# Biodegradable and Biocompatible Systems Based on Hydroxapatite Nanoparticles
## 1 Introduction  
Hydroxapatite (HAp, Ca\({}_{10}\)(PO\({}_{4}\))\({}_{6}\)(OH)\({}_{2}\)) is the main inorganic constituent of bone and teeth and therefore has been extensively studied as an artificial bone substitute [1; 2]. Moreover, composites prepared from synthetic HAp and collagen have a great potential to mimic and replace skeletal bones. Biomedical applications of calcium phosphate (CaP) based scaffolds take for example advantage of their ability to induce bone formation and vascularization and to cover varied uses such as bone augmentations, artificial bone grafts, maxillofacial reconstruction, spinal fusion, periodontal disease repairs and bone fillers after tumor surgery [2]. In fact, reconstructive tissue engineering is nowadays a highly active research area that involves the development of materials with outstanding properties. These can only be achieved by means of combining different components, processes and technologies. Scaffolds constituted by biodegradable polymers and HAp nanoparticles have a great potential due to the excellent combination of properties (e.g., enhancement of biocompatibility and reinforcing effect provided by the inorganic component) and the facility of preparation.  
Researches are nowadays extensively involved in the study of bionanocomposites based on HAp nanoparticles due to both their recognized applications in the biomedical field and their high potential for new specific uses [3]. In this way, development of new processing technologies, surface treatments, encapsulation techniques or incorporation of compatibilizers are topics that have made great progress in recent years and deserve special attention. The successful combination of desired properties of matrix polymers with those of fillers should lead to innovative materials having both appropriate in vitro and in vivo properties for their use in the biomedical field [4]. HAp biom nanocomposites can be obtainedfollowing different processes that are able to provide specific features for a specific purpose but, unfortunately, none of them can cover all possible requirements. Thus, materials that can be applied for example in tissue engineering should fulfill a wide range of specifications: degradation products must be biocompatible and non-toxic, the scaffold should have a 3D porous geometry, good mechanical properties and should provide an appropriate microenvironment for cells regenerating the tissue, supporting cell attachment, proliferation, differentiation, and neo tissue genesis [5].  
The present review is a survey of recent works concerning hydroxyapatite nanocomposites, being organized in sections that firstly deal with preparation methods of nanoparticles and derived biocomposites with applications in tissue engineering. Secondly, the modification/encapsulation techniques that provide functional bionanocomposites are considered. Specifically, the development of bionanocomposites with antimicrobial properties, doping with biomarkers for imaging and medical diagnosis, and finally encapsulation/adsorption of transfecting agents, proteins and drugs are mentioned [6; 7]. HAp has been revealed as a highly promising system for new therapies that take advantage of the capability of HAp to mask biomolecules, cross biological barriers and deliver drugs just onto the disease target.