*Nature-Inspired Processes and Structures: New Paradigms to Develop Highly Bioactive Devices… DOI: http://dx.doi.org/10.5772/intechopen.82740*

surface layer containing water molecules and relatively weakly bound ions (e.g., Ca2+, HPO4 <sup>2</sup><sup>−</sup>, CO3 <sup>2</sup><sup>−</sup>, etc.) [11] occupying non-apatitic crystallographic sites.

The hydrated surface layer is responsible for most of the properties of biomimetic apatites. The role of bone mineral in homeostasis in vivo could be explained by the high surface reactivity of biomimetic apatites in relation to surrounding fluids (which is probably directly linked to a high mobility of ionic species contained within this layer). The ions inside the hydrated surface can potentially be exchanged by other ions from the surrounding solution or by small molecules, which may be exploited for couplings with proteins or drugs. It is interesting to remark that during the aging of the nanocrystals, the typical non-apatitic features mentioned above tend to be progressive. This process that has been related to the progressive growth of apatite domains at the expense of the surface hydrated layer is called "maturation" [12].

The metastability of such poorly crystallized nonstoichiometric apatites, which steadily evolve in solution toward stoichiometry and better crystallinity, is thought to be linked to the maturation process. This evolution can be, for example, witnessed by the decrease of the amount of non-apatitic HPO4 <sup>2</sup><sup>−</sup> ions upon aging or else by the decreased potentialities to undergo ion exchanges [12].

Synthetic HA exhibits excellent biological properties such as biocompatibility, bioactivity, lack of toxicity, absence of inflammatory and immune responses, and relatively high bioresorbability. Improving their biomimetism, that is, by preparing them with dimensions, morphology, and nanostructure, can significantly enhance these properties and chemical characteristics that are similar to those found in biological apatites [9]. In the recent years, many different strategies have been employed in the preparation of synthetic nanosized HA crystals, with the most common method being stoichiometric titration of calcium hydroxide slurry with phosphoric acid up to neutrality.

Several methods have been successfully employed in the synthesis of nanocrystalline apatites, including wet chemical precipitation, sol-gel synthesis, coprecipitation, electrodeposition, vapor diffusion, and a number of others [13]. The physicochemical characterizations carried out on several synthesized apatites at low temperatures have shown that they have the typical features of biological apatite, such as the size domain, the low degree of crystallinity, and the existence of surface compositions different from the bulk [14, 15].

The method of ionic substitution has been proposed for improving not only the biomimetic features of apatite but also the biological performance of apatite-based materials. Many attempts have been made to synthesize HA that contains carbonate as a raw material for the manufacture of biomaterials. Carbonate can substitute for OH (A-type substitution) or for PO4 <sup>3</sup><sup>−</sup> (B-type substitution). A and B carbonated apatites can be distinguished by the different positions of the carbonate infrared absorption bands and by their different lattice constants. In biological apatites, CO3 <sup>2</sup><sup>−</sup> substitutes mainly for PO4 <sup>3</sup><sup>−</sup> in B-type apatite. Charge compensation by a Ca2+ vacancy, together with an H atom that bonds to a neighboring PO4 <sup>3</sup><sup>−</sup>, has been established to be the most stable arrangement. The incorporation of carbonate usually results in poorly crystalline structures with increased solubility, because it inhibits apatite crystal growth [16].

Divalent ions, such as magnesium and strontium, that replace calcium are particularly active during the first stages of the remodeling and regenerative processes. In particular, magnesium enhances skeletal metabolism and bone growth, so is associated with the first stages of new bone formation. Like carbonate, magnesium decreases with the aging of the bone and with increasing calcification. In synthetic HA, the presence of magnesium increases the chemical-physical mimesis of the mineral bone. In fact, magnesium affects the kinetics of HA nucleation on collagen,

*Bio-Inspired Technology*

**2. Biomimetic nano-apatites**

, Mg2+, K+

cations (especially Na+

embryonic (young) bone mineral crystals [10].

thetic analogues) can generally be described as

mechanisms must be taken into account.

