Bioactive silica–collagen composite xerogels modified by calcium phosphate phases with adjustable mechanical properties for bone replacement

The development of composites has been recognized as a promising strategy to fulfil the complex requirements of biomaterials. The present study reports on the modification of a novel silica–collagen composite material by varying the inorganic/organic mass ratio and introducing calcium phosphate cement (CPC) as a third component. The sol–gel technique is used for processing, followed by xerogel formation under specific temperature and relative humidity conditions. Cylindrical monolithic samples up to 400 mm3 were obtained without any sintering processes. Various hierarchical phases of the organic component were applied, ranging from tropocollagen and collagen fibrils up to collagen fibers, each characterized by atomic force microscopy. Focusing on the application of fibrils, various inorganic/organic mass ratios were used: 100/0, 85/15 and 70/30; their influence on the structure of the composite material was demonstrated by scanning electron microscopy. The composition was extended by the addition of 25 wt.% CPC which led to increased bioactivity by accelerating the formation of bone apatite layers in simulated body fluid. Synchrotron microcomputed tomography demonstrated the homogeneous distribution of the cement particles in the silica–collagen matrix. Compressive strength tests showed that the mechanical properties of the brittle pure silica gel are changed significantly due to collagen addition. The highest ultimate strength of about 115 MPa at about 18% total strain was registered for the 70/30 silica–collagen composite xerogels. Incorporation of CPC lowered the gel’s strength. By demonstrating differentiation of human monocytes into osteoclast-like cells, an important feature of the composite material regarding successful bone remodeling is fulfilled.