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.

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The obtained data failed to provide a consistent favourable effect on bone formation of the collagen coating over 3 months of implantation. It is concluded that the source of the collagen as well as the limited osseous environment overshadowed a possible effect of the applied implant surface modifications. Similarly, the tested nano-apatite surface coating did not improve peri-implant bone ingrowth into a gap-implant model.

The possibility to develop a bone implant with bioactive aspects and in situ drug-delivery properties, in order to provide local treatment in vivo, is a big challenge. Where conventional surface modifications for bone implants focused on the deposition of ceramic (mostly calcium phosphate, CaP) coatings, current surface engineering approaches attempt to incorporate active features to render bone implant surfaces capable to direct biological performance. Biomimetic apatite nanocrystals (nAp) represent, among the CaPs, an elective material for bone applications and their surface functionalization with drugs allows them to act as a drug-delivery vehicle. Since load-bearing bone implants are increasingly used in patients with compromised health conditions, surface engineering is important to warrant the performance of these implants under such conditions. In view of this, bisphosphonates (BPs) represent a treatment modality for a variety of disorders of bone metabolism associated to bone loss, including Paget's bone disease, osteoporosis, fibrous dysplasia and bone metastases. In this work, we have synthesized and characterized bioinspired nAp and evaluated their functionalization with alendronate. In vitro tests will be used to evaluate the efficacy of the functionalized compound to impede the formation of osteoclasts and to show that alendronate-functionalized nAp can significantly reduce osteoclasteogenesis. Finally, alendronate-functionalized nAp (FnAp) has been deposited on titanium implants via the electrospray deposition technique in order to develop inorganic-organic coatings for bone implants with improved functionality.