrogels is found to be limited especially in case of local illumination. To resolve such issues, a composite material design approach is adopted and the abovementioned polymer networks of hydrogels are combined with nano fibrillated cellulose (NFC). It is demonstrated that the NFC significantly increases the hydrogels’ stiffness (up to 275%) without compromising the failure strength or reducing the water content. Moreover, the NFC fibers effectively prevent formation of the swelling induced cracks in the NVP based hydrogels. The results of rheology experiments indicate that the dispersed NFC fibers form a network-like structure in the hydrogels precursor solution. Such a structure, as confirmed through the photorheology tests, serves as a light-scattering source when the solution is illuminated by UV light. The latter accelerates the kinetics of photopolymerization and consequently allows reducing the required concentration of photoinitiator. The enhanced and tunable properties of the PEGDM-NFC composite hydrogel, developed in this work, make it a promising implant material for NPR. The applicability and reliability of such a material design approach for NPR is examined using a bovine organ model. In order to meet the properties of the native tissue, the mechanical properties of the composite hydrogel are tuned. The performance of the optimized composite hydrogel versus the native NP tissue is evaluated in several functional tests such as extrusion, confined compression and swelling pressure...