# Biodegradable and Biocompatible Systems Based on Hydroxapatite Nanoparticles
## 3 Preparation of Bion nanocomposites Incorporating HAp
### Electrospun Scaffolds Incorporating HAp Nanoparticles  
Electrospinning is an easy and low cost technique able to produce micro/nanofibers through the application of a high electrical field [52]. A charged polymer jet is formed when the applied electrostatic charge overcomes the drop surface tension of a polymer solution slowly ejected through a capillary. The jet is subsequently collected on a grounded collector giving rise to porous scaffolds after their accumulation. Control of processing parameters (e.g., voltage, flow rate and tip-collector distance), solution properties (e.g., solvent, polymer concentration and viscosity) and environmental conditions (e.g., humidity and temperature) allows tailoring the morphology of the derived fibers [53]. Moreover, electrospinning allows easy incorporation of drugs and nanoparticles into the fibers by dissolving/dispersing them into the initial polymer solution. Electrospinning is therefore appropriate to get fibers with comparable dimensions to fibrils existing in the extracellular matrix (ECM), porous scaffolds suitable for cell colonization and materials with enhanced bioactivity. Cautious must be taken into account when toxic organic solvents are employed and, consequently, different emulsion processes have been considered [54; 55; 56]. Electrospinning of polymer solutions incorporating HAp nanoparticles may cause some problems like the decrease of mechanical properties and loss of structural stability of the final scaffold due to the lack of adhesion between the inorganic and the organic phases [57]. HAp nanoparticles are therefore usually grafted with an organic and biodegradable polymer to facilitate their adhesion with the polymer matrix. Different examples can be found as the successful electrospinning of poly(lactic-\(co\)-glycolic acid) and HAp grafted with polylactide (PLA) as compatibilizing agent [58]. A second option to improve interactions between the inorganic and the organic component is based on the coating of HAp nanoparticles with lipophilic compounds such as oleic acid [59]. Incorporation of HAp nanoparticles can influence the morphology and structure of electrospun scaffolds [60]. In general, fibers tend to be thicker and irregular with increasing HAp content. Nevertheless, a correct adjustment of electrospinning parameters may lead to homogeneous distributions of inorganic particles without formation of clumps inside or between the fibers. The effect of scaffolds having HAp nanoparticles on the grown of human bone-derived cells has been extensively evaluated [61]. Interestingly, the increase on HAp content stimulated the proliferation of osteogenic cells due to different effects: (a) upregulated expression of genes associated to gene expression [62]; (b) mitogenic effect of HAp crystals [63]; and (c) reinforcement effect that improves mechanical properties and lead materials with better capacities to support cell colonization [64]. The improvement of osteoconductivity is clear for example for chitosan based scaffolds [65]. Furthermore, polymeric fibers containing HAp have been revealed as promising candidates for guided bone regeneration [65]. Electrospinning of demineralized bone matrices (DMBs) has also been assayed as a natural material that contains the protein components of bone (e.g., adhesion ligands and osteoinductive signals, such as growth factors). Diameter size and distribution of fibers was strongly depended on the viscosity of the solution (1,1,1,6,6,6-hexafluoroisopropanol/trifluoroacetic mixture), which increased with the solubilization time (Figure 3). The process can be performed without the use of a carrier polymer [66] and can take profit of a subsequent treatment with glutaraldehyde vapors to improve mechanical performance. The ultimate strain of the crosslinked sample was 8.85% and the Young's modulus obtained from the linear region of the stress-strain curve 3.37 MPa. Embedding HAp particles inside electrospun nanofibers may render a low bioactivity and a decrease of mechanical properties [67].