ReviewSynthesis, biological and environmental applications of hydroxyapatite and its composites with organic and inorganic coatings
Introduction
Biomaterials have been attaining rising significance due to the desirable traits that they possess for the fabrication of novel implantable devices. As biocompatibility comes to the forefront of medicine and environmental conservation, the quantum of research for producing novel biomaterials and enhancing the features of the prevailing ones, is surging ahead in recent years. A novel biomaterial restores, repairs or replaces the injured tissue by direct incorporation into the that problem-laden organ with an aim of hauling up life expectancy [1]. In terms of properties, biomaterials are innumerable and hence, can be precisely tailored as per the demands of an application. The said properties comprise the physical, chemical, mechanical and structural ones.
The phenomenon of biomineralization facilitates the crystallization and deposition of a variety of minerals, in in vivo environment [2,3]. These deposits, say, calcium phosphate, can be both normal and abnormal. The former includes teeth and bones in vertebrates, while latter manifests as calculus, stones (both urinal & dental), and lesions associated with atherosclerosis [4,5]. Calcium phosphate, ideally represented as Ca10(PO4)6(OH)2 fits in to the apatite family [6]. There is an increasing demand for bone grafts owing to unfortunate accidents and diseases. Orthopaedics faces a common, but, serious challenge in healing such defects. If the patient happens to be younger, the implant that is to be incorporated into the body has to highly biocompatible with a considerably high lifespan. Thus arises the continuous need for ideal implants which also must be endowed with special properties like they being antimicrobial, antibiofilm, zero-cytotoxic, porous, hydrophilic and mechanically strong.
Hydroxyapatite (HAP) is one of the most common forms of calcium phosphate. In nature, bones are structurally complex formations comprising HAP to the highest extent of 70 %. Fibrillar collagen (Type I) accounts up to 20 % and the rest is water [4,7,8]. The form of apatite which is short of OH– ions is biologically predominant, especially in mammals. The other variant of apatite is rich in CO32– ions [7,9,10]. Synthetic HAP resembles the formula and features of its natural inorganic counterpart in living organisms. Synthetic HAP is a substance of variable properties as decided by its synthetic technique. HAP and HAP coated with various organic/inorganic materials find applications in environmental as well as biomedical domain [11]. High surface area, high porosity, crystalline nature and charged surfaces of these compounds make them suitable for use in adsorption and catalytic degradation of various pollutants [12].
HAP-containing biomaterials are continuously being tested and evaluated, to be used as bone graft, either wholly or just as a peripheral coat on the implants. The desirable features of synthetic HAP and its doped versions include high biological activity and biocompatibility. If it were used a drug carrier, they have to release the drug in a very sustained and controlled manner to prolong the expected therapeutic effects in the target cells of the host [13]. It can incorporate well with the natural bone devoid of any adverse toxicity, immune or inflammatory response [14]. Additionally, from an orthopaedic standpoint, the material must be osteoinductive, osteoconductive and osteointegrative [13]. The unique selling proportion of HAP is that it can accept numerous cations and anions as substituents, essentially becoming a solid solution. During the synthesis of HAP, the molar ratios of the other special substituents (like proteins, drugs, metal ions) can be varied giving rise to a multitude of options to suit their corresponding biomedical applications [6].
The current work reviews the contemporary methods and experimental studies employed in the synthesis of HAP and its diverse composites, aimed for terminal use in environmental and biomedical fields. It must, however, be noted that HAP has applications in diverse other fields also, as in, to fabricate fuel cells [15], fluorescent lamps [16] and adsorption matrices for toxic metals and radioactive wastes [17]. Primary of biomedical applications include surgical implants (including dental), fillers, scaffolds, bone augmentation and drug delivery vectors. The authors aim to offer an insight into the diverse synthetic approaches for the synthesis of HAP and HAP-containing composites, in the first section of the review. The second segment of the review details the various environmental and biomedical applications of HAP and its composites [6,18].
Section snippets
Various synthesis methods of hydroxyapatite
HAP can be synthesized by various methods such as dry methods, wet methods and high-temperature methods using chemical precursors and natural resources [19]. Each method yields HAP of different sizes, morphology, crystallinity and purity, depending on the conditions used during the synthesis process. Therefore, the properties of HAP substantially influence the biocompatibility, mechanical strength, and biological properties [20]. By properly manipulating the operating parameters, it is possible
Synthesis of HAP composites
In order to enhance the mechanical strength, biocompatibility and bioactivity or to provide some specific features, HAP is coated or doped with natural organic (chitosan, collagen, gelatin, fibrin, alginate, agarose and cellulose) or inorganic compounds (Fe, Pb, Zn and Ce) [70]. The same methods outlined in the above sections are used to synthesize HAP composites.
Removal of dyes using HAP/HAP composites
Rapid urbanization and industrialization led to the contamination of water bodies with a variety of pollutants which is a serious environmental concern. Among them, synthetic dyestuffs are the significant water pollutants which exists in the effluents of various industries like textiles, leather, paint, paper, food, and pharmaceuticals [91,92]. A considerable amount of synthetic dyes such as Acid Blue 113 (AB 113), Crystal Violet (CV), Congo Red (CR), Eriochrome Black T (EBT), Methyl Orange
Removal of heavy metals using HAP/HAP composites
With the rapid growth and development of industries, there is increased use of heavy metals resulted in the accumulation of metallic substances such as arsenic, cadmium, chromium, and lead in the aquatic environment [103]. Heavy metals are entering into surface and groundwater due to various industrial processes such as mining, electroplating, battery production, refining, and nuclear power plant [34,53,104]. They are non-biodegradable and toxic to living systems even at their low concentration
Applications of HAP/HAP composites as antibacterial agents
Bacterial infections are a major roadblock towards an effective treatment after an invasive surgery. The bacteria may gain direct entry onto the implant surfaces when the tissue/organ is grafted. When the immune machinery of the host fails to shield the surgical implantation, infection sets in the vicinity of the site of injury and leads to the failure of the surgery. It has been documented that implant-linked infection accounts for 5% and 6% in primary and revision cases, respectively. It
Applications of HAP/HAP composites in drug delivery
In spite of the radical developments in surgical approaches and antimicrobials, the leaky control of chronic osteomyelitis, specifically that caused by S. aureus resistant to methicillin (MRSA) and oxacillin, poses a grim reminder that more sustainable solutions need to be conceived [131,132]. There are numerous approaches used by doctors in the management of osteomyelitis. Intensive debridement at the local site and usage of antimicrobials are the most conventional. Wang, Calhoun, and Mader [
HAP coatings with anti-biofouling applications
Dental implants based on HAP are used during their incorporation in engineered bones and teeth. The main challenge in designing such constructs is the colonization of artificial surfaces by bacteria for a prolonged time. In order to prevent this, the HAP-based polymers or composites must be inherently antimicrobial or a suitable coating must be applied over such implants that can stand the test of time. The section describes a few research attempts to develop effective polymer coatings that
Conclusion
Hydroxyapatite, a vital component of vertebrate bone minerals is fabricated under controlled conditions, as it finds a multitude of crucial applications. The present review sums up the recent methods used for the synthesis of HAP and its composites from natural sources as well as pure forms of equivalent chemical compounds. There was substantial literature to prove that the ultimate characteristics of the composites depend on the source of phosphorus/calcium and the synthetic approach. The
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors wish to express their gratitude for the support extended by the authorities of Manipal Institute of Technology (MIT), Manipal Academy of Higher Education (MAHE), Manipal, Karnataka, India, in carrying out this work in Department of Chemical Engineering.
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