Hybrid materials with continuous mechanical property gradients that can be 3D printed

Here, we show tough materials with continuous composition and stiffness gradients, without interfaces between regions, using inorganic/organic hybrid materials that can also be 3D printed. Sol-gel hybrid materials have interacting and interpenetrating organic and inorganic co-networks and can deliver a synergy of the properties of those constituents. Their mechanical behaviour can be tuned through inorganic/organic content and cross-linking density. We describe hybrids of covalently linked silica-poly (tetrahydrofuran) (SiO2/PTHF) that show an unprecedented range of mechanical properties. SiO2/PTHF hybrids were formed with different silica contents, producing materials ranging from elastomeric to glassy, with a compressive stiffness at 10% strain of between 2 and 200 MPa: at an SiO2 content of 9 wt%, hybrids show a failure stress in compression of 8 MPa, at 70% strain, and tensile strength of 2 MPa at 80% strain. Gradual sol-gel gelation was employed to generate monoliths with composition and stiffness gradients without visible joins or points of weakness, and for development of ‘inks’ for additive manufacturing of 3D structures through direct ink writing. Monoliths with gradients were at least as strong, in tension and compression, as those made by casting a single sol. Cytocompatible materials with seamless stiffness gradients will have many applications: one is biomimicry of natural cartilaginous structures of the body, such as the intervertebral disc, which has a natural radial stiffness gradient.