Calcium phosphates of synthetic and natural origin: current status and future challenges

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Background

Bone joint defects/diseases represent a difficult and frequent clinical problem in medicine and are mainly caused by age, trauma, infections, tumor resection, or congenital/hereditary disorders. The current increase in life expectancy and the expanded frequency of injuries and diseases are considered the most important causes for the alarmingly large demand for orthopaedic endo-prostheses.

Although exhibiting excellent mechanical properties, low weight, high corrosion resistance, and good biocompatibility properties, Ti implants still require improvement in terms of osseointegration rate. In good congruence with this trend, the scientific community strives to explore innovative biomaterials with improved performances, capable to prevent failure and prolong the lifetime of osseous implants. Nowadays, research efforts are focused on the optimization of human body (“host”) ‒ implanted biomaterials interface, aiming to design reliable platforms for the development of bone implants. These are characterized by a rapid osseointegration rate and enhanced long-term functionality, in which the implants’ surface properties play a key role. Therefore, intentional implant changes in terms of morphology (i.e., roughness), chemistry, structure, and/or surface energy are usually applied to attain a strong and intimate bone-implant interface.

Certain materials in the calcium phosphate (CaP)-family are renowned for their excellent ability to induce cells differentiation towards an osteoproductive phenotype. When such capability is coupled with suitable degradation rate, the prospect to be timely replaced by the newly-formed bone tissue becomes feasible. Hydroxyapatite (HA) is one of the most investigated CaPs for osseointegrative applications due to its excellent biocompatibility, high biomineralization capacity, controlled degradation speed and good osteoconductivity. HA is widely produced by chemical protocols (i.e., hydrothermal synthesis, co-precipitation), employing various calcium and phosphorous commercially available reagents. In the last years, because of the demographic expansion and rapid economic growth, the search for alternative, sustainable solutions for the production of CaPs was also encouraged. Thus, CaPs can be obtained also by extraction from some important primary natural reservoirs: (i) biogenic products or organisms from the aquatic environment (e.g., egg-shells, corals, various types of sea-shells or fish bones), or (ii) terrestrial organisms (e.g., mammalian or bird bones.). Biological HA (BioHA), in comparison to synthetic one, is known to contain an array of trace-elements (e.g., Na, Mg, Sr, K) with definite bio-functional roles as well as an improved mechanical performance. In addition, BioHA demonstrated a high metabolic activity, a much more dynamic response to the environment, and less intense inflammatory reactions.

As the population ages, the number of bone surgeries is constantly increasing. There is still a great number of medical implants which undergo detachment from the host tissue because of inadequate biocompatibility and poor osseointegration. Therefore, the surface properties have a critical impact on the success or failure of an implant.

The purpose of surface modification is to keep the key properties of the bulk material while modifying just its surface behavior. Usually, modifications can either chemically or physically alter the existing surface, or completely change it by coating with either nanoparticles, or thin uniform films. Therefore, new solutions for modification of chemistry or physical topography of implants surface are constantly looked for to improve biocompatibility and reduce healing time.

The surface functionalization of implants represents an advanced development in implantology. CaPs of both synthetic and natural origin are the subject of recent researches with the aim to improve the biointegration. HA is one of the most well-known and studied CaPs, used as implantable bioceramic due to its chemical and structural similarity with human hard tissues. Bulk HA ceramics are brittle and exhibit poor mechanical properties, therefore, they cannot be used in orthopedic devices that must withstand substantial loadings. However, HA can be used in the form of nanoparticles or thin films on implants or areas where no loads are applied.

The surface modification of orthopaedic and dental implants with either nanoparticles or thin layers of CaP-based bioactive ceramics is expected to foster the development of a new generation of osseous implants, which will both (i) harmoniously couple the excellent mechanical properties of the substrate with the capacity of CaPs to induce fast osseointegration rates and (ii) reduce the risk of bio-functional coating mechanical failure reported for the commercially available thick layers (hundreds of micrometers to millimeters range). Moreover, different solutions to further improve the mechanical and biological properties of synthetic and biological CaP structures are being investigated, either by doping with different concentrations of ions, or by using alternative deposition techniques to plasma spray technology.

In particular, the topics of interest of this Research Topic include, but are not limited to:

• alternative methods used for the fabrication/extraction and characterization of synthetic and biological-derived calcium phosphates;
• doping of synthetic and biological-derived calcium phosphates;
• composite synthetic and biological-derived calcium phosphates;
• synthetic and biological-derived calcium phosphates (powders, nanoparticles and thin films) for orthopaedic and dental implants;
• in vitro and in vivo assessment of synthetic and biological-derived calcium phosphates;
• various applications of synthetic and biological-derived calcium phosphates.

Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 300 words) can be sent to the Editorial Office for announcement on this website.

Keywords: calcium phosphates, bioactive ceramics, biomaterials, nanoparticles, orthopedic impants

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