Composites of hydroxyapatite (HAp) are widely employed in biomedical applications due to their biocompatibility, bioactivity and osteoconductivity properties. In fact, the development of industrially scalable hybrids at low cost and high efficiency has a great impact, for example, on bone tissue engineering applications and even as drug delivery systems. New nanocomposites constituted by HAp nanoparticles and synthetic or natural polymers with biodegradable and biocompatible characteristics have constantly been developed and extensive works have been published concerning their applications. The present review is mainly focused on both the capability of HAp nanoparticles to encapsulate diverse compounds as well as the preparation methods of scaffolds incorporating HAp. Attention has also been paid to the recent developments on antimicrobial scaffolds, bioactive membranes, magnetic scaffolds, in vivo imaging systems, hydrogels and coatings that made use of HAp nanoparticles.

Hydroxyapatite (HAp) particles with very different surface charges and compositions (i.e. different Ca/P and CO₃ ²⁻/PO₄ ³⁻ ratios) have been obtained by varying the experimental conditions used during the chemical precipitation process. The DNA adsorption capacity and protection imparted against the attack of nucleases of HAp particles have been proved to depend on the surface charge while the buffering capacity is affected by the chemical composition. On the basis of both the surface charge and the crystallinity, the predominant planes at the surfaces of HAp particles have been identified. Atomistic molecular dynamics simulations of surfaces constructed with these planes (i.e. (001) and the two terminations of (010)) with the adsorbed B-DNA double helix have been performed to get microscopic understanding of the influence of the mineral in the biomolecule structure and the interaction energies. The results indicate that the DNA secondary structure is perfectly preserved on the (001) surface, this stability being accompanied by an attractive binding energy. In contrast, the (010) surface with PO₄ ³⁻, OH⁻ and Ca²⁺ ions in the termination induces significant local and global deformations in the double helix, repulsive OH⁻(HAp)⋯PO₄ ³⁻ (DNA) interactions provoking the desorption of the biomolecule. Finally, although the termination of the (010) surface with PO₄ ³⁻ and Ca²⁺ ions also deforms the double helix, it forms very strong attractive interactions with the biomolecule. These binding characteristics are in excellent agreement with the DNA adsorption and protection abilities experimentally determined for the HAp samples. Finally, the surface charge has been found less decisive than the chemical composition in the efficacy of the transfection process.

Encapsulation of DNA into hydroxyapatite (HAp) has been investigated using a rational approach that involves computer simulation and experimental techniques. The temporal evolution of the radial distribution functions derived from atomistic molecular dynamics simulations of Ca²⁺, PO₄ ³⁻ and OH⁻-containing aqueous solutions in the presence and absence of B-DNA has been used to conclude that the backbone of the double helix acts as a template for HAp growth. More specifically, results reveal the formation of calcium phosphate clusters at the first stages of the simulations, which subsequently re-organize to nucleate HAp. This effect is produced in the absence and, especially, presence, of DNA indicating that the biomolecules do not inhibit but even promote mineral growth. Furthermore, computer simulations suggest that the diffusion of the OH⁻ anions through the inorganic solution is the limiting step for the nucleation of the biomineral. Nanocapsules and crystalline nanorods of HAp containing DNA molecules inside have been prepared by mixing solutions containing Ca²⁺ and PO₄ ³⁻ ions with fish sperm DNA at high pH. The dimensions and morphology of such nanostructures have been examined by transmission electron microscopy, while the characterization of the biomineral has been focused on the identification of DNA inside HAp using infrared, X-ray photoelectron and UV-vis spectroscopies, as well as gel electrophoresis. The biominerals reported in this work are important for biomedical applications requiring the protection of DNA from aggressive environmental conditions.

Herein, a versatile bilayer system, composed by a polypropylene (PP) mesh and a covalently bonded poly(N-isopropylacrylamide) (PNIPAAm) hydrogel, is reported. The cell adhesion mechanism was successfully modulated by controlling the architecture of the hydrogel in terms of duration of PNIPAAm grafting time, crosslinker content, and temperature of material exposure in PBS solutions (below and above the LCST of PNIPAAm). The best in vitro results with fibroblast (COS-1) and epithelial (MCF-7) cells was obtained with a mesh modified with a porous iPP-g-PNIPAAm bilayer system, prepared via PNIPAAm grafting for 2 h at the lowest N,N′-methylene bis(acrylamide) (MBA) concentration (1 mM). Under these conditions, the detachment of the fibroblast-like cells was 50% lower than that of the control, after 7 days of cell incubation, which represents a high de-adhesion of cells in a short period. Moreover, the whole system showed excellent stability in dry or wet media, proving that the thermosensitive hydrogel was well adhered to the polymer surface, after PP fibre activation by cold plasma. This study provides new insights on the development of anti-adherent meshes for abdominal hernia repair.