Towards biologically inspired materials

The development of a synthetic bone substitute able to mimic both the chemical composition and morphology of bone's inorganic component is a priority for biomaterials research. Hydroxyapatite is a promising material for this application. The role of nanostructured apatites in the design and preparation of biomimetic and bioactive scaffolds is particularly crucial. These new materials characterised by nanodomain and modified chemical features show improved performance in terms of interaction with natural tissue, allowing and taking part in bone ingrowth. Thus interest has shifted from biocompatible to biomimetic and bioactive materials. The importance of non-stoichiometric/defective apatites similar to the mineral part of bone, in particular those containing HPO42-, CO32- and Mg2+, is explained. Attention is focused on innovative synthesis techniques that can yield powders with higher reactivity to build synthetic implants with a porous structure resembling that of spongy bone that can ensure complete penetration and consequent replacement of the bony tissue. The use of cellulosic sponges soaked with powder suspensions can give adequate porosity in terms of both size and distribution, and this appears to be a suitable method for the repair of both loadbearing and non-loadbearing osseous defects. Such sponges can also function as in situ drug delivery systems if they are charged with pharmacologically active compounds. So far the characteristics of artificial bone tissues have been shown to be very different from those of natural bone, mainly because of the absence of the self-organising interaction between apatites and the proteinic component that modifies the intrinsic features of each constituent. For that reason attention is currently focused on so called biologically inspired materials, in this case composites synthesised to exploit the ability of biological systems to store and process information at the molecular level. Following this new approach, nanosize bladelike crystals of hydroxyapatite have been nucleated in situ on self-assembling collagen fibres. The characteristics of the resulting composites are described.