and K+

lattice (Na+

Several solutions were introduced with increased functionality reducing energy and materials and with no impact on environment, exactly the targets faced by the actual technological challenges [1, 2]. Biomimicry has engaged several fields creating smart materials to solve those problems that nature has already solved. In that past 50 years, some examples of biologically inspired materials were developed. In particular, exploiting bioinspired technologies bone-like materials based on wood and tough ceramics based on mother-of-pearl were designed. Despite biomedical field, other kinds of materials were created such as self-cleaning structures based on flowers, underwater glues based on mussel adhesive, drag reduction based on dermal riblet on shark skin, flight mechanisms based on insect flight, etc. [3–6]. The most recent researches increasingly take inspiration from the nature trying to mimic complex behavior typical of natural structures; in particular, new synthesis methods enabling controlled crystal growth and organized structures at the multi-scale levels are paying close attention. In this way nature is studied not only to develop biomimetic material but also to mimic natural process to create new materials. A highly mimicked natural process is biomineralization useful to create biocompatible materials very close to natural tissue. Biomineralization is a natural process by which organisms form minerals and consists in a complex cascade of phenomena generating hybrid nanostructured materials hierarchically organized from the nanoscale to the macroscopic scale. This process is at the basis of load-bearing structures such as bones, shells, and exoskeletons and allows designing biocomposite with unique properties, not obtainable with any conventional approach, as the information's exchange with cells and the trigger of the bone regenerative cascade [7, 8].

In biology, calcium phosphates are the major inorganic constituents of bones, teeth, fish enameloid, deer antlers, and some species of shells [9]. Human hard tissues are composed principally of calcium phosphates with the exception of small portions of the inner ear. They are poorly crystalline carbonate-substituted nanosized apatites, with the exception of enamel, which has a high degree of crystallinity. Nanocrystalline apatites are nonstoichiometric (Ca/P ratio less than 1.67) and calcium (and OH)-deficient and may incorporate substituted ions in the crystal

is the stoichiometric hydroxyapatite phase that is the most stable and least soluble calcium phosphate at physiological conditions. The nanocrystalline apatites exhibited higher solubility compared with HA; the responsible are calcium and hydroxide deficiencies. If they are submitted to humid environment, they are able to mature; as a result, "mature" bone crystals in vertebrates are less soluble and reactive than

The chemical composition of nanocrystalline apatites differs significantly from that of HA. The global chemical composition of biological apatites (or their syn-

Ca10−x (PO4)6−x (HPO4 or CO3)x (OH or ½CO3)2−x with 0 ≤ x ≤ 2 (1)

The nanocrystalline apatites (whether biological or their synthetic analogues prepared under close-to-physiological conditions) could be probably described as the composition of an apatitic core (often nonstoichiometric) and a hydrated

Minor substitutions are also found in biological apatites that involve monovalent

, Sr2+, Zn2+, etc.), in contrast to HA [Ca10(PO4)6(OH)2], which

), for example. In this case, charge compensation

**34**

increasing it, and retards its crystallization, affecting the shape and size of mineral nuclei. The substitution of Ca2+ with Mg2+ into the HA structure leads to a continuous ion exchange from the outer hydrated layer to the well-crystallized apatite lattice, inducing a disordered state on the HA surface. Moreover, the incorporation of magnesium in surface crystal sites increases the number of molecular layers of coordinated water; all of these phenomena favor the adhesion of cells to the scaffold because the protein adsorption is increased. A greater osteoconductivity over time and higher material resorption, compared to stoichiometric HA, were detected in granulated Mg-HA powders that were implanted in a rabbit's femur, proving the increase of osteogenic activity in the presence of magnesium-substituted HA. A higher expression of specific markers of osteoblast differentiation and bone formation, which are associated with a lower osteoclastogenic potential, was revealed by studies of osteoblast gene expression profiles from Mg-HA grafts [17, 18].

The incorporation of strontium into the HA structure reduces bone resorption while enhancing osteogenesis; this effect improves physical stabilization of the new bone matrix, enhancing collagen synthesis, as shown in in vitro and in vivo studies. The incorporation of strontium ions into the HA lattice has been practiced in recent years, due to its potential as an anti-osteoporotic agent, and increasing effort is being dedicated to the development of strontium-containing bone cements [19].

Biomimetic HA powders can be synthesized and used as granules to fill bone defects of limited size, but if the regeneration of an extended bone part is necessary, the implantation of a 3D porous scaffold is required because the lack of mechanical stability and specific morphology of granulated bio-devices does not enable regeneration of extended bone segments; therefore, the porous scaffold must have, in addition to bioactivity and osteoconductivity characteristics, also biomechanical performance suitable for the specific implant site. The scaffolds must provide both the space for the new bone formation and the necessary support for the cells to proliferate and maintain their differential function. Furthermore, they should present suitable architectures for inducing the formation and maturation of well-organized tissue. The use of bioactive scaffolds aids the process of osteoconductivity that establish physical and mechanical integration with the surrounding bone, which in turn avoids micro-movements and the possibility of early mechanical loading in vivo [20].
