# Biodegradable and Biocompatible Systems Based on Hydroxapatite Nanoparticles
## 3 Preparation of Bion nanocomposites Incorporating HAp
### Bionanocomposites  
Combination of HAp with another biopolymer can help to overcome certain inherent limitations such as poor load bearing properties, difficulty to be processed in a determined shape and high mobility [35; 36]. Multicomponent materials are designed to provide the appropriate combination of properties for each specific application. However, typical phase separation problems, which can arise along time, should be avoided. Coating or grafting of HAp nanoparticles with polymers compatible with the polymer/copolymer matrix are good approaches. For example, polylactide (PLA) was easily grafted on the surface of nHAp particles through reaction with their surface hydroxyls and then incorporated in a poly(L-lactide-\(\alpha\)-glycolide) (PLGA) matrix in such a way that improved properties, tensile strength and dispersability were observed with respect to ungrafted systems [37].  
Chitosanan [poly-\(\beta\)(1\(\rightarrow\)4)-2-amino-2-deoxy-d-glucose] (CHI) is obtained by deacetylation in strong alkaline solutions and by enzymatic hydrolysis of chitin (poly-(1\(\rightarrow\)4)-N-acetyl-glucosamine), which is the second most abundant natural polymer. CHI is biocompatible, biodegradable and has interesting properties (e.g., antimicrobial and antioxidant activities) that justify its use in multiple fields such as biology, food nutrition, medicine (e.g., tissue engineering), and pharmacy [38; 39; 40; 41; 42]. CHI based tissues should mimic the behavior of the replaced tissue, being an ideal system with a practically insoluble outer surface and an inner surface (i.e., in contact with the tissue) capable to experience a progressive degradation and a complete resorption after finishing the regeneration process.  
Carboxymethyl-chitosan (CMC) has some advantages with respect to chitosan due to its higher solubility (e.g., in water, alkaline solutions and physiological conditions). More interestingly, CMC is capable to adsorb calcium ions and has a great influence on precipitation of biominerals [43; 44], even enhancing bone regeneration.  
nHAp particles have been easily embedded in CHI and CMC matrices, following a one-step co-precipitation method in water [45]. Selected biopolymers act as capping ligands able to control both nucleation and growth rate of HAp during the precipitation process while serve to give rise to a biocompposite network. In this sense, CMC was able to produce lower size and narrower size distribution of HAp particles and higher cell viability of the bionanocomposite caused by the higher surface of particles.  
The sol-gel method has been applied to produce new bionanocomposites based on HAp nanopowders doped with calcium fructoborate, which have promising applications as osteosynthesis materials [46].  
Nanocomposites of cellulose and HAp can be used as synthetic grafts since may accomplish many of the strict requirements requested for bone graft substitutes (i.e., mechanical support and moderate stiffness, capability of integrating with the host bone tissue, biocompatibility, enhancement of bone regeneration and biomechanical stability) [47; 48; 49; 50].  
Bacterial cellulose (BC) is having currently an increasing attention for tissue engineering applications due to its good specific properties (high water holder capacity, purity, crystallinity, mechanical strength and ultrafine fibrillar structure) [51]. Nevertheless, its high production costs hinder its commercial use. In order to get cheaper nutrient sources, BC has been produced fromagroindustrial waste, being also observed that HAp was deposited with relatively good uniformity between nanobiofibers giving rise to similar characteristics than biological apatite [51].