Abstract:The mechanical and biological properties of bone implants need to be optimal to form a quick and firm connection with the surrounding environment in load bearing applications. Bone is a connective tissue composed of an organic collagenous matrix, a fine dispersion of reinforcing inorganic (calcium phosphate) nanocrystals, and bone-forming and -degrading cells. These different components have a synergistic and hierarchical structure that renders bone tissue properties unique in terms of hardness, flexibility and regenerative capacity. Metallic and polymeric materials offer mechanical strength and/or resilience that are required to simulate bone tissue in load-bearing applications in terms of maximum load, bending and fatigue strength. Nevertheless, the interaction between devices and the surrounding tissue at the implant interface is essential for success or failure of implants. In that respect, coatings can be applied to facilitate the process of bone healing and obtain a continuous transition from living tissue to the synthetic implant. Compounds that are inspired by inorganic (e.g., hydroxyapatite crystals) or organic (e.g., collagen, extracellular matrix components, enzymes) components of bone tissue, are the most obvious candidates for application as implant coating to improve the performance of bone implants. This review provides an overview of recent trends and strategies in surface engineering that are currently investigated to improve the biological performance of bone implants in terms of functionality and biological efficacy. OPEN ACCESSCoatings 2012, 2 96

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Titanium (Ti) and its alloys are widely used to manufacture orthopedic and dental implants due to their excellent mechanical properties and corrosion resistance. Although these materials are bioinert, improvement of biological properties (e.g., bone implant contact) can be obtained by the application of a coating made of nanostructured apatite. The aim of this study was to investigate the applicability of the electrostatic spray deposition (ESD) technique for the deposition of nanostructured apatite coatings onto commercially pure (cp) Ti substrates at room temperature. To that end, poorly crystalline, nano‐sized, carbonate‐apatite plate‐like particles with dimensions similar to the nanocrystals present in bone were synthesized using wet‐chemical precipitation techniques and their physicochemical properties were subsequently characterized thoroughly. The apatite suspensions were optimized for the ESD process in terms of dispersion, aggregation, and stability. Furthermore, relevant ESD processing parameters, including nozzle‐to‐substrate distance, relative humidity in the deposition chamber and deposition time were varied in order to study their effects on coating morphology. Porous films made of agglomerates of nano‐sized apatite particles of ≈50 nm were generated, demonstrating the feasibility of the ESD technique for the deposition of thin apatite coatings with a nano‐sized surface morphology onto titanium substrates. The ability of these nanocrystals to bind therapeutic agents for bone diseases and the capability of ESD to produce coating at physiological conditions makes this work a first step toward the set‐up of coatings for bone implants based on surface‐activated apatite with improved functionality.