Sol-gel hybrid materials are made up of covalently bonded and interpenetrating networks of organic and inorganic components and produce a synergy of the properties of those components above the nanoscale. By altering the ratio of inorganic to organic content, the mechanical properties can be tuned. Here, a silica-poly(tetrahydrofuran) hybrid system was developed with the aim to form a graded stiffness structure that could imitate the radial variation in stiffness of the intervertebral disc and address the unmet clinical need of intervertebral disc replacement.
Hybrids were formed with a range of silica contents between 4 and 45 wt.%, varying from an elastomeric to a glassy material, with compressive stiffness between 2 and 200 MPa. High compressive strains are recoverable and mechanical properties were maintained on soaking up to 1.5 years and to 10000 cycles in compression. The hybrid surface was shown to support cell attachment and extract solutions containing the hybrid were non-cytotoxic.
A novel synthesis method was developed to join hybrid sols during their gelation, forming a single specimen with a variation in silica content along its length, producing a corresponding variation in stiffness. Samples joined in this way were at least as strong as single phase samples in tension and compression. This exploits the gradual gelation process of the hybrid sol, which can also be used to create a successful ink for 3D extrusion printing: porous scaffolds were formed in this way with 27.7 wt.% SiO2. Meniscus and intervertebral disc replacement prototypes were formed and tested under cyclic loading at rates for comparison with human disc data.