# Biodegradable and Biocompatible Systems Based on Hydroxapatite Nanoparticles
## 5 Conclusions  
The use of HAp nanoparticles for the development of new biodegradable and biocompatible systems is a rich and active field of research. HAp adsorption and encapsulation capabilities of ions, molecules and biopolymers are of outmost interest as HAp is a highly biocompatible substrate. On the other hand, HAp by itself can be part of biodegradable and biocompatible systems acting as a secondary agent that contributes decisively to provide some added value properties to the main  
Figure 15: Scheme showing the targeted antitumor mechanism of HAP-HA nanoparticles loaded with DOX. Mechanism of drug delivery into the cell nuclei and mitochondria. Adapted from [168], Elsevier, 2016.  
system. Furthermore, the possibility to act as a \"Trojan Horse\" when delivering drugs or genes to cells completes the extraordinary features of HAp.  
This review has summarized representative research branches where HAp plays a relevant role. First, biocomposites developed in combination with chitosan derivatives, are reported to enhance bone regeneration where HAp contributes improving the mechanical properties. Second, technologies such electrospinning demonstrate the huge range of biocompatible systems affordable by adjusting the working parameters. Moreover, electrospinning technique allows controlling the performance of the polymeric fibers that contain HAp that even can include bioactive substances. Third, considering the relevance of the fight against biofilm formation on the surface of implanted medical devices, any antimicrobial strategy that can bring long term protection from biofilm formation is attractive and should be explored. Therefore, HAp is a polyvalent substrate that can incorporate both ions and molecules with antimicrobial activity that can be applied as a coating of such implants. Fourth, it cannot be overestimated the impact of imaging technologies in the medical diagnostic field. We can only expect an exponential development of such techniques. Biocompatible nanosubstrates, such HAp, in combination with biomarkers are good candidates for exploring the root cause and the development of diseases. Fifth, even though gene therapy is still under development, non-viral transfecting agents based on HAp are receiving more attention due to the lack of secondary effects. Intense research to increase the transfection efficiency is still fruitful but results are not completely satisfactory. The study of molecular mechanisms related to dissolution and transfection using HAp will decisively contribute to gene therapy development. Finally, controlled drug delivery systems are expected to open new therapies by means of delivering drugs after crossing biological barriers and attacking the core of the disease. The capability of HAp of masking biomolecules converts it into an excellent \"Trojan Horse\" that can deliver the drug just onto the target.  
HAp attracts researchers from several fields, for that reason, a common platform of knowledge is necessary to be developed. This review contributes to such multidisciplinary approach to enlightening the HAp performance in the field of biodegradable and biocompatible systems.  
The authors acknowledge support from MINECO and FEDER (MAT2015-69547-R and MAT2015-69367-R), and the Generalitat de Catalunya (2014 SGR188).  
[PERSON], [PERSON], [PERSON], and [PERSON] equally contributed to the development of the review.  
The authors declare no conflict of interest.